Although studies on Plasmodium infections of wild birds have been reported frequently in the literature, our knowledge of the ecology of these parasites remains incomplete. A synthesis of data and ideas from these field studies, and recent experimental work led to the construction of the following hypothetical model for the ecology of avian malaria:
During the late spring, summer, and early fall susceptible birds (young of the year or previously uninfected adults) contract the infection from the bite of an infected mosquito on the breeding ground. The birds migrate or remain in the area and infections become latent over the winter. In the spring, migratory birds return to the breeding area and all birds commence reproductive activity. With the onset of migration and breeding activity, parasite populations become elevated in the birds. This relapse of malarial infections coincides with emergence of vectors. The mosquitoes obtain the parasite, passing it on to susceptibles in the population (whose numbers are simultaneously increasing as the result of reproduction), and the cycle continues. Under favorable conditions, transmission rates equal or exceed a level needed to replace mortality of infected birds. Under unfavorable conditions the parasite is maintained by the bird reservoir, the population of susceptibles increases, and transmission is postponed until favorable conditions return and transmission to the expanded population of susceptibles replenishes the supply of infected adults. Such a cycle, in which the parasite, vector, and susceptible host populations reach a maximum in an apparently favorable sequence, with provision for occasional failure of transmission, has obvious survival value.
A more complete understanding of the ecology of avian malaria will be achieved with the investigation of specific problem areas defined in this model. The model may prove of additional value in suggesting an ecological approach to our understanding of the epidemiology of human malarias. It may also have applicability in other disease systems where bird-mosquito relationships are similar, such as certain of the arboviruses.