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IT鈥橲 not just farm animals that can contaminate water supplies with a
parasite that causes severe diarrhoea, claim researchers.

Diminutive Californian ground squirrels can shed as much cryptosporidium as
cattle, say Edward Atwill and his colleagues at the University of California,
Davis. But others doubt that wild animals pose much of a risk.

Even in healthy people, cryptosporidium can cause diarrhoea, stomach cramps
and vomiting for days or weeks. For immunocompromised people, such as AIDS
patients, it can be deadly. There is no effective treatment.

The parasite is a threat because it can survive the chlorine treatments used
to disinfect drinking water. Water treatment plants in Britain are now having to
install filters that can remove most spores, or oocysts, but this is not
compulsory in the US.

When an outbreak of cryptosporidiosis struck San Francisco in 1994,
politicians and activists accused cattle ranchers of letting parasites from cow
manure get into the water. They suggested banning cattle from areas where the
oocysts could pollute water sources, a policy Atwill supports.

But Atwill thinks squirrels, skunks, field mice, raccoons and coyotes are
also a risk. He鈥檚 been checking these animals for Cryptosporidium parvum, the
type of cryptosporidium that infects animals and humans. Atwill鈥檚 team counted
the number of oocysts in the faeces of ground squirrels from Californian
grasslands and woodlands over several months. The numbers were surprisingly
high.

Sixteen per cent of ground squirrels were infected and squirrel populations
can shed as many as 10 million oocysts per hectare per day, the team found.
People can be infected by fewer than 100 oocysts. 鈥淵our typical little squirrel
sheds more crypto than a whole adult cow,鈥� says Atwill. 鈥淚t looks like they are
a very infected population.鈥�

Cryptosporidium remains a big issue in California. One environmental group is
even trying to force people who work on cattle ranches to disinfect themselves
before going home to prevent the parasite spreading. But Ralph Phillips, a
member of the team, says the findings suggest hikers and even people picnicking
in the countryside would also need to be disinfected.

Atwill thinks the results imply cattle aren鈥檛 entirely to blame for the
cryptosporidium problem. 鈥淲e鈥檙e not necessarily better off kicking out all the
humans and leaving the land for wildlife,鈥� he says.

But Tony Sturdee of Coventry University is sceptical. 鈥淰irtually all wild
mammal populations carry the parasite and shed oocysts to a greater or lesser
degree,鈥� he says. But unless animals defecate near rivers, or heavy rain washes
faeces into streams, the oocysts probably won鈥檛 get into water supplies, Sturdee
says.

Inquiries into British outbreaks have suggested farm animals are often to
blame, he says. What鈥檚 more, while it was thought that C. parvum was unusual in
infecting a wide range of mammals, the latest studies suggest that there are
subspecies specific to different animals. Atwill has yet to confirm that the
crytosporidium infecting squirrels can indeed infect people. 鈥淲e don鈥檛 have any
volunteers yet,鈥� he says.