Schistosomiasis

doi:10.1016/j.idc.2012.03.004

Infectious Disease Clinics of North America

Volume 26, Issue 2, June 2012, Pages 383-397

https://doi.org/10.1016/j.idc.2012.03.004 Get rights and content

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The parasites

Adult schistosomes are macroscopically visible worms, with a white-grayish, cylindrical body approximately 1 to 1.5 cm in length.1, 2, 3 Taxonomically, they belong to the class of Trematoda (flukes) and the phylum of Platyhelminthes (flatworms). The worms live in the bloodstream of the human host, where they can survive for up to 30 years with an average of 3 to 5 years. This is a remarkable evolutionary achievement, which requires intricate evasion mechanisms to all forms of innate and

Transmission

The transmission cycle of schistosomes is shown in Fig. 2. The female worms produce hundreds (Schistosoma mansoni and S haematobium) to thousands (S japonicum) of eggs per day, which are 100 to 180 μm in size and have a typical shape and spine that varies per species. Each egg contains one ciliated larva, the miracidium, which secretes enzymes that help the eggs to migrate from the blood vessels through the tissues into the lumen of the bladder (S haematobium) or the intestine (S mansoni and S

Distribution and epidemiology

Humans are host to six Schistosoma species, which are morphologically very similar. Their distribution is primarily determined by the presence of the intermediate hosts. The different location of the species in the human body leads not only to a different excretion path, but also to different pathology. Three species are of global or regional importance (Fig. 3): S japonicum (China and Southeast Asia) and S mansoni (Africa, Arabia, and South America) dwell in peri-intestinal venula, and cause

Acute pathology

Minutes after the cercarial infection, local urticaria may appear that usually last for a few hours but sometimes persist for days as more papulous lesions (Fig. 4). This “swimmers itch” is mainly seen after primary infections, especially in tourists and migrants. A similar syndrome can also be caused by animal trematodes in temperate climate zones, including North America and Europe.

One to four weeks after infection, the migrating and maturing schistosomula can cause a systemic

Chronic pathology

Particularly in local populations, the main morbidity is caused by years of chronic infection in which not the worms but the eggs are the culprit.1, 2, 3, 15 As described, during their migration most eggs are trapped in tissues surrounding the intestinal or urinary systems, or in the liver and spleen after being evacuated by the bloodstream. The enzymes excreted to assist their penetration cause subsequently inflammatory and granulomatous immune reactions, characterized by eosinophilic

Diagnosis

The gold standard for the diagnosis of Schistosoma infection is the microscopic detection of eggs in stools or urine.1, 2, 3, 24 The typical size and shape allow an easy distinction from other helminth eggs, and among schistosome species (Fig. 9). Concentration methods and examination of several specimens may be needed to detect light infections.²⁵ For epidemiologic purposes, the eggs can be counted in a fixed amount of urine or feces, allowing an estimation of intensity of infection.1, 2, 3

Treatment

Until the 1970s, the treatment of schistosomiasis was almost as difficult and toxic as it still is today for trypanosomiasis and leishmaniasis.10, 33 Because pathology was limited in many cases, the decision to treat or not could be a real dilemma and wide-scale application for control purposes was a hazardous undertaking. In the early 1980s, praziquantel, an acylated quinoline–pyrazine active against all schistosome species in a single oral dose, came on the market.³³ It paralyzes and kills

Control

The control and prevention of schistosomiasis used to be largely based on the chemical destruction or environmental management of snail populations, a difficult, costly, and mostly inefficient method (Fig. 10).1, 3, 10 Some countries, most notably Japan in the 1950s, have nevertheless been able to eliminate schistosomiasis by combining this approach with water supply, sanitation, and environmental interventions.³⁸ Nowadays, the emphasis has entirely shifted to preventive chemotherapy of

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