Parasitic Helminths

Helminths, or worms, are soft-bodied invertebrate animals. Their adult forms range in size from a few millimeters to a meter or more in length, but their immature stages (eggs, or ova, and larvae) are of microscopic dimensions. Relatively few species of helminths are parasitic for humans, but these few are widely distributed. It has been estimated that 30% of the earth's human inhabitants harbor some species of parasitic worm.

There are two major groups of helminths: the roundworms, or nematodes, and the flatworms, or platyhelminths. The latter are again subdivided into two groups: the tapeworms (cestodes) and flukes (trematodes). A summary of the major characteristics of these groups is given here.

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Figure 32.2 Acanthamoeba trophozoite (bottom center) and cysts (refractile objects at top right) isolated from the contact lens of a patient who required a corneal transplant because of the infection. The tiny objects throughout the background are cells of Escherichia coli on which the amebae feed when grown in culture.

Figure 32.2 Acanthamoeba trophozoite (bottom center) and cysts (refractile objects at top right) isolated from the contact lens of a patient who required a corneal transplant because of the infection. The tiny objects throughout the background are cells of Escherichia coli on which the amebae feed when grown in culture.

Refractile Objects Cell Culture

Roundworms (Nematodes). Roundworms are cylindrical worms with bilateral symmetry. Most species have two sexes, the female being a copious egg producer. These ova hatch into larval forms that go through several stages and finally develop into adults. In some instances, the eggs of these worms are infective for humans when swallowed. In the intestinal tract they develop into adults and produce local symptoms of disease. In other cases, the larval form, which develops in soil, is infective when it penetrates the skin and is carried through the body, finding its way finally into the intestinal tract where the adults develop. In the case of Trichinella (the agent of trichinosis), the larvae are ingested in infected meat, but penetrate beyond the bowel and become encysted in muscle tissue. One group of roundworms, the flaria, are carried by arthropods and enter the body by way of an insect bite. (See table 32.2.)

Flatworms (Platyhelminths). Flatworms are flattened worms that also show bilateral symmetry. Some are long and segmented (tapeworms); others are short and nonsegmented. Most are hermaphroditic.

Tapeworms (Cestodes). Tapeworms are long, ribbonlike flatworms composed of individual segments (proglottids), each of which contains both male and female sex organs. The tiny head, or scolex, may be equipped with hooklets and suckers for attachment to the intestinal wall. The whole length of the tapeworm, the strobila, may have only three or four proglottids or several hundred. Eggs are produced in the proglottids (which are then said to be gravid) and are extruded into the bowel lumen. Often the gravid proglottids break away intact and are passed in the feces. All tapeworm infections are acquired through ingestion of an infective immature form, in most cases larvae encysted in animal meat or fish (e.g. Diphyllobothrium latum, see colorplate 54). Usually development into adult forms occurs in the intestinal tract, and the tapeworm remains localized there. In one type of tapeworm infection, echinococcosis, the eggs are ingested, penetrate out of the bowel, and develop into larval forms in the deep tissues (see colorplate 55).

Flukes (Trematodes). Some flukes are short, ovoid or leaf-shaped, and hermaphroditic; others are elongate, thin, and bisexual. The flukes are not segmented. They are usually grouped according to the site of the body where the adult lives and produces its eggs, that is, blood, intestinal, liver, and lung flukes. Some of these infections are acquired through the ingestion of larval forms encysted in plant, fish, or animal tissues. In others, a larval form (swimming freely in contaminated water) penetrates the skin and makes its way into deep tissues.

Table 32.2 summarizes the important helminths that cause disease in humans.

Laboratory Diagnosis

Almost all parasitic diseases, whether intestinal or extraintestinal, are diagnosed by finding the organism in appropriate clinical specimens, usually by microscopic examination. Intestinal infections are generally limited to the bowel, and therefore, fecal material is the specimen of choice. In extraintestinal infections, the diagnostic stage of the parasite may be

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Bacterial Vaginosis Facts

Bacterial Vaginosis Facts

This fact sheet is designed to provide you with information on Bacterial Vaginosis. Bacterial vaginosis is an abnormal vaginal condition that is characterized by vaginal discharge and results from an overgrowth of atypical bacteria in the vagina.

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  • yusef
    What are refractile bodies in helminths?
    8 years ago

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