Other parasites Hymenolepiasis

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Hymenolepis nana is the dwarf tapeworm and the only human tapeworm that does not require an intermediate host. Nevertheless, rodent strains for H. nana are infectious for humans including pet rodents such as rats, mice and hamsters. It occurs worldwide, with high childhood prevalences (> 10%) reported from Argentina, Peru, Brazil, Egypt, Pakistan and Zimbabwe.41 Transmission is mostly fecal-oral, either from person to person or in food and water, with high rates in children from orphanages.139

Adult worms are only 15-45 mm in length with the spherical/ovoid eggs measuring 30-45|im in diameter, which do not survive beyond 11 days and are sensitive to heat and drying. In children who ingest eggs, the larval stage develops in the intestinal villi in 4-5 days, then break into the lumen and develop into adult worms, which produce ova in the feces within a month or less. The larval stage of H. nana is a cyst-like structure about 250 |im in diameter and is called a cysticer-coid. As for other intestinal parasites, transmission is enhanced by poor hygiene and overcrowding, but control programs have found mass treatment more effective than improved hygiene.41

Most infected children are asymptomatic. High worm burdens of over 3000 may be associated with abdominal pain, loose stools and growth retardation. Disseminated larval infection has been described but is rare. Diagnosis is based on the characteristic eggs in stool. A single dose of praziquantel 20mg/kg is effective. Niclosamide 60-80 mg/kg per day for 5 days or paromomycin 45 mg/kg per day for 7 days are alternatives, but compliance is more difficult to ensure. Retreatment 10 days later may be necessary to ensure eradication of cysticercoids in tissues.

Enterobiasis

Enterobius vermicularis is the common pinworm or threadworm of the large bowel. Unlike most other helminthic infection, enterobiasis may be more common in temperate, developed countries than in poor, tropical, developing countries. For example, rates in Australian schoolchildren have been reported as 43% compared to 12% in India. However, it is worldwide in distribution and is almost exclusively transmitted person to person. The organism is a small white roundworm with the adult female worm 9-12 mm long with the male worm only 2.5 mm long and rarely observed. The worm's lifespan is 5-13 weeks for females and only 7 weeks for males, but reinfection is common. The oval eggs laid by the gravid female on the anus are 50 x 25 |im and the diagnosis is based on observation of these typical eggs from sticky tape applied to the perianal skin and then examined under a microscope. Adult pinworms or their eggs are rarely found in the feces, so this tape test for demonstrating pinworm eggs on the perianal skin is the most reliable technique.

Humans are the only host for E. vermicularis, and worms inhabit the lumen of the cecum and appendix. The gravid adult female migrates to the perianal skin where she deposits her eggs resulting in intense anal pruritus, itching of the perianal skin leads to the contamination of the fingernails with the infective eggs. Each mature female worm can produce up to 11000 eggs. Most infections are asymptomatic, but perianal pruritus is the most common symptom of enterobiasis. Eosinophilia is uncommon, since the organism does not generally invade tissues. The incubation period is 3-4 weeks.

There is no definitive evidence that pinworms cause appendicitis, since most pinworms encountered in the appendix are an incidental finding, occurring in as many normal as inflamed appendices after surgery. However, it is impossible to exclude the possibility that chronic inflammatory changes due to local pinworm infection may contribute to some cases of appendicitis. Large numbers of larval pinworms have been shown to cause eosinophilic enterocolitis in an adult.140 The most common site of infection (86.5%) is the lumen of the appendix, where the worms invoke no histological reaction. Other sites of infection include the abdominal and pelvic peritoneum and female genital tract.141 Little girls sometimes present with ectopic pinworm infection of the vagina, which can cause hysterical reactions.

The treatment of choice is albendazole or meben-dazole, but ivermectin, pyrantel and levamisole are also effective. Since reinfection is very common, it is wise to readminister the drugs 2

weeks later and treat all household members, particularly children. Prevention of enterobiasis rests largely on treating infected children, keeping nails short and possibly improving personal hygiene, although infection is not an indicator of poor hygiene.

Schistosomiasis

There are seven human species of this trematode, including S. haematobium which affects the renal tract and S. mansoni the gastrointestinal tract. It has been estimated that 220 million people in 74 countries are infected by schistosomiasis, and 20 million have severe disease. Eggs of S. mansoni are passed in the feces, and hatch in warm water; the ciliated larvae swim and penetrate fresh-water snails (Biomphalaria). After a sporocyst stage, by 4-5 weeks of infection, the larval phase results in thousands of cercariae (300 x 60|im) which penetrate human skin in water and enter peripheral lymphatics or veins and are carried to the lungs and mature in portal vessels. Adult worms migrate to the liver and mesenteric veins, where they may survive for 2-5 years or longer, producing eggs 25-28 days after cercarial infection.

Eggs cause granuloma formation resulting in localized colitis and hepatitis. The acute phase of S. mansoni infection may cause allergic symptoms (Katayama syndrome) which are rarely recognized in children. Most chronic infections are light and asymptomatic, but with heavy infections up to half of the eggs become trapped in the mucosa and submucosa of the colon, resulting in granuloma-tous reactions with significant blood loss. The host's inflammatory reaction to eggs carried to the liver in the portal veins leads to portal hypertension. Severe disease with hepatosplenomegaly affects about 10% of S. mansoni cases in endemic areas, taking 5-15 years to develop.

