Principles of Epidemiology and Microbiology
Lesson 2: Public Health Microbiology
Section V: Protozoa
2-28
2-28. NEMATODES (ROUNDWORMS)
Nematodes are cylindrical worms with tapered bodies, resembling earthworms in appearance. They range in length from about 2 mm, in the case of Strongyloides stercoralis, to about 40 cm., in the case of Ascaris lumbricoides (the intestinal roundworm). Females of Dracunculus medinensis may even attain the length of
120 cm. The sexes are usually distinct, the females being larger than the males. With the exception of the
filariae, these parasites do not require an intermediate host. They are transmitted from one host to another primarily by contamination of fingers, food, fomites, and soil with infected feces. The life cycle consists of three stages: eggs or embryos, larva stages, and adult worms. The eggs of most nematodes are passed from the definitive host in feces and, as they mature, rhabditoid larvae develop which soon become infective stage larvae. Mature eggs, when ingested by a suitable definitive host, disintegrate releasing the infective larvae which find their habitat in the new host systems; some of them maturing in the lungs before finally seeking their habitat in the intestine. A notable exception occurs in the life cycle of the hookworms where the infective stage larvae are capable of entering the host by active penetration of the skin. Then they migrate via the circulatory system to the lungs and ultimately find their habitat in the small intestine. Another exception, the filarial worms instead of producing eggs, produce larvae that swarm into the blood where mosquitoes while taking a blood meal from the host, ingest these larvae and later transmit them to a new host. Larvae produced by the guinea worms rupture from blisters formed on the skin of the host and escape into water where they are ingested by fresh-water crustaceans (copepods) that are in turn swallowed by new definitive hosts. The effect of the various hematodes on man may vary from mild, unnoticed symptoms to severe illness and death. Table 2-10 (above) lists the principal nematodes affecting man and the usual site of infestation.
