Horses, cows, pigs, and, perhaps less worrying for Britons, koalas are among other carriers of bacteria that pose an unknown risk to humans, say scientists at the Moredun Research Institute near Edinburgh.

They warn that far too little is known of the threat posed by bacteria of the chlamydia family, even though one is responsible for the most common sexually transmitted disease in Britain, with 71,125 mainly young people being diagnosed in England, Wales and Northern Ireland in 2001, and as many as 200,000 more young women unaware they were carrying the bug.

This form of chlamydia, C. trachomatis, spread though close contact between adults, is threatening a health crisis - it can cause severe pelvic pain and infertility, unless treated with antibiotics.

Our proximity to pets, livestock, and wildlife could infect us with related species, David Longbottom and Lesley Coulter warn in the Journal of Comparative Pathology. Two animal forms of chlamydia are known to spread to humans.

One is C. abortus, which causes miscarriages and abortions in sheep or goats; pregnant women are at risk too, and there have been public warnings about contact in the lambing season.

C. abortus has also been linked to occasional cases of respiratory illness in staff in labo ratories. C. psittaci can spread to humans from the parrot family, turkeys, and seagulls. It causes anything from mild flu-like symptoms to acute pneumonia. Bird fanciers, staff in aviaries, and vets, have been infected.

This avian chlamydia was responsible for 1,620 cases in Britain over five years to 2001, but the Moredun authors say this is probably an underestimate since psittacosis is difficult to diagnose.

There is laboratory evidence that C. felis can spread from cats and cause conjunctivis in people. A Japanese study found that antibodies to the bacteria might be in 1.7% of the public and in nearly 9% of vets specialising in small animals.

Nearly half of stray cats tested for the same research had C. felis infection, as did more than one in six pets.

Other forms might spread from pigs and cattle, while another human member of the family, C. pneumoniae, linked to asthma, arthritis and Alzheimer's disease, as well bronchitis, pneumonia, and heart disease, has been found in frogs, koalas, and horses. originated in animals.

Moredun, an institute largely funded by the Scottish executive and a world leader on animal disease, belongs to an international network trying to research the subject.