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‫ا‪.‬د‪ .‬سطوحي أحمد سطوحي محمد‬
‫استاذ صحة الحيوان والبيئة‬
‫عميد الطب البيطري‪-‬الوادي الجديد‬
Epidemiology
 Is
the study of disease in
population and factors that
determine its occurrence and
spreading.
 Epizoology
 Epornities
For simplicity,
Epidemiological and Clinical
approach
It is useful to distinguish
epidemiological from
clinical approaches to
disease management:-
Clinical approach: is focused on individual
animal and is aimed to diagnose a disease
and then treating it. This involves: physical examination
 differential diagnosis
 lab. Tests.
 Possibly response to treatment
to narrow the list of differential diagnosis to a
single diagnosis.
 Epidemiological
approach: It is
based on observing differences and
similarities between diseased and
non-diseased animals in order to
understand what factors may be
increasing or reducing the risk of
disease.
Role of epidemiological and/or
clinical approach

In practice, clinicians unwittingly use a
combination of clinical and epidemiological
approaches in their day-to-day work. If the
problem is relatively clear-cut then an
epidemiological approach plays a very minor role.
If the condition is new or more complex then the
epidemiological approach is preferred since it will
provide a better understanding of what makes
individual susceptible to disease and once these
factors are known-the measures required to control
the disease become better defined.
Aim of studying epidemiology
Prevention and controlling
epidemics through studying
the causative agents and
factors influencing their
infectivity,
resistance,
pathogenicity, as well as
their spreading.
Objects (Uses) of epidemiology
1.
2.
Determine the origin of
disease whose its cause is
known
Investigation and control of
diseases whose its cause
initially unknown.
3-Collection of information on
the ecology and natural
history of the disease.
4-Planning and monitoring of
disease control programs.
5-Evaluation of the economic
effects of a disease and its
control.
1- Determination of disease
origin which its cause is known
Diagnosis of any disease depends on:1. Clinical symptoms
2. Lab. Tests
During investigation the epidemiologists
has to get answer on the questions:1. Why the outbreak is occurred?
2. Why the No. of cases get increased?
For example:
cases of actinobacillosis could be
increased in a group of cattle after
grazing a thorny materials. This
grazing is usually associated with
an increase abrasions in the buccal
mucosa, which could increase the
animal
susceptibility
to
actinobacillus ligniresi infection.
2- Investigation and control of a
disease whose its cause initially is
unknown
There are some examples of disease
control based on epidemiological
approaches before the causative
agent has been identified:1. CBPP, was eradicated from USA
before Mycoplasma mycoides has
been isolated.
2. Rinderpest: slaughter policy was
applied to control the disease before
its cause has been identified.
3-Getting information on the ecology
and natural history of a disease
Ecology: is the study of ecosystem:-
nature of the disease-producing agent.
B. Environmental factors affect the agent.
The environment may affects survival
rate of the infectious agents and their
hosts.
Study of ecosystem is very important
during the disease control programs
A.
Example: The causative agent of
liptospirosis (L.interorgans), is
maintained in rats. So, any
control programs should directed
to eradicate the rats.
 Insects, ticks, other arthropods,
snails.
4- Planning and monitoring a
disease control program
Controlling of a disease in population
must based on:1. The No. of animals get diseased.
2. Factors associated with its
occurrence.
3. Facilities required to control the
disease.
4. The costs and benefits involved.
To control the disease, routine
collection of data is of urgent
need to decide which strategies
are being successful. The plan
depends on the nature of
causative agent and method of
spread
Example
FMD and air:During spreading of FMD in UK on
1968, the epidemiologists notes the
importance of wind-born virus
particles in the disease transmission.
Thus restriction of animal movement
was one of the important points to
facilitate disease control.
5- Evaluation of the economic effects
of a disease and its control
The cost of disease control should be
balanced against the economic loss
due to the disease. So, economic
analysis is required.
From epidemiological point of view, it
may be economic to reduce high level
of a disease in a herd and uneconomic
to reduce low level of a disease.
Example
If 15% of cows in a herd are affected by
mastitis, productivity will be severely
affected and a control program will be
likely economic.
On the other hand, if less than 1% of a
herd is affected, the cost of further
reduction of the disease may not
result in a sufficient increase in the
milk productivity.
Types of epidemiological
investigation
There are 4 approaches to
epidemiological investigation:-
1.

Descriptive epidemiology:- It is
usually the first part of any
investigation. Observing and recording
the disease. It means observation in
the field in which the following
questions should be answered:What is the event or phenomenon
(disease).
Which animals are involved (distribution).
 When does phenomenon take place
(time).
 Where does it takes place (place
distribution)?.
 How and why did it occur?.
Several data are collected from observation,
which lead to hypothesis on the disease
event and its possible determinants.

2- Analytical epidemiology
It means analysis of observations obtained
using diagnostic and statistical tests.
Specific mathematical methods are used
including :A. Determining strength of epidemiological
association.
B. Determining
importance
of
epidemiological association.
C. Determining statistical significance of
association. eg. Chi-square test.
Final goal is controlling disease in the
present time and preventing its reoccurrence in the future.
3-Experimental epidemiology
Designing specific population experiment
to test epidemiological hypothesis.
Epidemiological experiments include
vaccine trials, drug trials and various
clinical trials.
In many cases, veterinary research is
proceed directly from descriptive to
experimental stage without quantitative
analysis of naturally occurring disease.
4- Theoretical epidemiology
It consists of representation of
the
disease
using
mathematical “models” that
attempts to simulate natural
patterns
of
disease
occurrence.
Components of epidemiology
The first stage in any investigation is
the collection of relevant data. The
main aim of this stage is to
formulate causal hypothesis. The
ultimate goal is to control the
disease, reduce productivity losses
and to improve animal welfare.
Investigation may be:-
1.




Qualitative investigation: Ecology of
disease should including:Disease distribution
Mode of transmission
Maintenance of the causative agent
Factors that directly or indirectly
cause the disease.
2- Quantitative investigation:-
This means collection of data
concerning the No. of cases get
diseased to analyses these data
and get numerical values.
This includes surveys, monitoring,
modeling
and
biological
and
economic evaluation of disease
control.
Patterns of disease occurrence
There are 4 commonly
recognized patterns of disease
in population:-
1- Sporadic disease
Sporadic disease characterized by:1. Rarely
2. Irregularly
3. Haphazard
4. Usually producing small localized
outbreaks.
So, it is usually a single case or a cluster
of cases in an area not infected before
or rarely infected.
Example: case of rabies was recorded in
1969 in UK after 6 months
quarantined.
Scientists refers this type of disease
occurrence to the following:1. Infection
is
in-apparently
and
occasionally some animals show
signs of the disease.
2. Infection is almost absent and disease
is only noted after admission of
diseased one to the area.
3. Infection
may be maintained in
another
animal
species
and
occasionally
interspecies
2- Endemic diseases
This type is characterized by:1. Frequently occurs in a population.
2. Constant present in a population.
3. It occurs with a predictable
regularity in a population with a
relatively minor fluctuation over
time.
When describing the disease,
epidemiologist should
specified:1. The affected population.
2. The disease location.
According to the proportion of the affected
animals, the frequencies of endemic
disease may be:a) Holo-endemic: if most animals are
infected
b) Hyper-endemic: if high % of animals are
infected
c) Meso-endemic: if moderate % of animals
get infected.
d) Hypo-endemic: if small % of animals are
3-Epidemic diseases

1.
2.
3.
The disease is Ch. by:Sudden onset
Unexacting (unpredictable)
High frequencies.

