Outbreak of Acute Schistosomiasis: Brief Exposure and Severe Morbidity

Abstract & Commentary

By Brian Blackburn, MD, Clinical Assistant Professor of Medicine, Division of Infectious Diseases and Geographic Medicine, Stanford University School of Medicine, is Associate Editor for Infectious Disease Alert.

Dr. Blackburn reports no financial relationships relevant to this field of study.

Synopsis: A group of travelers to Tanzania experienced a high attack rate of acute schistosomiasis after swimming in a freshwater pond. Resultant short-term morbidity was high, although no patients had developed clinical evidence of chronic infection within the first year of follow-up.

Source: Leshem E, et al. Acute schistosomiasis outbreak: clinical features and economic impact. Clin Infect Dis. 2008;47:1499-1506.

Schistosomiasis is the most important trematode infection worldwide, transmitted to humans through skin contact with infested freshwater.1 Endemic primarily to the tropics, transmission is particularly intense through much of sub-Saharan Africa. Although chronic schistosomiasis can lead to portal hypertension or serious urinary tract pathology, these complications are seen mostly in long-term residents of endemic areas and are rare in travelers.1 In contrast, acute schistosomiasis (also known as Katayama fever) is often seen among non-immune travelers, usually within 1-2 months of infection.2 Traditionally, this syndrome has been characterized by fever, cough, rash, eosinophilia, hepatosplenomegaly, and other clinical findings.2

Although several outbreaks of acute schistosomiasis have been reported, most have involved prolonged freshwater exposure and few have examined the short-term morbidity of acute schistosomiasis.3-6 Leshem et al, therefore, undertook an investigation of a cluster of cases from a group of 34 Israeli travelers that had been on a luxury safari trip to Tanzania. After the index case of acute schistosomiasis was identified, an epidemiologic investigation revealed that 27 persons in the group had been exposed to a freshwater pond at a tented lodge in Northern Tanzania. This was the only freshwater exposure for all of the travelers during this trip, and all 27 exposed persons had a single, brief exposure to the pond (mean, 39 minutes). Twenty-two (81%) of the 27 developed acute schistosomiasis, and none of the seven unexposed individuals developed schistosomiasis. Infection status was determined using stool (and urine) ova and parasite examinations, serology, and clinical findings.

Swimming time in the pond tended to be higher for infected than for non-infected (but exposed) persons (44 vs 12 minutes, p = 0.06). Cercarial dermatitis was reported shortly after exposure in three (14%) of the 22 infected persons. Symptoms experienced by those with acute schistosomiasis included: cough (78%), fever (68%), fatigue (58%), rash (37%), diarrhea (37%), and abdominal pain (26%). Three (14%) infected patients were asymptomatic. Rash and fever appeared earliest (mean, 3-4 weeks after exposure), whereas cough and gastrointestinal symptoms appeared last (mean, 5-6 weeks after exposure). Fatigue was the symptom that persisted the longest (mean 6-7 weeks), whereas cough and diarrhea lasted 3-4 weeks, and rash and fever only 1-2 weeks. Among the 22 infected travelers, elevated ALT was found in 45% and eosinophilia in 72%; the latter persisted for over a year in two persons. All infected patients were treated with praziquantel, and eight also received corticosteroids. None experienced any late complication of schistosomiasis at 12-month follow-up.

Among the 22 infected travelers, 258 medical encounters resulted (mean, 11.7 per person) and four persons were hospitalized. Most (86%) patients missed time from work or school (mean, 8 days each), and a mean of 15 leisure activities were also missed per person. Among infected persons, health-related quality of life (assessed using a standardized questionnaire) was significantly lower than US norms for three months after infection, after which they recovered from infection.


This outbreak of acute schistosomiasis was characterized by high attack rate among swimmers with a single, brief exposure to a freshwater pond in Tanzania. Although many schistosomiasis outbreaks have been reported, few have demonstrated high infection rates after such a brief exposure, and in this cohort, infected travelers were exposed only 30 minutes longer than their uninfected counterparts.

Most infected patients were symptomatic with acute schistosomiasis, and the study provides a useful description of the relative frequency of symptoms and their time course. Although underreported in other outbreaks, cough was the most commonly reported symptom. Interestingly, fever was absent in nearly one-third of symptomatic patients, despite being previously considered a hallmark of acute schistosomiasis. The severity and persistence of fatigue and the impact on quality of life also was significant.

The optimal therapy for acute schistosomiasis has yet to be determined. Although all infected patients in this outbreak received praziquantel, long-term follow-up data are not available. Praziquantel is not active against schistosomula (the immature form of the parasite), and it is known that patients treated only with praziquantel during acute schistosomiasis can develop chronic disease if they are not retreated in the following months.7 Artemesinins may be a better choice early in disease, as they do have activity against schistosomula, but clinical data to support this are lacking. In addition, although 42% of the symptomatic patients in this cohort received corticosteroids, their role in acute schistosomiasis also remains poorly defined. Notably, some patients in this cohort had persistent eosinophilia up to a year after treatment. Demonstrating cure after treatment also remains a challenge, especially given the difficulty of identifying the parasite in stool or urine samples, and the persistence of antibody for years or longer after cure.

This study demonstrates the importance of patient education regarding the mechanisms of transmission of this preventable disease and the severe morbidity that is possible with acute schistosomiasis. Travelers should be advised to avoid swimming or bathing in untreated freshwater sources in endemic areas (even on luxury trips), no matter how brief the exposure, particularly in sub-Saharan Africa.


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