We estimate that at least 421,000 envenomings and 20,000 deaths occur worldwide from snakebite annually. These figures may be as high as 1,841,000 envenomings and 94,000 deaths. On the basis of the estimation that the total number of snakebites is two to three times the number of envenomings, we estimate that 1,200,000–5,500,000 snakebites may occur globally. The vast majority of the estimated burden of snakebite is in South and Southeast Asia, sub-Saharan Africa, and Central and South America, as identified in previous estimates of the global burden. Despite accounting for nearly one-fourth of the global snakebite incidence, mortality due to snakebite is relatively lower in Central and South America when compared to other high incidence regions. Mortality may be lower because of better snakebite management systems, including the development of locally effective antivenoms, in many Latin American countries. The lower estimates of snakebite incidence in sub-Saharan Africa are probably a reflection of under-reporting from many parts of this region; we found it particularly difficult to find reliable data for this region, especially for East Africa. India, with its population of over a billion people, accounted for the highest estimated number of bites and deaths for a single country.
The most often quoted currently available estimates of the global burden of snakebite [3
] are subject to the major limitation that the methodology of estimation is not given and so cannot be reproduced. The formalization of methods for the assessment of disease burden provides a framework for standardized methodology [81
]. In an attempt to provide a more contemporary and accurate picture of the global problem, we developed and applied a method to obtain an estimate of the disease burden due to snakebite. Our global estimates were based on regional estimates that were, in turn, derived from data available for countries within a defined region.
The true global incidence of snakebite, envenomings, and its associated mortality are difficult to estimate. The overwhelming majority of bites occur in rural areas of resource-poor countries. Reporting and record-keeping in such situations are generally poor. Snakebite varies seasonally and geographically within countries; i.e., high incidences are reported during agricultural activity [88
]. Many estimates from these countries are based on hospital returns or incomplete central databases, and are bound to be underestimates, because many victims do not seek hospital treatment and prefer traditional remedies [5
]. Some may die at home, with their deaths unrecorded [8
]. Studies from rural Nigeria and Kenya have reported that only 8.5% and 27% of snakebite victims, respectively, sought hospital treatment [7
]. This situation may be common to many middle- and low-income countries where health-seeking behaviour, health beliefs, and access to health care are not optimal. Thus, most of the available data on snakebite should be regarded as underestimates.
Conversely, many of the few published community surveys of snakebite have been performed in areas where the problem is endemic and perhaps a major public health problem, and incidence and mortality figures then extrapolated to represent the entire country or region; this would lead to an overestimation of the burden. To circumvent this problem we applied the higher rates identified for a particular country only to the rural population of that country if the rates were reported at subnational level. This would have resulted in considerable underestimation, because some of the published subnational studies have reportedly been conducted in regions where snakebite is not considered a high priority public health problem [59
]. It is for these reasons, and because the paucity of data prevented more precise estimations, that we decided to present our estimates as a range by calculating both high and low estimates of snakebites and related mortality.
The most important issue we faced was the paucity of good-quality published data, particularly from nationwide community-based studies. Despite this deficiency, we have given priority to national estimates derived from the national health systems and related services over rates available from subnational studies to try to avoid overestimation of national incidence or mortality rates. Data generated from hospital-based studies were used in a few instances where no other data were available, provided that the catchment population was known.
We considered only data reported after 1985 to make the estimates as current as possible. More than 70% of the data that were utilised in arriving at the estimates were, in fact, reported after 1995. Most regions lacked either population-based studies or surveillance systems that might measure snakebite incidence at the population level. In some cases, many countries in a region lacked data. For example, data were available for only one country in eastern sub-Saharan Africa, where snakebite is known to be an important public health problem. Population-based studies of incidence and mortality are urgently needed to describe the epidemiology of snakebite in these areas. In some countries that encompass large geographical areas and have large populations such as India, China, Indonesia, and Russia, country estimates had to be calculated on the basis of single or few regional incidence or mortality figures. This also means that relatively small changes in the incidence rates could lead to considerable differences in the estimation of the total burden in terms of the number of envenomings and deaths. The proportion of the population that is rural, and therefore exposed to the risk of snakebites, can also considerably affect estimates for these countries.
On the rare occasions where multiple studies were reported for the same country, we selected the most conservative incidence and mortality rates for our low estimates. We followed this principle when extrapolating data for countries within a region. That is, where no data were available for a country in which bites were likely to occur, we selected the most conservative rate available from a country within the region as the rate for the country without data [92
]. This was done to avoid bias toward overestimation of the incidence and mortality in the region where the study was conducted. This is especially relevant for countries such as China, India, and Indonesia which have very large populations. When calculating our high estimate we used only the higher figures of incidence to calculate burden for the rural population of a given country (where snakebites are commoner) and considered the lowest incidence figures when calculating the burden in the urban population in that country. Thus, even our high estimates could be considered conservative.
Most previous estimates of snakebite morbidity and mortality appear to have been derived from studies that were done in such high incidence areas within regions or countries, and extrapolated to whole countries and regions. Our approach minimised this effect, although in some regions, such as the Caribbean, lack of data still meant that we were forced to use very high rates in our calculations of the high estimate. These methodological differences may have played a substantial role in accounting for the differences between the previous estimates and our low estimate (). The case for this is further strengthened by the fact that those estimates are closer to our high estimate. However, this is difficult to assess given that a clear methodology is not available for any of the previous estimates.
