Transmission of infectious diseases with several host species
Animal hosts such as dogs and rodents are the reservoir from which the Leishmania parasite is transmitted to humans. Yet, killing these reservoir animals may not always be a good idea. In fact, it can increase the incidence of human disease, as was shown with mathematical transmission models that were developed in collaboration with researchers at the University of Fortaleza and of the field station in Teresina, Brasil. The models describe the transmission of parasites with multiple reservoir hosts. The key to understand the apparently paradox result lies in the transmission of the disease. The Leishmania parasite is exclusively transmitted by sandflies, small blood-sucking insects of the genus Phlebotomus. The population size of the vector flies mainly depends on the availability of breeding sites (sufficiently moist places on the ground with rotten plant material), but are widely independent on the availability of hosts from which they take their blood meals. Reducing the population size of the sandflies' favourite hosts concentrates the number of flies on the remaining hosts and thus intensifies transmission within the animal reservoir population. Furthermore, sandflies are forced to look for alternative sources of a blood meal, which directs them to domestic animals and also to humans. Depending on the degree of destruction of the animal reservoir, both the infectious fraction of sandflies and the fraction of sandflies approaching humans may increase and, thus, the incidence of human disease may go up in spite of the efforts of fighting the disease.
German text (PDF; 1.3 MB)