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Cestodes: Intestinal and Extraintestinal Tapeworm Infections, Including Echinococcosis and Cysticercosis
Infections by cestodes (tapeworms, or flat, segmented worms) in the United States, Canada, and other affluent countries are rare but do occur in specific at-risk populations related to geographic, behavioral, or migration risk characteristics. Clinical suspicion is needed to detect quiescent infections or asymptomatic disease.
Some of the tapeworms affecting humans are endemic in the United States, Canada, and other developed nations. Practically, these can be divided clinically into tapeworm infections that are purely intestinal-dwelling ( Taenia saginata , beef tapeworm; Taenia solium , pork tapeworm; and Diphyllobothrium latum , fish tapeworm), purely tissue invasive (echinococcosis), or with both phases (pork tapeworm and cysticercosis). All other human tapeworm infections are usually not clinically significant but reflect poor socioeconomic or immigration status. Rarely, transmission of tapeworm infections introduced by means of infected food handlers under unhygienic circumstances (a public health issue) has been reported. Table 46.1 shows the common tapeworm infections affecting humans, their usual distribution, and clinical significance.
TABLE 46.1
Distribution and Usual Clinical Significance of Tapeworms Affecting Humans
| Parasite | Distribution | Usual Clinical Significance |
|---|---|---|
| Taenia saginata (beef tapeworm) | T. saginata is common in cattle-breeding regions worldwide with humans being a definitive host and cattle the intermediate host. Areas with the highest (i.e., >10%) prevalence are Central Asia, the Near East, and Central and Eastern Africa. Areas with low (i.e., 1%) prevalence are Southeast Asia, Europe, and Central and South America. Pre-patent period: 3-5 months Life span: up to 25 years Length of worms: 4-8 m |
Adult tapeworms live in the gastrointestinal tract of the human host. Eggs are excreted in the stools, and motile tapeworm segments can also be expelled from the bowels. The beef tapeworm does not cause invasive disease in humans, but in regions where distribution overlaps with pork tapeworm, must be distinguished from the latter, which does cause tissue infections in people. All ages, races, and genders are susceptible to infection that is acquired by eating larvae-infected undercooked beef meat. |
| Taenia solium (pork tapeworm) | T. solium is endemic in Central and South America, Southeast Asia, India, the Philippines, Africa, Eastern Europe, and China, with humans being a definitive host and pigs the intermediate host. Areas of highest prevalence include Latin America and Africa. In some regions of Mexico, prevalence of infection may reach 3.6% of the general population. Pre-patent period: 3-5 months Life span: up to 25 years Length of worms: 3-5 m |
Adult tapeworms live in the gastrointestinal tract of the human host. Eggs are excreted in the stools, and motile tapeworm segments can also be expelled from the bowels. The pork tapeworm larvae causes invasive disease in humans affecting soft tissues and the brain (cysticercosis). All ages, races, and genders are susceptible to infection that is acquired by eating larvae-infected undercooked pork meat or by ingestion of pork tapeworm eggs. |
| Diphyllobothrium latum (fish tapeworm) | In North America, D. latum infections have been previously reported in fish from the Great Lakes. There are six Diphyllobothrium species known to reside in Alaskan lakes and rivers, and some saltwater species may also be seen in North America. Diphyllobothrium infections are not species specific, and widespread reports describe infection in North American fish-eating birds and mammals. Humans are a definitive host, and crustaceans, followed by fish, are intermediate hosts. The incidence in the USA has been declining recently. Pike, perch, and salmon are among the fish most commonly infected. Reports are commonly made of D. latum infection in humans residing in Europe, Africa, and the Far East. Pre-patent period: 3-5 weeks Life span: up to 25 years Length of worms: 4-10 m |
Adult tapeworms live in the gastrointestinal tract of the human host. Eggs are excreted in the stools, and motile tapeworm segments can also be expelled from the bowels. The fish tapeworm does not cause invasive disease but due to its length and potential to interfere with vitamin B12 absorption can cause a number of nonspecific symptoms. All ages, races, and genders are susceptible to infection that is acquired by eating undercooked, infected fish flesh. People preparing fresh fish, implements used to prepare fish (e.g., knives and cutting boards), and raw or undercooked fish meals (e.g., sushi, sashimi, ceviche) may be associated with a higher risk of infection. |
| Dipylidium caninum | D. caninum is a cosmopolitan tapeworm infection of dogs with inadvertent human infections occurring through ingestion of the intermediate host, a flea that has fed on the tapeworm eggs contaminating the animal's fur or dog feces. Human infections have been reported in Europe, the Philippines, China, Japan, Argentina, and North America. Pre-patent period: 3-4 weeks Life span: <1 year Length of worms: 10-70 cm |
Adult tapeworms live in the gastrointestinal tract of the inadvertent human host, usually a child. Perianal irritation may occur with the passage of motile segments of the tapeworm; small “grain of rice–like” motile segments may be seen in the stools. The proglottids are motile when passed and may be mistaken for maggots or fly larvae. |