In modern epidemiology,
Epidemic disease: is a disease occurs in
excess of the expected level.
It occurs when the population have been
subjected to one or more factors that
are not present previously. These
factors work together to break down
natural barriers of the body.
4- Pandemic disease
It is a wide spread epidemic that usually
affects large populations and many
countries may be affected. Theses
diseases usually cause high financial
losses.
Examples: Rinderpest, FMD, SAHS (Animal
diseases).
Avian influenza, SARS, Human plague,
Cholera.
5- Other disease patterns
Short term patterns (Diurnal):Appearance of these diseases related to
some events.
Examples:
 Some chicken diseases related to
photoperiod
 Diseases related to vector availability
during night period.
A.
B- Seasonal patterns
 Most
of vector-borne diseases
 Poisonous plants
 Calf mortality.
C- Cyclic patterns
 Diseases
rise and decline with a
fairly constant periodicity for some
years. This may be due to
fluctuation in herd immunity or
factors related to the agent itself
or its reservoir.
Example: Rift valley fever
D- Secular changes
 Diseases
occur gradually over
long period of time.
epidemic curves
This curve consists of a bar chart showing
time on x axis and the number of new cases
on the y-axis. The shape of this curve can
provide important information about the
nature of the disease under investigation.
An epidemic occurs when there is a rapid
increase (usually heralded ‫ )نذير‬by an
exponential rise in the number of cases in
time) in the level of disease in a population
followed by a subsequent decline as
susceptible animals are exhausted.
This epidemic usually arise from the
introduction of a novel pathogen (or
strain) to a previously unexposed
population. So, epidemics may be
described as being either:1- Common source epidemic.
2- propagating epidemic.
1-Common source epidemic
 When
a large No. of animals
are infected at a given time.
This occurs when these
animals are exposed to a
common source of infection
such as contamination of
water, food, air, fomites, etc.
The epidemic of leukemia cases in
Hiroshima following the atomic bomb
blast would be a good example of a
common point source epidemic. The
shape of this curve rises rapidly and
contains a definite peak at the top,
followed by a gradual decline. The down
slope of the curve may be very sharp if
the common source is removed or
gradual if the outbreak is allowed to
exhaust itself.
2-Propagating epidemic
occurs when a case of disease serve
as a source of infection for
subsequent
cases
and
those
subsequent cases, in turn, serve as
sources for later cases.
 For simplicity, initial or primary
cases excrete infectious agents thus
infect, directly or indirectly other
susceptible
individuals
which
constitute secondary cases


1.
2.
3.
4.
The No. of the affected animals is
increased gradually over a specific time.
It depends on:Mode of transmission.
Infectivity and free living longevity of the
agent (survival rate).
Social distance between susceptible
hosts.
Proportion of susceptible and infected
animals in the population.

Theoretically, the epidemic curve of a
propagated epidemic has a successive
series of peaks reflecting increasing
number of cases in each generation.
The epidemic usually wanes (‫)يتضائل‬
after a few generations, either because
the number of susceptible falls below a
critical level, or because intervention (
‫ )التدخل‬measures become effective.

Usually the epidemic started as
a common (point) source, then
took on the characteristics of a
propagative epidemic over
time.
Factors affecting shape of epidemic
curve
Shape of the curve and time scale
depends on:1. Incubation period of the disease
2. Infectivity of the agent.
3. Proportion of susceptible animals in
the population.
4. Distance between animals (animal
density).
So,
A highly infectious agent, with a
short incubation period,
infecting an overcrowding
susceptible population,
producing a sharp rising in a
relatively small time scale.
As epidemic proceed,
The proportion of susceptibility
decrease either as a result of:1. Death of infected animals
2. Increasing immunity following
infection.
So, epidemics can’t continue because
there are insufficient susceptible
animals available for infection
Factors affecting pattern of
infection
The outcome of the detectable infections
depends on:1.
Pathogenicity and virulence of the M.Os.
Pathogenicity means the ability of DPA to set up
specific pathogenic effects (induce a disease
or morbid state). This term is used to
describe the whole species or a group of
M.Os.
Virulence is not different from pathogenicity but
it used to point out the differences in
pathogenicity within a group.

Pathogenicity and virulence

1.
2.
Are measured by:The ratio of clinical: sub-clinical
cases: The higher pathogenicity, the
higher clinical cases than the subclinical cases.
Case mortality: No. of deaths from
certain disease as a percentage of the
No. of cases. A higher virulence, a
higher mortality rate.
Factors affecting pattern of
infection
Antigenic power of the M.Os: This means
ability of M.Os to initiate antibodies.
The antigenicity could be measured by:A. The second attack frequency: this means
how frequent one gets two or more attacks
of a certain disease. The more frequency
of re-infection with a M.Os, the lower
antigenic power of it.
B. Age specific attack rate: The attack rate of
most M.Os is normally drop by age
specially for higher antigenic power M.Os
(from young to adolescence to old age).
2.
Factors affecting pattern of
infection
Duration of infectious state
(period of communicability):
This means the period of time
during which the DPA may be
transferred directly or indirectly
from an infected animals to
susceptible one.
3.
Factors affecting pattern of
infection
Ease of communicability of the
disease:
For disease transmitted directly, droplet
nuclei is a measure of
its
communicability within a restricted
population and is called as second
attack rate (S.A.R.). This rate is of
great importance in studying the
outbreaks and is expressed as follows:4.
S.A.R.
NO. of cases in an outbreak-1ry (index) case (s)

S.A.R=________________________________
Total No of susceptible animals- 1ry case
(s)
S.A.R=No of Secondary cases
__________________
No. of susceptible
S.A.R.
Example: in a flock of 100 animals, one gets the
infection of x-disease. Two weeks later, another
30 animals showed the disease. The Veterinary
records showed that 39 animals had the disease
before. Calculate the S.A.R.
S.A.R=
30
_____________________X= 50%
100-(39+1)

