Elsevier

The Lancet

Volume 348, Issue 9037, 9 November 1996, Pages 1274-1278
The Lancet

Articles
Schistosomiasis in Lake Malawi

https://doi.org/10.1016/S0140-6736(96)01511-5Get rights and content

Summary

Background

In 1992 two US Peace Corps volunteers (PCVs) developed central nervous system schistosomiasis due to infection with Schistosoma haematobium following recreational water exposure at Cape Maclear on Lake Malawi, an African lake considered by many to be free of schistosomiasis. To determine the transmission potential and risk for aquiring schistosomiasis in Lake Malawi, a cross-sectional survey of resident expatriates and visitors to Malawi was done during March and April, 1993.

Methods

A volunteer cohort of expatriates and visitors representing a cross-section of Malawi's foriegn population answered detailed questions about freshwater contact and provided blood specimens to determine the seroprevalence of S haematobium and S mansoni by ELISA and immunoblot analyses. A survey for vector snails was conducted along Lake Malawi's southwestern shore.

Findings

The study population of 955 included 305 US citizens and 650 non-US foreign nationals. 303 of the study population had serological evidence of current or past schistosome infection. Seroprevalence was 32% (141/440) among expatriates whose freshwater exposure was limited to Lake Malawi; S haematobium antibodies were found in 135 of 141 (96%) seropositive specimens. Risk of seropositivity increased with the number of freshwater exposures at Lake Malawi resorts. Although many resort areas in the southwestern lake region posed a significant risk, Cape Maclear was the location most strongly associated with seropositivity (OR 2·9, 95% CI 1·6–5·1). Schistosome-infected Bulinus globosus, the snail vector of S haematobium in Malawi, were found at Cape Maclear and other locations along the lakeshore.

Interpretation

S haematobium infection is highly prevalent among expatriates and tourists in Malawi. Recreational water contact at popular resorts on Lake Malawi is the most likely source of infection. Transmission of schistosomiasis is occurring in Lake Malawi, a previously under-recognised site of transmission.

Introduction

Schistosomiasis (ie, snail fever or bilharzia) is a parasistic infection caused by trematodes (flukes).WHO estimates that at present 200 million people are infected and 600 million are at risk world-wide. Three major species of schistosomes infect human beings: Schistosoma haematobium primarily causes genitourinary disease, whereas S mansoni and S japonicum primarily cause intestinal and hepatosplenic disease. Human beings are infected through skin contact with free-swimming cercariae that develop in freshwater snails

Schistosomiasis has been endemic in Malawi for several decades.1 Surveys have identified schistosomiasis in villages near Lake Malawi, but this finding has generally been attributed to local streams and rivers rather than the lake itself.1 The open waters and shorelines of Lake Malawi have long been considered risk-free with respect to schistosomiasis transmission.2, 3 As a consequence, tourist guides have described Lake Malawi as one of the last “schistosomiasis-free” freshwater lakes in Africa.4

In 1992 two US Peace Corps volunteers (PCVs) in Africa developed severe schistosomiasis of the central nervous system due to S haematobium, a manifestation of this infection that is considered extremely rare.5, 6 Sporadic reports of schistosomiasis among visitors to Malawi7, 8, 9, 10, 11, 12, 13, 14 suggest that some of these cases may have resulted from recreational freshwater exposure at Cape Maclear National Park (figure 1).

The two cases of central nervous system schistosomiasis within a short space of time suggested that transmission of schistosomiasis might be occurring among expatriates and tourists in Malawi. The possibility of a change in the epidemiology of schistosomiasis in Malawi, particulary in the southern lake region, prompted us to begin an investigation of the problem in 1993. We now report our findings.

Section snippets

Objectives

The principal objectives of the investigation were to determine if transmission of schistosomiasis was occurring in the heavily touristed southern region of Lake Malawi, and to assess the risk to tourists and expatriates of acquiring schistosomiasis during recreational freshwater exposure at popular resort areas in the southern lake region.

Epidemiologic survey

Participants were enrolled in Malawi in a retrospective, cross-sectional cohort study and serosurvey during March and April, 1993. Participants were enrolled

Seroepidemiologic investigation

955 participants were enrolled in the study (500 male, 455 female). The paricipants' countries of origin (33% USA, 20% UK, 11% South Africa, 26% other African countries, 5% other European countries, 4% Asia, and 2% South America) generally reflected the overall distribution of expatriates in Malawi (table 2). The US cohort included 96 of 101 PCVs and more than 50% of all Department of State officials in Malawi.

The overall seroprevalence of ASA was 33% (303 of 919 from whom serum was available)

Discussion

This study highlights the magnitude of schistosomiasis among expatriates and visitors to Malawi, and demonstrates, for the first time, that recreational freshwater exposure in Lake Malawi is an important source of transmission. Among 955 study participants, the seroprevalence of ASA was 33%; age-specific seroprevalence peaked at 54% among teenagers, who we thought likely to spend the most time in the lake. The high (12%) prevalence among children under 5 years suggests that transmission is

References (21)

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