Comparison between the Current and Previous Estimates of Snakebite Envenomings and Deaths
Another plausible reason is that we used very recent incidence and mortality data from India and Pakistan that are considerably lower than previous estimates. Our estimate of about 81,000 envenomings and nearly 11,000 deaths due to snakebite in India is much lower than the 200,000 bites and up to 50,000 deaths quoted in previous estimates [63
]. Given the population of over one billion in India, this had a substantial effect on our global estimates. The figures we report from India are based on health insurance schemes operated by many state governments such as the “Kisan Jeevan Kalyan Yojana” (Ian Simpson, personal communication). These schemes compensate the farming community for a variety of accidents, including snakebite. The level of compensation is substantial, ranging from US$250 to US$1,250. We feel that these figures may therefore be a more reliable estimate than hospital records, because the victim's family has an incentive to report the deaths due to snakebites, reducing the assumed impact of unreported deaths. It is possible that unexplained deaths are attributed to snakebite, but victims are examined and mortality certified by a doctor as being due to snakebite.
Estimation of the total number of snakebites (both with and without envenoming) is difficult because of the scarcity of literature that differentiates the two and variation in the distribution of venomous snakes in the regions; few community-based studies address this. The true burden of snakebites may not be reflected in hospital data, because a considerable proportion of people with asymptomatic bites may not seek treatment at hospitals; in some settings, snakebite victims may preferentially attend traditional healers. It was also not possible to ascertain whether “all bites,” especially in community surveys, included bites of nonvenomous snakes and dry bites of venomous snakes. These non-envenoming bites would arguably not contribute much to the burden of disease, although the opportunity cost of the bite may affect the victims and the households adversely. We estimated the total number of snakebites using data on the proportion of snakebites with envenoming from studies in different parts of the world. The proportion envenomed varied considerably, most probably because of both the effects of different snake species and variation in methodology. This heterogeneity means that we have only a crude estimate of the total number of snakebites. Our main focus was to estimate snakebite envenoming, as it is envenoming that causes most of the burden due to snakebite: requirement for antivenom, hospital and intensive care unit care, and surgery; complications; permanent sequelae; and even death.
The WHO mortality database was the main source of information used for estimation of the mortality due to snakebite, providing nearly 70% of the data used. The reliability of the data reported to this database by countries was assessed on the basis of two criteria: the HDI and the coverage of vital registration of deaths. Only code X20 of ICD10 was considered snakebite to minimise inclusion of deaths caused by other venomous animals. Code X20 does not differentiate between deaths due to snakes and lizards. However, lizard bites, unlike venomous snakebites, are not known to be fatal; in fact, in the last 50 y only a few cases of envenoming and no deaths have been reported as due to lizard bites. Thus, from an epidemiological point of view lizard bites are legible and have no public health implications.
In any estimation, assumptions have to be made and the robustness and validity of the estimate depends on how well the assumptions are met. The assumptions we made included considering a country as free of snakebites (and associated mortality) if there was no literature (published or unpublished) indicating the occurrence of snakebite since 1985, and assuming that the mortality data reported to the WHO mortality database to be accurate and representative of the national data for countries with a high HDI and a high vital registration coverage of deaths. We did not make any adjustments to arrive at the final estimate of deaths for countries with less than 100% vital registration coverage to prevent an apparent unrealistic precision to the estimate. The most recent incidence or mortality rate reported after 1985 was assumed to be the current rate for a country.
We estimated the numbers of snakebites and deaths only. No reliable data were available on the long-term physical and psychological consequences of surviving snakebite, but as most snakebite victims are in the economically productive age group, the economic impact of disability is likely to be high. Although the socioeconomic burden of snakebite cannot be stressed strongly enough, in this study we did not attempt to quantify this burden. Global health resource allocation is often based on DALYs (disability-adjusted life-years), and other socioeconomic markers rather than on the number of patients and deaths, despite the limitations of each of these measures of burden. Future assessments of the burden of snakebite will draw greater attention to the problem, which in turn will help win resources to tackle this neglected issue.
In conclusion, data from several new sources and the development and application of a scientifically robust method that can be replicated has enabled us to generate a revised estimate of the global disease burden due to snakebite, although the inadequacy of available data and the consequent need to rely upon extrapolation mean that this estimate is still far from perfect. The burden is considerable, especially in South and Southeast Asia, sub-Saharan Africa, and Central and South America. Given the high burden, the paucity of reliable snakebite data, particularly in some of these areas, is both surprising and worrying. The fact that snakebite varies geographically and seasonally, that it is mainly a rural tropical phenomenon where reporting and record keeping is poor, and that health-seeking behaviour is diverse with traditional treatments being sometimes preferred to Western medicine, all contribute to the difficulties faced when studying its epidemiology.
To address this problem, population-based studies of incidence and mortality in countries that appear to have the highest case load and mortality rates are urgently required to clarify the situation. The quality of reporting and recordkeeping on morbidity and mortality due to snakebite in health facilities should be optimised. Data sources should include traditional medical practitioners and rural health workers, because high morbidity and mortality due to snakebite can be encountered in geographically isolated communities [9
]. Accurate data on the epidemiology of snakebite, globally, will facilitate prioritisation of scarce health care resources for prevention and treatment of this neglected health problem.