Disease occurrence
(Host, agent, and environment)
Occurrence of the disease in an
individual depends on interplay
of three factors:-
Host
 The
host: Is the animal (or
human) that may attract a
disease. Age, genetic makeup,
level of exposure, and state of
health all influence a host’s
susceptibility to developing
disease.
The agent
 is
the factor that cause the
disease
(bacteria,
virus,
parasite,
fungus,
chemical
poison, nutritional deficiencies,
etc)- one ore more agents may
be involved.
The environment

includes surroundings and conditions
either within the host or external to it,
which cause or allow disease
transmission
to
occur.
The
environment may weaken the host and
increase its susceptibility to disease or
provide conditions that favour the
survival of the agent.
Is the level of the disease the
same in all populations?
The level of disease in a
population depends on
interplay of three factors:
Individual factors
Individual factors: What types of
individuals susceptible to infection
Susceptibility depend on: age, sex, breed, coat colour and so
on.
So, determination of the influence of
individual characteristics on the risk of
disease is of great importance.

For example, it was recorded that the
mortality rate for drowning varied
among children where the rate was
highest in those aged 1-4 years: an age
when children are mobile and curious
about everything around them, even
though they don’t understand the
hazards of deep water or how to survive
if they fall.
Place (Spatial factors)

The spatial pattern of disease is
typically a consequence of
environmental factors. Environmental
factors include aspects of climate
(Temperature, humidity, rainfall) as well
as aspects of animal management
(management of animals in certain area
of a country may result in high rates of
disease that may not be seen in other
areas).

Examples, geographical distribution of
BSE incidence risk in British cattle
revealed that the amount and type of
concentrate feeds fed to cattle is
thought to have been responsible for
high density of disease in the south of
the country, compared with the north.
Time (Temporal factors)
How does disease frequency change
over time, and what other factors are
associated with these changes? We
have to distinguish between animal
referent time and calendar time:

Animal referent time: refers to the
timing of disease in relation to defined
events that occur during an animal
lifetime. For example, increased risk
of milk fever during the first 7 days of
lactation. Here, time is measured in
relation to a calving event.
Calendar time: refer to the absolute
timing of events. We may talk of
the number of milk fever cases
that occur in August, and
compare those numbers with the
number that occur in December.
Causation of diseases
A.
1.
2.
Some technical terms:Infection: Invasion of a host (living
organism) by a disease producing
agent (DPA).
Infectivity: ability of DPA to establish
itself in a host (ID 50=No. of DPA
required to infect 50% of susceptible
host under controlled conditions.
Some technical terms
C.
Pathogencity: ability of
the agent (DPA) to
produce the disease in a
range of hosts under a
range of environmental
conditions.
Some technical terms
 Virulence:
It is the measure of a
disease-severity caused by a
specific agent (DPA). it is
commonly quantified using LD50
(i.e. No. of agent required to kill
50% of exposed susceptible host
under controlled conditions).
Carrier state

A host capable to disseminate the
agent but typically does not show
evidence of clinical disease. If it
is really infected, called truecarrier state. There are many
carriers named according to the
stage of the disease:-
Types of the carriers
I.
II.
Incubatory carrier: A host get
infected but in the pre-clinical
stage.
Convalescent carrier: a host
get infected but in postclinical stage
Antigenic variation
Biological situations where an agent
evade the host defenses by
changing its antigenic
characteristics. Example,
influanza virus can change its
antigenic structure to evade the
immune system of the body.
Periods of infection
1.
2.
3.
Pre-patent period: time between
infection and first detection of the
agent.
Incubation period: time between
infection and fist appearance of the
clinical signs.
Communicability period: time during
which the infected host is capable to
transmit the agent to susceptible
hosts
Periods of communicability
Incubation period
1st detection of
agent
Infection
Pre-patent
period
Convalescent carrier
Symptoms
Symptoms
disappearance
No M.Os
Disease determinants
 All
factors that contributes to
disease
occurrence.
Some
diseases are produced by
multiple
factors
called
collectively “ Determinants”
 General
classification
of
determinants:-
A- primary causes
 This
is a specific factor and the
disease not occur without it
B- Secondary causes
 Non
specific factor and play
an important role in the
disease occurrence. It is
called contributory or
predisposing factor. Its
importance vary widely from
disease to the other.
I- Primary determinants

1.
A.






It is classified to main major parts:Intrinsic factors: The causative agent is an
integral part of the host itself. It includes the
following:Hereditary or gene transmitting diseases as:Bull dog calves.
Umbilical hernia in calves.
Epistaxix in horses.
Cryptochidism in horses.
Imperforated anus in dogs.
Creeper in poultry (short wings and long
bones)
Intrinsic factors
B.




Metabolic and hormonal diseases:Bloat in cattle: Clover at certain
stage of growth make frothy
tympany. The exact cause still
unknown.
Hypocalcaemia in cattle.
Transit tetany.
Ketosis
Intrinsic factors
C.


Behavioral disorders:Weaving (vices in horses).
Feather pecking and
cannibalism in poultry
2- Extrinsic (primary)
determinants

A.


B.
The causative agent is not an integral part
of the host. It includes the followings:Non living agents:
Physical agents (trauma, bite of insects,
fractures, horning, etc.)
Chemical agents: (organic and non
organic poisons, poisonous plants,
allergic agents as well as photosensitizing
agents.
Living agents as bacteria, …
II- Secondary causes


1.
i.
These are the predisposing factors which
enabling and reinforcing factors.
The secondary causes may be:Intrinsic factors: inherited or acquired and
includes:Age: resistance of the body is generally
increased by the age in most diseases as
coccidiosis in turkey, virus hepatitis and
leukosis in fowl. On the other hand, resistance
of the host may be decreased with the age as
in red water disease caused by Babesia bovis.
Calves up to 12 months are usually resistant.
Also John’s disease which usually occur in
aged animals
Intrinsic (secondary) factors
ii.
Sex: susceptibility of some DPA may be
enhanced by the sex organs and
considered a predilection seats to them as
Brucella abortus which invade uterus,
udder, testicles.
On the other hand,
different habits of animal may influence
disease occurrence. Example liptospirosis
is more common in male dog because it
may smell and lick the uro-genital organs
of bitches where the urine carry the DPA.
Intrinsic (secondary) factors
Species, breed and strain: There is a wide
variation of animal species to the DPA. So
susceptibility to different disease wary widely.
Example: FMD can infect cattle while horses
are resistant.
Moreover, predilection seats of DPA in animal may
vary from species to the other. Example L.
monocytogenes can infect wide range of
animal species:
In cattle; sheep , goats and pigs: cause
meningio-encephalitis.

In birds: causes lesions in the heart muscle.

