Hymenolepiasis

Hymenolepiasis is infestation by one of two species of tapeworm:

• Hymenolepis nana
• Hymenolepis diminuta

Alternative names are:

• Dwarf tapeworm infection
• Rat tapeworm

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Causes, incidence, and risk factors

  Hymenolepis worms live in warm climates and are common in the southern USA. The eggs of these worms are ingested by insects, and mature into a life form referred to as a "cysticercoid" in the insect. In H. nana, the insect is always a beetle. Humans and other animals become infected when they intentionally or unintentionally eat material contaminated by insects. In an infected person, it is possible for the worm's entire life-cycle to be completed in the bowel, so infection can persist for years. Hymenolepis nana infections are much more common than Hymenolepis diminuta infections in humans because, in addition to being spread by insects, the disease can be spread directly from person to person by eggs in feces. When this happens, H. nana oncosphere larvae encyst in the intestinal wall and develop into cysticercoids and then adults. These infections were previously common in the southeastern USA, and have been described in crowded environments and individuals confined to institutions. However, the disease occurs throughout the world. H. nana infections can grow worse over time because, unlike in most tapeworms, H. nana eggs can hatch and develop without ever leaving the definitive host.

A study in Connecticut found that one third of rats sold in pet stores were infected with H. nana and concluded that these and other rodents sold in pet stores pose a potential threat to public health.

Life Cycle of H. diminuta

The risk of human infection from ‘’H. dimunuta’’ is very low, since its main host is the rat. Also known as the "rat tapeworm", H. diminuta adults live and mate in the bowels of rats. Eggs of H. diminuta are excreted by the rats as droppings, which are frequently consumed by beetles. Once inside the beetle, the eggs mature into a “cysticercoid”. The juvenile tapeworms claw their way out of the beetle gut into the circulatory system by means of their three pairs of hooks. There, they wait for a rat to ingest the host beetle, where they mature to adult form, lay eggs and restart the entire cycle. ^[1]

Beetle Manipulation

‘’H. diminuta’’ has an effective mechanism for interspecies transfection. Beetles are more likely to ingest rat droppings that are infected with tapeworm eggs. These droppings produce a fragrance (although it is not known if it is produced specifically by the eggs or the droppings) that attracts the beetle. ‘’H. diminuta’’ also sterilizes its beetle host. This is so the beetle does not waste energy in its reproductive system, allowing ‘’H. diminuta’’ to further exploit the beetle’s metabolic resources. ^[1]

Screening for activity against H. nana

H. nana in mice

Used because - Human infection—easily maintained in mice - Armed scolex similar to other pathogenic tapeworms - Corresponds to other tapeworms in its sensitivity to standard anthelmintics

Methods

  Mature worms collected from infected mice
  Terminal gravid proglottids removed, crushed under coverslip—eggs removed

Eggs containing hooklets (mature) counted 0.2 ml stock soln. containing 1000 eggs/ml given to each mouse. Adult worm develops- 15-17 days. Test drug given orally – autopsied on 3rd day Std. drug given Intestine examined under dissecting microscope for worms/ scolex Response – no. of mice cleared.

Symptoms

It is not clear that hymenolepiasis necessarily have any symptoms. The symptoms of hymenolepiasis are traditionally described as abdominal pain, loss of appetite (anorexia), itching around the anus, irritability and diarrhea. However, in one study of 25 patients conducted in Peru, successful treatment of the infection made no significant difference to symptoms.^[2] Some authorities report that heavily infected cases are more likely to be symptomatic.^[3][4]

Signs and tests

Examination of the stool for eggs and parasites confirms the diagnosis. The eggs and proglottids of H. nana are smaller than H. diminuta. Proglottids of both are relatively wide and have three testes. Identifying the parasites to the species level is often unnecessary from a medical perspective, as the treatment is the same for both.

Treatment

Praziquantel as a single dose (25 mg/kg) is the current treatment of choice for hymenolepiasis and has an efficacy of 96%. Single dose albendazole (400 mg) is also very efficacious (>95%). Niclosamide has also been used.

A three-day course of nitazoxanide is 75–93% efficacious. The dose is 1g daily for adults and children over 12; 400mg daily for children aged 4 to 11 years; and 200mg daily for children aged 3 years or younger.^[2][5][6]

Prognosis

Cure rates are extremely good with modern treatments, but it is unclear that successful cure results in any symptomatic benefit to patients.^[2]

Complications

• abdominal discomfort
• dehydration from prolonged diarrhea

Prevention

Good hygiene, public health and sanitation programs, and elimination of rats help prevent the spread of hymenolepiasis.

Source

• Hymenolepiasis. Medline Plus.

References

1. ^ ^{a b} Zimmer, Carl (2001). Parasite rex: inside the bizarre world of nature's most dangerous creatures. New York: Simon & Schuster. ISBN 0-7432-0011-X. 
2. ^ ^{a b c} Chero JC, Saito M, Bustos JA, et al. (2007). "Hymenolepis nana infection: symptoms and response to nitazoxanide in field conditions.". Trans R Soc Trop Med Hyg 101 (2): 203–5. doi:10.1016/j.trstmh.2006.04.004.
3. ^ Chitchang S, Plamjinda T, Yodmani B, Radomyos P. (1985). "Relationship between severity of the symptom and the number of Hymenolepis nana after treatment.". J Med Assoc Thai 68: 423–26.
4. ^ Schantz PM. (1996). "Tapeworm (cestodiasis).". Gastroenterol Clin North Am 25: 637–53.
5. ^ Ortiz JJ, Favennec L, Chegne NL, Gargala G. (2002). "Comparative clinical studis of nitazoxanide, albendazole and praziquantel in the treatment of ascariasis, trichuriasis, and hymenolepiasis in children from Peru". Trans R Soc Trop med Hyg 96: 193–96. PMID 12055813.
6. ^ Reomero-Cabello R, Guerro LR, Munez-Gracia MR, Geyne Cruz A. (1997). "Nitazoxanide for the treatment of intestinal protozoan and helminthic infections in México.". Trans R Soc Trop Med Hyg 91: 701–3.