Types of Parasitic Nematodes

Nematodes are small worms found in water, soil, plants and animals, and there are roughly 10,000 known species throughout the world. While some nematodes are free-living, others are parasitic and need other organisms (called hosts) to keep themselves alive. Once attached to their host, they divert nutrients and feed off of blood, tissues or pieces of cells to facilitate their own growth. While in some cases these parasitic nematodes can help control pests, in other cases they can cause damage, illness or death to the host organism.

Science aquaculture fish parasite. (Image: defun/iStock/Getty Images)

Human and Animal Parasitic Nematodes

Human parasitic nematodes like hookworm, whipworm, Ascaris (parasitic roundworm), filarial worms, eyeworms, trichinella, tapeworms and flukes are estimated to infect as many as 3 billion people. Each of these worms has adapted to its environment to find a unique way of entering its human host. Some enter our bodies by burrowing directly through the skin from soil or water, while others make their way to our intestines in the food we eat.

Many of these parasitic nematodes also infect animals, livestock and pets. For example, eyeworm attacks baboons as well as humans, and many species closely related to Ascaris infect dogs, cats, cattle, chickens, pigs and horses. In some cases, animals may be intermediate hosts where the nematodes enter and grow for a period of time as larvae and then become dormant cysts. If a human eats the infected meat, the cysts become active larvae again and grow into adult worms. This is the case with tapeworms that infect cows, fish or pigs and then latch onto the intestinal wall of the human that consumes them. Similarly, the trichinella roundworm lives and mates in the intestines of pigs, rats and other animals and, when those animals are eaten by other carnivores (humans or other animals), the parasite is passed on, causing a disease called Trichinosis. Heartworm is another common animal parasitic nematode that infects pets.

Plant Parasitic Nematodes

Plant parasitic nematodes invade the roots of plants and position themselves to divert nutrients away from the plant toward their own growth. There are two types of plant parasitic nematodes. Ectoparasites feed from the outside of plant tissue and endoparasites enter the plant tissue in order to feed. These parasites destroy the plant by damaging its vascular tissue and interfering with the transport of nutrients or by creating open wounds that leave it susceptible to other pathogens. One type of plant parasitic nematode called root knot (species Meloidogyne) causes an estimated 80 billion dollars in crop damage annually. Other plant parasitic nematode species include root-lesion nematodes (Pratylenchus), pin nematodes (Paratylenchus), ring nematodes (Criconemella), stubby-root nematodes (Trichodorus and Paratrichodorus), daggar nematodes (Xiphinema) and "mint nematodes"(Longidorus).

Insect Parasitic Nematodes

Insect parasitic nematodes are called entomopathogenic. They are free-living as adults but infect a host insect during their larval stage. They remain in the host insect until they grown to juvenile stage, and then they exit the insect by rupturing a hole in the host's cuticle. While some insects survive this exit, most die. Insect parasitic nematodes are sometimes used as biological control agents because they can be produced and used in mass numbers to attack and kill insect pests such as blackflies and mosquitoes.

Eight other families of insect parasitic nematodes contain species that attack and use insects as hosts, including Allantone-matidae, Neotylenchidae, Mermithidae, Diplogasteridae, Heterorhabditidae, Sphaerulariidae, Rhabditidae, Steinernematidae and Tetradonematidae.

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