In doge: showing canine distemper like
symptoms.
iii.
Intrinsic (secondary) factors
iv.
Body
confirmation:
Body
configuration of the host may
predispose to infection. Example:
crush region of Marino sheep is
fissured by deep folds of skin
where the dirt and faeces are get
accumulated,
fermented
and
become a focus of infection.
Intrinsic (secondary) factors
v.



Metabolism and hormonal balance: Immunity of
the animals may be influenced by some hormones
as follow:Estrogen may stimulate phagocytosis.
Cortisone may inhibit immigration of polymorph
nuclear leukocytes from the blood.
Estrogen, increase epithelial cell desquamation,
increase glycogen content which under goes
fermentation, increase lactic acid production,
reduce vaginal pH to < 4, leading to inhibition of
most DPA. This is the cause why females are more
resistant to Vibrios and Trichomonas during
estrous cycle. Moreover, even if females are get
infested by these parasites, spontaneous recovery
as opposed to bulls who get chronic infection
Intrinsic (secondary) factors
State of nutrition: No doubt the
well nourished animals are more
resistant to infection than those
under fed. Example: ring worm
infection, John’s disease and
others.
On the other hand, well nourished
animals and good conditioned
animals may predispose to some
diseases as enterotoxaemia.
vi.
Intrinsic (secondary) factors
vii.


Degree of function and fatigue:
Stress
may
predispose
to
infection as it inhibits the
immunity. Examples of stress:Transportation.
Infestation with Fasciola hepatica
which increase hepatic necrosis,
get
infected
by
clostridium
causing black disease.
Intrinsic (secondary) factors
viii.






Physiological state: All events of animal life as
puberty, pregnancy, parturition, lactation and
coming to lay in fowls have an effect on disease
initiation as:Brucellosis: established only after puberty
causing abortion at 6th months of pregnancy or
later.
Trichomonas fetus: cause abortion during first 2
months.
Milk fever: occurs 2nd day before to 10th day after
delivery.
Hypomagnesaemia: occurs in the 1st 2 months
after calving.
Staphylococcus mastitis: occurs only in lactating
animals.
Corynebacterium pyogenes: occurs only in dry
2- Extrinsic (secondary)
determinants

1.
2.
3.
These includes environmental factors
and not integral part of the host. As:Bad management, diet, housing.
Climate and weather.
Animal’s care: as manure disposal,
general sanitation of the house, overcrowding,
isolation
between
susceptible and infected animals, etc.
Occurrence of diseases

A.
B.

Infectious diseases: diseases which caused
by living agent:Communicable diseases: if the disease is
capable of transmission from one animal
to other.
Contagious diseases: if the infectious
disease is transmitted by direct contact
between infected and susceptible hosts.
What is the difference between infection &
contamination?
Requests for preparation of
communicable diseases

1.
2.
3.
4.
5.
6.
There are 6 requests:Presence of infective agent.
Presence of the reservoir host.
An outlet (portal of exit) from
infected or reservoir host.
A suitable mode of transmission.
A portal of entry.
Presence of susceptible host.
Factors affect disease occurrence ( in
relation to causative agents)
Virulence:
The virulence of the DPA is largely
different from one strain of a species
to the other. There are some Ag as
Vi-antigen of S. typhi is the
responsible for smooth form of
bacteria.
1.
Factors affect disease occurrence ( in
relation to causative agents)
Infective stage:
Some DPA is required to develop into
special infective stage to be
established in the host tissues and
produce a disease. E.g.. In
helminthes, eggs have to develop to
larval stage to infect the host
2.
Factors affect disease occurrence ( in
relation to causative agents)
Number of the organism:
To establish the disease, the initial No.
of the attacking organism has to be
large enough ( Salmonellae). On the
other hand, some DPA can infect the
host even at a very small dose (T.B.).
3.
Factors affect disease occurrence ( in
relation to causative agents)
Portal of entry into the host:
The more portal of entry, the more
frequency of the DPA to cause the
disease. E.g.., T.B. and Brucellae may
enter the host body through conjunctiva,
resp. tract, digestive tract, skin, coitus.
On the other hand, rabies and Cl. Tetani
are mainly enter the body through the
wounds.
Moreover,
Trypanosome,
Theileria, leshmania, etc has to enter
4.
Factors affect disease occurrence ( in
relation to causative agents)
5.
Establishment of the pathogen in the
host: DPA may establish itself in a:-

Localized in a predilection seats as C. renal
in kidney; St. agalactia in udder; Coccidia in
intestine; Trematodes in liver, and
Trypanosomes in blood.
Localized and affect other sites: Cl. Tetani is
localized but produce exotoxins which affect
the whole body.

Factors affect disease occurrence ( in
relation to causative agents)
6)
A.
B.
Production of the disease by more than one
species: Most of the diseases are produced by
certain species of the causative agent. On the
other hand, some diseases may be initiated by
more than one species as:C. pyogenes alone may cause localized
abscesses in the udder, but in collaboration
with St. dysagalactia may cause summer
mastitis.
Cl. Oedematious multiply in the damaged
liver of sheep (by liver flukes) causing black
disease.
Factors affect disease occurrence ( in
relation to causative agents)
7.
Pathogenic role taken over
by one organism on removal
of another: It is clearly known
that mould and fungi infection
is occur by long treatment of
bacterial
infection
by
antibiotics.
Factors affect disease occurrence ( in
relation to causative agents)
8.



Variation in animal pathogens:
This occurs due to adaptation of
the DPA or due to mutation as:
Mutation.
Drug resistance (bacteria).
Insecticide resistance.
Host-parasite relationship

1.
2.
The fate of invading
organisms depends on two
factors:The defense mechanisms of
the body
The pathogenicity or virulence
of the invader.
Microbial factors