Diagnosis is based on finding eggs in the feces, but stool concentration methods and numerous immunological techniques (ELISA, immunoblot-ting) are more sensitive for milder infections. Treatment is with praziquantel 40 mg/kg as a single dose. Prevention involves avoidance of water sources containing cercariae and promotion of latrine use. Control programs for schistosomia-sis involve mass chemotherapy, destruction of snails, environmental sanitation, prevention of water contact, health education and the future development of vaccines.

Fasciolopsiasis

The giant intestinal fluke Fasciolopsis buski is a class of flatworm called trematodes, which includes schistosomes. It is estimated that more than 40 million people have food-borne trematode infections. Those affecting the liver and lungs will not be considered here. The adult worm of F. buski measures up to 7.5 cm in length and attaches to the mucosa of the proximal small intestine. It has a lifespan of about 6 months and begins producing eggs about 3 months after infection. These are excreted in the stool and measure 130 |im in length. Up to 25 000 eggs may be excreted daily.

After several weeks in fresh water, the larvae hatch from the eggs and penetrate snails, where they undergo further development. Cercariae emerge from the snail after 1-2 months and encyst on a wide variety of aquatic vegetation. When humans ingest these raw, they develop infection, which lodges in the duodenum. The severity of symptoms correlates with the number of parasites, but heavy infections may cause nausea, vomiting, abdominal pain, edema, eosinophilia and poor nutrition. The diagnosis of fasciolopsiasis is based upon stool microscopy identifying the eggs. These are indistinguishable from those of the liver fluke, Fasciola hepatica. However, the intestinal fluke has a short lifespan and infection does not persist beyond 9 months from the time of departure from and endemic area.

The treatment of choice is praziquantel 25 mg/kg three times daily for 1 day. The prognosis is generally excellent, except for heavy infection in children with intestinal obstruction of edematous malnutrition. The infection can be prevented by cooking aquatic vegetation or immersing plants or nuts in boiling water. The use of human feces as fertilizer in aquaculture is a major cause of human infection.

Trichostrongylus

Species of Trichostrongylus (wireworms) mostly affect ruminants, but they affect humans in close proximity to cattle, particularly in Africa,

Australia, India, Iran and South America. The small adult worms (5-10 mm long) attach to the wall of the duodenum and jejunum after ingestion, and excrete eggs similar to those of the hookworm. Light infections are asymptomatic, but abdominal pain, diarrhea and eosinophilia may occur with heavy infections. Treatment is as for hookworm, and prevention involves thorough washing or cooking of vegetables.

Parastrongylus costaricensis

This nematode was discovered in Costa Rica in 1971 and appears to be mainly confined to Latin America and the Caribbean. The natural host is the cotton rat but adult worms (20 mm long) have been found in the cecum of children with appendicitislike illnesses with eosinophilia. Infection occurs by ingesting infected slugs. Diagnosis before surgery is based upon eosinophilia, radiology or serology, but larvae are not excreted in stool and no chemotherapy is effective.

Cyclospora cayetanensis

Cyclospora has emerged recently as an important protozoal pathogen causing diarrheal disease.142 It appears to be endemic in most parts of the Americas, Africa and Asia, with infections occurring in travelers, as well as waterborne and food-borne outbreaks.143-145 Children in tropical areas appear to have a high prevalence, and the disease peaks in the hot rainy season.146,147 A Venezuelan study documented prevalences in adult AIDS patients and children with diarrhea of 9.8% vs. 5.3%, which was more common than S. stercoralis (4.2% vs. 1.5%, respectively) but less common than C. parvum (35.2% vs. 9.8%, respectively) and had a peak prevalence in children between 2 and 5 years of age.148 Other childhood diarrheal prevalences have been similar in Nepal and Guatemala, but higher in Peru and Haiti (up to 18-20%). The incidence in Peruvian periurban children was 0.21-0.28 episodes/child-years.84 A Haitian study found a prevalence of 11% in AIDS diarrhea, with symptoms identical to isosporiasis and cryp-tosporidiosis.149

Since other pathogens are often found, it is difficult to characterize the illness exactly, but prolonged watery diarrhea with weight loss is typical, often associated with fever. Mucosal biopsies show inflammatory changes with partial villus atrophy with crypt hyperplasia.150 Stool oocytes are sparse in infected immunocompetent children with diarrhea (< 10/field) but may be higher in AIDS patients. The modified Ziehl-Neelson stain of the acid-fast trichrome technique is used for diagnosis.151 Oocysts have been found in the environment in sewage, vegetable washings and animal feces.152,153 A Peruvian study in childhood found that only 20% of cases with oocysts were symptomatic and co-trimoxazole treatment for 3 days decreased excretion from 12.1 to 4.8 days.154 An adult Nepali trial of travelers confirmed this, but suggested that 7 days of co-trimoxazole was more effective (clearance rate of 94% for 7 days vs. 29% for 3 days).155

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