Sep. 10, 2011 One and a half million people per year are poisoned by snake venom in Sub-Saharan Africa. An IRD researcher recently analysed around 100 surveys and medical reports published over the past 40 years. No large-scale study of the situation had hitherto been conducted and public health authorities had underestimated the size of the problem. This means that currently only 10% of victims are treated, owing to a shortage of antivenoms* and lack of awareness among health care practitioners. Yet the clinical complications can be very serious, even fatal. A bite from a cobra or mamba can bring on death by asphyxia -- due to respiratory paralysis -- within 6 hours of the incident. Venom injected by the ocellated carpet viper, common in the African savannah, can cause hemorrhages leading to the victim's death in a few days.
This new study provides authorities with more detailed and reliable figures which should enable them to readjust their health-care services in better tune with needs.
For snakes the best form of defence is attack. Some show complete ruthlessness when they sense they are under threat. They all have their methods. The Gaboon viper, for example, injects its venom very deep into the muscles with its 5 cm long fangs. The spitting cobra blinds its victims with its venom. And although only one out of two snake bites is venomous, these reptiles are still a real danger for humans. The number of incidents is considerable, especially in Sub-Saharan Africa where they represent a sizeable public health hazard, though neglected by the health authorities.
One and a half million envenomings
Every year over 300 000 people living South of the Sahara have to be treated after a snakebite, as the IRD researcher has shown. However, many cases are not reported, given the difficulty of access to health centres and the frequent reliance on traditional medicine. That figure therefore does not reflect the complete number of envenomings and specialists consider it to represent only between one-third and one-fifth of the real amount. This new study thus indicates up to one and a half million victims per year. Mortality resulting from a bite -probably also underestimated- can reach 7 000 and limb amputations range from 6 000 to over 14 000 per year.
Although many one-off studies have improved estimates, no large-scale survey had hitherto been conducted. Aiming to make up for the gaps, the IRD specialist performed a meta-analysis, a critical review of existing scientific works, taking the representativeness1 and heterogeneity of their results into account. He therefore studied in fine detail 100 or so scientific articles, conference proceedings and clinical reports published between 1970 and 2010. This thorough survey yielded much more reliable figures for the number of patients who were victims of snake bites.
Working in the fields: a high-risk activity
These studies also highlighted the situations most likely to favour biting incidents: 95% of bites occur in rural areas, particularly in plantations. The people most at risk are therefore agricultural workers. And in Africa, agriculture is the main economic activity.
Urban areas are not spared from such incidents either, even if the incidence of bites is from 10 to 20 times lower than in rural settings. In some regions therefore, in the rainy season, envenomings represent over 10% of hospital admissions.
In the most dangerous African snake species, two types of venom can be distinguished: the neurotoxic venom of the cobras and mambas, and the hemorrhagic and necrotic type of the vipers -including the ocellated carpet viper, the most widespread in the savannah. In other words, the first type causes respiratory paralysis, which can kill the victim by asphyxia between 1 and 6h after the bite. The second induces oedema and necroses in the limbs plus hemorrhage which can prove fatal in just a few days. The effective treatment remains intravenous injection of antivenom as swiftly as possible after the bite, in order to neutralize the toxic substance.
A vicious circle
The availability of these antidotes is currently restricted and only 10% of venomous snake bites are treated. Shortage of data has meant that up to now the problem remained underestimated by the health authorities. Moreover, these remedies are costly and the duration of their effect is short -3 to 5 years- which discourages people from keeping up stock supplies. In such situations, it is difficult to define the budgets and allocate funds for snakebite management and for setting up the necessary schemes for awareness-building among health-care professionals. Without training for such medical personnel in the use of antivenoms, treatment can give disappointing results, subsequently deterring people from using them. These chain reactions reduce demand. Manufacturers then hesitate before producing antivenoms they cannot be sure of selling. In the end this process diminishes their accessibility. The number of doses sold has been divided by ten in Africa since the 1980s, falling from 200 000 per year to less than 20 000 in the early 2000s.
This survey presents some realistic figures for needs in antivenom. The researcher's results indicate the need for an estimated 500 000 doses each year. The health authorities of these countries can now use these data as a basis for improving the quality of health care provided for victims and for organizing a counting and surveillance system.
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- Jean-Philippe Chippaux. Estimate of the burden of snakebites in sub-Saharan Africa: A meta-analytic approach. Toxicon, 2011; 57 (4): 586 DOI: 10.1016/j.toxicon.2010.12.022
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