Pathogenicity is the ability of the organism
to cause the disease. However, virulence is
the quantitative of an organism to cause the
disease
either
by
toxigenicity
or
invasiveness. So virulence is measured in “
No. of MOs or micrograms of toxins
required to kill a given host when
administered by certain rout”. It is usually
expressed as LD50 i.e. No of MOs or
micrograms of toxins must be administered
to kill 50% of animals under lab. conditions.
Virulence factors
1.
2.
Adherence factors: some DPA have
Pilli, help their adhesion to the host
cells, mm. So they start disease
processing as E.coli.
Invasiveness: Is the ability of the DPA
to invade the host tissues, multiply
and spread rapidly. Capsulation of the
DPA is an example of invasion as it
protect it from phagocytosis as in
pneumococci.
Virulence factors
3.
A.
B.
Toxigenicity: toxins are bacterial products
having a direct harmful action on the host
tissues. There are 2 types of toxins:Exotoxins: are extracellular, diffuse freely
into the surrounding medium as Cl.tetani,
Cl. Perfringens and Cl. Botulinum (G +ve
bacteria)
Endotoxins: Usually produced by G -ve
bacteria as E.coli, Salmonellae, shigella etc.
These toxins are liberated only by
disintegration of bacterial cells after death
or by treatment with acids or alkalis.
Virulence factors
4.
A.
B.
Production of certain enzymes:
Enzyme MW63: produced by all
pathogenic staph. This enzyme protect
the organism from phagocytosis by
forming fibrin barriers around the
organism and in the same time it
inactivate the antibacterial substances in
the blood.
Fibrinolysin or kinase: Secreted by
Strep.pyogenes (streptokinase).
Promote spreading of lesion by breaking
down the fibrin (barriers) around the
Virulence factors
C.
D.
E.
Depolymerizing enzymes: as mucinase,
lipase,
protease,
nuclease
which
breakdown the tissues of the host.
Collagenase
produced
by
Cl.
Perfringens play a role in gas gangrene.
Hemolysins and leukocidins: dissolve
RBC
(hemolysin)
and
leukocytes
(leukocidins).
Hyaluronidase: Spreading factor as it
dissolve hyaluronic acid (cement like
substances) that bind the cells together.
So, it allows the pathogens to spread
(Strep. Pyogenes).
Resistance factors

I.
A.
Protective mechanisms of the animal
body. There are two main classes of
defenses those are:Non-specific protective mechanisms
(1ry defenses)= innate immunity:
Operating non specifically against most
invading organisms and including:
Mechanical
barriers:
Multiple
mechanisms are present to prevent and
destroy invaders. These physical
barriers including:-
Resistance factors
I- Mechanical barriers:1- The skin: intact skin un-penetrable by most of DPA.
After injury a portal of entry can help invaders to
enter host tissues, multiply and causing infection.
As after trauma, fracture, insect bites.
 Decontamination of the skin is important and occur
in 3 ways: Mechanically: desquamation of surface cells remove
some superficial contaminants.
 Biologically: Some resident flora living on the skin
as Stapl. epidermidis diphteroids etc. produce
antibiotic like substances and compete with other
organisms on the available nutrients.
 Chemically: sweat is normally acidic which is
unsuitable for growth of most pathogens. Moreover
this acidity of sweat is due to lactic acid.
Resistance factors
I- Mechanical barriers:2- M.Ms: mms are protected by mucous
secretion which traps the invading
pathogens. Body get rid of mucous with its
content by many routs acc. To its location
(via stools in the digestive tract and resp.
tract; via cough from resp. tract by the help
of ciliated epithelium.
 GIT: saliva contain hydrolytic enzymes,
stomach
acidity,
may
inactivate
pathogens. In small intestines, proteolytic
enzymes and active macrophages may
attack invaders.

Mechanical barriers



Vagina: acidic pH maintained by lactobacilli
fermentation of glycogen from desquamated cells
making
acid barrier (pH <4.3), prevent infection with yeasts,
anaerobes and G –ve bacteria. As estrogen hormone is
responsible for glycogen content of these cells, this acid
barrier is absent in childhood and old age. So, acute
vulvovaginitis is therefore usually seen in these two age
groups.
Eyes: Tear is unsuitable medium for invaders
Tears carry organisms to tear ducts and to pharynx, then
swallowed.
Tears containing lysozymes which is a protective agent
against most pathogens
Urinary tract: M.Os are swept out with urine.
Urine pH is unsuitable for mos
2- Phagocytosis

1.
2.
3.
4.
All invaders that enter lymphatic, lung or
blood stream, are engulfed by any of a
variety of phagocytic cells which includes:Polymorph
nuclear
leukocytes
(microphages) and mononuclear cells:
these may be wandering in the tissues
(histocyte) or in blood (monocytes) or fixed
in tissues as Kupfer’s cells in the liver.
Polymorph
nuclear
neutrophils
granulocytes.
Eosinophils
Macrophages.
3- Body secretions

1.
2.
3.
4.
5.
Contain many antimicrobial
substances as:Gastric secretions: highly acidic
Vagina lactic acids.
Nasal hairs.
Saliva
Wave like motion of tracheal cilia.
II- Secondary defenses


1.
2.
Highly specific against invaders. It is
called parentral defenses or
immunity.
So, it is clearly known that infection
is controlled by two factors:Host resistance (R).
Virulence of the DPA (V).
Transmission and maintenance of
infection

A.
B.
A complete knowledge of the infectious
agents ( survival, induction of a disease,
transmission to a susceptible host,
replication) is essential for selection of
the most applicable control technique
which involve:Mode of transmission and maintenance.
Ecological conditions that favor survival
of DPA.
Methods of disease transmission

There are two main methods
of disease transmission:-
1.
Horizontal (lateral) transmission.
2.
Vertical transmission.
Horizontal (lateral) transmission.

1.
The DPA is transmitted from an
individual to another either directly or
indirectly:Direct transmission: This occurs when
a susceptible host contacts an infection
either physically or with its infected
discharges (urine, pus, blood, stool, etc
(transmission of canine distemper with
infected urine).
Horizontal (lateral) transmission
2.
Indirect transmission: This occurs
through an intermediate vehicle,
living or inanimate
The living vehicle may be termed as
vector.
The non-living vehicle may be the air,
water, utensils (fomites).
Vertical transmission

A.
B.

This means that the disease is transmitted
through the generations and may include”Hereditary: DPA is carried out within the
genome of one of the parents (retrovirus).
Congenital: disease is acquired at birth from
uterus, or any other part of the reproductive
system.
It may occurs at any stage of embryonic
development causing either abortion or
teratoma. This may include the following:-
Congenital disease
1.
2.
3.
4.
Germinative: this occurs in the ovary or
in the ovum itself (chicken leucosis or
salmonellosis.
Trans-placental (To the embryos via
placenta): this occurs by some DPA
which can pass the placental barriers as
Pan leucopenia virus.
Ascending infection.
At parturition.
Disease transmission
DPA
Vertical
Lateral
Living (vector)
Direct
ysical contact
Indirect
Hereditary
Congenital
Inanimate (fomites)
Infected discharges
Germinative
Transplacental
Ascending At parturitio
Hosts
 It
means plant, animal or
arthropod that is capable of
being infected with DPA and so
give sustenance to it. In the host
the DPA may replicate, develop,
grow or all.
 Types of the host:-
Types of the host
1.
Definitive host (final host): It is a
parsitological term describing a
host in which the organism
undergoes its sexual phase of
reproduction ( Taenia pisiformis
in dogs & plasmodium spp. In
mosquitoes.
Types of the host
2.
Maintenance or primary host: A
host that maintains DPA in the
endemic area ( dogs infected with
distemper
virus).
DPA
is
completely dependent on the 1ry
host for long-term existence so
called maintenance host.
Types of the host
3.
Secondary host: A host that is
involved in the life cycle of
DPA outside typical endemic
area. Example, cattle carry
strain of FMD virus that
specific in buffaloes.
Types of the host
4.
Intermediate host:
A host in which the DPA
(usually parasite) undergoes
asexual forms of reproduction
(Cysticercus pisiformis in
rabbits).
Types of the host
5.
Paratenic
host:
It
is
a
parsitological term and refers to a
host in which the DPA (parasite)
is completely ingested without a
further development. E.g. fish
containing Diphyllobothrium spp.
Larvae which preyed by a large
fish.
Types of the host
Incidental
(dead-end
or
accidental) host:A host that doesn’t usually
transmit PDA to another
animals ( bulls infected with B.
abortus).
6.
Types of the host
Reservoir host:
A host in which the DPA is normally lives
and multiplies. So, it is a common
source of infection to other animals.
E.g. cattle are reservoir of blue tongue
for sheep and Salmonella pullorum in
ducks.
 Exception: soil is a reservoir host for
Anthrax and Clostridia spp.
7.
Vectors


