11 Parasites and pathogens
This chapter introduces parasitism and disease within wildlife populations. It
addresses how an infection affects a population’s dynamics and how it spreads
through a population. The veterinary aspects of infection, special to each parasite
and host, are not dealt with here. Instead we look at examples of how parasites
and disease regulate populations, structure communities and affect conservation of
endangered species, reduce the potential yield of harvested populations, or are of use
in controlling pests.
Parasites feed on living hosts and (unlike predators) do not always kill the hosts.
Some parasites have many hosts, others are species specific. Parasites and pathogens
can best be divided into two classes: microparasites, which include viruses, fungi,
and bacteria, and macroparasites, such as arthropods (e.g. fleas, ticks), nematodes,
and cestodes (e.g. tapeworms). Microparasites and macroparasites have a roughly
equivalent kind of effect upon their hosts and so can be lumped together as
parasites. The debilitating effect of the parasite upon the host is termed disease. (At
the end of the book we include a glossary of terms most often used in parasitology
and epidemiology.)
All animals support many species of parasites. For example, the American robin (Turdus
migratorius) has at least 62 macroparasite species, the European starling (Sturnus
vulgaris) has 126 helminth species alone, the African buffalo over 60 species, and
we, ourselves (Homo sapiens), as many as 149 species (Windsor 1998). Many of these
species live with their hosts through a substantial portion of the life of the host,
causing some minor debilitation. These parasite species are adapted to their hosts,
and the hosts are adapted to the presence of the parasite. Such parasites are said to
be endemic. The disease caused by this type of parasite is called enzootic. (Note the
special use of the term endemic in this context. In another context a species is endemic
when it is confined naturally to one location such as an island or a habitat.)
Endemic parasites cause chronicimpacts on a host, that is, low-level, persistent,
non-lethal debilities or diseases. In contrast to endemic parasites, there are others
that cause epizooticdisease (in animals) or epidemicdisease (in humans). These
parasites cause relatively short-term, major, and often fatal debilities. As a result
of human impacts and global climate change on ecosystems, we are experiencing
the appearance of new diseases, sometimes termed emerging infectious diseases.
Enzootic and epizootic diseases have different effects on ecosystems, endangered species,
and introduced pests. Parasites may lower the standing biomass of a host population.
Hence, they are disadvantageous if the host population is to be conserved or har-
vested, and advantageous if the host population is to be controlled.
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11.1 Introduction and definitions
11.2 Effects of parasites
