OF CABBAGES AND CHLORINE: CHOLERA IN PERU
The report by Swerdlow and colleagues in this issue (p 24) should put an end to two rumours about the cholera epidemic that began in January, 1991, in Peru. The first is that the remarkably low case-fatality rates reported--less than 1%--were not so much a credit to the Peruvian health-care system as a result of overreporting; some sceptics were suggesting that many, perhaps most, of the thousands of reported cases were in fact diarrhoeas with an aetiology less virulent than cholera. Swerdlow et al, studying the epidemic in Trujillo, Peru's second city, in its early phases when a panic overreaction by doctors was perhaps most likely, found that 79% of the patients tested were infected with Vibrio cholerae O1. The second rumour, reported in a news item in Nature last year,(n1) is that lack of chlorination of many water supplies in Peru was a deliberate decision by the authorities, and was based on studies by the US Environmental Protection Agency showing that chlorine may create a slight cancer risk by reacting with organic matter in water to form trihalomethanes. one study suggested that this risk might account for up to 700 cases of cancer each year in the USA; to put this figure in perspective, the Latin American cholera epidemic claimed nearly 4000 lives in its first year. Whatever the reasons for the lack of adequate disinfection of water supplies in Lima, the reasons in Trujillo proved to be more prosaic and more typical of hundreds of other towns throughout the developing world--lack of chlorinators and chlorine, and a shortage of funds to buy them.
Since 1976 when, for the first time since Snow(n2) and Koch,(n3) it was suggested that cholera might not always be primarily water borne,(n4) the scientific community has rediscovered the importance of other transmission routes.(n5) Interest has focused most recently on aquatic reservoirs of the bacillus and, in the case of Peru, on the role of ceviche, a dish made with raw fish. The results of the Trujillo study suggest that raw seafood played a minor part, if any, but confirm the importance of drinking water as a vehicle for cholera transmission. However, it is not clear whether contamination of the water supply, or of stored water in the home, was mainly responsible. Adequate chlorination of the supply helps to control infection from both sources.
A potential transmission route that has been blamed for some cholera epidemics is irrigation of vegetables with raw wastewater, a practice prevalent in many arid parts of the world, including Peru.(n6) Consumption of such vegetables, cabbages in particular, seems to have been a risk factor for the disease, but only one of several. Nevertheless, if the authorities in Peru and other developing countries are to try to make the practice safer, by first treating the wastewater in waste stabilisation ponds, as recommended by the World Health Organisation,(n7) recent research raises a new cause for concern. A possible aquatic reservoir for V cholerae is the mucilaginous sheath of blue-green algae, Anabeana spp,(n8) a species commonly found in stabilisation ponds. other zooplankton and phytoplankton may also provide a haven for the organism, even a microhabitat in which the vibrio can thrive. Data from Peru(n9) have shown a similarity between the seasonal pattern of the cholera epidemic and the seasonality of V cholerae non-O1 isolated from sewage lagoons in Lima. The possibility that sewage treatment lagoons could fail to remove V cholerae O1 from wastewater, and even cause an increased concentration in the effluent, needs urgent research.
(n1.) Anderson C. Cholera epidemic traced to risk miscalculation. Nature 1991; 354: 255.
(n2). Snow J. Snow on cholera. New York: Hafner, 1965.
(n3.) Koch R. An address on cholera and its bacillus. BMJ 1988; ii: 403-07, 453-59.
(n4.) Feachem RG. Is cholera primarily water-borne? Lancet 1976; ii: 957.
(n5.) Feachem RG. Environmental aspects of cholera epidemiology. III. Transmission and control. Trop Dis Bull 1982; 79: 1-47.
(n6.) Mara DD, Cairncross S. Guidelines for the safe use of wastewater and excreta in agriculture and aquaculture. Geneva: World Health Organisation, 1989.
(n7.) World Health Organisation. Health guidelines for the use of wastewater in agriculture and aquaculture. WHO Tech Rep Ser 778.
(n8.) Islam MS, Drasar BS, Bradley DJ. Long-term persistence of toxogenic Vibrio cholerae O1 in the mucilaginous sheath of a blue-green alga, Anabaena variabilis. J Trop Med Hyg 1990; 193: 133-39.
(n9.) Ventura G, Roberts L, Gilman R. Vibrio cholerae non-O1 in sewage lagoons and seasonality in Peru cholera epidemic. Lancet 1992; 339: 937-38.
CP Copyright Lancet 1992
Source: Lancet, 7/4/92, Vol. 340 Issue 8810, p20, 2p