1.
2.
It is an invertebrate (usually arthropods)
that transmit infectious agent to
vertebrates. It is an animate transmitter of
infectious agents. On the other hand the
inanimate transmitter are called fomites.
The vector usually transmit the infection
either through:Mechanical
Biological
Mechanical vector
 They
are the invertebrates that
physically carries DPA to its 1ry or
2ry host. E.g. mosquitoes and fleas
transmitting myxomatosis virus
between rabbits. The infectious
agent neither multiplies nor
develops in the mechanical vector.
Biological vector

A.
B.
C.
A vector in which the infectious agent
undergoes multiplication or development or
even both in this vector. According to what is
going on inside the host, the transmission of
the DPA may be:Developmental transmission: Essential part of
life cycle is spent inside the vector e.g.
plasmodium malaria in mosquitoes.
Propagative: when DPA is multiply in the
vector (louping ill virus in ixodid ticks).
Cyclopropagative: where development and
multiplication of DPA is occur in the vector
Factors associated with disease
spreading

1.
2.
3.
There are 3 important factors for
disease spreading:Characteristics of the host
Characteristics of the pathogen
Effective contact
1. Characteristics of the host

A.
It includes two main points:Susceptibility: It means ability of the host to
be infected. It may be limited to a single
species or group of species. Example: cows
are susceptible to cattle plague while equines
are not. On the other hand, equines are
susceptible to African horse sickness while
cows are not. Moreover, rabies can infect
several animal species. In the same species,
susceptibility may vary widely from genus to
other, from age to other age or even from sex
to other sex.
Characteristics of the host
B.


Infectiousness:- this term refers to:Duration through which the animal is
infective.
The relative amount of the DPA that
infected host can transmit. In this respect
the animal is not infectious as soon as it is
infected (show the following diagram).
Periods of communicability
Diagram:
Incubation period
1st detection of
agent
Convalescent carrier
Symptoms
Symptoms
disappearance
Infection
Pre-patent
period
Period of infectiousness
No M.Os
2. Characteristics of the
pathogen

A.
There are 3 important characters of the
pathogen that affect disease spreading:Infectivity: Ability of the DPA to invade
and establish itself in the host. It relates to
the amount of organism that requires to
initiate infection. This varies largely from
pathogen to the other. When the pathogen
is capable of infecting many species, its
dose of infection varies widely. Example,
campylobacter jejuni can infect chicken
with 5X100 while seagulls requires
Characteristics of the pathogen
B.
Virulence: Quantitative measure of
pathogenicity and expressed by
LD50 (No. of M.Os than can infect
50% of animals under test or No. of
Micrograms of toxins to kill 50% of
animals under test. There is no
doubt the repeat passage of M.Os
through the same host tend to
increase its virulence.
Characteristics of the pathogen
C.


Stability: It is the length of time through
which the pathogen can remain infective
outside the host. The organism may be:Stable: where it can live for long time as
T.B., spore forming bacteria, Clostridia
species.
Labile: where it can live for short time
outside the host as all vegetative forms
of bacteria.
Effective contact

A.
B.
C.
It describes the conditions under which the
infection can occur. It depends on:Stability of the organism: Organisms which
can withstand adverse conditions, increase its
stability and increase the incidence of disease
occurrence.
Route by which organism leave the infected
host (faeces, urine, pus, blood, milk, etc.
Route by which the organism enter the host:
ingestion,
inhalationbiologically
transmitting diseases
Effective contact
 Contact
may be very short as
seasonal transmitting disease,
vector borne diseases. On the
other hand it may very long ( to
years) as Anthrax.
Factors affecting transfer of DPA
1.
2.
3.
4.
5.
6.
7.
Portal of exit and portal of entry.
Availability of the host
Movement of the host
Presence of the pathogens in the
susceptible animal environment.
Ability of the pathogens to survive outside
or inside the host.
Existence and movement of disease vector,
IM host, Transport host, mechanical carriers
and fomites.
Movement of the pathogen itself.
1. Portal of entry and exit of the
pathogen to and from the host
 DPA
which can enter and leave the
host by several routes, can spread
more easily than others. On the
other hand, there are some routes
of entering and leaving animal
body which facilitate disease
spreading than other routs.
2. Availability of the susceptible
host
 Overcrowding
of susceptible host
in a given area may increase the
chance of disease spreading.
Moreover, epidemics may result
from availability of large No. of
host in a restricted area subjected
to very infective and virulent
organism.
3. Movement of the host

The infected or carrier host acts as a source of
infection So its movement will distribute the
DPA to other susceptible hosts. World-wide
diseases are result from the movement of
infected hosts/and or their products from
country to the other. However, transportation
of livestock by airplanes, by sea railways or by
any other means, usually give the time for the
infected animals to show the disease (if they are
in the incubation period before their arrival.
Movement of the host

1.
2.
3.
4.
Moreover, movement of the reservoir hosts
(wild animals or birds) may spread the
infection. The degree of infection
spreading depends on:No. of infected moved.
Rote of dissemination.
Duration of the disease.
Availability of suitable vectors in diseases
transmitting indirectly.
Movement of the host

A.
B.
C.
So, the source of infection is the sick animals
which either:Acute infected animals (maximum excretion
of DPA).
Chronic diseased animals: very important
source of infection due to prolonged period of
excretion of DPA.
Apparently healthy animals: either in the
incubation period or chronic carriers
(important source as they are freely moved.)
4- Presence of the pathogen in
the host’s environment


A.
DPA may reach the host environment
through the secretion and excretion of
infected animals. These DPA may be
transmitted to the susceptible hosts either
directly (???) or indirectly (???).
Transfers of DPA between animals may be
increase due to:Poor hygiene: in animal dwellings,
utensils, clothes, etc.
Transfers of DPA between animals
may be increase due to:B.
C.
D.
Contact between animals from
different farms, different age group,
different species etc.
Using protein of animal origin for
animal’s feeding.
Pasture: is a main source of DPA
spreading as the causative agent find
their ways to pasture from infected or
5-Ability of the pathogens to
survive outside or inside the hosts.

