Introduction

Microparasites have been broadly defined to include viral, bacterial and protozoan pathogens (Anderson and May 1979). Mathematical models for microparasites characteristically divide the host population into susceptible, infectious and recovered and immune hosts (Anderson and May 1991). This reflects a bias towards studies of microparasites of vertebrates (especially viruses, rather than bacteria and protozoa). In this chapter we review observed epidemiological patterns in the population dynamics of microparasites in free-living animal populations. Many microparasites are characterized by their tendency to produce a sustained immunological response in infected vertebrate hosts; recovered hosts are usually considered to be resistant to further infection. Ecological studies of microparasites in wild animal populations can only occasionally differentiate between the different epidemiological classes within the host population.

More frequently tissue samples have to be collected so that serological tests may be used to differentiate between susceptible and recovered, or resistant animals. Similarly, not all pathogens produce a sustained immunological response, in some cases the pathogen is always fatal (for example, many bacterial and viral infections of invertebrates), in other cases recovered individuals simply re-enter the class of susceptible hosts. Furthermore, it is possible for infected individuals to show no overt symptoms of the disease. In some cases these individuals harbour latent infections which eventually develop and exhibit the characteristic symptoms of the disease in old, malnourished or stressed hosts. In other cases, the infected individuals show no clear symptoms, but are fully capable of transmitting the pathogen to susceptible hosts of the same or another species.

Introduction

The parasitic helminths and arthropods (ticks and fleas) are usually classified as macroparasites. The definition embraces the nematodes, cestodes, trematodes, acanthocephalans and the lice, fleas and ticks (Anderson and May 1979). In these species, reproduction usually occurs via the transmission of free-living infective larval stages, although many species have complex life cycles involving several obligatory host species. Macroparasites do not usually have direct reproduction within their definitive hosts, but asexual reproduction occurs in the intermediate hosts of the digenean trematodes and some cestodes and nematodes. When compared with the microparasites (the protozoans, viruses and bacteria), the macroparasites are relatively large, have long generation times and are immunologically characterised by a diversity of antigens. Infections by macroparasites are generally chronic leading to morbidity rather than mortality (see Lloyd this volume). It is therefore important to consider the sub-lethal effects of macroparasites on host fitness rather than more obvious lethal effects as both have important consequences for the dynamics of the interaction between the host and parasite populations.

Despite the very large number of parasites known to infect wild animals, relatively few ecologists have investigated the role of parasitic helminths and arthropods in regulating host abundance or distribution. This is surprising when one considers the numerous studies that have investigated the regulatory role of other natural enemies such as parasitoids and predators (Crawley 1992). On the one hand, ecologists and wildlife managers have traditionally seen parasites as benign symbionts living in equilibrium with their host. When environmental factors disturbed this careful balance, then the epidemic outbreaks of macroparasites could cause massive host mortalities, but these were thought to be unusual exceptions to this general rule.