In forested landscapes of the eastern and central United States, the white-footed mouse is typically one of the most abundant vertebrates. Imagine a recently hatched larval tick waiting on the forest floor, nearly immobile, waiting for a potential vertebrate host to approach. If the tick is born in a habitat that favors white-footed mice, and/or in a year of high mouse density, the tick has a high probability of obtaining its first blood meal from a mouse. Because a high percentage (#) of white-footed mice carry the spirochete bacterium that causes Lyme, it is very likely the tick will be infected. When it molts from a larvae into an infected nymph during the spring or summer, it will be dangerous to humans.
Two situations should reduce a questing larval tick's probability of encountering a white-footed mouse. One is a reduction in the population density of mice, the other an increase in the number of non-mouse hosts in the forest. When host diversity is high, there is a lower probability that ticks will feed on a white-footed mouse host. Larval ticks are less likely to become infected with B. burgdorferi when they feed on other vertebrate animals, such as chipmunks, lizards, or ground-dwelling birds. When tick's obtain their larval blood meal without becoming infected, they are not dangerous to humans when they feed as nymphs the following year.
The second situation, termed the Dilution Effect by Ostfeld and Keesing (2000 a, b), occurs when high host diversity dilutes the impact of white-footed mice, reducing mouse-tick interactions and subsequent disease risk. Empirical and theoretical support for the dilution effect is growing. An assessment of major tick host species at our New York study sites revealed that Lyme disease risk is lower when diverse host communities are present. Conversely, disease risk escalates in species-poor communities (LoGiudice et al. 2003). Computer simulation models (Van Buskirk and Ostfeld 1995, 1998, Schmidt and Ostfeld 2001) suggest mechanisms behind the dilution effect. Human-induced environmental changes, such as landscape fragmentation and predator suppression, can inadvertently increase disease risk by reducing biodiversity.
The dilution effect appears to be a general phenomenon, not restricted to the Lyme disease system. For the dilution effect to apply to a vector-borne zoonosis, the following conditions must hold:
- The vector must be a generalist that parasitizes at least several host species, including humans
- Hosts parasitized by the vector must vary strongly in their reservoir competence, such that some are highly infective and others are dilution hosts
- Vectors must acquire the pathogen via blood meals rather than relying predominantly on transovarial transmission
- The most competent reservoir host(s) must be dominant members of the host community, feeding a high proportion of the tick population. A corollary of condition (4) is that host species with lower reservoir competence will tend to occur only in more diverse communities.
The extent to which these conditions are met is the subject of ongoing assessments.