Important Species

Five species are important parasites of humans: Echinococcus granulosis and Taenia solium are dangerous as larvae; Taenia saginata, T. solium, Hy-menolepsis nana, and Diphyllobothrium latum live as adults in the intestine. Several other species can also infect humankind, but usually have other hosts. For example, Dipylidium canium, the dog tapeworm, can spread to children who accidentally eat fleas. Larval stages of several species of the genus Spirometra, normally parasitic in other vertebrates, can cause a dangerous condition called sparaganosis if a person swallows them in their copepod hosts. The Oriental custom of treating wounds or inflamed eyes with a poultice of fresh frog flesh can permit larvae to become established in the patient.

T. saginata, the beef tapeworm, inhabits human intestines around the world, although it is no longer common in developed countries. Cattle and other bovids are the intermediate hosts. If a cow or water buffalo eats grass contaminated with feces and eggs, the eggs hatch in the animal's intestine into a larva that migrates through the intestinal wall and forms a bladderlike sac, a cysticercus, in the muscles. Humans acquire the worm by eating raw or poorly cooked beef, as in steak tartare. Infections are sometimes asymptomatic, but many people experience mild to severe abdominal discomfort and a few have convulsions and develop problems of malnutrition. Prevention is by meat inspection, improved rural sanitation, and proper cooking. In the last instance, as is the case with many other parasites, higher prices for fuels in poor countries are often accompanied by increasing incidence.

T. solium, the pork tapeworm, is much less common than the beef tapeworm, but potentially is a considerably more dangerous parasite. It occurs around the world, except where Islamic or Jewish customs restrict pork consumption. The life cycle resembles T. saginata, except that wild and domestic swine are the intermediate hosts. Human infection usually results from eating poorly cooked pork; sausages can be especially dangerous. People can also serve as intermediate hosts if they ingest eggs in food or water or from soiled hands. The resulting larval infection, cysticercosis, can be very serious and even fatal, especially if cysticeroids develop in the brain. Adults cause symptoms like those of the beef tapeworm. Inspection and thorough cooking or freezing of pork are important for prevention, and adult infections should be treated to avoid the danger of cysticerosis.

H. nana, the dwarf tapeworm, is only 1 to lj inches long. It occurs around the world, including the southern United States, and is a common parasite of domestic mice was well as humans. Infection results from eating larvae in fleas or in the grain beetle Tenebrio. Autoinfection is also common. In this direct life cycle, eggs hatch in a person's intestine, and the larvae attach to the intestinal wall to mature. Heavy infection can cause severe diarrhea, abdominal pain, and convulsions, especially in young children.

D. latum, the broad fish tapeworm, was described in 1602 and recognized as a distinct species in 1758, but its complex life cycle was not fully worked out until 1917. D. latum is an old parasite of humankind; eggs have been discovered in pre-Christian archaeological sites in northern Germany. This large tapeworm is found in the Baltic region, the Alps, the lower Danube, in much of European Russia, in scattered places in Central Asia, the Far East, Africa, and Alaska, and in the Great Lakes area of North America. The worm was probably introduced into the United States and Canada by Scandinavian immigrants. D. latum has a complex life cycle, with the first larval stage in small freshwater crustaceans and the second stage, the pleurocercoid, in the muscles of fish of the trout, pike, and perch families. People become infected by eating raw or undercooked or undersalted fish. Cooks preparing gefilte fish or pickled or salt fish dishes become infected as they sample the product to see if it is properly seasoned. Adult worms produce as many as a million eggs a day. If the host defecates in or near water, motile larvae emerge that seek a crustacean to complete the cycle. A related species with a life cycle involving marine fish and sea lions has afflicted inhabitants of coastal Peru and Chile since pre-Columbian times.

D. latum may exceed 35 feet in length and, like the beef and pork tapeworms, can thrive for many years in its host's gut. Symptoms are similar to those of other tapeworms, but in rare cases this worm can produce a form of anemia by robbing the host of vitamin Bi2. There seems to be a genetic component to this complication, with Finnish populations most vulnerable. Treatment is effective, and prevention consists of better sanitation and cooking fish to kill the pleurocercoid larvae.

K. David Patterson

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