In many cities, pigeons—to take one urban animal—are reviled as flying vermin. They whitewash ledges and pick at filthy crumbs in the gutter. And, yes, these, dubbed by some as "rats with wings," do carry diseases that humans can catch. But so do innumerable wild creatures outside city limits, the animals we eat—even our beloved pets.
Pigeons are guilty of transmitting fungal and bacterial diseases, primarily via their droppings, which pose the greatest risk to those with weakened immune systems. But cast against the recent spread of infectious zoonotic diseases—such as H5N1 bird flu, severe acute respiratory syndrome (SARS) and human immunodeficiency virus (HIV)—experts question the degree of concern over the disease-bearing potential of the birds that have colonized cities the world over.
In principle, any animal can carry a disease that humans could catch. But Marm Kilpatrick, an ecologist at the Consortium for Conservation Medicine in New York City, which studies human-induced environmental change, species health and biodiversity, wrote in an e-mail: "In reality, the vast majority [about 99.999 percent] of pathogens that are carried by animals won't infect people."
Even so, zoonotic diseases represent a growing proportion of emerging infectious diseases; two British studies calculated that about 75 percent of emerging infectious diseases are zoonotic. (By comparison, about 60 percent of all human pathogens can infect animals.)
Real rats (the ground-hugging kind) aren't innocent by any means: Research links them with the reemergence of bubonic plague and typhus. But bats (of whom "winged rats" is more apropos) may be giving the unpopular rodents a run for their infamous reputation. Long associated with rabies, bats gained new notoriety in the 1990s after outbreaks of the Hendra and Nipah viruses killed both humans and livestock in Australia and Southeast Asia, respectively. A few years later SARS terrified the world by taking flight on commercial airlines. The virus left a trail leading back to the live animal markets in China, first to civet cats and subsequently to bats, the latter vector now believed to be the true starting point for the virus.
And, despite increasing urbanization throughout the world, people and wildlife are sharing more infections. In the Hendra and Nipah outbreaks, habitat fragmentation and increased contact between wild bats and domestic animals have been implicated. Bushmeat, particularly that of our close cousins the chimpanzee, has caused Ebola outbreaks in Africa.
In the U.S., prairie dog owners caught monkey pox from their pets. And the reforestation of Northeastern states over the past century has allowed deer populations to boom, spreading Lyme disease.
By comparison, pigeons' potential for spreading bird flu seems rather minimal. So far most of nearly 220 human deaths caused by the pathogenic H5N1 strain of bird flu have been traced to contact with poultry. And the strain has yet to arrive in North America. If a similar one were to emerge here, the result could be disastrous for industrial farm workers before anyone else, according to Gregory Gray, director of the University of Iowa's Center for Emerging Diseases.
"Exposure to domestic birds has changed markedly," he says. In the nation's confined animal feeding operations (CAFOs)—the industrial operations that have replaced family farms with at minimum 9,000-chicken or 750–large pig facilities—agricultural workers spend much more time in close contact with animals than a farmer would have 50 years ago.
These are potentially the mixing pots for the next flu pandemic, Gray argues. When an outbreak occurs undetected in a facility, viruses can mutate as they cycle through large flocks or herds. Gray and his colleagues have shown farmers, veterinarians and meat processors all had high swine influenza infection rates, and avian veterinarians carry more bird flu.
In 1983 a low-grade bird flu virus, perhaps left by ducks, spread into chicken warehouses in Pennsylvania. There, it mutated from a minor infection to become what Robert Webster, the virologist at the scene, called "Ebola for chickens."
This outbreak took two years and the destruction of 17 million birds to control. Webster links some of its spread to New York City's live bird markets, where chickens are packed into cages in close quarters with ducks and geese, natural carriers of bird flu.
Webster believes these markets pose a greater risk than CAFOs in the developed world where so-called "biosecurity" procedures to keep diseases out have been tightened since the emergence of H5N1. "Live bird markets are the breeding place for all pandemic strains in my opinion," he says, and, despite attempts to purge it, avian influenza continues to show up in American live bird markets.
But for those whose daily animal interaction doesn't extend beyond shooing squirrels or feeding the dog, the prospect of zoonotic disease shouldn't keep them awake at night. "Most people should be more afraid to walk into a doctor's office during flu season," says Pennsylvania State University avian pathologist Patty Dunn.
As for pigeons: research has shown that even those infected with bird flu actually transmit very little. And they carry so little West Nile virus in their bloodstreams that they are unlikely to infect mosquitoes who could then infect humans, Kilpatrick says, making the birds more likely to slow an epidemic than spread one.