Survival rate for long period outside the
host body, increase its chance to meet
susceptible host. On the other hand,
survival of the pathogens in apparently
healthy host is another mean of
continuing its existence. These DPA
may leave through secretion and
excretion to find their ways to the
susceptible animals.
6- Existence and movement of disease
vector. IM hosts, Transport host,
Mechanical carriers and fomites.



Vector: invertebrate carry the parasite either
mechanically or biologically to susceptible host.
Transport host: A host carry eggs or larvae
without playing part of their development and
transmit the infection when it is ingested by
definitive host as Syngamus trachea in the earth
worm.
Mechanical carriers: animate or inanimate
(fomites) carry DPA on its surface. It may be
man, animals, arthropod vector, wind or water
7- Movement of the pathogen itself
 The
pathogen posses its own
power of movement and so,
travels from host to host e.g.
sheep nostril fly, ticks, mites,
lice and fleas.
Measures of disease
occurrence
Quantification of diseases can simply
done on the basis of counts of
individuals infected, or dead.
1. Prevalence “P”:- It is the amount of
animals get infected in a population at
a specific time without distinction
between old and new cases. So, it
could be determined weekly, monthly
or annually.
Prevalence is usually expressed as a
point prevalence, which mean the
amount of disease in a population at
a particular point of time. So,
P= No. of individuals get infected at a particular point of time
---------------------------------------------------------------------------No. of all individuals in the population at risk
2- Incidence “I”
Is the No. of new cases get infected in the
population through specific period of
time. It measures the flow of the disease
in the population.
During calculation of “I”, two essential
components are important:a) The No. of new cases.
b) The period of time through which the
new cases are occur.
Incidence is calculated as a cumulative
incidence, the proportion of new cases get
infected through a specific period of time
to the No. of healthy individuals at the
beginning of that period.
No. individuals that become diseased during a particular period of time
CI= ---------------------------------------------------------------------------------No. of healthy individual in that population at the beginning of that period
CI can take the values between 0 and 1.
Example.
In a herd of 100 horses, 20 animals get infected
with strangles in the first week of November
and 10 more cases were recorded during the
second week. Calculate the incidence during
the first week and the second week as well.
Moreover, calculate the cumulative incidence
after two weeks of recording?.
“I” during the first week= 20 X 100
------------= 20%
100
“I” during the second week= 10 X 100
------------= 12.5%
80
CI= 30/100 x100= 30%
The longer the period of
observation, the greater the
cumulative incidence. As we
see during the last example.
Example
Last year, a herd of 121 cattle were tested for
tuberculosis using tuberculin test and all
animals were negative. The animals were retested one year later where 25 cases tested
positive.
CI over 12 months= 25/121= 0.21
This means that if the disease proceed by the
same way, each animal in that herd has a
21% chance of becoming infected over the
12-month period.
Relationship between Prevalence
and Incidence
Prevalence depends on the duration of the
disease “D” and the disease incidence
“I”.
So, P= I X D
This means that a change in prevalence can
be due to:a) Change in Incidence
b) Change in disease duration.
c) Change in both.
According to the equation used for
calculation of “I” and “P”, CI includes
new cases in the enumerator
So, Incidence can predicts what will
happening in the future. So it is useful
for making decision on the preventive
measures as TTT, or vaccination.

On the other hand, Prevalence does not
distinguish between old and new cases.
But it describes the probabilities of having
the disease among a group of individuals
at a point of time
Anyway, both “I” and “P” can be used to
make comparisons between risk factors
such as when comparing prevalence of
disease in vaccinated and non-vaccinated
animals.
Calculation of one parameter
from the other
 Since
P= I X D so, any
component of the equation could
be determined if the other two are
available.
 In turn I= P/D
Example
On a survey for leptospirosis among dairy
farmers, 34% of them showing serological
reaction titre of 1/24. The persistence of
antibodies at that titre was maintained for
10 years.
So, P= 34%
D= 10
I rate per year= 34/10= 3.4%.

3- Other measurement factors
Mortality rate= No. of dead cases at a given time
---------------------------------------total No. of animals at risk
Case fatality rate= No. of dead animal at a given time
---------------------------------------------No. of sick animals at the same period
Example
A veterinarian investigate a disease in a herd at
July 1st 1998 and the following data were
recorded: Total herd size was----------------------600
 Clinically ill animals were---------------100
 Clinically ill animals between July 98 and July 99---200
 Total dead animals from july98 and July 99---------120
 Calculate, P, I MR, and case fatality rate





P on July 98= 100/600 X 100= 16.7%
I on July 99= 200/500 X 100= 20% (per year)
MR (July 99)= 120/600 X 100= 20%
CFR (July 99)= 120/300 X100= 40%
How the disease can be
controlled

1.
2.
3.

1.
2.
It includes the following:Prevention of disease occurrence
Prevention of disease spreading
Treatment of curable cases.
Controlling of disease may be:Voluntarily by the owner.
Compulsory by the state (orders, rules,
regulations) as :-

1.
2.
3.
4.
5.
These obligations cover a wide range
as:Notification
Isolation
Recording livestock movement
Regulation and controlling production of
the biological products.
Regulation of animal and animal product
importation.
A- Diseases caused by intrinsic
factors
Inherited diseases:Controlling is based on good breeding
to eliminate spreading of a particular
defect or predisposition
Animals carry an inherited diseases are
not used for breeding.
a)
b- Metabolic and hormonal
diseases
Although the pathogenesis of these
diseases is still unknown, but their
controlling is mainly based on good
management and feeding for each
single case.
Example: pregnant toxaemia in ewes.
c- Behavioural diseases
Avoiding the bad management
which is main cause of these
diseases.
B- Diseases caused by extrinsic
factors

See the second term
Fate of invading organisms
The infecting organisms may be:1. Destroyed by the host tissues.
The virulence of the organism and
body resistance are in fighting all
time So,
 If the body resistance overcome the
organism virulence, the invading
organism will be destroyed.
 Chronic diseases.