| Hymenolepis nana (dwarf tapeworm) | H. nana is a cosmopolitan intestinal tapeworm usually infecting rodents (mice or rats). The intermediate host, a beetle, is not required to complete its lifecycle in definitive hosts. Ingestion of tapeworm eggs by a definitive host, including humans, can reestablish an adult tapeworm infection. Pre-patent period: 2-3 weeks Life span: many years due to autoinfection Length of worms: 2.5-4 cm |
Often associated with environments with poor sanitation, the dwarf tapeworm causes few clinical problems with nonspecific abdominal complaints, loosening of the stools, perianal irritation, and the possible presence of small motile segments visible in the stool or on undergarments. |
| Hymenolepis diminuta | The rat tapeworm requires a grain beetle as an intermediate host, so is most common in grain-producing areas of the world or where grain or other dry foods are stored. Human infections are uncommon. Pre-patent period: 3 weeks Life span: <1 year Length of worms: 20-60 cm |
Often associated with environments with poor sanitation, the rat tapeworm rarely infects humans and causes few clinical problems with nonspecific abdominal complaints, loosening of the stools, and perianal irritation and the possible presence of small motile segments visible in the stool or on undergarments. |
| Echinococcus granulosis and Echinococcus vogeli | E. granulosis is a tapeworm of canines (dogs) with other vertebrates, most commonly sheep, as the intermedia te host. Ingesting tapeworm eggs passed in the stools of infected dogs infects humans. Once common in all sheep-raising areas of the world (Asia, Europe, the Americas, Africa, and Oceania), animal husbandry practices are resulting in effective disease control in most affected areas. | The larval tapeworm infection in humans causes hydatid disease with large, complex cystic masses occurring most commonly in the liver, but any organ can be affected, presenting with mass effects. Cysts can rupture, causing allergic reactions and anaphylactic shock. Cysts can also present with secondary bacterial infection. Pulmonary cysts can spontaneously rupture and be expelled through the mouth. If intraperitoneal rupture of a primary cyst occurs, multiple secondary cysts can develop in the peritoneal cavity. |
| Echinococcus multilocularis | E. multilocularis is a tapeworm of canines, with foxes and wolves being particularly important definitive hosts with other vertebrates (small rodents such as voles, lemmings, and mice) as intermediate hosts. Humans become infected by ingesting tapeworm eggs passed in the stools of infected canines. This is a rare form of echinococcosis that occurs predominantly in Arctic regions of Europe and North America. It can be prevented by carefully washing strawberries, cranberries, blueberries and other foods that may be contaminated with canine feces. | The larval tapeworm infection in humans causes alveolar hydatid disease with large, complex externally budding cystic masses, occurring most commonly in the liver. Due to its rapid growth and budding characteristics, this form of echinococcosis behaves more like a malignancy displacing liver tissue than an indolent infection, such as cystic hydrosis. |
Humans acquire infective tapeworm larvae or eggs through ingestion of contaminated soil or infected food or by accidentally swallowing the intermediate vectors such as the flea or beetle. Echinococcus spp. are tapeworms of carnivorous mammals, usually dogs, that serve as the definitive hosts for the parasite. The extraintestinal larval stages of Echinococcus spp. cause cystic mass lesions, most often in the liver of sheep, deer, or moose, but can affect virtually all other organs as well. Humans are an inadvertent and usually “dead-end” intermediate host.
Epidemiology and Demographics of Human Tapeworm Infections in Developed Nations
Intestinal Tapeworms
In the United States and other developed nations, tapeworm infections are uncommon in the general population and are found more frequently in defined migrant, ethnic, and certain cultural groups with specific risks, such as exposures in endemic regions of the world, dietary practices, food choices, and methods of food preparation. Occupational risk exposures include certain animal contacts, veterinary care, and animal control. Humans act as the definitive host for the beef, pork, and fish tapeworm, becoming infected through the ingestion of encysted larvae in the muscles of various animals. Humans can also be infected by ingesting tapeworm eggs of T. solium and Echinococcus spp., both leading to cystic tissue disease, or Hymenolepis nana , leading to development of an adult worm and patent infection.
In the typical lifecycle of tapeworm infection, ingesting the cyst-infected meat releases the larval forms in the gastrointestinal tract. The larvae attach to the mucosal lining of the small intestine by the head, or scolex . The tapeworm grows from each scolex, forming proglottids containing both male and female reproductive organs. Adult tapeworms (beef, pork, and fish) may reach lengths of 4.5-10 m. The proglottids may detach from each other, forming a short chain of segments (strobila) containing fertilized eggs. These segments and eggs are passed in the stool. If the scolex and neck of the tapeworm remain attached to the host gut mucosa, it can continue to produce proglottids for many years.
The beef tapeworm ( T. saginata ) infection is most common in people who eat raw or undercooked beef dishes. Recent immigrant groups from areas endemic to T. saginata in cattle-rearing Africa (i.e., Ethiopia), cattle and llama-rearing areas in Latin America, and reindeer-rearing areas in the Northern Hemisphere are at greatest risk.
