The Class Nematoda contains over half a million described species, which are variously free-living, or parasitic on plants, invertebrates or vertebrates. In animals, nematodes are found in almost every organ system, including the gastrointestinal and respiratory tracts, circulation, skin and nervous system, where they feed on the blood and secretions of the host, and often cause disease. The life cycles of parasitic nematodes show enormous diversity, and their immature stages may variously live free in the environment or use intermediate hosts such as molluscs and insects. The host range can be very narrow or extremely broad. The relationship between hosts and their nematodes, like other parasites, is dynamic and disease can be severe or undetectable depending on parasite burdens, virulence and host response.
The Class Nematoda includes important parasites of wild and domestic animals, as well as humans. Gastrointestinal nematodes are so far the only parasites shown to regulate wild vertebrate populations (of grouse and reindeer), and are also major causes of disease and production loss in farmed livestock. Members of the superfamily Filarioidea, meanwhile, cause important human diseases such as elephantiasis and river blindness. Some nematodes of animals can occasionally infect humans to cause disease. Control of nematodes in domestic animals is largely achieved by strategic use of anthelmintic drugs, but must also take account of parasite population dynamics. Control strategies are continually being refined, and drug resistance increasingly encourages us to consider alternative methods.