OVERVIEW: What every clinician needs to know

Parasite name and classification

Cestodes, or tapeworms, include multiple species of flat worms that can reside in the human gastrointestinal tract. The species that most commonly cause human disease include Taenia saginatum, Taenia solium, Diphyllobothrium latum and Hymenolepis nana.

What is the best treatment?

Praziquantal is the treatment of choice at varying doses:

– T. saginatum, T. solium, D. latum – 10mg/kg orally once

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– H. nana – 25mg/kg orally once, then repeated one week later

Albendazole or praziquantal can be used for neurocysticercosis.

What are the clinical manifestations of infection with this organism?

  • Many infected patients are asymptomatic. The most common gastrointestinal symptoms include nausea, diarrhea, epigastric discomfort or decreased appetite. There can also be nonspecific complaints including anxiety, fatigue or dizziness.

  • Whole adult worms, or segments (proglottids) may be seen in the stool.

  • One of the classic, although rare, manifestations of D.latuminfection is vitamin B12 deficiency and megaloblastic anemia. In these cases, fatigue, and peripheral and central neuropathy including parasthesias and ataxia may develop.

  • Most of the time there are no key physical exam findings. Patients may appear fatigued, or have mild to moderate abdominal pain on palpation.

  • In patients with megaloblastic anemia, classic findings of this condition may be present including glossitis, ataxia and decreased sensation, especially in the lower extremities.

Do other diseases mimic its manifestations?

  • Other infectious causes of gastroenteritis, including viral or mild bacterial infections, can mimic cestode infection.

What laboratory studies should you order and what should you expect to find?

Results consistent with the diagnosis (provide discussion of interpretation):

  • In some cases there can be a mild peripheral eosinophilia

  • A classic finding in D. latum infection can be Vitamin B12 deficiency, caused by the worm, and which in a minority of cases, results in megaloblastic anemia.

Results that confirm the diagnosis

  • The diagnoses of these infections are made by identifying proglottids or eggs in the stool using microscopy.

  • Because shedding is intermittent, repeat sampling or concentrating the specimen before examination is often necessary to increase sensitivity.

  • The species of Taenia can be distinguished by proglottid morphology.

What imaging studies will be helpful in making or excluding the diagnosis of cestodes?

  • Imaging studies are generally not helpful in making the diagnosis of intestinal cestode (tapeworm) infection.

  • If T. solium infection is complicated by cysticercosis (see complications section below for details), plain X-ray ($) of the muscles may reveal calcifications.

  • In cases of neurocysticercosis, brain computed tomography (CT) or magnetic resonance imaging (MRI) can help elucidate the number and location of cysts ($$$-$$$$).

  • ($ = 60-125, $$ 125-500, $$$ 500-1,000, $$$$ > 1,000)

What complications can be associated with this parasitic infection, and are there additional treatments that can help to alleviate these complications?

D. latum infection can result in vitamin B12 deficiency and resultant megaloblastic anemia as noted above. Patients may need vitamin B12 replacement after infection is eradicated.

If the eggs of T. solium are ingested, cysticercosis results which manifests as calcified cysts in the muscle and other tissues. If cysts occur in the central nervous system (CNS), they can cause complications such as seizures if located in the brain parenchyma or hydrocephalus if obstructing the ventricles.

Treatment of patients with cysticercosis depends on the clinical manifestations of the disease. Different treatments will apply to cysts confined to non-CNS areas, such as skeletal muscle, asymptomatic cases many not require treatment, and excision can be performed for solitary lesions.

Central nervous system disease

Antiepileptic therapy should be initiated immediately if a patient presents with seizures due to neurocysticercosis. Surgical shunting or other measures to decrease intracranial pressure are required for patients with hydrocephalus. A combination of corticosteroids to decrease inflammation and either albendazole 400mg po bid or praziquantel 33mg/kg orally three times daily on day one, then 17mg/kg orally three times daily.

What is the life cycle of the parasite, and how does the life cycle explain infection in humans?

