Schistosomiasis in African Travelers
By Carol A. Kemper, MD, FACP
Source: Schwartz E, et al. J. Travel Med. 2005;12:13-18.
Schistosomiasis may be the most under-recognized infectious disease in developing countries in both travelers and immigrants, especially because nearly half of infections are asymptomatic. In recent years, an increased number of cases have been reported in developed countries, either because of increased travel and/or increased identification of cases. A European surveillance group catalogued over 800 cases in 2003—mostly in persons from sub-Saharan Africa. About one-fourth of these were adventure travelers.
Adventure rafting and diving tours to schistosomiasis endemic areas, such as Malawi and Victoria lakes, the Dogon Country, and the Omo River, which cuts across the southwestern corner of Ethiopia before emptying into Lake Rudolf in Kenya, are increasingly popular. Schwartz and colleagues describe a series of river rafting trips down the Omo River, and examined the risk factors for schistosomiasis. Two rafting trips in particular—a group from the United States in 1993 and an Israeli group in 1997—garnered the most intensive examination because both groups had high rates of infection, compared with other similar rafting groups.
In the US group, of 17/18 rafters available for assessment, 10 (59%) were seropositive for S. mansoni. Among the 10 infected, fever (60%), cough (40%), and abnormal liver tests (30%) were common findings. Ova were demonstrated in stools in only 5/10 (50%) seropositive individuals, and egg counts were low. In the Israeli group, 18/20 (90%) rafters available for testing were seropositive for S. mansoni. Among the 18 infected, eosinophilia (83%), fever (44%), and abnormal liver function tests (17%) were common, but cough was less common (11%). Symptoms generally occurred within 3-6 weeks of exposure.
In contrast, Schwartz et al examined a total of 84 US rafters from 12 other similar expeditions in 1994-1995, and found that none were infected. Assessment of a small convenience sample from a group of 133 Israeli rafters from 13 similar expeditions in 1995-1998 found only 1 seropositive individual (10%). Most of these rafting trips were done on either the first or second upstream leg of the Omo River (~300 Km each), and lasted about 8-15 days. Schwartz et al determined that the 2 rafting tours with high rates of infection had required re-routing because of bad road conditions, and took rafters an additional ~200 km downstream. The Omo River in this area is wider, slower moving, and more populated. In addition, large numbers of infected snails have been found in 1 of the Omo tributaries in this area. The US rafting company has since stopped tours to the Omo, and the Israeli river rafting company has stopped using this route.
Since 43% of infected persons in this series were asymptomatic but are at risk for late sequelae of infection, Schwartz et al advocate serologic screening of all travelers exposed to fresh water in these endemic areas. Egg counts and peripheral eosinophilia are insensitive and inadequate markers for infection. Fortunately, because the parasite load is low in many of these infected individuals, significant end-organ dysfunction is uncommon.
Schistosomiasis may be the most under-recognized infectious disease in developing countries in both travelers and immigrants, especially because nearly half of infections are asymptomatic.
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