The parasitic helminth worms are major macroparasites of animals. The intestinal nematodes of humans, for example, all of which are transmitted directly, are perhaps the most important human intestinal parasites, both in terms of the number of people infected and their potential for causing ill health. There are also many types of medically important animal macroparasites with indirect life cycles. For example, the tapeworms are intestinal parasites as adults, absorbing host nutrients directly across their body wall and proliferating eggs that are voided in the host's feces. The larval stages then proceed through one or two intermediate hosts before the definitive host (in these cases, the human) is reinfected. The schistosomes, as we have seen, infect snails and vertebrates alternately. Human schistosomiasis (bilharzia) affects the gut wall where eggs become lodged, and also affects the blood vessels of the liver and lungs when eggs become trapped there too. Filarial nematodes are another group of long-lived parasites of humans that all require a period of larval development in a blood-sucking insect. One, Wucheria bancrofti, does its damage (Bancroftian filariasis) by the accumulation of adults in the lymphatic system (classically, but only rarely, leading to elephantiasis). Larvae (microfilariae) are released into the blood and are ingested by mosquitoes, which also transmit more developed, infective larvae back into the host. Another filarial nematode, Onchocerca volvulus, which causes 'river blindness', is transmitted by adult blackflies (the larvae of which live in rivers, hence the name of the disease). Here, though, it is the microfilariae that do the major damage when they are released into the skin tissue and reach the eyes.

In addition, there are lice, fleas, ticks and mites and some fungi that attack animals. Lice spend all stages of their life cycle on their host (either a mammal or a bird), and transmission is usually by direct physical contact between host individuals, often between mother and offspring. Fleas, by contrast, lay their eggs and spend their larval lives in the 'home' (usually the nest) of their host (again, a mammal or a bird). The emerging adult then actively locates a new host individual, often jumping and walking considerable distances in order to do so.

Plant macroparasites include the higher fungi that give rise to the mildews, rusts and smuts, as well as the gall-forming and mining insects, and some flowering plants that are themselves parasitic on other plants.

Direct transmission is common amongst the fungal macroparasites of plants. For example, in the development of mildew on a crop of wheat, infection involves contact between a spore (usually wind dispersed) and a leaf surface, followed by penetration of the fungus into or between the host cells, where it begins to grow, eventually becoming apparent as a lesion of altered host tissue. This phase of invasion and colonization precedes an infective stage when the lesion matures and starts to produce spores.

Indirect transmission of plant macroparasites via an intermediate host is common amongst the rust fungi. For example, in black stem rust, infection is transmitted from an annual grass host (especially the cultivated cereals such as wheat) to the barberry shrub (Berberis vulgaris) and from the barberry back to wheat. Infections on the cereal are polycyclic, i.e. within a season spores may infect and form lesions that release spores that infect further cereal plants. It is this phase of intense

Figure 12.2 A cuckoo in the nest. Reproduced by permission of FLPA/Martin B. Withers.

Figure 12.2 A cuckoo in the nest. Reproduced by permission of FLPA/Martin B. Withers.

multiplication by the parasite that is responsible for epidemic outbreaks of disease. On the other hand, the barberry is a long-lived shrub and the rust is persistent within it. Infected barberry plants may therefore serve as persistent foci for the spread of the rust into cereal crops.

Plants in a number of families have become specialized as parasites on other flowering plants. These are of two quite distinct types. Holoparasites, such as dodder (Cuscuta spp.), lack chlorophyll and are wholly dependent on the host plant for their supply of water, nutrients and fixed carbon. Hemiparasites, on the other hand, such as the mistletoes (Phoraradendron spp.), are photosynthetic but have a poorly developed root system of their own, or none at all. They form connections with the roots or stems of other species and draw most or all of their water and mineral nutrients from the host.

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