Microparasites

Probably the most obvious microparasites are the bacteria and viruses that infect animals (such as the measles virus and the typhoid bacterium) and plants (e.g. the yellow net viruses of beet and tomato and the bacterial crown gall disease). The other major group of microparasites affecting animals is the protozoa (e.g. the tryp-anosomes that cause sleeping sickness and the Plasmodium species that cause malaria). In plant hosts some of the simpler fungi behave as microparasites.

The transmission of a microparasite from one host to another can be in some cases almost instantaneous, as in venereal disease and the short-lived infective agents carried in the water droplets of coughs and sneezes (influenza, measles, etc.). In other species the parasite may spend an extended dormant period 'waiting' for its new host. This is the case with the ingestion of food or water contaminated with the protozoan Entamoeba histolytica, which causes amoebic dysentery, and with the plant parasite Plasmodiophora brassicae, which causes 'club-root disease' of crucifers.

Alternatively, a microparasite may depend on a vector for its spread. The two most economically important groups of vector-transmitted protozoan parasites of animals are the try-panosomes, transmitted by various vectors including tsetse flies (Glossina spp.) and causing sleeping sickness in humans and nagana in domesticated (and wild) mammals, and the various species of Plasmodium, transmitted by anopheline mosquitoes and causing malaria. In both these cases, the flies act also as intermediate hosts, i.e. the parasite multiplies within them.

Many plant viruses are transmitted by aphids. In some 'nonpersistent' species (e.g. cauliflower mosaic virus), the virus is only viable in the vector for 1 h or so and is often borne only on the aphid's mouthparts. In other 'circulative' species (e.g. lettuce necrotic yellow virus), the virus passes from the aphid's gut to its circulatory system and thence to its salivary glands. Here, there is a latent period before the vector becomes infective, but it then remains infective for an extended period. Finally, there are 'propagative' viruses (e.g. the potato leaf roll virus) that multiply within the aphid. Nematode worms are also widespread vectors of plant viruses.

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