A warmer-than-average winter is thought to be partly responsible for a doubling in the number of cases of ornithosis since January.
The main risk is severe respiratory infections contracted from unwittingly breathing in dried and powdered bird faeces. It is feared that such infections are under-reported and under-diagnosed, and a national working group of scientists is being established to investigate the problem.
The scientists warn that infections, often carried in the birds' intestines, can kill or cause severe respiratory problems after the remotest of contacts with birds.
"We simply don't know the scale of the problem because there are no figures, and we do need to do a lot of research. There is very poor public awareness about this problem," said Dr Tim Wreghitt, consultant virologist at the Public Health Service Laboratories, Addenbrooke's Hospital, Cambridge, and an international expert on chlamydia infections.
"Chlamydia psittaci is probably the most important and it causes a respiratory infection in humans which can be fatal and is often associated with neurological conditions.
"Some time ago we had a 60-year-old woman who died of an infection that she probably acquired from a dead pigeon that her cat had brought into the house.
"She probably breathed in dust from the dead faeces that were infected. A local GP was off work for four weeks after acquiring an infection from pigeons nesting outside the door of his surgery. Another man became ill after removing nests on his house prior to painting it."
Birds carry the infection in their guts and once they have it they never lose it, even though they themselves may be unaffected by it. Up to 60 per cent of some species may have the bacteria, which is usually passed on when humans unknowingly breathe in the dried and powdered faeces.
Dr Wreghitt said there had been a doubling in reported cases in the first three months of this year: "There has been a 100 per cent increase, but we don't know why, although my theory is that the warmer winter has meant that more diseased birds have survived, increasing the likelihood of contact with humans and of infection being passed on."
An increase in the numbers of wild birds in towns and cities may also be a factor. Populations of birds including pigeons and gulls have increased significantly in many urban areas over the past few years.
A team of biologists from the University of Wales, Cardiff, are planning to investigate the whole issue of infections passed from birds to humans and are setting up a working party. It would look at a variety of other diseases passed on to man by birds, including salmonella. The group's Professor Peter Haskell said: "There is a lot of concern about this area. We had a conference of specialists recently and we think it is something that needs to be looked into in depth."Reuse content