ENVIRONMENTALLY DRIVEN EPIZOOTICS

Environmental conditions can be the driving force behind an epizootic. Environmental changes may favor growth of a particular species, which results in increased contact rates and spread of a disease. We examin e this particular phenomenon in SI and SIS models and use it to explai n the possible disease outbreaks in nature. Either infected individual s recover from the disease (SIS model) or suffer disease fatalities (S I model). Epizootic models for a single population are examined where contact rate depends on population size. A reproductive number R is de fined that depends on environmental carrying capacity. The single-popu lation models are coupled to form three different two-species models w ith intra- and interspecies contact rates that depend on the populatio n sizes of both populations. The stability results show that it is pos sible for the disease to drive one of the populations to extinction, t he one with disease fatalities. The surviving species serves as a rese rvoir for the disease. Single- and two-species epizootic models are ex amined in a particular case where the contact rates are assumed to be constant. This leads to a new definition for the contact rate. A compl ete global analysis is possible in these latter cases. The results are compared and contrasted with the models with variable contact rates. The prototype for the models is the spread of disease in wildlife popu lations, which includes such diseases as plague or Lyme disease.