Considerable insights into disease dynamics can come from focusing on pairwise interactions between one host species and one disease-causing species. However, to understand the spread and evolution of disease more fully, we also need to take into account interactions between: parasite / pathogen species — including those mediated by hosts, vectors and the environment; host species — including those mediated by parasites / pathogens
Infection with one type of disease agent can reduce or increase the incidence of infection with another, for instance, because:
- Host immune response alters. For example, the first infection may make the host more immuno-competent against the second (e.g. development of cross-immunity to closely-related influenza strains). Alternatively, the first infection may make the host more likely to acquire the second (e.g. people in the later stages of HIV infection are more susceptible to a range of other disease agents).
- Transmission is reduced or increased. For example, research by Andrew Stephenson and Matt Ferrari has shown that infection of wild gourds with a moderately virulent viral pathogen deters the beetle vectors of a fatal bacterial pathogen. An important consequence of which is that cultivated, transgenic squash, which are resistant to viruses, can then be more likely to suffer mortality due to bacterial infection.
Nature of the interaction depends on the host
The nature of parasite-parasite interactions can vary with characteristics of the host, including:
- Gender. For instance, Hudson and collaborators discovered that infection with one type of gut helminth (Passalurus ambiguus) increases infection intensity of another (Mosgovoyia pectinata) in male but not in female European rabbits (Oryctolagus cuniculus).
- Age. Taking another example from rabbits, the negative impact of one gut helminth (Trichostrongylus retortaeformis) on another (Graphidium strigosum) has been found to vary with host mass, a correlate of age.
By incorporating empirical data on differences between hosts into models of disease dynamics, we are able to better predict the emergence, spread and persistence of disease in host populations.
Parasite-mediated host-host interactions
When host species share a common parasite or pathogen, disease dynamics in one host species can be affected by disease dynamics in the other. Examples currently being studied by CIDD researchers include:
- The dilution effect — vectors feed on non-competent hosts; these hosts act as a "parasite sink" so transmission is reduced.
The impact of parasites and pathogens on host population dynamics can depend on how the environment independently affects variables such as:
- Host health and immunocompetence
- Parasite survival outside the host
- Survival of disease vectors
We are investigating these kinds of interactions in a variety of wildlife disease systems.