The adult pork tapeworms and cystic forms of the infection (cysticercosis) are found among people exposed to undercooked pork or in contact with the excreted eggs of T. solium. This is most common in immigrants and refugees from endemic areas in the developing world.
Infection with the fish tapeworm, D. latum and other Diphyllobothrium species, occurs when eating raw, smoked, pickled, or undercooked fish. Those at risk are the Inuit, fishermen, Jewish home cooks (uncooked gefilte fish), and consumers of sushi and sashimi, ceviche, or caviar made from infected fresh water fish and roe. The highest risk fish are raw salmon and other anadromous fish (fish ascending rivers to spawn): American shad, blueback herring, short-nose sturgeon, striped bass, and steelhead trout. In Europe, these fish include pike, perch, and turbot.
H. nana and Hymenolepis diminuta are rodent tapeworms with rare human infections in developed nations. Human infections are through accidental ingestion of rodent feces containing the tapeworm egg ( H. nana ), or larvae-containing fleas and grain beetles ( H. nana and H. diminuta ) . Young children in rodent-infested environments are at risk of H. nana . Person-to-person transmission of H. nana occurs through fecal-oral contamination.
Dipylidium caninum infection usually occurs in children living in households with exposures to a dog, where accidental ingestion of an infected flea can result in a human infection.
Tissue Tapeworms
In the United States, areas endemic to Echinococcus granulosus exist in sheep-raising areas of California, Utah, and in Alaska, where dogs, wolves, and other canines are frequently infected. Immigrants from the Mediterranean region, the Middle East, and South America are also at greater risk. Alveolar hydatid disease caused by Echinococcus multilocularis is endemic in Alaska and the Arctic regions, central North America involving some parts of Canada, the United States, central Europe, Siberia, and northern Japan.
Cysticercus cellulosae are the tissue lesions caused by larval dissemination of T. solium. This disease occurs in Mexico, Central and South America, Africa, India, China, Eastern Europe, and Indonesia. Cysticercosis infection of the central nervous system is a common cause of seizures among immigrants from Latin America to the United States.
People are usually infected with T. solium when the eggs passed in the stool are ingested in contaminated food or water. Person-to-person transmission in household settings has been documented in non-endemic areas. Autoinfection by refluxing T. solium eggs within the human gut may be a means of larval infection. The larvae are found in subcutaneous connective tissue. The second most common site is the eye, followed by the brain, muscle, heart, liver, and lungs. Cysts may cause irreparable damage to the eye.
Sparganosis has been reported from many countries of the world but is most common in eastern Asia and rarely elsewhere. It is almost impossible to identify spargana to the species level, making it unclear as to which species actually infect humans. Infection in humans is caused by swallowing the first intermediate infected copepod hosts in contaminated drinking water or eating raw or undercooked amphibians, reptiles, or mammals, which are second intermediate hosts in endemic areas. The spargana can migrate to virtually any part of the body and grow up to 6 cm in length. Asymptomatic soft tissue or internal organ infection is most common in humans. Surgical removal, if necessary, is the usual treatment.
Clinical Presentation of Tapeworm Infections
Intestinal tapeworm infections usually asymptomatic. Nonspecific abdominal pain, cramps, and diarrhea have been described. Discovery of infection occurs when a motile segment or a “tape” of proglottids is passed during a bowel movement or spontaneously appears in the undergarments. Perianal or vaginal irritation may also occur. Chronic D. latum infection may rarely cause vitamin B12 deficiency and megaloblastic anemia.
Tapeworm infections are confirmed by the presence of eggs or segments found in stool specimens during parasitological examination.
Tissue infection with Echinococcus spp. ( E. granulosis or E. vogeli ) causes hydatid disease and most commonly presents with a mass effect of the slow-growing cystic lesions, usually in the liver, taking decades to reach symptomatic size. Primary cysts may reach 25 cm or more in diameter. Rupture of a cyst can release antigenic material, causing an acute allergic or anaphylactic reaction. Secondary bacterial infection of liver cysts can present as a liver abscess. Previous leakage of a liver cyst into the peritoneal cavity may cause multiple secondary cystic growths throughout the abdominal cavity. Pulmonary echinococcal cysts are usually asymptomatic, but if they rupture into the bronchial tree they can cause hemoptysis and coughing out of daughter cysts that look like grape skins. Echinococcal cysts in other organs (e.g., skin, bone, brain, eye, and visceral organs) have been described. Alveolar echinococcal infections ( E. multilocularis ) present as a rapidly progressive solid tumor mass in the liver. Due to their aggressive presentations, these infections clinically behave like a malignancy.
Tissue infection with cysticerci of T. solium infection is usually asymptomatic, even when the cysts are in the brain. Neurocysticercosis is a common cause of seizure disorder in Mexico, Central and South America, Africa, India, China, Eastern Europe, and Indonesia. A rare form of central nervous infection, called racemose cysticercosis, is associated with grape-like cystic growths in the brain causing obstructive hydrocephalus, arachnoiditis, and cerebellitis.
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