Cestodes - The Tapeworms
Intestinal Parasite Infections From Tapeworms -- Description, Diagnosis, Treatment
Cestodes (or tapeworms) are parasites of the GI tract whose life cycle requires at least one intermediate host. They are long, flat worms that have a scolex or sucker located at the head. The cestodes that cause significant human diseases include Taenia saginata, Taenia solium, Hymenolepsis nana and Diphyllobothrium latum.
T. saginata, the most common tapeworm in humans, is found wherever raw or undercooked beef is eaten, especially in Africa and the Arab world. In certain cultures where raw beef is considered a delicacy, the number of tapeworms harbored may be considered a status symbol. Cattle become infected in areas where raw sewage drains onto pastures. Chemical treatment of the soil does not kill the ova, which may live in the soil for up to 6 months.
Life cycle. Humans are the definitive host of T. saginata, and infection occurs after ingestion of beef containing viable larvae. The larvae are released with digestion and mature in the upper small intestine. Adult worms may inhabit the GI tract for as long as 25 years. The adult worms release eggs that are passed through the feces onto soil and vegetation. Herbivorous animals ingest the eggs, which penetrate the GI tract and migrate to the skeletal muscle.
Clinical manifestations. The majority of T. saginata infections are asymptomatic. The most common symptoms are epigastric pain, nausea, vomiting, increased appetite, weight loss, and a spontaneous emerging of proglottid on the anus. Intestinal obstruction is a rare complication.
Diagnosis, treatment, and prevention. The diagnosis is made by finding typical eggs in the host stool. The eggs of T. saginata and T. solium are indistinguishable; however, the two may be differentiated through examination of gravid proglottids pressed between two glass slides. If more than 13 uterine branches are present on each side of the proglottid, the infection is consistent with T. saginata.
The treatment of choice for T. saginata infection is a single 5- to 10-mg/kg dose of praziquantel. Two hours after treatment, a purgative should be given to eliminate ova. Prevention of T. saginata infection occurs at two levels. First, soil contamination in areas of cattle grazing must be prevented. Second, beef should be adequately cooked to kill all larvae.
T. solium infection occurs in all areas where raw or partially cooked pork is eaten. It is common throughout Mexico, South America, and southern Europe. T. solium infection is distinctly uncommon in the United States.
Life cycle. The life cycle of T. solium is remarkably similar to that of T. saginata with the primary difference being the intermediate host, the pig. Adult worms may inhabit the human upper GI tract for as long as 25 years.
Clinical manifestations. T. solium infection is usually asymptomatic. The most common GI symptoms include abdominal pain, nausea, vomiting, and diarrhea. One of the most serious complications is cysticercosis, an infection of the larval forms of T. solium, which invade the liver, lungs, peritoneum, subcutaneous tissue, brain, eye, muscle, and heart.
Diagnosis, treatment, and prevention. The diagnosis of T. solium infection is made by the identification of typical eggs in the host’s stool or by the use of adhesive tape placed on the anus. The treatment of T. solium infection is the same as that for T. saginata. Prevention of T. solium infection can be achieved with adequate cooking of all pork and pork products.
H. nana, or the dwarf tapeworm, is found throughout the world, especially in tropical and subtropical areas, and wherever poor sanitary conditions are present. Infection is most common in children, with contaminated soil passed from hand to mouth being the most frequent mode of transmission.
Life cycle. H. nana eggs are ingested from contaminated soil and develop into the cysticercoid stage. The cysticercoids attach to the mucosa of the small intestine and mature into adult worms. The adult worms lay eggs that may undergo maturation in the small bowel or pass through the host feces.
Clinical manifestations. Most H. nana infections are asymptomatic; when symptoms occur, they are usually limited to vague abdominal discomfort. Infections with a heavy worm load may lead to nausea, vomiting, weight loss, diarrhea, and more severe abdominal pain.
Diagnosis, treatment, and prevention. Diagnosis is based upon the identification of typical eggs in the host feces. The treatment of choice for H. nana infection is a single 25-mg/kg dose of praziquantel. Prevention of H. nana infection can be achieved with improvement in sanitary conditions and personal hygiene.
D. latum infection is found in areas where freshwater fish such as pike, walleye, trout, and salmon are regularly consumed. In North America, D. latum occurs in the Great Lakes region, the western United States, Canada, and Alaska. Other areas with a high prevalence of D. latum infection include Chile, Argentina, the Baltic states, Russia, Scandinavia, and the lake areas of central Europe. Endemic areas are also reported in central Africa and parts of Asia.
Life cycle. The adult most commonly attaches itself to mucosa in the ileum, although it has been reported to inhabit the jejunum and (rarely) the colon and gallbladder. The life cycle of D. latum requires two intermediate hosts. The eggs develop in freshwater and hatch into coracidia. The coracidia must be ingested by small crustaceans (copepods) within 24 hours or death occurs. Within the copepod, the coracidia develop into procercoid larvae. When the copepod is eaten by a freshwater fish, the larvae migrate from the fish’s intestinal tract to the muscle.
Within the muscle, the larvae develop into infective plerocercoids. The fish is then eaten by a larger carnivorous fish (i.e., pike, salmon). When humans ingest the host freshwater fish, larvae develop into adult worms within the host intestine and attach to the mucosa. The adult worm lays up to 1 million eggs per day.
Clinical manifestations. Most infections with D. latum are asymptomatic; however, abdominal pain, nausea, increased appetite, and diarrhea may occur. Intestinal obstruction has been reported in heavy worm infections. Megaloblastic anemia occurs in up to 1% of D. latum infections and is associated with the presence of worms in the jejunum. Competition between the host and the worm for vitamin B12, leading to its deficiency, is the cause of the megaloblastic anemia.
Diagnosis, treatment, and prevention. The diagnosis can be made by the identification of typical operculated eggs in the host feces. The treatment of choice for D. latum infection is the same as that for T. saginata and T. solium. The megaloblastic anemia should be corrected with vitamin B12 replacement. Infection with D. latum can be prevented with adequate cooking of freshwater fish.
Source: Dr. David Bernstein, M.D.