Hymenolepiasis is infestation by one of two species of tapeworm: Hymenolepis nana or Hymenolepis diminuta. Alternative names are Dwarf tapeworm infection and Rat tapeworm infection. The disease is a type of helminthiasis which is classified as a neglected tropical disease.
Causes
Hymenolepis worms live in the intestines of rats and are common in warm climates. Hymenolepis is generally found in the feces of rats which is consumed by its secondary hosts: beetles. The worms 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 if left untreated. 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.
Hymenolepis diminuta
The risk of human infection from H. diminuta 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.
Beetle Manipulation
H. diminuta has an effective mechanism for interspecies transfection. Beetles prefer to ingest rat droppings infected with tapeworm eggs, because of their odor. It is not known if the odor is produced specifically by the eggs or the droppings. H. diminuta also sterilizes its beetle host, if female. This is so the beetle does not waste energy in its reproductive system, allowing H. diminuta to further exploit the beetle’s metabolic resources.
Hymenolepis nana
Hymenolepis nana is a tapeworm, belonging to the class Cestoidea, phylum platyhelminthes. It consists of a linear series of sets of reproductive organs of both sexes; each set is referred to as a genitaluim and the area arount it is a proglottid. New proglottids are continuously differentiated near the anterior end in a process called strobilation. Each segment moves toward the posterior end as a new one takes its place and, during the process, becomes sexually mature. The proglottid can copulate with itself with others in the strobilla, or with those in other worms. When the segment reaches the end of its strobila, it disintegrates on route, releasing eggs in a process called apolysis.
Life Cycle
Hymenolepis nana is the only cestode that is capable of completing its life cycle without an intermediate host. It can, however, pass through an intermediate host as well. The most common intermediate host for H. nana are arthropods (ex. Flour beetles). When eggs are ingested by the definitive host, they hatch and release a six hook larva called the oncosphere (hexacanth) which penetrates the villi of the small intestine and develop into a cysticercoi.
Infection
Transmission of H.nana occurs via the fecal-oral route. It also occurs by accidental ingestion of insect containing the cysticercoid.
Screening for activity against H. nana
H. nana in mice Used because
- Human infectionâ€"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 coverslipâ€"eggs removed
- Eggs containing hooklets (mature) counted
- 0.2 ml stock soln. containing 1000 eggs/ml given to each mouse.
- Adult worm develops â€" 15â€"17 days.
- Test drug given orally â€" autopsied on 3rd day
- Std. drug given
- Intestine examined under dissecting microscope for worms/ scolex
- Response â€" 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. Some authorities report that heavily infected cases are more likely to be symptomatic.
Symptoms in humans are due to allergic responses or systematic toxaemia caused by waste products of the tapeworm. Light infections are usually symptomless, whereas infection with more than 2000 worms can cause enteritis, abdominal pain, diarrhea, loss of appetite, restlessness, irritability, restless sleep, anal and nasal pruritus. Rare symptoms include anorexia, increased appetite, vomiting, nausea, bloody diarrhea, hives, extremity pain, headache, dizziness and behavioral disturbances. Occasionally epileptic seizures occur in infected children.
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.
Pathology and clinical manifestations of hymenolepiasis
Hymenolepis nana lodges itself in the intestines and absorbs nutrients from the intestinal lumen. In human adults, the tapeworm is more of a nuisance than a health problem, but in small children, many H. nana can be dangerous. Usually it is the larva of this tapeworm that causes the most problem in children. The larva will burrow into the walls of the intestine, if there are enough tapeworms in the child, severe damage can be inflicted. This is done by absorbing all the nutrition from the food the child eats. Usually a single tapeworm will not cause health issues. Hymenolepis nana usually will not cause deaths unless in extreme circumstances and usually in young children or in people who have weakened immune systems. In some parts of the world, individuals that are heavily infected are a result of internal autoinfection.
Treatment
The 2 drugs that have been described for the treatment of hymenolepiasis are praziquantel and niclosamide. Praziquantel, which is parasiticidal in a single dose for all the stages of the parasite, is the drug of choice because it acts very rapidly against H. nana. Although structurally unrelated to other anthelminthics, it kills both adult worms and larvae. In vitro the drug produces vacuolization and disruption of the tegument in the neck of the worms, but not in more posterior portions of the strobila. Praziquantel is well absorbed while taken orally, it undergoes first-pass metabolism and 80% of the dose is excreted as metabolites in urine within 24 hours. Preventing fecal contamination of food and water in institutions and crowded areas is of primary importance. General sanitation and rodent and insect control (especially control of fleas and grain insects) are also essential for prevention of H. nana infection. Repeated treatment is required for Hymenolepis nana at an interval of 7-10 days.
Praziquantel as a single dose (25Â mg/kg) is the current treatment of choice for hymenolepiasis and has an efficacy of 96%. Single dose albendazole (400Â mg) is also very efficacious (>95%). Niclosamide has also been used.
A three-day course of nitazoxanide is 75â€"93% efficacious. The dose is 1g daily for adults and children over 12; 400 mg daily for children aged 4 to 11 years; and 200 mg daily for children aged 3 years or younger.
Prognosis
Cure rates are extremely good with modern treatments, but it is unclear that successful cure results in any symptomatic benefit to patients.
Complications
- abdominal discomfort
- dehydration from prolonged diarrhea
Prevention
Good hygiene, public health and sanitation programs, and elimination of infected rats will help to prevent the spread of hymenolepiasis.
Sources
- MedlinePlus Encyclopedia Hymenolepiasis