Of the eighteen Orders in the phylum Nematoda, seven contain nematodes that are parasites or associates of invertebrates, and six include species that are parasites of vertebrate animals.
Nematodes are reported as parasites and associates of many invertebrate animals, especially in the Annelida, Mollusca, and Arthropoda. In some cases, the invertebrate functions as the intermediate host in a life-cycle that includes parasitism of a vertebrate. In other cases, the invertebrate, usually an insect, functions as a vector between vertebrate hosts, or the nematode is passively transported by the insect. Several interesting plant-parasitic nematodes fall into this latter group and, significantly, they are closely related to nematode species that are parasites of insects. A considerable research effort has been applied toward using nematode parasites of insects as biological control agents, e.g., for mosquitos and blackflies (Maggenti, 1981).
Some of the nematode associates of insects are important because they vector bacteria that kill the insect. The nematode invades (or is consumed by) the insect, and bacteria are released into the insect hemolymph. When the insect is dead or near death, growth and subsequent development of nematodes occur as they utilize essential steroids supplied by the insect (Maggenti, 1981). These nematodes are also used extensively in the biological control of insects and are particularly effective against those insects that pass through at least one life stage in the soil.
Some 5,000 species of nematodes are estimated to be parasites of vertebrate animals and humans. These species are often characterized in a larger group of worm parasites as helminths. Nematode parasites of domestic vertebrate animals are managed by strategies that include control of secondary hosts or vectors and the use of chemical anthelminthics. Helminth infections of wild animals are, of course, not managed, except by attrition of infected individuals. As human demography patterns change in California, and throughout the world, the interface between the ranges and habitats of wild and domestic animals change and overlap. Consequently, the pattern of exposure of domestic animals to helminth infections is also changing, and new associations continue to be reported; for example, the incidence of heartworm (Dirofilaria immitis) infection in dogs is currently increasing in California.
In general, the nematode parasites of California wildlife have only been studied descriptively. There is much interesting biology to investigate. Studies on the diplotriaenid nematode parasites of the air sacs of California swallows indicate that the birds carry a substantial biomass of nematode parasites on their annual migrations. The studies raise interesting ecological questions regarding flight efficiency and energetics, and also provide models for considering the distribution of parasites.
Both freshwater and marine fish are subject to nematode infections. The impact of the infections on the health and longevity of fish in nature is generally unknown. Frequently, nematodes are observed in the tissues of fish purchased by consumers. The nematodes are usually killed during cooking, but certainly the transfer of live fish parasites to humans can occur during consumption of sashimi and other raw fish products. Generally, these nematodes will not establish a permanent infection in humans, but they may cause intestinal disorders in attempting to do so.
There are other well-known examples of the transfer of nematodes to humans. In most cases, the incidence of infection is relatively low due to regulatory inspection of food products, public education, and cooking of food. An example is trichinosis caused by the nematode Trichinella spiralis. Humans become infected by Trichinella by eating raw or undercooked pork.
The nematode parasites of humans cause a variety of disease conditions and symptoms, ranging from lack of energy and vigor to blindness and malformations. Pinworms, hookworms, and roundworms are extremely common intestinal helminth infections of humans; worldwide, roundworms are probably the most common, but in the U.S., pinworms predominate. Pinworm transmittal generally occurs through ingestion of fecal-contaminated material, and infection occurs commonly in children.
Other helminth infections are vectored as filarial worms by insects such as mosquitos, or the filaria may penetrate directly through the skin from water or soil. Filarial worms cause such diseases as river blindness (Onchocerca volvulus) and elephantiasis which are major health problems in some third-world countries. In the United States, most helminth infections of humans are controlled by public health programs, public education, vector control, intermediate host control, and anthelminthic drugs. However, changing demographic patterns, including the immigration of new California residents from third-world countries, has resulted in the introduction of unfamiliar helminth infections into the state. Frequently, the faculty in the Departments of Nematology are consulted by public health officials for identification of unfamiliar nematodes.
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