researchParasite interactions
- Overview
- Parasite-parasite interactions
- Nature of the interaction depends on the host
- Parasite-mediated host-host interactionst
- Environmental effects
Overview
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
Parasite-parasite interactions
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, modeling by Grenfell and collaborators has revealed that in the early decades of the 20th century in the U.K., whooping cough epidemics reduced the transmission of measles, and vice versa, as children died or were quarantined.
CIDD researchers are investigating how the spread and evolution of disease is affected by positive and negative interactions between parasites and pathogens infecting the same host individuals. This work has implications for control strategies in human, livestock and wildlife diseases.
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 helmith (Trichostrongylus retortaeformis) on another (Graphidium strigosum) has been found to vary with host mass, a corrrelate 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.
» CIDD Dilution Effect Workshop, 2005
Environmental effects
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. For example, Cattadori, Hudson and coworkers have found that the effect of parasites on grouse populations depends on climatic variables.