Snakebite envenoming

It is very difficult to determine the number of snakebites that occur worldwide, but the best available evidence suggests that each year as many as 2.7 million people develop a clinical illness following a bite from a venomous snake. In the field of clinical toxinology, which studies snakebite and other illnesses caused by venomous and/or poisonous organisms, the term used for such an injury is “envenoming” (American “envenomation”), and patients suffering the effects of venomous snake bite are said to have been “envenomed”. Of the 2.7 million people envenomed each year, between 81,000 and 138,000 die as a result, and as many as 400 000 of the survivors are left permanently disabled.

There are four primary criteria that define an illness as a neglected tropical disease (NTD), and snakebite envenoming satisfies each of them:

• First, there is a significant burden of mortality and morbidity (as detailed in the answer to question 1) – snakebite envenoming is a high-impact disease.
• Secondly, a majority of incidents of snakebite occur in the world’s tropical and sub-tropical regions, and it particularly impacts the poor – snakebite is a tropical disease.
• Thirdly, snakebite is amenable to treatment, as well as prevention – the impact of snakebite can be mitigated now if we make the effort to do so.
• Finally, the overall level of investment in research addressing snakebite, from prevention to diagnosis to treatment and rehabilitation, is exceptionally low in comparison to its impact – snakebite is a neglected disease.

The majority of snakebite envenoming cases occur in tropical and sub-tropical regions of the developing world. As many as 46 000 snakebite deaths happen in India alone; sub-Saharan Africa, tropical Asia, New Guinea, and Central and South America are also snakebite hot spots. Underscoring the fact that snakebite is a disease of the developing world are the incredible statistics from Australasia – although Australia itself is world famous for its diversity of highly venomous organisms, on average only 2 people die from snakebite there each year. In nearby New Guinea, on the other hand, an almost identical snake fauna (with, if anything, a lesser diversity of venomous species) claims more than 1000 lives each year. This is largely due to the differing socioeconomic and development levels of the two countries, and emphasizes the greater impact of snakebite envenoming in poor, less-developed communities.

Like most NTDs, snakebite affects particularly the poorest members of society. Agricultural workers (including working children aged 10–14 years) and people living in poorly constructed homes face the highest risk and often have limited access to education, health care and even footwear. When poor people are bitten, the luxury of modern health care is usually far removed from their reality.

The socioeconomic impacts of snakebite for survivors and their families can be dire.

In rural India, where 75% of people live on average monthly household incomes of just 5000 rupees (US$ 78), immediate treatment costs after snakebites of more than 350 000 rupees (US$ 5400) have been reported. The follow-on economic losses, due to loss of income or being forced to sell livestock, homes or land in order to meet additional costs, can be as high as 400 000 rupees (US$ 6215). Elsewhere in the developing world, the story is similar: those lucky enough to survive a venomous snakebite are often financially crippled by the cost of treatment, as well as physically crippled by the effects of the venom. Given that a majority of bite sufferers are among the most productive members of society (young, working age men and women), disablement following snakebite can prevent survivors from earning an income, making them a lifelong financial burden for their family members. In addition, snakebite survivors may suffer disfigurement and post-traumatic stress disorder, which can result in their becoming social pariahs.

The snake species implicated in serious snakebite injury vary from region to region, a fact that increases the challenge of mitigating the burden of snakebite worldwide. In India, the “big four” species are the Indian cobra (Naja naja), the Russell’s viper (Daboia russelii), the Indian saw-scaled viper (Echis carinatus) and the Indian krait (Bungarus caeruleus), although other species are also implicated in many life-threatening bites.

Throughout the rest of tropical Asia, a similar cohort of culprits occurs, and cobras (Naja ssp.), kraits (Bungarus ssp.) and Russell’s vipers (where they occur) are responsible for the lion’s share of serious bites.

In Africa, different types of cobras (Naja ssp.), multiple species of saw-scaled/carpet viper (Echis ssp.), and puff adders (Bitis arietans) are major culprits, although in some areas mambas (Dendroaspis ssp.) are also responsible for numerous bites, along with other species of viper (particularly other members of the genus Bitis).

In Central and South America, a range of pit-viper species, particularly lanceheads (genus Bothrops and related genera) and rattlesnakes (genus Crotalus) are responsible for the most bites, although coral snakes (genus Micrurus) are common in some areas and capable of inflicting life-threatening envenomations.

