A potentially fatal liver virus could be spread in sweat during contact sports, say Turkish researchers.
People involved in contact sports are urged to be cautious
They are calling for more compulsory testing for Hepatitis B in sports such as wrestling.
The study of 70 male Olympic wrestlers, published in the British Journal of Sports Medicine, found eight who had the virus in their sweat.
However, British experts say that catching it through sweat, rather than via blood, remains extremely unlikely.
Hepatitis B causes a serious, chronic infection in approximately 5% of adults who catch it.
This infection can last throughout life, and cause severe liver problems such as cirrhosis and liver cancer, with some patients dying as a result.
The usual advice to the public is that the biggest risks of catching the virus are through unprotected sex and through infected blood.
The latest study, from Celar Bayar University in Izmir, Turkey, suggests that in certain circumstances, the sweat of an infected person could carry a similar level of virus to blood.
Their research focused on the sport of wrestling, and a selection of competitors aged between 18 and 30, who answered a questionnaire on the effects of their sport on their bodies.
Many of them said that they frequently had bleeding or weeping wounds during training or competitions, offering a more conventional method for virus transmission.
None of the wrestlers had an active hepatitis B infection, but blood tests revealed that nine of the 70 had the virus in their blood. Eight of these also had the virus in their sweat.
Some sporting bodies already hold mandatory testing for HIV among athletes taking part in contact sports, but the researchers say that hepatitis is a bigger issue, as levels of the virus tend to be higher in the blood, making it easier to pass on.
Dr Mark Thursz, a consultant hepatologist advising the British Liver Trust, strongly agrees that all contact sports offer a genuine threat of Hepatitis B transmission - but not via sweat.
"A single drop of blood can hold millions of viable viruses. Viral particles have been found in sweat but we don't know if they could infect someone, so I think the risk is quite low.
"It's hard to imagine wrestling happening without some blood being spilled, and we know the risk of transmission this way is substantial.
"I would recommend that anyone involved in contact sports is tested for the virus, and even if they test negative, is immunised against it, as there is a very effective vaccine."
He said Hepatitis B infected blood could be extremely potent: "I recall reading about an orienteering event in Sweden in which a single person with the virus scraped through some brambles on the route, scratching their skin - and infecting other competitors who later came the same way and scraped the same brambles."