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Helminths: Pathogenesis and Defenses.


Wakelin D.


In: Baron S1, editor.


Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Chapter 87.

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University of Texas Medical Branch at Galveston, Galveston, Texas


Helminths - worms - are some of the world's commonest parasites (see Ch. 86). They belong to two major groups of animals, the flatworms or Platyhelminthes (flukes and tapeworms) and the roundworms or Nematoda. All are relatively large and some are very large, exceeding one meter in length. Their bodies have well-developed organ systems, especially reproductive organs, and most helminths are active feeders. The bodies of flatworms are flattened and covered by a plasma membrane, whereas roundworms are cylindrical and covered by a tough cuticle. Flatworms are usually hermaphroditic whereas roundworms have separate sexes; both have an immense reproductive capacity. The most serious helminth infections are acquired in poor tropical and subtropical areas, but some also occur in the developed world; other, less serious, infections are worldwide in distribution. Exposure to infection is influenced by climate, hygiene, food preferences, and contact with vectors. Many potential infections are eliminated by host defenses; others become established and may persist for prolonged periods, even years. Although infections are often asymptomatic, severe pathology can occur. Because worms are large and often migrate through the body, they can damage the host's tissues directly by their activity or metabolism. Damage also occurs indirectly as a result of host defense mechanisms. Almost all organ systems can be affected. Host defense can act through nonspecific mechanisms of resistance and through specific immune responses. Antibody-mediated, cellular, and inflammatory mechanisms all contribute to resistance. However, many worms successfully avoid host defenses in a variety of ways, and can survive in the face of otherwise effective host responses.

Copyright © 1996, The University of Texas Medical Branch at Galveston.

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