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 30 60 Frequency % 60 50 40 30 20 10 Frequency 25 20 15 10 5 Frequency % 50 40 30 20 10 0 1 2 3 4 5 6 7 8 9 10 11 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 Parasites per host Parasites per host Parasites per host
Frequency Distribution % Number of Parasites per host
Overdispersion 70 60 50 Frequency % 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 Numbers Death Emigration
Deaths Births Population 12 1200 3500 10 1000 3000 Number of deaths 8 6 4 Births 800 600 400 2500 2000 1500 2 200 1000 0 1 2 3 4 5 6 7 8 9 Time 0 1 2 3 4 5 6 7 8 9 Time 500 0 1 2 3 4 5 6 7 8 9 10
Constant Death Rate (60% of larvae die/month L t =L t-1 (1-0.6) Larvae (L) Slope dl / dt 0 1 2 3 4 5 100 40 16 6.4 2.56 1.06 91.63 36.84 14.55 5.86 2.34 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: R 0 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.
R 0 defines the threshold between persistence and extinction of an infection. If R 0 < 1 If R 0 = 1 If R 0 > 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 Standard assumption of the evolution of virulence theory Virulence
Host Parasite models between local and meanfield Pair-wise Approximation: differential equations for pair densities eg, P SI (t) =prob randomly chosen pair is in state SI event r(si II) = transmission rate z P SI z (z 1)P SI q I /SI # neighbours (fixed) conditional prob that I is a neighbour of an S site in an SI pair