Transcript Sept. 8

Lecture 4
1.
2.
3.
4.
5.
Announcements
Email me if you did NOT receive a
message from me last Friday.
BlackBoard kinda works but we will stay
with our own site for now:
www.uky.edu/~gcbrown/ent561
Will try to get the lecture notes on sooner.
No quiz on Thursday
Online Reading:
http://www.crid.or.cr/digitalizacion/pdf/eng/
doc14606/doc14606-10.pdf
Epidemic Components
• 1. The etiologic agent = microorganism that can cause
disease.
– Pathogenicity = its ability to cause disease;
– Virulence = the severity of infection, expressed as:
• morbidity = incidence of disease
• mortality = death rate of the infection.
• 2. The method of transmission = means by which the
agent goes from the “source” to the host. We are only
concerned with vectored pathogens here.
• 3. The host – Depends on 3 things:
– Accessibility
– Susceptibility
– Transmissibility
Most studies of arthropod-vectored
disease study:
•
Time Trends – Four kinds
–
–
–
–
•
The secular trend - occurrence of disease over years
The periodic trend – temporary change in a secular trend,
The seasonal trend - seasonal changes in disease occurrence
The epidemic occurrence of disease.
Place Considerations
– where the individual was when disease occurred;
– where the individual was when he or she became infected;
– where the vector became infected with the etiologic agent.
• Infected Person
– Individual characteristics: age, sex, occupation, personal habits,
underlying disease, immunization history, etc.
– Social characteristics: socioeconomic status, geographical
distribution, public works & policies, disease rates in other group
members
Example: Rocky Mountain Spotted Fever
Etiological Agent – Rickettsia rickettsii
Vector – Dog & wood ticks
Example: Rocky Mountain Spotted Fever
Periodic Trend
Secular trend of the disease
www.cdc.gov/ncidod/dvrd/rmsf/Epidemiology.htm
Example: Rocky Mountain Spotted Fever
Seasonal Trend
Example: Rocky Mountain Spotted Fever
Epidemic Time Period
Example: Rocky Mountain Spotted Fever
Geography
Age
Parasite types in Medical
Entomology
• Micro vs macroparasites – Micro are
microorganisms (viruses, bacteria, protozoa,
etc.). Macro are arthropods.
• Ectoparasites vs endoparasites –
– Ecto – parasitize from outside the host, infestation
normally remains topical and peripheral
– Endoparasites – parasitize from inside the host,
Microparasites can produce systemic infestation.
Disease is normally systemic.
• Facultative vs. obligate –
– Obligate are totally dependent on host.
– Facultative have parasitic and free living forms.
Vectors and bloodfeeding
• Most vectors are blood-feeders:
– Normally the pathogen does not benefit the
vector, detrimental effects sometimes occur.
– The pathogen receives many benefits
(acqusition, transport, re-insertion).
– Principal problems for the pathogen from this
system:
• Must overcome ingestion by the vector (next slide)
• Must be able to infect a new host from a small
inoculum (slide following)
• Totally dependent on the vector-host population
dynamics and interaction.
1. Pathogen ingested
2. Pathogen may multiply here (plague bacteria, Leishmania)
3. Passage through gut wall (filarial worms, Borellia) or multiplication in
epithelial layer (rickettsiae, arboviruses), or cysts (malaria)
4. Transport by hemolymph to tissues (some arboviruses multiply here
too)
5. Tissue concentration in (a) salivary glands (b) reproductive system
Infection
6. Contaminated mouth (fluids, mouthparts, escape through body wall)
7. Infected feces
8. Infective fluids (e.g. coaxial gland fluids in ticks)
9. Crushing/ingestion of vector
Definitive vs. Intermediate Hosts
Definitive
• Host in which
gametocycte
union occurs for
the parasite.
• AKA – Primary
host
Intermediate
• Host in which
asexual
reproduction
occurs for the
parasite.
• AKA – Secondary
host; Alternate
host
Example: Lymphatic Filariasis
• Human – Definitive Host, Mosquito the
intermediate host
Opposite Example: Malaria
• Mosquito definitive host, human is the
intermediate host
More about hosts
• accidental host one that accidentally harbors an
organism that is not ordinarily parasitic in the particular
species.
• amplifying host one that increases parasite numbers
&/or no. infectious vectors. Typically short-lived
infections.
• dead-end host the disease cannot be transmitted from
the infected host to another animal.
• predilection host the host preferred by a parasite.
• reservoir host an animal (or species) that is infected by
a parasite, and which serves as a source of infection for
humans or another species.
• transfer host, transport host one that is used until the
appropriate definitive host is reached, but is not
necessary to complete the life cycle of the parasite.
About Vectors – Host Selection
• ‘phagic’ – feeds successfully; ‘philic’ –
attracted to.
• Anthropophagic – feed successfully on
humans; Anthropophilic – attacks humans
• Zoophagic – feed on vertebrates other
than humans. Two big subsets
– Mammalophagic
– Ornithophagic
Modes of Transmission
• Two main modes:
– VERTICAL – vector to vector in subsequent
life stages or generations
– HORIZONTAL – passage of parasites
between vectors and hosts
Vertical Transmission Subtypes
• Transstadial – between subsequent life
stages
• Transgeneration – between subsequent
generations (several methods)
• Venereal – passage of parasites occurs
between male and female during mating.
Rare in vectors
Horizontal vs Vertical
Transmission
Compare Figs. 3.3 & 3.2 in your
text
1. Which are the dead
end hosts?
2. Where are the
bridge vectors?
3. Where is the
amplification cycle?
4. Where is the
enzootic
transmission cycle?
Interseasonal Maintenance
• Most seasonal disease from arthropod-vectored
pathogens in temperate zones require interseasonal
maint.
• Principal mechanisms:
– Continued transmission – no seasonality, tropical
diseases, disease of parasites that continuously live with
hosts
– Infected Vectors – most encephalitis viruses
– Infected hosts – Reservoirs go dormant, parasite has a
dormant stage in host.
– No interseasonal maintenance – occasional local
extinction
• Migratory hosts
• Weather fronts
Principles of Disease Vector
Management
• Surveillance
– Vector, disease prevalence, pathogen burden
• Prevention
– Personal
– Public
• Control
– Vector-based
– Pathogen-based (pharmacologic)
Surveillance
• Very good general section on this subject
in your text:
pp: 29 – 32
Prevention
• Personal
– Exclusion agents (clothing, screening, bed nets, sealing
cracks/crevices, etc.)
– Chemical agents (repellents)
– Private source controls
– Sanitation & Personal Hygiene
– Source Avoidance
• Public
– Public service announcements/education
– Vector breeding site controls/sanitation
– Disaster Management Planning
READ:
http://www.crid.or.cr/digitalizacion/pdf/eng/doc14606/doc1460610.pdf
A Key Concept in DV Management
is Risk
• Most DV Management is Govt. Funded
• They fund efforts in response to perceived
risk
• Risk = Cost of a Bad Outcome
TIMES
Probability of that Outcome
Occurring
The main problem is there are
multiple risks
• Multiple “Bad Outcomes” including
– Economic
– Environmental
– Social
– Political
• Each outcome has it’s own cost and
probability
• Major Complication: Disease risk is
inversely correlated with vector control.