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Chapter 3Infectious Diseases Related To Travel
Schistosomiasis
Susan Montgomery
INFECTIOUS AGENT
Schistosomiasis is caused by helminth parasites of the genus Schistosoma. Other helminth infections are discussed in the Helminths, Intestinal section earlier in this chapter.
MODE OF TRANSMISSION
Waterborne transmission occurs via penetration of larval cercariae in contaminated bodies of freshwater.
EPIDEMIOLOGY
An estimated 85% of the world’s cases of schistosomiasis are in Africa, where prevalence rates can exceed 50% in local populations. Schistosoma mansoni and S. haematobium are distributed throughout Africa; only S. haematobium is found in areas of the Middle East, and S. japonicum is found in Indonesia and parts of China and Southeast Asia (Map 3-14). Two other species can infect humans: S. mekongi, found in Cambodia and Laos, and S. intercalatum, found in parts of Central and West Africa. These 2 species are rarely reported causes of infection.
All ages are at risk for infection with travel to endemic areas and freshwater exposure. Swimming, bathing, and wading in contaminated freshwater can result in infection. Human schistosomiasis cannot be acquired by contact with saltwater (oceans or seas). The distribution of schistosomiasis is very focal and determined by the presence of competent snail vectors, inadequate sanitation, and infected humans. The geographic distribution of cases of schistosomiasis acquired by travelers reflects travel and immigration patterns. Most travel-associated cases of schistosomiasis are acquired in sub-Saharan Africa. Sites in Africa frequently visited by travelers are commonly reported sites of infection. These sites include rivers and water sources in the Banfora region (Burkina Faso) and areas populated by the Dogon people (Mali); Lake Malawi; Lake Tanganyika; Lake Victoria; the Omo River (Ethiopia); the Zambezi River; and the Nile River. However, as visitors travel to more uncommon sites, it is important to remember that most freshwater surface water sources in Africa are potentially contaminated and can be sources of infection.
The specific snail vectors can be difficult to identify, and snail infection with human schistosome species must be determined in the laboratory. The types of travelers and expatriates potentially at increased risk for infection include adventure travelers, Peace Corps volunteers, missionaries, soldiers, and ecotourists. Outbreaks of schistosomiasis have occurred among adventure travelers on river trips in Africa.
CLINICAL PRESENTATION
Incubation period is typically 14–84 days for acute schistosomiasis (Katayama syndrome), but chronic infection can remain asymptomatic for years. Penetration of cercariae can be associated with a rash that develops within hours or up to a week after contaminated water exposures. Acute schistosomiasis is characterized by fever, headache, myalgia, diarrhea, and respiratory symptoms. Eosinophilia is present, as well as often painful hepatomegaly or splenomegaly.
The clinical manifestations of chronic schistosomiasis are the result of host immune responses to schistosome eggs. Eggs secreted by adult worm pairs enter the circulation and lodge in organs and cause granulomatous reactions. Eosinophilia may be present. S. mansoni and S. japonicum eggs most commonly lodge in the blood vessels of the liver or intestine and can cause diarrhea, constipation, and blood in the stool. Chronic inflammation can lead to bowel wall ulceration, hyperplasia, and polyposis and, with heavy infections, to periportal liver fibrosis. S. haematobium eggs typically lodge in the urinary tract and can cause dysuria and hematuria. Calcifications in the bladder may appear late in the disease. S. haematobium infection has been associated with increased risk of bladder cancer.
Rarely, central nervous system schistosomiasis may develop; this form is thought to result from aberrant migration of adult worms or eggs depositing in the spinal cord or brain. Signs and symptoms are related to the site of the granulomas in the central nervous system and can present as transverse myelitis.
DIAGNOSIS
Diagnosis is made by microscopic identification of parasite eggs in stool (S. mansoni or S. japonicum) or urine (S. haematobium). Serologic tests are useful to diagnose light infections where egg shedding may not be consistent and in travelers and others who have not had schistosomiasis previously. Antibody tests do not distinguish between past and current infection. Available test sensitivity and specificity vary, depending on the antigen preparation used and how the test is performed.