In New Guinea, the taipan (Oxyuranus scutellatus) is responsible for the most bites, followed by the death adders (Acanthophis spp.) – both groups of snakes occur also in parts of Australia, but bites in that country are very rare.

It is often said that prevention is better than cure, and a majority of snakebite accidents are readily preventable. In many parts of the world where snakebite envenoming is very common, walking barefoot through snake-infested areas is a major cause of snakebite and as many as 80% of all snakebites occur on the legs or feet, below the knee. These could be prevented if people had access to cheap, practical and protective footwear, and were educated to understand the importance of wearing their shoes.

Snakes are typically shy creatures that spend most of their time hiding, and piles of human rubbish and old construction materials make perfect shelter sites. Cleaning up and keeping the grass short in areas in close proximity to dwellings, along pathways, and in places where children play, are measures that can reduce the likelihood of snakebites. Similarly using a light when walking outside at night is essential in areas where some snakes forage after dark. Even the short walk from the house to an outdoor latrine can be fatal if you step on a snake in the dark.

Many homes in areas where snakebite is endemic are constructed in such a way that snakes can gain access. Sealing houses so that snakes cannot enter the dwelling can help to prevent snakebites, but if this is not possible, the simple act of sleeping on a raised platform with a well-tucked in mosquito net can prevent not only mosquito-borne diseases but also snakebites.

Most of the methods currently proposed as first aid for snakebites are either ineffective, dangerous, or delay proper medical treatment. Even methods that have been recommended by medical authorities in one part of the world may not be appropriate for use in other places because of the differences in the types of effects that different snake venoms can have.

Methods such as the use of narrow tourniquets, excision or incision of the wounds or the affected limb, the use of chemicals such as potassium permanganate or the application of heated glass containers to produce suction, can all be dangerous as well as ineffective and should not be used. Other types of first aid, such as the use of the Belgian black stone, are simply ineffective and waste valuable time.

The best approach is to immobilize the bitten person and keep them from moving around at all. The application of pressure at the bite-site via a pressure pad, a compression bandage applied to the bitten limb, or both, may be effective in delaying the onset of systemic effects, but is only recommended for bites from snakes with predominantly neurotoxic venom, and not for those that cause significant local tissue damage.

Place envenomed patients on a makeshift stretcher and transport them to medical care without delay. If possible place them on their side to help protect their airway and breathing.

Administration of the appropriate antivenom is the most effective treatment for life-threatening snakebite. The best response to antivenom is likely when it is administered as soon as possible following a bite, as many of the effects of snake venom are irreversible, including some neurotoxic, cytotoxic, haemorrhagic and haemotoxic effects.

Snake venoms are also highly variable in their toxic components and effects. This variation exists not only among species but often also within a single species among geographical regions, age and size classes, or between the sexes. As a result, antivenom raised against the venom of one species or population may not be effective in the treatment of envenoming by snakes from other species or even other populations of the same species. There are often barriers that complicate the task of ensuring early access to appropriate antivenom to snakebite victims, including lack of appropriate transport networks in developing countries, lack of unbroken cold chain-based distribution systems that can maintain the integrity of antivenom (which may require refrigeration) in rural and remote areas, and general lack of infrastructure associated with healthcare facilities. Surmounting these challenges is essential to effective control of snakebite envenoming.

Reducing the burden of snakebite in the tropical developing world is a massive task, with many complicating factors, some of which have been detailed above.

The first step is securing the requisite resources via commitments from WHO Member States, donors and other funding bodies. Supporting the development of functional health systems and other infrastructure in the worst affected regions will not only help reduce the number of snakebite casualties, but contribute to the capacity to fight the myriad other diseases that afflict these areas.

Education, both about the prevention of snakebite, and about appropriate treatment in the event of an accident, is a crucial piece of the puzzle.

Increased investment in research that can deliver tangible and sustainable solutions is essential, particularly with respect to developing and rolling out better diagnostic tools and therapeutic treatments, and improved control tools and strategies.

As with other NTDs, snakebite is a complex disease with many factors that contribute to its overall impact – mitigating these impacts requires commitment from multiple stakeholders.