Nematodes 101

Nematodes are nonsegmented, cylindrical helminths. In English, this means that nematodes are round worms. There are SO MANY nematodes (most are free-living), but only some affect the human host.

Infections caused by nematodes are among the most prevalent, affecting nearly all humans at one time in our lives. The typical nematode, both larva and adult, is surrounded by a flexible, durable outer coating - the cuticle. Imagine like the skin of snakes. The cuticle not only protects the worm but may also be involved in the active transport of water, electrolytes, and organic compounds.

You will learn three groups of nematodes:

1. Intestinal Nematodes: these are adult worms that live in the intestine of humans. We will cover the soil-transmitted helminths and other nematodes of minor medical importance (e.g., Anisakis, Capillaria). Because of its uniqueness, we gave Strongyloides a separate lesson.
2. Filarial Nematodes: these are adult worms that live in tissues (different locations), but larval forms can be in tissues or in blood. All filarial nematodes do part of its life cycle in an intermediate host that functions as a vector for the disease.
3. Zoonotic Nematodes: these are adults that live in the intestine of animals (definitive hosts). Humans become infected as accidental (also called paratenic) hosts, and larval forms cause most of the problems. Toxocara and Ancylostoma braziliensis will be described together in Zoonotic Nematodes I, and other zoonotic nematodes (e.g., Angiostrongylus) which are more rare, will only be briefly described in Zoonotic Nematodes II.

Nematodes are very similar to humans - they eat, they have organs, and they reproduce.

Digestive tract: nematodes have a complete functional gut - including an oral (e.g., buccal) cavity, an esophagus, a midgut, and a hindgut with anus. Nematodes excrete solid and fluid wastes. In addition, a number of specialized exocrine glands open into the lumen of the digestive tract (e.g., in hookworms, cephalic glands secrete an anticoagulant).

Neuro-muscular system: the nervous system consists of a dorsal nerve ring or a series of ganglia that give rise to the peripheral nerves. Nematodes have a well-developed muscular system that allows for fluid, serpiginous movements. The longitudinal muscular system lies just underneath the cuticle, and their origins and insertions are in cuticular processes.

Reproductive apparatus: the adult female nematode has a large portion of her body devoted to reproduction (e.g., ovaries, vagina, uterus), and so is the male with a single testis, vas deferens, seminal vesicle, ejaculatory duct and a cloaca. After fertilization, gravid adult females lay eggs. Reproductive organs often help identify these helminths under the microscope.