Tropical Medicine: A Clinical Text, 8th Edition, Revised and Expanded

Intestinal Cestode Infections

Intestinal Cestode Infections These worms, although varying in size from 10 mm to 20 meters, have certain characteristics in common. They are flattened and ribbonshaped . A minute head (scolex) attaches firmly to the intestinal mucosa by means of suckers, hooklets, or suctorial grooves. A short neck leads to a body made up of 100 to 4,000 segments (proglottids). Nutrients are obtained by diffusion through the body wall; there is no intestinal canal. The worms are hermaphrodites, each proglottid having a complete set of male and female organs. The proglottids of different species may be distinguished by their shape and the appearance of the uterus. The eggs contain an embryo with six hooklets (hexacanth embryo or oncosphere). All species except Hymenolepis nana require a secondary intermediate host in which the larvae develop in a cystoid structure, usually in muscle. Human infection is usually acquired by ingesting raw or undercooked meat or fish. Drug treatment is with praziquantel. Taenia saginata The beef tapeworm may grow to a length of 12 to 20 meters, the body being composed of 1,000 to 2,000 elongated segments. The head is attached to the jejunal mucosa by four suckers, and the uterus has more than 13 lateral branches. The eggs, which are usually excreted within intact gravid segments, are spherical and have a thick wall that is radially striated. The intermediate hosts are cattle, which ingest the eggs deposited on grazing land. Human infection is acquired by eating raw or undercooked beef or beef products, and also following contamination of hands while preparing raw meat in the kitchen. The diagnosis is confirmed by pressing a proglottid between two glass slides, and counting the lateral segments. PAGE 165 ................. 18086$ CH13 07-15-11 13:50:09 PS 166 intestinal cestode infections Although this long tapeworm, which may stretch from the jejunum to the rectum, seldom causes serious symptoms, the passing of segments and the wriggling of gravid proglottids through the anus causes distress. Infection is not regarded with equanimity by the almost universally infected cattle herdsmen of the Rift Valley of Kenya, and infected cattle are a cause of serious economic loss. Taenia solium This tapeworm closely resembles T. saginata. Its main importance is that both adult and larval forms parasitize man. Cysticercosis, the larval infection, will be discussed in the next chapter. The head of the adult worm is armed with two rows of hooklets in addition to the four suckers , and the uterus has less than 12 lateral branches. The eggs of beef tapeworms and those of pork tapeworms are indistinguishable. Domestic pigs are usually the intermediate hosts, but dogs are sometimes infected, and in areas such as southeast Asia where canine meat is eaten by humans, dogs play a significant secondary role in transmission. The fear of regurgitation of ova after treatment with the consequent development of cysticercosis is largely unfounded. Hymenolepis nana Hymenolepis nana, the dwarf tapeworm, measures only 25 to 40 mm in length. The head is equipped with four suckers, and a single row of hooklets mounted on a retractile rostellum. The body is composed of 100 to 200 squat segments. Infection is widespread in mice and is usually transmitted to man through the ingestion of food contaminated with mice feces. A cysticercoid larva develops in the human intestinal mucosa, matures in 10 to 12 days, and then the adult worm breaks into the lumen of the intestine to attach to the ileal mucosa. Characteristic oval eggs, with a double membrane between which four to eight filaments arising from the poles may be seen, are passed in the feces. No intermediate host is required, and autoinfection is frequent. Most infections remain silent, but heavy worm loads in children may cause abdominal pain, diarrhea, vomiting, and pruritus ani. The PAGE 166 ................. 18086$ CH13 07-15-11 13:50:09 PS intestinal cestode infections 167 treatment of choice is praziquantel, which causes destruction of both worms and larvae. Endemic regions include the Mediterranean, Egypt, Sudan, India, and Latin America. Hymenolepis diminuta is another very similar tapeworm of rats and mice, but differs from H. nana in requiring fleas, beetles, or other insects as intermediate hosts. Human infection occurs when children swallow these insects in grain. Diphyllobothrium latum Diphyllobothrium latum, the fish tapeworm measuring up to 10 meters in length, is also widespread along certain river systems in the tropics. The head is spatulate and equipped with two...