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???

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Frequency Distributions

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Most hosts have few or no parasites- some hosts have very many parasites.

Overdispersion

Birth

Immigration

Death

Emigration

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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

Transmission

Virulence

Standardassumptionof the evolution of virulence theory

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

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(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,