2-Elimination by secretion and
excretion of the host
The amount of the organisms excreted
from the host differ according to the
disease course:a. Per-acute: No invading organisms are
escaped from the dead animals.
b. Chronic:
Invading
organisms
are
eliminated in a large doses over a long
period of time.
The route through which organism leave the

Nature of the causative agent.
II.
The organ or organs harbor the infective
organisms. The following are the general
routes through which the disease producing
agent leave the body host:Pus
Urine
Droplets
Uterine discharges
Faeces
Vaginal discharges
I.
In case of rabies: (affect CNS and escape from Salivary
glands)
3-In case of fatal diseases
Disease producing agents are
destroyed with the carcasses specially
if the latter is hygienically disposed.
 In per-acute disease, the host
resistance of the host is so quickly
destroyed and the invading organisms
are multiplies in all parts of the body as
anthrax.

4- The agent and host in an
impasse
This case occurs when the V equal the R
So,
a) The organism unable to cause the disease.
b) The host is unable to eliminate the
organism.
This case may continue or it may terminated
either by:a. Resistance overcome V, elimination.
b. V overcome R----Disease occurs

In case of TB

The bacilli may become walled off by
dense tissues in some organs and the
symptoms are stopped. In such case the
animal is still infected (living organism
still present in the animal tissues and may
break up the capsule and flare-up of the
disease. Sometimes, the disease may be
cured but the organism is still discharged
i.e. convalescent carrier.
Occurrence of Infectious
diseases
Diseases which can be transmitted
from infected to susceptible
animals are called communicable
or infectious diseases.
 Infectious diseases:
 Contagious diseases.
 Infection:
 Contamination.

For occurrence of communicable diseases,
six requests are required those are:1.
2.
3.
4.
5.
6.
Presence of the DPA.
Presence of the reservoir and
source.
Presence of susceptible host.
An outlet (portal of exit) from
reservoir.
Suitable mode of transmission.
Portal of entry.
Factors affecting disease occurrence
1.
2.
Virulence: Presence of certain antigen as
Vi-Ag of S. typhi or Ag associated with
smooth or rough form of organisms. The
DPA are largely differ of their virulence
between them.
Infective stage: Some DPA requires a
special infective stage to be established in
the susceptible host and producing the
disease. E.g. Most helminthes passes eggs
which should hatched to produce the
infective stage, larvae.
Factors affecting disease occurrence
3.
4.
Number of the organisms: To initiate the disease,
certain diseases requires high doses of the
organism to establish the disease E.G.
(salmonellae). Other diseases may establish the
disease even in low No. as M tuberculosis.
Portal of Entry: The more portal of entry, the more
frequency to establish the disease. T. bacilli and
Brucellae may gain enter through many routs as
m.m; resp. tract, digestive tract, skin and coitus.
On the other hand, Rabis, Cl. tetani and most of
vector borne pathogens ( Trypanosoma, Thileria,
malaria, Lishmania) may have only one route to get
enter the animal body.
Factors affecting disease occurrence
5.
A.
B.
C.
D.
Establishment of the DPA in the host: The
following are some ways in which the DPA infect
the host:Localized: as C. renal ( kidney); Strp. agalactia
(udder); Trypanosomes (blood); Coccidia
(Intestine); Liver flukes (liver).
Generalized: as all diseases causing bacterimia or
septicemia as B. anthracis; FMD; Canine
distemper; ND, etc..
Some DPA remain localized but produce exo-toxins
affecting other tissues as Cl. tetani (tetanus) and
Cl. Septique (braxy).
Some organisms may start as localized infection
but at certain stage become invasive and
generalized as Salomellae, Brucella and
Erysiplothrix.
Factors affecting disease
occurrence
6.
A.
B.
Production of disease by multi-factors:
Some diseases may be produced by:collaboration of different species of
DPA.
An organism may multiply due to tissue
damage caused by other organism. as,
C.
pyogenes
causing
localized
abscesses and in presence of Strp.
Dysagalactia
may
cause
summer
mastitis. Moreover, Cl. oedematious only
multiply in the damaged hepatic cells by
liver flukes causing black disease.
Factors affecting disease
occurrence
7.
8.
A.
B.
C.
D.
Pathogenic role taken over by DPA on
removal of another: It was recorded that
moulds and fungi infection may be
exaggerated after an antibiotic treatment of
bacteria.
Variation in animal pathogens: It only occurs
under lab. Or may field conditions as:Mutation of pathogens.
Drug resistance
Insecticides resistant ticks.
Resistant trypanosomes
Factors affecting pattern of diseases

1.
A.
B.
The outcome of invasion of the DPA to
susceptible host depends on:Pathogenicity and virulence: they are
measured by:The ratio of clinical and subclinical cases: A
higher pathogenicty, a higher proportion of
clinical cases and the lower proportion of
subclinical cases.
Case mortality: No. of deaths are quite high
in more pathogenic and more virulence DPA.
Characteristics of the host
A host susceptibility and infectiousness
determine its ability to spread infection.
A. Susceptibility: ability of the host to be
infected. Invading organism can infect a
single animal species or a group of
animal species.
Examples:Cows susceptible to cattle plague while
equines are not. On the other hand,
African hose sickness may infect
equines while cows are not. Moreover,
all mammals are susceptible to rabis.A

Characteristics of the host
Infectiousness: This term refers to:
Duration period during which the animal is
infective.

The relative amount of an infectious agent that
an animal can transmit.
An animals not infectious as soon as it infected.
Prepatent
period:
Between
infection
and
availability of DPA.
Incubation period: Between infection and clinical
symptoms. So, inapparent has prepatent but
not incubation period.
Generation time: between infection and maximum
infectiousness.
B.
Periods of communicability
Incubation period
1st detection of
agent
Infection
Pre-patent
period
Convalescent carrier
Symptoms
Symptoms
disappearance
No M.Os
Characteristic of the pathogens
1.
Infectivity: amount of organism that
require to initiate infection. This vary
widely from species to another. Not
only but also the infective dose of an
organism differ from an animal
species to the other. Example,
infective dose of Campylobacter
jejuni for chickens is 500 organism
while in seagulls is more than 107
Characteristic of the pathogens
2.
3.


Virulence: ability of organism to cause
the disease. Repeated passage through
the same animal species tend to
increase the virulence.
Stability: is the length of time through
which the organism remain infective
outside the host. The organism may be:Stable: can live for long time as T.B. and
spore forming bacteria.
Labile: survive for short period as
viruses, leptospira.
Effective contact

1.
2.
3.
It depends on:Stability of the organism: Highly resistant
organism to adverse environmental conditions,
increase its stability, increase the incidence of
disease spread.
Route by which organism leave infected host: if
it leaves by feces, urine, milk, exhaled droplets,
give chance for more spreading. On the other
hand, vector borne disease, rabbis, have special
way so decrease disease spreading.
Route by which organism enter susceptible host:
Ingestion, inhalation leads to high disease
spreading while vector borne pathogens are not.