A strikingly high percentage of individuals in this study tested positive for Schistosoma antibodies after brief freshwater exposure in Africa. Travellers often believe that schistosomiasis is only transmitted in stagnant water, and the students were told by locals that rafting did not pose a risk of Schistosoma infection. More than half of the students (55 %) in our study stated that their only freshwater contact was through rafting, although we cannot rule out the possibility of infection via domestic water. Cercariae can be infectious for two to three days and can be transmitted via domestic water if untreated surface water is used (2, 12). Another study of 69 people with freshwater contact in the Upper Nile detected schistosomiasis in 17 % of the participants (13).
Only a minority of seropositive individuals reported symptoms in the period after exposure that could potentially be related to chronic schistosomiasis, such as urinary tract, genital and intestinal disorders, or fatigue as a result of chronic inflammation, with the vast majority being asymptomatic (Table 2). Cercarial dermatitis, which can occur after penetration of the skin by cercariae, has been reported in 7–36 % of travellers with schistosomiasis (7, 11). Only 5 % of individuals in the current study were able to recall symptoms of this condition. Only one person who was seropositive reported symptoms consistent with acute schistosomiasis.
Since Schistosoma antibodies can be detected in the blood for several years after the infection has resolved or been treated (14), an indeterminate number of students with positive serology might already have cleared the infection spontaneously. As the worms can potentially survive for several decades, however, it is likely that many still had an active infection, and all seropositive students were therefore recommended treatment with praziquantel. The main reason for treating schistosomiasis in travellers is the risk of neuroschistosomiasis. Praziquantel is considered very effective against schistosomiasis, but is unlicensed and expensive in Norway. The recommended dose varies from 40 to 60 mg/kg, split into two doses taken on a single day (2, 7, 15, 16). Praziquantel is effective only against adult worms, and since parasite maturation is assumed to take up to 12 weeks, it is recommended to postpone treatment until at least three months after freshwater exposure, or to repeat the treatment three months after the last exposure if treatment was carried out at an earlier stage. The treatment has few side effects and is considered safe to use during pregnancy.
Tests other than serological assays had little or no diagnostic value in this study. Microscopy for the detection of eggs in urine and faeces is used as a routine diagnostic test for schistosomiasis, but has low sensitivity in travellers because of their brief exposure and hence low worm burden and egg production (6), consistent with our findings in this study.
Serological testing is the most sensitive diagnostic method available to travellers and also has high specificity (> 95 %). It is therefore suitable for detecting low-grade infection in individuals who have not previously been exposed to or treated for schistosomiasis. The main limitation of the test is its inability to distinguish active versus previous infections. It is also not 100 % sensitive, and some cases of schistosomiasis may be detected by microscopy or PCR despite negative serology (6, 17). Both serological assays and attempts to detect eggs in urine or faeces will give negative results until the parasites are sexually mature and have started egg production. A definitive diagnosis therefore cannot be made until at least 12 weeks after the last exposure. Serological diagnostic testing has previously been performed at laboratories outside Norway and has been very expensive. An affordable Schistosoma antibody test is now available at the newly established reference laboratory for parasite serology at the University Hospital of North Norway, Tromsø.
Oslo University Hospital and Haukeland University Hospital now offer real-time PCR analysis for the detection of Schistosoma spp. in urine, faeces and serum. PCR testing is more sensitive than microscopy (17–19) and is not dependent on a highly trained microscopist. PCR testing of serum can detect infection as early as a few weeks after exposure, and has high sensitivity provided that repeat sequences in the Schistosoma genome are used as the target region (17, 18, 20, 21). PCR can be positive for more than a year after treatment has been completed, owing to the presence of DNA from retained eggs (17).
Another interesting diagnostic method is antigen testing, in which the detection of circulating anodic antigen (CAA) in serum appears particularly promising with respect to diagnosing travellers and assessing treatment responses (19). The test is not yet commercially available, and its practical utility is currently uncertain.
This observational study has a number of limitations that may have affected the results. The fact that many years had passed between exposure and diagnosis in some individuals, might have led to poor recall and thus less accurate descriptions of symptoms. It would have been helpful to have the results of other diagnostic tests, including microscopy and PCR analysis of urine and faeces, for all participants; however, many opted out of these additional tests as serological analysis is easy to perform and is the most sensitive method. We did not collect information about any prior stays in endemic areas by the students, and therefore cannot rule out the possibility that some may have lived in endemic areas and have been exposed to schistosomiasis previously, which could result in a positive serology result. It would also have been useful to obtain data from those students who were not exposed to freshwater, as a negative control, but due to limited resources and the high costs of serological testing this was not possible.
The findings of this study emphasise that contact with freshwater is common among travellers to Africa, and that the incidence of schistosomiasis can be high even among individuals with only brief freshwater exposure. It is likely that the infection is underdiagnosed in Norway since many do not develop symptoms. We generally recommend serological analysis alone when testing for schistosomiasis in asymptomatic travellers with no previous exposure. Serological testing should be supplemented with PCR and/or microscopy for individuals with symptoms and those who have spent longer periods of time in endemic areas. Upon diagnosis of schistosomiasis, a single day of treatment with praziquantel 40–60 mg/kg/day is recommended. Both diagnostic testing and treatment should be performed at least three months after the last possible exposure. Post-treatment confirmation of serological status is not indicated, as antibodies can be detected long after an infection has been cleared. Local purchase and use of praziquantel shortly after exposure is common among travellers, but is not recommended as early treatment is ineffective. Schistosomiasis is a potentially serious disease, and travellers should be warned to avoid skin contact with freshwater in Africa and in endemic areas of Latin America and Asia.