It is important to consider screening asymptomatic people who may have been exposed during travel. These people may benefit from treatment. Additional information and assistance with diagnosis may be obtained through CDC’s Division of Parasitic Diseases and Malaria (www.dpd.cdc.gov/dpdx).
TREATMENT
Schistosomiasis is uncommon in the United States, and the inexperienced physician should consult an infectious disease or tropical medicine specialist for diagnosis and treatment. Praziquantel is used to treat schistosomiasis. Praziquantel is most effective against adult forms of the parasite and requires an immune response to the adult worm to be fully effective.
PREVENTIVE MEASURES FOR TRAVELERS
No vaccine is available. No drugs for preventing infection are available. Preventive measures are primarily avoiding wading, swimming, or other contact with freshwater in disease-endemic countries. Untreated piped water coming directly from freshwater sources may contain cercariae, but filtering with fine-mesh filters, heating bathing water to 50°C (122°F) for 5 minutes, or allowing water to stand for ≥24 hours before exposure can eliminate risk for infection.
Swimming in adequately chlorinated swimming pools is virtually always safe, even in disease-endemic countries. Vigorous towel-drying after accidental exposure to water has been suggested as a way to remove cercariae before they can penetrate, but this may only prevent some infections and should not be recommended as a preventive measure. Topical applications of the insect repellent DEET can block penetrating cercariae, but the effect depends on the repellent formulation, may be short-lived and cannot reliably prevent infection. Repellents with active ingredients other than DEET have not been evaluated, so there is no evidence of their effectiveness of preventing cercarial penetration.
BIBLIOGRAPHY
- Bierman WF, Wetsteyn JC, van Gool T. Presentation and diagnosis of imported schistosomiasis: relevance of eosinophilia, microscopy for ova, and serology. J Travel Med. 2005 Jan–Feb;12(1): 9–13.
- Corachan M. Schistosomiasis and international travel. Clin Infect Dis. 2002 Aug 15;35(4):446–50.
- Doenhoff MJ, Cioli D, Utzinger J. Praziquantel: mechanisms of action, resistance and new derivatives for schistosomiasis. Curr Opin Infect Dis. 2008 Dec;21(6):659–67.
- Grobusch MP, Muhlberger N, Jelinek T, Bisoffi Z, Corachan M, Harms G, et al. Imported schistosomiasis in Europe: sentinel surveillance data from TropNetEurop. J Travel Med. 2003 May–Jun;10(3): 164–9.
- King CH, Sturrock RF, Kariuki HC, Hamburger J. Transmission control for schistosomiasis—why it matters now. Trends Parasitol. 2006 Dec;22(12): 575–82.
- Meltzer E, Artom G, Marva E, Assous MV, Rahav G, Schwartzt E. Schistosomiasis among travelers: new aspects of an old disease. Emerg Infect Dis. 2006 Nov;12(11):1696–700.
- Morgan OW, Brunette G, Kapella BK, McAuliffe I, Katongole-Mbidde E, Li W, et al. Schistosomiasis among recreational users of Upper Nile River, Uganda, 2007. Emerg Infect Dis. 2010 May;16(5): 866–8.
- Nicolls DJ, Weld LH, Schwartz E, Reed C, von Sonnenburg F, Freedman DO, et al. Characteristics of schistosomiasis in travelers reported to the GeoSentinel Surveillance Network 1997–2008. Am J Trop Med Hyg. 2008 Nov;79(5):729–34.
- Ross AG, Bartley PB, Sleigh AC, Olds GR, Li Y, Williams GM, et al. Schistosomiasis. N Engl J Med. 2002 Apr 18;346(16):1212–20.
- Ross AG, Vickers D, Olds GR, Shah SM, McManus DP. Katayama syndrome. Lancet Infect Dis. 2007 Mar;7(3):218–24.
- WHO Expert Committee. Prevention and control of schistosomiasis and soil-transmitted helminthiasis. World Health Organ Tech Rep Ser. 2002;912:1–57.
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