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Page last updated at 09:06 GMT, Tuesday, 22 February 2011
Snake-bites: a growing, global threat

An eastern diamondback rattlesnake (c) Barry Mansell / NPL

A cryptic shape glides across the forest floor, searching, tasting the air, hunting for its next victim.

Over 2m-long, this predator packs a powerful punch; a venom capable of killing up to five people with a single bite.

This ability makes the eastern diamondback rattlesnake, like many species of venomous snake, among the most feared and maligned of all creatures.

New research suggests that the problem of snakes accidentally biting people could be worse than previously thought, with deadly consequences for both snakes and their human victims.

The latest estimates suggest that around 5.5 million people are bitten by snakes each year, resulting in around 400,000 amputations, and between 20-125,000 human deaths.

But these estimates are unlikely to represent the true scale of the problem, one that experts fear is causing a neglected, and potentially growing global health issue.

Efforts are now underway to try learn more about the issue, mitigate its impacts, and find ways that snakes and people can more peacefully coexist.

Millions of bites

Dealing with snake-bites can be extremely difficult.

It is often impossible to know exactly which snake species was responsible for a bite; identification is particularly difficult in developing countries as the majority of snake-bites are inflicted at night in rural communities that do not have electricity or artificial lighting.

Then the victim, who often lives in an impoverished remote place, has to find suitable medical treatment before the toxic venom leads to permanent disfigurement or death.

Rattlesnakes are more likely to flee than attack a human
Spectacled cobras strike defensively with their mouths closed

Late last year, an international team of researchers from Costa Rica, Australia, Brazil and the UK published an academic paper calling for a more integrated approach to dealing with snake-bites.

The review, published in the journal Toxicon by Jose Maria Gutierrez from the University of Costa Rica, David Warrell from the University of Oxford, UK and colleagues, calls for more work to be done to evaluate just how many people snakes bite each year, and the extent of their injuries.

For example, though the researchers estimate that more than five million people may be bitten each year by snakes, they point to other research which uses random questionnaires to ask local people directly about their experiences rather than relying on hospital admission records.

Results from those studies suggest the incidence, mortality and long term disability caused by snake-bites may be much higher than suggested by official statistics.

For example, results published last year by the University of Toronto, based on interviews within rural communities, suggests a million people are bitten by snakes in India alone each year.

The scale of the problem caused by snake-bites in India has been documented by a BBC film crew for the Natural World programme One Million Snakebites.

King cobra (c) Kaylan Varmar
The world's largest venomous snake, the king cobra

India is well known for its reptilian fauna and is home to the world's largest venomous snake, the king cobra, which can grow to 18ft long and has a bite containing enough venom to kill 20-30 people.

However, living in dense jungle, the king cobra rarely comes into contact with humans.

Instead, the country's main snake-bite offenders, known as the "big four", comprise the spectacled or Indian cobra (Naja naja), Russell's viper (Daboia russelii), saw scaled viper (Echis carinatus) and common krait (Bungarus caeruleus).

Attracted by the rats that feed on crops, food stores and human waste, these venomous snakes can all be found living in close proximity to humans.

Conservationists and health professionals alike are concerned that, as human populations grow, harmful encounters between people and snakes will increase.

One way to mitigate the problem of snake-bites is to find ways for people and snakes to live more harmoniously.

A saw-scaled viper strikes (c) BBC

The saw-scaled viper is a prolific biter

The BBC documentary One Million Snakebites details how one expert in India, Romulus Whitaker, is trying to engage and educate local communities about the snakes living around them.

By helping local communities to understand and respect the snakes they share their land with, passionate herpetologist Whitaker hopes that snakes such as the threatened king cobra will no longer be persecuted.

Whitaker has worked with the Irula tribe, who have a long tradition of snake catching, to create a cooperative that now supplies snake venom to laboratories across the country to create lifesaving antivenom, the only effective treatment for snake-bites.

Milking the venom of a saw-scaled viper (c) BBC
Milking the venom of a saw-scaled viper

Yet antivenoms are difficult to make; snake venoms are complex mixtures of compounds unique to species, so each antivenom must contain different antibodies to fight off the particular toxin.

In sub-Saharan Africa, home to iconic venomous species as carpet vipers, black mambas, puff adders and boomslang, the World Health Organization (WHO) has previously described a "crisis" in antivenom production.

That is caused in part by the high cost of antivenom treatments, which is seen as the main barrier to their wider use in developing countries.

Soon to be published guidelines for the manufacture and quality control of antivenoms, prepared by the WHO, should help facilitate and standardise that process, bringing down its cost.

The global burden of snake bites becomes clear when comparing snake bite incidence and mortality with other tropical diseases:
Up to 5.5 million snake bites cause up to 125,000 deaths a year
More than 217,000 cases of Chagas disease cause 14,000 deaths
More than 178,000 cases of cholera cause 4000 deaths
Around 2000 cases of yellow fever cause 100 deaths

Researchers including Drs Gutierrez and Warrell have also helped launch the Global Snake Bite Initiative, details of which were published last year in the medical journal The Lancet.

The initiative aims to help governments and public health bodies recognise the real burden of snake bites, and produce the best antivenoms and guidelines for their use.

Stronger venom

The pressure is on; because tantalising new research also suggest that the snakes themselves may be becoming more venomous.

In the natural world, snakes and their prey appear to be locked into an "evolutionary arms race".

Researchers in North America have found that California ground squirrels and rock squirrels have blood that has evolved to neutralise the specific toxins found in the venom of some rattlesnakes.

But snakes too are responding, including the eastern diamondback rattlesnake (Crotalus adamanteus).

A study published last month in Toxicon analysed which genes code for the complex proteins that make up the toxins in snake venom.

California ground squirrel (c) Bagginz
Some mammals have evolved resistance to snake venom

Dr Darin Rokyta of Florida State University, Tallahassee, US and colleagues found that all these genes in the eastern diamondback rattlesnake are undergoing "positive selection", the first study to show such a change.

That means that snakes acquire new genes coding for new venoms as they evolve.

"To find nearly all of the genes for a single trait being continually developed in this way is truly remarkable," says Dr Rokyta.

The cause is unclear, but it could be to overcome the defences evolved by the snake's prey.

But while snake-bites are unlikely to become more dangerous to their animal victims, which can acquire new defences, they could become more venomous to people.

"From the perspective of humans, who are not competing in that arms race, the possibility exists that toxicity would increase over time," says Dr Rokyta.

And if snake venom becomes more toxic, conflict between snakes and people will only intensify.

Natural World: One Million Snakebites broadcasts on BBC TWO at 2100 GMT on Tuesday, February 22.

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