Forensic DNA test conclusively links snake bite marks on people to species

Starting with a simple DNA swab taken from fang marks on people bitten by snakes, an international research team correctly identified the species of the biting snake 100 percent of the time in a first-of-its-kind clinical study, according to data presented today at the American Society of Tropical Medicine and Hygiene's (ASTMH) Annual Meeting.

The study, conducted at three medical facilities in Nepal, found that if snake DNA could be isolated from the bite wound, the test identified the species of snake responsible every time.

"These findings represent a significant step toward improving care for patients in areas of the world where snakebites constitute a massive but neglected health risk," said Franois Chappuis, MD, PhD, chief of the division of tropical and humanitarian medicine at Geneva University Hospitals, Switzerland, who directed the study and presented the results. "This DNA test may hasten more effective bedside diagnostics for snakebite victims, giving them a better chance of surviving and making a full recovery."

Bites from venomous snakes are common in many parts of the world and an especially serious unresolved health problem to millions of people living in South and Southeast Asia, as well as Africa and Latin America. Although there are no reliable numbers at the global scale, a study published in 2008 estimated at least 421,000 cases of envenomation and up to 94,000 deaths occur worldwide from snakebite each year. However, experts warn that these figures may underestimate the real problem, which is believed to affect several million people bitten by venomous snakes annually and hundreds of thousands who die or survive disabled, suffering from amputation or deformed limbs as a result of unavailable or delayed treatment.

In some villages of the study area of southeast Nepal reported today, a community-based survey in 2002 revealed an estimated 1,162 snakebites and 162 snakebite deaths per 100,000 people per year, one of the highest rates ever reported. Likewise, a recent survey of more than one million deaths in India, published in the journal PLoS Neglected Tropical Diseases in 2011, found that 46,000 people die every year in India from snakebites. This figure is about 20 times the official death toll recorded in Indian hospitals, most likely because less than one in four fatal snakebite cases had received any hospital treatment, the study revealed.

"People bitten by snakes in South Asia often do not seek treatment at a medical facility, and if they do, the vast majority don't take the snake to clinics, although it is often killed, and can't identify the species that bit them," Chappuis said. "Yet knowing the species of snake is critical to determining the best course of treatment."

In Nepal, for example, cobras and kraits are among the most common venomous snakes. Their venoms attack the nervous system and can be fatal. Patients bitten by either snake have similar symptoms, such as nausea, headache, drowsiness and limb as well as respiratory paralysis. However, the toxins of these snakes and their mechanisms of action differ, and so does the response to antivenom and other drugs. For example, the locally available antivenom has limited or no efficacy against krait envenomation, which often leads to useless repeated doses resulting in increased cost and--possibly--adverse side effects, explained Chappuis. Also, bites by cobras and kraits lead to different complications that the attending physicians need to prevent or be prepared for. Therefore, positive identification of the biting species becomes important to effective treatment.

Preliminary results presented today show that among 194 bite-site DNA samples collected in Nepal during the study, 87 were from a venomous species. The spectacled cobra accounted for 42 bites and the common krait for 22. In 21 cases, the patients brought the dead snake with them, and in those cases expert examination of the snake and the DNA test, performed by different investigators, independently provided the correct identification. Snake DNA could be obtained from about one in four bite wounds. According to the researchers, one factor that excluded samples was if the patient attempted folk or home remedies tampering with the bite site prior to arriving at the medical clinic.

The research team also correlated clinical symptoms with the species biting patients. They found that krait bites happened more often at night, indoors, or while people were sleeping, while cobra or pitviper bites were more likely to cause swelling at the bite site. The technique also identified several species of non-venomous snake involved in bites.

"The proportion of non-venomous snakes that bite people is actually rather high," said Ulrich Kuch, PhD, of the department of tropical medicine and public health in the Institute of Occupational, Social and Environmental Medicine of Goethe University (Frankfurt, Germany), and developer of the snake DNA identification test. "Snakebite patients are kept in the hospital for 24-hour observation, which strains healthcare resources in many countries. By identifying species of nonvenomous snake, it is also possible to assist the local people and avoid hospital stays for those not at risk of envenomation."

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Forensic DNA test conclusively links snake bite marks on people to species