More than 400,000 people are poisoned by snakebites worldwide each year and 20,000 of them die, with most cases occurring in the poorest countries, researchers say.

In an article published in U.S.-based open access Public Library of Science Medicine, the researchers said the burden from snakebites was highest in South and Southeast Asia and sub-Saharan Africa.

Data on snakebites is far from comprehensive as most of them happen in places with poor healthcare systems and where record-keeping is generally poor or non-existent.

For this study, the researchers trawled through 3,256 published articles and extracted data on 68 countries.

They said, according to a conservative estimate, there are 421,000 cases of envenoming, or venom delivered through biting, each year and which lead to at least 20,000 deaths.

But the actual figures could be several times higher.

"These figures may be as high as 1,841,000 envenomings and 94,000 deaths. Based on the fact that envenoming occurs in about one in every four snakebites, between 1.2 million and 5.5 million snakebites could occur annually," wrote the team led by Janaka de Silva at the University of Kelaniya in Sri Lanka.

India has the highest figures — with 81,000 envenomings and 11,000 deaths each year, followed by Sri Lanka with 33,000 envenomings, Vietnam (30,000), Brazil (30,000), Mexico (28,000) and Nepal (20,000).

The researchers stressed that snakebite was a badly neglected problem in many countries as they often go unreported.

"Many victims do not seek hospital treatment and prefer traditional remedies. Some may die at home, with their deaths unrecorded," they wrote.

"Studies from rural Nigeria and Kenya have reported that only 8.5 percent and 27 percent of snakebite victims, respectively, sought hospital treatment."