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Cholera Burden of Disease Estimates The number of cases of cholera and the number of cholera deaths which occur annually is difficult to estimate with certainty. Although cholera is a reportable disease, most cases are not reported to the national authorities or to the World Health Organization (WHO). About 200,000 cases and about 5,000 deaths are reported to WHO in most years, but these numbers both underestimate the true numbers and in many cases are misleading because several countries with high rates of endemic cholera do not report any, or very few cases. The Institute of Medicine provided an estimate of 3 to 5 million cases annually and about 120,000 deaths annually.1 However, this estimate was published in 1986 and many changes have occurred in the epidemiology of cholera since then. In an attempt to provide a reasonable estimate of the number of cases of cholera which occur annually, a list of countries in Asia and Africa with their estimated total populations was prepared on an Excel spread sheet. The numbers for the population of each country was obtained from a UN web site.2 For each country, in the region, an estimate of the annual incidence was used to calculate the number of cases for that country. Cholera has a rate that is variable from year to year and from season to season; thus, the numbers used as the incidence rate is the rate for a “typical” or average year. For the incidence, only the cases that should be treated at a medical facility were included and the incidence would not include persons who might have very mild infection and not require treatment. To calculate the total number of infections (including mild cases and those with no symptoms), one would multiply the numbers shown by a factor of 2 to 10. To estimate the number of cholera deaths, the number of estimated cases was multiplied by the case fatality ratio (CFR) for each continent among those who came for treatment. For Africa, I estimated a CFR of 4% and for Asia, I used an estimated CFR of 2%. The CFR is based on the number of deaths that occur among patients who come to a treatment facility but this does not include the patients who die without treatment at a facility. However cholera occurs suddenly and is able to kill a patient in a matter of a few hours. Many patients are not able to reach hospital in time and they die without treatment at a facility. In fact most cholera deaths occur among people who do not reach a treatment facility. Thus, the estimate of cholera deaths includes a “multiplier” factor in order that the estimate includes the “out of hospital deaths.” For Africa I have used 2 as the multiplier and for Asia, I used 1 as the multiplier. While this multiplier number is highly speculative, they are probably conservative. Most likely, the numbers of persons who die without treatment at a facility is even higher. This is because cholera tends to occur among persons who live in remote areas and among populations who are poor, less educated, and have less access to health care. Since the estimates are just that – estimates; I would welcome any input to help refine the estimates of incidence or case fatality rates. If there are suggestions for changing the incidence rate or the case fatality ratio for a country, please provide the basis for the changes. Since these estimates apply to a country, a low case fatality rate at a very good health facility would not indicate a change in the national estimates. However, if a country has significantly improved its infrastructure or if the infrastructure has deteriorated, we should change the estimate for case fatality ratios in the model. I would also welcome comments about the approach to these estimates. Let me know if there are ways the model can be refined. David A Sack |
