Thursday, February 3, 2011

Snakebites in Sub-Saharan Africa

Herpetologist and ecologist W. Gibbons (1983) described one of his students being bitten by a canebrake rattlesnake in South Carolina, USA. The student was taken to the hospital emergency room, the attending doctor examined the patient, and disappeared for a time. Gibbons' concerned about the situation, searched for the doctor and found him in his office reading the snakebite treatment section of the Boy Scouts of America Handbook. Even in the USA, physicians are often poorly prepared to deal with snake envenomations - imagine what it is like in Africa or India. What follows is based upon a forth coming paper by Jean-Philippe Chippaux that examines the snakebite burden in sub-Saharan Africa.

The number of envenomations and deaths from snakebites has become a contenceious issue. Recent analysis suggests snakebites represent an important but neglected public health problem in poor rural populations in many developing countries and it has been estimated that one million snakebites occur every year in Africa, primarily in sub-Saharan Africa, resulting in 100,000 to 500,000 envenomations and 10,000 to 30,000 deaths. However, incidence of snake bite and mortality data are inaccurate because epidemiological and health data are not reliable. Venomous animal bites and sting are usually not reported and it becomes difficult to establish budgets and allot funds for envenoming management and health staff training. As a result, envenoming is often not included in budgets by health authorities who consider therapeutic solutions unavailable and resources inaccessible. The cost of antivenoms and their short shelf life discourage clinics and hospitals from keeping sufficient inventory, which in turn reduces requests for antivenom. Moreover, healthcare workers tend to be ignorant when it comes to the directions for antivenom that produces disappointing results, and lowers the probability they will use them in the future. Manufacturers hesitate to produce antivenoms they may not be able to sell. This cycle has resulted in the reduction of antivenom accessibility, particularly in Africa, where the sale of antivenom has declined from approximately 200,000 vials per annum in the 1980s to less than 20,000 at the beginning of the years 2000. Attempts to estimate snakebite incidence were made, with the two most recent papers reporting high incidences of snakebites. However, the results were disputed by some authors who regard them as incomplete and skewed. According to the critics, the methods used for data collection were not specified, and/or the extrapolations were not justified.

Chippaux examined the literature for the years 1970–2010 and analysed data using meta-analysis to take into account the heterogeneity between the studies and their respective weight. Incidence, mortality and the population at risk were estimated after stratification according to the environment (urban or rural) and survey methodologies (national, hospital or community studies). He found the incidence of snakebite was inversely correlated with population density. The number of envenomations was estimated at 314,078, 95% of these occurred in rural areas; the remainder occurred in cities. The annual mortality was estimated at 7,331 (5,148–9,568); 97% occurred in rural environments. The number of amputations per year ranged from 5,908 to 14,614. As in most countries, the demographic most at risk was young men engaged in agricultural or pastoral laborers. Household surveys indicated that actual incidence and mortality were likely 3–5 times higher. The difference maybe explained by treatment seeking behavior. However, Chippaux suggests the incidences and mortalities reported reflect the number of patients who attend modern health facilities, giving underestimated figures of the burden of snakebites in sub-Saharan Africa but realistic numbers for the amount antivenom needed.

Estimates of snakebite incidents and mortality with snakebites came from 3 different sources: two were based on the notifications of cases and deaths by the health services in urban and rural areas; they are similar and consistent. Household surveys were the third source of data and suggested that the actual incidence and mortality, including patients who do not attend modern health centres, was 3–5 times greater than the data reported by health services.The data also confirms that only a fraction of patients go to health facilities - instead they turn to traditional healers. [Many of these people are probably bitten by harmless snakes.] However, they may secondarily go to the hospital for treatment, particulalry if the poisoning is severe and traditional treatment is not effective. Household survey data are thought to be more representative of the actual number of bites and mortality, but  surveys are expensive and time consuming. Thus, estimation of actual incidence and mortality remains speculative. Relatively few countries have this data available, and investigations are still needed to improve our knowledge of envenomations in sub-Saharan Africa.

Chippaux, J.-P. 2011 (in press). Estimate of the burden of snakebites in sub-Saharan Africa: A meta-analytic approach. Toxicon doi:10.1016/j.toxicon.2010.12.022.

Gibbons, W. 1983. Their Blood Runs Cold. The University of Alamaba Press 164 pp.

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