Transcript Lecture_3_Jan 08, 2015

Parasites transmitted by vectors
Often very specific vector-parasite relationships
Biomphalaria sp. - Schistosoma mansoni
Anopheles sp. – Plasmodium falciparum
Simulium sp. – Onchocerca volvulis
Some more general
Rhodnius sp / triatoma sp. - Trypanosoma cruzi
TRANSMISSION OF PARASITES BY VECTORS:
Biological Transmission
I.
A. Cyclopropagative Transmission
The parasite undergoes cyclical changes and multiplies within the
vector, i.e., there are both developmental changes and multiplication
of the parasite.
B. Cyclodevelopmental Transmission
The parasite undergoes cyclical changes within the vector but does
not multiply, i.e., there are only developmental changes of the parasite
without multiplication.
C. Propagative Transmission
The parasite multiplies within the vector without any cyclical
changes, i.e., the parasite increases in number within the vector but
does not undergo any developmental changes.
II. Mechanical Transmission
This is similar to a "flying syringe" where transmission from one host
to another is accomplished because the parasite contaminates the
mouthparts of an arthropod and is physically carried to another host.
EPIDEMIOLOGY TERMS
A. Epidemiology
This literally means "as it falls upon the people." A good working
definition is the ecology of disease, i.e., all aspects of the pathogen,
host(s), environment, social conditions, etc. that contribute to or
influence the maintenance of a disease.
B. Endemic
A disease pathogen is present in an area and is expected to be there.
C. Epidemic
The presence of a disease is at levels higher than what normally is
expected.
D. Pandemic
An epidemic that is worldwide in scope.
ADDITIONAL TERMS IN PARASITE ECOLOGY/EPIDEMIOLOGY
A. Prevalence: Number of hosts infected divided by the number of hosts
examined at a point in time.
B. Incidence: Number of new cases of infection (disease) in a given time period
divided by the number of uninfected and susceptible hosts at the
beginning of the time period.
C. Intensity: Number of parasites in a given host (Mean Intensity = the total
number of parasites recovered divided by the number of infected hosts).
D. Density: Number of parasites per unit area, weight, or volume of tissue (e.g.,
number of parasite eggs per gram of feces).
E. Overdispersion: A general rule in parasite infections where relatively few
hosts harbor the majority of all parasites in a population. In contrast, an
underdispersed parasite population would mean that all hosts have the same
number of parasites.
Anthropohhilic: associated with humans
Anthroponoses: humans are only known host
Etiologic agent: organism that causes disease
Etiology: Study of the course of the disease
Disease: symptoms in host caused by infectious organism
Zoonotic disease: disease that moves from animals to humans
Many human diseases are considered zoonotic….. WHY?
Swine flu. Avian flu, SARS, HIV-AIDS, plague, ebola, bovine TB, lyme
disease, west nile, rabies, hantavirus anthrax, Lassa fever
Many reside in other animals (reservoir hosts) and therefore are difficult
to control/eradicate
Infection takes place
Parasite enters potential host
Parasite searches for suitable location- responds to host signals
Migrates/transported to specific tissue and establishes
Parasite begins its life cycle in host
Host may begin to show symptoms
Symptoms are general or may be indicative of a specific disease
(general fever vs blindness caused by Onchocerca)
The distribution, periodicity, severity of disease is a field unto itself.
What are possible outcomes of the infection???
Frequency Distributions
70
60
30
60
25
50
20
40
40
30
Frequency %
Frequency
Frequency %
50
15
10
30
20
20
10
5
10
0
0
0
1
2
3
4
5
6
7
Parasites per host
8
9
10
11
1
2
3
Parasites per host
4
5
1
2
3
4
5
Parasites per host
6
7
8
Frequency Distribution %
Number of Parasites per host
Overdispersion
70
60
Frequency %
50
40
30
20
10
0
1
2
3
4
5
6
7
8
9
10
11
Parasites per host
Most hosts have few or no parasites- some hosts have very many parasites.
Birth
Immigration
Parasite
Death
Numbers
Emigration
Births
Population
12
1200
3500
10
1000
3000
8
800
2500
Births
Number of deaths
Deaths
6
2000
600
1500
4
400
2
200
0
0
1000
500
1
2
3
4
5
Time
6
7
8
9
1
2
3
4
5
Time
6
7
8
9
0
1
2
3
4
5
6
7
8
9
10
Constant Death Rate
(60% of larvae die/month
Lt=Lt-1 (1-0.6)
Larvae
(L)
Slope
dL / dt
0
100
91.63
1
40
36.84
2
16
14.55
3
6.4
5.86
4
2.56
2.34
5
1.06
0.97
Make things more complicated:
Include age-dependent death rate
Development, Migration and Infection
Mathematical models can become quite complicated.
The Objective is to be able to understand what is happening
in a population.
Increasing---stable---decreasing
Basic Reproduction Ratio:
R0
The average number of offspring produced throughout the
reproductive life-span of a mature parasite that themselves survive
to maturity in the absence of density-dependent constraints to
population growth.
Ro of any infection is defined for a given environment and a given
host community.
If a child has measles and that child is responsible for the infection
of 20 other children then the Ro in this community is 20.
If Ro=1 then we expect that the child to infect only one other
person before (s)he recovers and loses infectiousness.
R0 defines the threshold between persistence
and extinction of an infection.
If R0< 1……
If R0= 1……
If R0> 1……
This threshold assumes great importance when planning
control programs. If Parasite eradication is the objective then
the basic reproduction ratio must be reduced and maintained
below 1.
Many factors affect Ro in communities:
Nutritional status affects duration of infectious period
Environmental conditions affect mortality of infectious stages
Same parasite with different vectors will be different
Standard
assumption
of the evolution
of virulence theory
Transmission
Virulence
Host Parasite models between local and meanfield
Pair-wise Approximation: differential equations for pair densities
eg,
PSI(t) =prob randomly chosen pair is in state SI
event
r(SI  II ) =
transmission
rate

z
PSI 

(z  1)PSI qI /SI
z
# neighbours
(fixed)
conditional prob that
I is a neighbour of an S
site in an SI pair