Parasite life cycle
  • Humans are the only definitive hosts in which T. saginatum and T. soliumcan complete its life cycle.

  • Eggs passed by infected humans can be ingested by cattle (T. saginatum) or pigs (T. solium).

  • The eggs then hatch and the organism migrates through the intestinal wall and hematogenously to the muscles, forming cysterci.

  • Humans then become infected by eating undercooked meat that contains cysterci.

  • In the intestine, protoscolices are released from the cysts and attach to the intestinal wall, gradually adding proglottids over time that contain eggs.

  • The proglottids break off and are passed in the stool, releasing eggs.

  • Direct ingestion of eggs from fecal-oral contamination or autoinoculation leads to cysticercosis (see complications above)

  • Taenia infections are found worldwide, and are increased in regions where cattle or swine are kept in close proximity to human waste.

  • Intestinal tapeworm infection is not present in communities that do not eat raw or undercooked meat.

  • Cysticercosis, another manifestation of T. soliuminfection described above (in Complications), can occur from contamination with human feces, and can occur in people who do not eat meat.

  • D. latumis predominately seen in Northern Europe and Japan, in populations that eat raw or undercooked freshwater fish (sushi, ceviche, smoked or pickled fish). However, cases have been exported to other regions as fish are shipped worldwide.

  • Adult parasites live in the intestinal tract and shed eggs and proglottids in the stool.

  • Once in water, coracidia hatch from the eggs and enter small crustaceans

  • The coracidia develop into larvae in the crustacean, which in turn is ingested by a small freshwater fish.

  • The larvae migrate to the muscles, and these fish are eaten by larger predator fish.

  • The larvae can then migrate to the muscles of these larger fish.

  • After humans ingest the raw or undercooked flesh of these fish, the parasite matures into the adult tapeworm in the intestine.

  • Cases are decreasing in some areas as sewage treatment separates human feces from freshwater.

  • The growing popularity of raw or undercooked fish dishes has at the same time increased the pool of people at risk worldwide.

  • H. nanais the most common human tapeworm.

  • It is the only tapeworm that does not require an intermediate host.

  • Infected humans pass eggs in their stool.

  • If ingested by another person due to contaminated food or water the eggs develop into larvae, and then adult tapeworms, in the ileum.

  • Infection is most common in India, Thailand, Egypt, Sudan and Central and South America.

Prevention and Infection control issues
  • For all the intestinal cestodes (tapeworm infections) there is no effective prophylactic treatment or vaccine.

  • Good sanitation and thorough cooking of meat and fish are the primary prevention measures.

How does this organism cause disease?

  • Adult tapeworms in the intestine cause minimal inflammatory response.

  • There may be mild elevations in peripheral eosinophils or intraluminal IgE, but this is not sufficient to clear the organism or stimulate a brisk immune response.

  • Cysts of T. solium in cysticercosis does provoke a stronger response initially with neutrophils, eosinophils and macrophages. This becomes quiescent as the cysts encapsulate and calcify, but can increase again as the cysts degenerate, leading to sometimes very destructive local immune responses with dire clinical consequences particularly for cysts located in the CNS.

WHAT’S THE EVIDENCE for specific management and treatment recommendations?

Del Brutto, OH. “Neurocysticercosis: a review”. ScientificWorld Journal. 2012. pp. 159821(Good overview of the epidemiology, pathogenesis and treatment of neurocysticercosis.)

Flisser, A, Avila, G, Maravilla, P. “Taenia solium: current understanding of laboratory animal models of taeniosis”. Parasitology. vol. 137. 2010. pp. 347-57. (Reviews the current knowledge about the pathophysiology of Taenia infections based on animal modeling.)

Scholz, T, Garcia, HH, Kuchta, R, Wicht, B. “Update on the human broad tapeworm (genus diphyllobothrium), including clinical relevance”. Clin Microbiol Rev. vol. 22. 2009. pp. 146-60. (Overview of human infection with fish tapeworms.)