Hundreds of sea otters on the coast of California are dying of toxoplasmosis, a disease caused by the Toxoplasma gondii parasite that is carried harmlessly by up to 70 per cent of cats and passes into the environment in their faeces.

When the parasite leaches into the ocean, it infects sea otters, triggering a brain disease. Toxoplasmosis is now responsible for 17 per cent of sea otter deaths, and signs of infection have been found in 38 per cent of living otters and 52 per cent of dead ones. Patricia Conrad, of the University of California, Davis, said that the disease was now one of the most significant threats to the southern sea otter, Enhydra lutris nereis. The species was hunted almost to extinction in California in the 19th century and now numbers about 2,500.

“This is now one of the greatest challenges facing sea otters,” Dr Conrad told the conference. “Tourists are drawn to watch them frolicking in the surf, but this behaviour that makes them so appealing puts them at risk of being exposed to infectious disease agents that come from the land.”

Cats pick up the T. gondii single-celled protozoan parasite by eating wild birds and mammals that are infected. The cats are not themselves harmed by the parasite, but it reproduces in their bodies and produces egg-like oocysts that are shed in the animals’ faeces.

The parasite can infect human beings, and about one in four people carries it, but it does not usually cause disease. The exceptions are pregnant women, in whom it can cause miscarriage and birth defects, and people with compromised immune systems, such as Aids patients. It can cause a brain infection that is often fatal.

Dr Conrad said that the only way to control toxoplasmosis infection would be to curb it at source, and that meant keeping domestic cats indoors so they could not eat infected rodents and birds.

“I know that’s not fun,” said Dr Conrad, who owns four cats.

Times Recommends

• 'Great Schlep' may seal deal for Obama
• Hindu 'storm troopers' flex their muscles
• Ahtisaari: deserved reward for (failed) plan