Sunday, April 15, 2007

Pigeons Revisited


# 666


Twice in the past week I've seen references to pigeons in the news, followed by assurances that pigeons aren't thought to be carriers of the H5N1 virus. 


This latest example comes from Bahrain.


30 pigeons die


MANAMA: Nearly 30 pigeons have died of a mysterious disease in Hamad Town over the past five days. Bahraini Yusif Abdulla, who is raising pigeons among other birds at his residence, suspected them to be victims of bird flu. However, the vets believe pigeons are not normally infected with the deadly virus, he told our sister paper Akhbar Al Khaleej.



Pigeons are ubiquitous in many urban settings.  They inhabit parks, building ledges, and rooftops in some of the most crowded cities in the world.  In some places, like the Piazza San Marco, they are a tourist attraction.   People raise pigeons, and fanciers often race them.


In urban settings, people interact with pigeon more than any other bird.  And of course, since the H5N1 virus has been shown to be viable in bird feces for days, this adds another potential route of infection.  


Early on in the Avian Flu crisis, concerns were raised about whether pigeons would be carriers of the virus, and a great many experts denied they were much of a threat.  Pigeons, they claimed, were resistant to the disease.


One such pronouncement came from this article on pigeons and the bird flu virus, published in SEED magazine on May 22, 2006, entitled The Invincible, Flu Immune Pigeon.

"Generally, you can't even infect pigeons, even with high doses [of the flu]," said David Swayne, director of the USDA's Southeast Poultry Research Laboratory. "If you give them high doses, occasionally you can get infected pigeons, but they usually don't shed very much virus."


Swayne has made it his business to shoot obscenely large doses of avian flu into pigeons' noses, using three strains of H5N1 in his experiments: The first, a strain found in Hong Kong in 1997, failed to infect even one pigeon, even when he gave them a far higher dose than they would ever encounter in nature. His other two strains were both found in birds isolated in Thailand in 2004, one a dead pigeon, the other, a dead crow. He nasally administered a high dose of the Thai strains to six pigeons each. Only one of these birds died. Six showed signs of infection but never became sick, and Swayne couldn't even detect the virus in the other five.


This would seem to put the issue of infected pigeons to rest. But there are other accounts that seem to contradict these assumptions.



In February of last year, a 14 year old pigeon handler in Iraq reportedly died of the H5N1 virus, and in West Jakarta, a 39 year old man died in May, after reportedly cleaning pigeon feces from blocked roof gutters at his home.



In the April 2006 edition of the CDC's Journal of Emerging Infectious Diseases, in an article entitled  Avian Influenza H5N1 in Naturally Infected Domestic Cats, they write of a domestic cat that died after eating an infected pigeon.  Other pigeons had reportedly died in the area.


We report H5N1 virus infection in a domestic cat infected by eating a pigeon carcass. The virus isolated from the pigeon and the cat showed the same cluster as the viruses obtained during the outbreak in Thailand. Since cats are common house pets, concern regarding disease transmission to humans exists.


And in a report from last December, Robert Webster, arguably the world's leading authority on the H5N1 virus, had this to say.



Leading virologist Robert Webster told Reuters his laboratory infected sparrows, starlings and pigeons with strains of the H5N1 virus isolated in Vietnam, Thailand and Hong Kong recently.


His team confirmed the birds shed the virus in their stools and can therefore infect poultry.


"They were infected and shedding the virus in their faeces and from their respiratory tracts. The sparrows died, so they are not as big a threat," Webster said on the sidelines of a conference on avian flu and other infectious diseases in Singapore.


"The bigger threats are the starling and the pigeon. The starling didn't die, but shed plenty of virus," said Webster of the St Jude Children's Research Hospital in the United States.


The virus replicated very well in the starling and less well in the pigeon, he added.


Obviously, we have some conflicting opinions here.


It would seem that pigeons can carry the virus, although how much of a threat they pose is still a matter for debate.    This is obviously news that pigeon fanciers, and many city managers, would rather not hear.


The good news is, so far, we've not seen many infections linked to pigeons.  One would think, if they were a major threat, we'd have seen more evidence.  


I've no idea if the pigeons in Bahrain are infected with the H5N1 virus. Hopefully, despite their doubts, they will conduct tests and release the results.  


However, simply dismissing pigeon deaths because `vets believe pigeons are not normally infected with the deadly virus', would seem to be gambling on a popular belief, more than relying on science.