Nematode Parasites of Animals

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.

Humans

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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