The diversity of parasites

The language and jargon used by plant pathologists and animal parasitologists are often very different, and there are important differences in the ways in which animals and plants serve as habitats for parasites, and in the way they respond to infection. But for the ecologist, the differences are less striking than the resemblances, and we therefore deal with the two together. One distinction that is useful, though, is that between microparasites and macroparasites (Figure 12.1) (May & Anderson, 1979).

Microparasites are small and often intracellular, and they multiply directly within their host where they are often extremely numerous. Hence, it is generally difficult, and usually inappropriate, to estimate precisely the number of microparasites in a host. The number of infected hosts, rather than the number of parasites, is the parameter usually studied. For example, a study of a measles epidemic will involve counting the number of cases of the disease, rather than the number of particles of the measles virus.

Macroparasites have a quite different biology: they grow but do not multiply in their host, and then produce specialized infective stages (microparasites do not do this) that are released to infect new hosts. The macroparasites of animals mostly live on the body or in the body cavities (e.g. the gut), rather than within the host cells. In plants, they are generally intercellular. It is usually possible to count or at least estimate the numbers of macroparasites in or on a host (e.g. worms in an intestine or lesions on a leaf), and the numbers of parasites as well as the numbers of infected hosts can be studied by the epidemiologist.

micro- and macroparasites

Figure 12.1 Plant and animal micro- and macroparasites. (a) An animal microparasite: particles of the Plodia interpunctella granulovirus (each within its protein coat) within a cell of their insect host. (b) A plant microparasite: 'club-root disease' of crucifers caused by multiplication of Plasmodiophora brassicae. (c) An animal macroparasite: a tapeworm. (d) A plant macroparasite: powdery mildew lesions. Reproduced by permission of: (a) Dr Caroline Griffiths; (b) Holt Studios/Nigel Cattlin; (c) Andrew Syred/Science Photo Library; and (d) Geoff Kidd/Science Photo Library.

Figure 12.1 Plant and animal micro- and macroparasites. (a) An animal microparasite: particles of the Plodia interpunctella granulovirus (each within its protein coat) within a cell of their insect host. (b) A plant microparasite: 'club-root disease' of crucifers caused by multiplication of Plasmodiophora brassicae. (c) An animal macroparasite: a tapeworm. (d) A plant macroparasite: powdery mildew lesions. Reproduced by permission of: (a) Dr Caroline Griffiths; (b) Holt Studios/Nigel Cattlin; (c) Andrew Syred/Science Photo Library; and (d) Geoff Kidd/Science Photo Library.

Cutting across the distinction between micro- and macroparasites, parasites can also be subdivided into those that are transmitted directly from host to host and those that require a vector or intermediate host for transmission and therefore have an indirect life cycle. The term 'vector' signifies an animal carrying a parasite from host to host, and some vectors play no other role than as a carrier; but many vectors are also intermediate hosts within which the parasite grows and/or multiplies. Indeed, parasites with indirect life cycles may elude the simple micro/macro distinction. For example, schistosome parasites spend part of their life cycle in a snail and part in a vertebrate (in some cases a human). In the snail, the parasite multiplies and so behaves as direct and indirect life cycles: vectors a microparasite, but in an infected human the parasite grows and produces eggs but does not itself multiply, and so behaves as a macroparasite.

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