Question

1. We went over the SIR model in lecture. You do not need to know the equations or make any calculations to answer the following question, you just need to know the principles we discussed. A reservoir population for an infectious disease is a population in which the disease naturally lives and reproduces with sustained transmission. Usually, we expect a pathogen to evolve to a moderate level of virulence based on Paul Ewald’s Trade-off Hypothesis. Use the SIR Model framework and the behavioral changes induced by the rabies virus to explain why the rabies virus maintains a high levels of virulence in populations of social mammals such as bats or raccoons.

a. Pick one of the African hemorrhagic fevers we discussed in lecture (Lassa Fever or Ebola) and use your example to demonstrate why understanding the ecology of the reservoir species can help predict future outbreaks.

Answer 1

Since it is already known that the SIR model is a mathematical model for epidemics we will have to consider its implication in the case of rabies virus and when it particularly infects social animal populations.

The SIR model consists of S= susceptible I= Infected & R=Recovered, individuals, modeled using differential equations. The core Idea of the model is to predict the density of the population required in order to an epidemic to emerge or to know know what are the other ecological principles of the virulent factors involved.

Before extrapolating the implications of the SIR model to the social population of animals we need to first understand that according to the group of individuals involved in this SIR modeling (as described earlier) the infected group is the one that actually encounters the infection by a particular pathogen (in our case rabies virus) and then transmits it to the hosts/individuals that have a weaker immune system i.e the susceptible individuals.

Since we have to consider the virulence factor of the rabies virus here in the social animals, by definition such animals which live in a group/herd or keep interacting we are talking about an increase in the susceptible hosts/individuals for this virus. This is because if there are infected individuals present in the population and due to the absence of a vaccine the recovered host/individuals will be negligible. Further, the chances increase for the infected individuals to transmit the rabies virus much more frequently to the susceptible hosts. This is the reason why the rabies virus maintains high levels of virulence in populations of social mammals such as bats or raccoons.

This example can be further extrapolated to the African hemorrhagic fevers such as Lassa Fever, Lassa fever is caused by the Lassa virus which infects the humans when they come in contact with urine or feces of the infected rats (Mastomys rats) or if there is a direct contact with the infected blood, urine or feces of the infected individual. Remember in the earlier example we had pointed out that if the susceptible population density of the individuals is higher (as in this case, being a third world country, Africa has a huge population) and devoid of proper vaccination approach (leads o decrease in the recovered individuals and the Infected population can easily spread the virus giving the social nature and interactions of humans) the SIR could be used to predict that the Lassa virus can have a huge virulence factor amongst the African people/population.

Thus this is how understanding the ecology of the population may aid in predicting the outcomes of pathogen infection and epidemics.  

Hope this helps...!