13 rowers on the 40-member U.S. team come down with stomach illnesses at the World Junior Rowing Championships in Rio de Janeiro -- a trial run for next summer's Olympics -- and the team doctor suspects it was due to pollution in the lake where the competition took place.

13 American teenage rowers and four members of staff were struck down by a stomach bug at the World Junior Rowing Championships in Rio de Janeiro -- a trial run for next summer's Olympics.

The 40-strong USA team was the hardest hit at the international regatta, which finished at the weekend.

The U.S. team doctor said the vomiting and diarrhoea had probably been caused by the polluted water at Rodrigo de Freitas Lake, where the event took place.

However the tournament's information centre manager, Marcelo Neves, disagrees, saying he's been swimming in the water for 30 years, he's never contracted any diseases and he doesn't know anyone who has.

Other teams in the competition also reported illnesses, according to World Rowing, the sport's governing body.

Swiss rower Katharina Strahl, who won gold at the event, and U.S. coach Susan Francia have reportedly voiced their concerns about the water quality just under a year before the Games begin.