Murray Jenkinson, of Alexandra, asks :-

I and many others have to walk under a bridge in whose steelwork live hundreds of pigeons. Could these droppings we have to walk through be a danger to health, especially if someone slipped and fell into it, and for children in bare feet, or with cuts in their skin, or with respiratory problems?

Heather Brooks, of the University of Otago's Department of Microbiology and Immunology, responded.

Your question about the health hazards posed by pigeon droppings is important as pigeons inhabit almost every city in the world. Sixty different disease-causing microbes have been identified in pigeon droppings but only five of these really pose a danger to human health, as the others are rarely or never transmitted from pigeons to humans.

One of the five (Chlamydia psittaci) is a bacterium which infects the parrot family and some other bird groups. In humans, symptoms of psittacosis range from mild and ‘flu-like to severe pneumonia. The remaining microbes on the ‘bad list’ are all fungi. One yeast-type fungus, (Cryptococcus neoformans) has been isolated on several occasions from pigeon droppings in Dunedin by microbiologists at the University of Otago. This and some of the other fungi occasionally cause life-threatening infections. Another yeast on the list (Candida parapsilosis) is known to infect the skin.

If, at this point, you feel like demanding the city council exterminates the entire population of pigeons in your town, don’t – for three reasons! First, many animals (including pets) carry harmful microbes. Second, the risk of acquiring an infection from a pigeon appears to be very low compared to the number of person-pigeon contacts. Third, normal exposure is highly unlikely to lead to disease if you are healthy, as the majority of infections have occurred in people with impaired defences or who have been exposed to large quantities of droppings or dead pigeons.

With regard to the pigeon-infested bridge at Alexandra, there is an increased risk of infection if bare skin is in contact with the droppings, particularly if there are cuts or abrasions, or if the person’s defences against infection are compromised. It would be a good idea if the pigeons could be discouraged from nesting in the bridge.