Ecology of Infectious Diseases: the Role of Biodiversity
Richard S. Ostfeld
Institute of Ecosystem StudiesPI: R. Ostfeld, coPIs: F. Keesing (Siena College); K. Schmidt (IES)
Infectious diseases are those caused by microbial agents, such as viruses, bacteria, and protozoans. Zoonotic diseases are infectious diseases in which the microbial agent is maintained typically in animal populations and only occasionally transmitted to humans. In some cases, transmission of zoonotic diseases is direct, for example via the bite or scratch of an infected animal (e.g., rabies) or via inhalation of aerosols containing animal excreta (e.g., hantavirus disease). In other cases, transmission is indirect, occurring via arthropod vectors. Lyme disease is one example of a vector-borne zoonotic disease. The tick vectors of this disease hatch free of the Lyme disease agent (a spirochete bacterium), but may acquire the bacterium if they feed on an infected host. The ticks are highly generalized in host preference, being found parasitizing dozens to hundreds of mammalian, avian, and reptilian species. However, they acquire the disease agent efficiently from only one or two species, particularly the white-footed mouse, which is also the most abundant and widespread member of many vertebrate communities. Once a tick acquires the Lyme bacterium, it maintains the infection and remains capable of infecting humans for life. Our studies show that the greater the density of white-footed mice, the greater the density of infected ticks after a one-year lag.
Results of both modeling and empirical studies suggest that increases in species diversity within communities of vertebrate hosts will dilute the effect of white-footed mice, causing more tick meals to be taken from inefficient disease reservoirs, and resulting in lower infection prevalence in ticks. This in turn will result in lower risk of exposure to Lyme disease. We call this effect of increasing species diversity The Dilution Effect. The Dilution Effect can be contrasted to what might be expected when vectors acquire disease agents efficiently from many hosts, in which case high diversity may result in a Rescue Effect, by buffering effects of the loss of particular hosts. A large number of disease systems appear to be characterized by generalized vectors and a vertebrate host community consisting of few competent reservoirs for the disease. Thus, we expect that the Dilution Effect may be common. If so, maintaining high biodiversity will be important in decreasing human exposure to zoonotic diseases.
References:
Van Buskirk, J. and R. S. Ostfeld. 1995. Controlling Lyme disease by modifying density and species composition of tick hosts. Ecological Applications 5:1133-1140.
Van Buskirk, J. and R.S. Ostfeld. 1998. Habitat heterogeneity, dispersal, and local risk of exposure to Lyme disease. Ecological Applications 8:365-378.