parasites transmitted by vectors often very specific vector-parasite relationships biomphalaria sp....
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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 TransmissionThe parasite undergoes cyclical changes and multiplies within the vector, i.e., there are both developmental changes and multiplication of the parasite.B. Cyclodevelopmental TransmissionThe 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 TransmissionThe 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 TransmissionThis 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. EpidemiologyThis 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. EndemicA disease pathogen is present in an area and is expected to be there.
C. EpidemicThe presence of a disease is at levels higher than what normally is expected.
D. PandemicAn 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???
0
10
20
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1 2 3 4 5 6 7 8 9 10 11
Parasites per host
Fre
qu
ency
%
0
5
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30
1 2 3 4 5
Parasites per host
Fre
qu
ency
0
10
20
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1 2 3 4 5 6 7 8
Parasites per host
Fre
qu
ency
%
Frequency Distributions
0
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20
30
40
50
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70
1 2 3 4 5 6 7 8 9 10 11
Parasites per host
Fre
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%
Most hosts have few or no parasites- some hosts have very many parasites.
Overdispersion
0
2
4
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12
1 2 3 4 5 6 7 8 9
Time
Nu
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f d
eath
s
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1 2 3 4 5 6 7 8 9
Time
Bir
ths
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1 2 3 4 5 6 7 8 9 10
Deaths Births Population
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
Host Parasite models between local and mean-field
Pair-wise Approximation: differential equations for pair densities
PSI(t) =prob randomly chosen pair is in state SI
z
(z 1)PSIqI /SI
conditional prob thatI is a neighbour of an Ssite in an SI pair
event
z
PSI =
transmission rate
# neighbours(fixed)
r(SI II )
eg,