Scientists studying pigeons have found that the often reviled urban birds that dominate city squares around the world carry two disease-causing bugs that make them a public health hazard.

The findings of the study by a team of researchers in Spain show that although these bacteria can be harmful to humans, they appear to cause no harm to the birds themselves.   

As a result, pigeons -- often dubbed 'rats with wings' by those who suspect them of spreading disease -- can act as living 'reservoirs' for some harmful bugs, the scientists said.

'Animals that live in close contact with humans can be dangerous reservoirs of human pathogens,' wrote Fernando Esperon from the Animal Health Research Centre in Madrid, who led the study. 'These birds may therefore pose a public health risk to the human population.'    

Inhabitants of cities from London to Venice to New York to San Francisco tend to have a love-hate relationship with the millions of urban pigeons that dominate city plazas, street-side cafes and monuments. Their droppings plaster Trafalgar square in London, St Mark's square in Venice, and Times Square in New York, where they peck endlessly at crumbs or leftover food.

For this study, which was published in the BioMed Central journal Acta Veterinaria Scandinavica, Esperon and colleagues analysed 118 pigeons captured using gun-propelled nets from urban areas of Madrid to find out the prevalence of certain bacteria known to cause disease in humans.

They found a bug called Chlamydophila psittaci in 52.6 percent of the pigeons captured, and another bug called Campylobacter jejuni in 69.1 percent.   

Psittacosis infection in humans often starts with flu-like symptoms and can develop into life-threatening pneumonia. And according to Esperon, bugs from the campylobacter species are one of main causes of acute diarrhoea across the world.

'In fact, in many countries such as Britain, Canada, Australia and New Zealand, Campylobacter jejuni infection causes more cases of acute diarrhoea than infection by salmonella species,' he wrote.

Like other bugs, salmonella bacteria can cause fever, diarrhoea, nausea and vomiting in those infected with it.

The scientists said that although the birds themselves did not seem to get sick from the bacteria, they could potentially pass them on the humans. 'These data should be taken into account for pigeon population management,' they wrote. - Reuters