Travel-related Schistosomiasis Acquired in Laos

Twelve Israeli travelers acquired schistosomiasis in Laos during 2002–2008, and 7 of them had acute schistosomiasis. The patients were probably exposed to Schistosoma mekongi in southern Laos, an area known to be endemic for schistosomiasis. Four possibly were infected in northern Laos, where reports of schistosomiasis are rare.


The Study
The study was conducted at the Center for Geographic Medicine at Sheba Medical Center and was approved by the institutional review board. Travel-related schistosomiasis was defi ned as a case of schistosomiasis confi rmed by serology or ova detection in a traveler who had been exposed to freshwater in Laos. Travelers were thoroughly questioned regarding freshwater exposures during the index trip and any previous trips to schistosomiasis-endemic areas.
Serologic diagnosis conducted at the Israel Ministry of Health (MOH) Central Laboratories in Jerusalem was based on the soluble egg antigen ELISA test (IVD Research Inc., Carlsbad, CA, USA), a nonspecies-specifi c method. Consequently, most samples (11/12) Figure. Map of Laos. The area in which Schistosoma mekongi is known to be endemic is highlighted in light blue. The area highlighted in light yellow shows both the known area and the area predicted by Attwood's paleogeographic models (1) to be inhabited by Neotricula aperta (freshwater snails), the known intermediary host for S. mekongi. Two foci of travel-related schistosomiasis are also highlighted with red stars. The dark blue line shows the route of the Mekong River.
Mean patient age was 24 years (range, 6-42 years). Seven patients were exposed to freshwater in both southern and northern Laos (4,000 Islands and the town of Vang Vieng, respectively); 1 patient was exposed only in southern Laos (Figure). Notably, 4 patients reported freshwater exposure exclusively in northern Laos (Vang Vieng). Three of the 4 reported no travel in southern Laos; 1 patient (Table 1, patient 10) visited southern Laos but was not exposed to freshwater. Exposure occurred during the months of February-April for 9 of the 12 patients.
Seven patients had a diagnosis of acute schistosmiasis. Fever (86%), headache (86%), urticarial rash (71%), and cough (71%) were the most prevalent acute schistosmiasis symptoms ( Table 2). Four patients reported chronic gastrointestinal symptoms (abdominal pain and discomfort, diarrhea or loose stools). One patient described a pruritic papular rash that appeared hours after exposure and resolved within a few days (suspected cercarial dermatitis). The patient was asymptomatic upon evaluation at our clinic ( Table 1, patient 12).
Diagnosis was made by positive serology in all 12 patients. Eleven serum samples were sent to CDC for specia-tion; 9 patients had positive immunoblots for S. japonicum (Table 1). One patient had a positive immunoblot for S. haematobium; this result was judged to be a cross-reaction because the patient had never visited S. haematobium-endemic areas.
S. mekongi/japonicum ova were detected in stool samples of 1 of 7 patients who submitted such samples for ova detection (Table 1). Issues of technical profi ciency and expertise precluded a defi nite conclusion regarding speciation according to egg size. Laboratory fi ndings in 5 patients with acute schistosmiasis were signifi cant for marked eosinophilia ( Table 2).
All infected patients were treated with praziquantel at >12 weeks postexposure to avoid treatment failure (8). Of the acute schistosmiasis patients, 3 of the 7 required corticosteroid treatment during the acute illness.

Conclusions
Acute schistosmiasis, which is considered to be a hypersensitivity reaction that usually develops a few weeks after Schistosoma infection, is best studied in nonimmune travelers rather than in continuously exposed local popula- tions. We report 7 cases of acute schistosmiasis presumably caused by S. mekongi infection acquired in Laos. Acute schistosmiasis is reportedly not a species-specifi c phenomenon but may develop after infection with any Schistosoma spp. (8), a view strengthened by this report. Symptoms of acute schistosmiasis caused by S. mekongi, although a small number of cases, appear similar to symptoms of acute schistosmiasis caused by S. mansoni or S. haematobium (7) ( Table 2). The only symptom significantly more prevalent in acute schistosmiasis caused by S. mekongi was headache.
Most Schistosoma infections in travelers are acquired in Africa (8,9). In our clinic, travel-related schistosomiasis acquired outside Africa was diagnosed only in travelers exposed in Laos (8). This exposure is probably due to the popularity of water-related adventure activities among travelers to Laos.
S. mekongi-endemic areas in Laos have presumably included only the southern reaches of the Mekong River (Figure) (1,2,5). However, this assumption may refl ect a serendipitous effect because schistosomiasis in Laos was fi rst diagnosed in immigrants originating from this region. These early schistosomiasis cases led early epidemiologic surveys to the region of Khong, where most subsequent studies were performed (5,10). Since these epidemiologic surveys were conducted, S. mekongi infections acquired in northern Laos have been described only anecdotally (11)(12)(13).
In this report, we describe 4 patients with schistosomiasis apparently acquired in northern Laos (Figure) after exposure to freshwater exclusively in Vang Vieng; that is, they reported no other freshwater exposure during their visit to Laos. However, because of lack of species-specifi c serology and the inability to fi nd Schistosoma ova in these patients' stool samples, we can not determine which Schistosoma spp. caused their infection.
Most of these patients were infected during February-April, Mekong's early low-water period, indicating a seasonal infection pattern similar to that of local populations (5). The increased risk of schistosomiasis during the late dry season should be conveyed to travelers during pretravel consultations.
Our observation of Schistosoma infection in the 4 travelers exposed exclusively in Vang Vieng has several limitations. First, the diagnosis was based on positive serology and not on ova detection. Cross-reactivity of nonhuman Schistosoma spp. with S. japonicum in serologic studies (e.g., S. sinensium or S. ovuncatum) could have caused seropositivity in our patients. Second, these 4 travelers ( In other world regions (e.g., Lake Malawi in Africa), Schistosoma-infected travelers have served as sentinels alerting local authorities to previously unsuspected foci of transmission (14). The cases of schistosomiasis in travelers thought to be exposed only in northern Laos, an area where dam building may have changed local conditions, mandates a systematic revaluation of S. mekongi distribution in Laos.  (7). All values are no. (%) except as indicated. †p<0.001.