That cholera is a waterborne disease has long been known (4–6). Furthermore, surface water has been linked with transmission of cholera since the pioneering work of Snow in 1854 (7). Demonstration of the potential for water to transmit cholera was provided by Koch, who, after Pacini first described the Vibrio (8), isolated and characterized the bacterium, which he named Vibrio comma, and was able to find it in pond water used by an Indian community suffering a cholera epidemic (9).

The association of pathogenic vibrios with zooplankton was reported in 1973 by Kaneko and Colwell (10) and of V. cholerae with copepods by Huq et al. in 1983 (11). Commensal occurrence of Vibrio spp. in the copepod gut was demonstrated by Sochard et al. in 1979 (12). A few years later, preferential attachment of V. cholerae to copepod surfaces, egg cases, and the copepod oral region was reported by Huq et al. (11). Extensive data have since been accumulated showing that planktonic copepods play a major role in the multiplication, survival, and transmission of cholera (13–17). That environmental V. cholerae O1 can cause cholera has been established by molecular genetic evidence (18).

During spring and late summer in Bangladesh, phytoplankton blooms occur, followed by zooplankton, with heaviest blooms occurring in September and October (13, 19). Each year, the seasonal zooplankton blooms, in turn, are followed by cholera outbreaks (11, 13). It has been determined that a single copepod, depending on species and size, can carry up to 10⁴ cells of V. cholerae (11, 17). Thus, a copepod bloom can result in the number of V. cholerae per ml of water comprising an infective dose, based on findings from human volunteer studies, showing that ≈10⁴ to 10⁶ V. cholerae O1 can produce clinical cholera (20). Patchiness in copepod distribution, often species specific in the aquatic environment (21), can result in significant variability in the number of copepods in water taken directly from a pond or river for drinking.

Village populations of Bangladesh depend on untreated surface water for household use, especially during times of flooding (22). Surface water from ponds and rivers is used by some villagers as a source of drinking water for reasons of taste, convenience, or a local belief that “quality” water is “natural,” i.e., not chemically treated (15, 22). Furthermore, with the current arsenic crisis in Bangladesh, as many as half of the wells drilled in the late 1960s as the answer to Bangladesh's severe surface water pollution problem have been found to be contaminated with arsenic in amounts that exceed 50 ppb, with some concentrations even 10 times higher in contaminated areas (23–26). According to a recent study conducted in Araihazar in Bangladesh by WHO, >30 million people are exposed to unsafe levels of arsenic in their drinking water, and 20% of the arsenic contaminated tube-well water users switched back to untreated surface water (27). In addition, studies showed that tube well water contained 10⁴ to 10⁶ total bacteria by acridine orange direct count (28) and a high incidence of zooplankton, as well as coliforms and other bacteria (29). These findings indicate that surface water has again become important as a source of household water and for drinking when no other safe water is available.

Although boiling water before drinking is effectively the better practice, because it will kill all waterborne pathogenic microorganisms, it is not used routinely in the villages because fuel wood in rural Bangladesh is both in very short supply and costly. Moreover, during severe flooding, which frequently occurs in Bangladesh, there are geographical areas that experience reduction in the quality of life to mere survival, when even the barest necessities are difficult to obtain and building fires to boil water is simply not possible.

It is common practice in villages in Bangladesh to use cloth, frequently a flat, unfolded piece of an old sari, to filter home-prepared drinks. In laboratory experiments employing electron microscopy, we found that inexpensive sari cloth, folded four to eight times, provides a filter of ≈20-μm mesh size, small enough to remove all zooplankton, most phytoplankton, and all V. cholerae attached to plankton and particulates >20 μm. Laboratory studies showed that sari cloth folded at least four times retained the V. cholerae cells attached to plankton, effectively removing >99% (>2 logs) of V. cholerae (30). Nylon net with a mesh size of ≈150 μm was also successfully used in this study to compare its effectiveness in preventing cholera because it has been used to remove cyclops from drinking water. Cyclops is a crustacean copepod common in Africa and carrier of the guinea worm larvae that causes dracunculiasis (31, 32).

Results of the pilot study indicated that filtration reduced the number of cases of cholera when nylon net or sari cloth was used, compared with those who did not filter their water. That is, ≈38% reduction in cholera cases among filter users was achieved. However, the sample for the pilot study was too small for statistical significance. Hence, a larger study was undertaken.

In the full study, both the nylon filtration group (P < 0.02) and the sari filtration group (P < 0.005) experienced significantly lower cholera rates than the control (Fig. 2 and Table 4). Both filters were capable of removing copepods that are >150 μm in size, as well as particulate matter. In the comparison of nylon versus sari filtration, no significant difference was noted (P > 0.3) even though the sari filtration was observed to be more successful in lowering the rate of cholera. Hence, effectiveness in removing copepods was demonstrated. From the generalized linear model, we estimated that the sari group had a cholera rate ≈52% of the control, or cholera was reduced by about half. It should be noted that the years in which this study was undertaken were relatively low in cholera incidence, therefore, the cholera rates for all three study arms were low when compared with historic data (Tables 3 and 4).

Several conclusions can be drawn from this study, which was carried out by a highly interdisciplinary team, including sociologists, physicians, field extension agents, microbiologists, epidemiologists, ecologists, statisticians, and environmental scientists. First, significant reduction in cholera was achieved by filtering out zooplankton, namely copepods, and colonial phytoplankton from household water, both by nylon and sari filtration. From the laboratory and field studies, it was found that sari filtration removed all zooplankton, most of the phytoplankton, and all particulates >20 μm. The nylon filter removed the zooplankton, larger in size, and hence, was almost equally effective. Because cholera is dose dependent (20), by reducing the number of V. cholerae cells by filtration, we were able to achieve significant reduction in the number of cholera cases. Severity of the disease in those cases that did occur among the filtration population also appears to have been reduced. This observation will be confirmed in ongoing studies. Nylon material was also effective in filtering out copepods, but sari cloth is much less expensive, very effective, and readily available to all villagers in Bangladesh. Although sari cloth was used in Bangladesh and found to be effective, with even a visual difference in the quality of water being easily discernible, other material may be similarly functional and can be used in other parts of the world, where untreated water is used for domestic purposes and cholera is endemic. Interestingly, during the period of the study, meetings with the community health workers revealed that a large number of mothers using filtration perceived a positive decline in the incidence of diarrhea within their families. Therefore, efforts are currently being made to quantitate this finding. The importance of such a perception by a village mother would be significant in disseminating the message for effective implementation of the recommended procedure, when a filtration program is initiated. Other waterborne diarrheal diseases endemic in the Bangladesh villages are being analyzed to determine whether similar reduction was achieved and those results will be communicated separately.

In designing the full study, the study villages were not randomly selected for reasons of minimizing communication between villages. The villagers in the study used either surface water or tube-well water for drinking, but none of the villages used entirely one or the other source. Future studies will need to separate these factors to determine any effect on cholera rate. The type of storage of water in the home was not examined specifically, but will need to be addressed in future studies to determine whether the effectiveness of filtration can be improved. It was not the intention to culture water after filtration and storage in the home, because the supply of water for the household was replenished daily and instructions were given biweekly to all study households (including controls) on proper cleaning and storage of water. Visits by field workers every 2 weeks were concluded to be adequate to ensure compliance with filtration. Because each cholera case was followed up to determine probable source, in most cases the villagers had visited other, nonstudy villages where cholera had occurred. Thus, it was concluded that the biweekly field worker visits were effective. To prove this statistically, however, will require a separate, far more extensive study. Nonetheless, these factors will be addressed in future studies to improve the effectiveness of filtration and to address limitations of the research reported here.