Parasitic Nematodes

Nematodes are group of pseudocoelomate animals, classified under the separate phylum Nematoda. Their digestive tract is a tube like structure with openings at both ends, thus they are often descried as organisms with a tube within a tube. Although there are beneficial species, the number of parasitic nematodes is relatively more. Of the total 28,000 scientifically known nematodes, it is estimated that nearly 16,000 are harmful in some way or the other. Scientists are of the opinion that there may be around one hundred thousand nematodes.

Parasitic Nematodes: An overview

Both parasitic and beneficial nematodes are characterized by presence of a false body cavity, hence the term pseudocoelomate is used to define these invertebrate animals. The body encloses a pseudocoel, which is lined by three distinct layers. It is believed that the body cavity is lost as a result of mutation in successive generations. Coming to parasitic nematode species, they may attack insects, plants or animals. For better understanding, parasitic nematodes in humans and plants are discussed in the following paragraphs.

Human Parasitic Nematodes
Several parasitic nematodes in humans are identified, which attack the muscles, intestines and other bodily tissues. The modes of entry include bite by an infected vector, unintentional ingestion of eggs through consumables and lastly, penetrative entry via skin. Examples of human parasitic nematodes are:

• Roundworm: Represented scientifically as Ascaris lumbricoides, this intestinal parasite is a pinkish colored, large nematode species (35 cm in length). Infection by roundworms occurs after ingesting contaminated food or through soil.
• Whipworm: Another in the list of human parasitic nematodes is the whip-like species, whipworm (Trichuris trichiura). It is transmitted from contaminated soil, and remains asymptomatic in mild intestinal infections.
• Pinworm: Of the different nematode species, pinworm (Enterobius vermicularis) causes the highest number of infections in the United States. Small in size (about 13 mm in length), pinworm infection occurs after ingestion of eggs.
• Hookworm: Under hookworm, there are two species of parasitic nematodes (Ancylostoma duodenale and Necator americanus). As per medical data, they are the second leading intestinal parasites found in humans. At the time of infection, the larvae penetrate human skin.
• Trichina: Infection of trichina (Trichinella spiralis) is called trichinosis. This parasite nematode is present in pork meat, and infection of the same is resulted after consumption of undercooked pork meat. The larvae from the stomach later migrate to the muscular tissues.

Plant Parasitic Nematodes
Believe it or not, a scoop of garden soil contains hundreds of nematodes. Thus, plant attack by parasitic nematodes usually occurs in the underground root system, where the roots are in direct contact with the soil parasites. They may make their way to the upper portion and feed on different plant parts. Common parasitic nematodes for plants are:

• Root-knot Nematode: This is the most destructive nematode genus (Meloidogyne), in which some species are capable of infecting more than 1,000 plant cultivars. After infection in lawn grass and other plants, they feed in specific root areas and cause formation of root galls.
• Stem and Bulb Nematode: Unlike the root knot nematode, this group of parasitic nematodes affect the upper plant parts, like leaves, bulbs and steams. They are classified under the genus Ditylenchus, and species name varies according to the strain.
• Cyst Nematode: In comparison to root-knot nematode, cyst nematodes parasitize in a narrow range of host plants. A commonly identified cyst nematode is Globodera pallida that exclusively attacks potato plants.
• Lesion Nematode: The lesion nematodes are classified under the taxonomic genus Pratylenchus. As the name goes, this type of plant parasitic nematode causes necrotic lesions in various part of ornamental plants, fruits, coffee, corn and other crops.
• Citrus Nematode: Identified by the scientific name, Tylenchulus semipenetrans, the citrus nematode is a serious parasite for citrus fruit plants. In severe cases, its symptoms mimic nutrient deficiency, and cause slow decline in the fruit yield.

This was all about parasitic nematodes in humans and plants. The parasitic nematode life cycle differs according to species. In general, the life cycle of beneficial and parasitic nematodes encompasses six stages. Starting with the egg stage, it hatches into larva, followed by three other immature stages, before entering the matured adult stage.