Nematodes are unsegmented worms. Nematodes can be found in every environment, in every region of the world. They also can occur in tremendously high numbers. For example, one decomposing apple found on the ground, contained 90,000 nematodes . Nematodes are probably the most abundant animals living on Earth. About 80,000 species have been described in scientific literature; researchers estimate that about 1 million living species exist. They range in length from one millimeter for terrestrial and freshwater species to over 9 meters.4

Nematodes can be free-living or parasitic. Free-living nematodes have slender, elongated bodies that taper at both ends. They are not segmented like earthworms, but smooth-bodied. Parasitic nematodes have a variety of shapes, many sac-like. Parasitic nematodes can infest plants, animals and humans. Diseases and conditions caused by nematodes range from mild to severe. For example, the condition elephantiasis is caused by nematodes in the lymph system, It causes swelling in the arms and legs by blocking lymph vessels.1

Although nematode-caused diseases attract public attention, most nematodes are not harmful. Free-living roundworms live everywhere; soil-, freshwater, and marine environments feeding primarily on bacteria. As many as a billion roundworms per acre have been counted in the top 1 inch of fertile soil. Free-living nematodes aerate soil, consume detritus, and circulated mineral and organic components of soil and sea sediments.4

Classification of Nematodes

Members of the phylum Nematoda are grouped into two classes: Adenophorea and Secernentea. Trichinella is a parasitic member of Adenophorea class. Many members of Secernentea live in plants, vertebrates, and insects. Hookworms, pinworms, and filaria worms belong to this class.

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Nematode Infections in Man and Animals

These infections are mostly associated with chronic, debilitating conditions and, relatively frequently, with acute disease. Nematode parasites usually develop from eggs in the environment to an infective free-living larvae. Infection may occur by ingestionand skin penetration, or be transmitted by insect.2

Parasitic Nematodes in Fishes

Parasitic nematodes constitue on of the earliest known groups of helminths in fishes. They infect freshwater, marine and brackish-water fish species and sometimes cause substantial dmage to the host. Although parasitic nematodes can infect almost organs in a fish, the majority of currently known species have been described from the intestine. Most nematodes infect fish as adults, but a large proportion of them occur as larval stages. These are usually parasites of fish-eating (pischivorous) birds, mammals, or reptiles, or less frequently predatory fishes.3


  1. The worm book: the complete guide to worms in your garden. Loren Nancarrow, Janet Hogan Taylor
  2. Advances in Parasitology, Volume 43. John R. Baker, Ralph Muller, David Rollinson
  3. Fish Diseases and Disorders: Protozoan and metazoan infections. P. T. K. Woo, John F. Leatherland
  4. Five kingdoms: an illustrated guide to the phyla of life on earth. Lynn Margulis, Karlene V. Schwartz

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