Posted by Influenza is primarily a disease of birds. Most emerging infectious diseases in humans are started out as diseases of animals, what are called zoonoses. We worry about zoonoses for that reason. It is one of the hardwired tendencies of any species to think of their own survival first — that’s natural — but humans are only one species amongst many. And while we worry about viruses we might catch from animals, the animals are also getting sick. It’s not just influenza we share with birds. Birds suffer from other diseases they can pass on to humans, too, and one of these is West Nile virus (WNV) infection, one of a group of insect borne encephalitis viruses that infect both birds and humans. Since arriving in North America in 1999, WNV has killed almost 1000 people but done considerably more damage to the bird population. A new paper in Nature tries to estimate how much damage.
West Nile virus or a similar disease could wipe out many of the U.S.’s backyard birds, profoundly changing some of the country’s most familiar wildlife and ecosystems.
That’s the finding of a new analysis of 26 years of data from the national Breeding Bird Survey?data that reveal the dramatic effects of the 1999 arrival of West Nile virus in the U.S.
Lead author Shannon LaDeau of the Smithsonian Migratory Bird Center and her colleagues found that species that thrive near humans suffered extremely high death rates from the disease.
Up to 45 percent of crows died after the virus arrived, with robins, chickadees, and eastern bluebirds not far behind.
Some of these populations had been increasing before the virus hit, which is a good indication that West Nile caused the declines, the authors write.
The disease may not completely wipe out bird populations on its own, the scientists add, but it is an alarming addition to existing population threats such as climate change and habitat loss.
“They’re our backyard species, and we haven’t been watching them as much as we’re watching the other species, because people consider them safe,” LaDeau told National Geographic News. (National Geographic News)
The virus is spread from bird to bird — and from bird to human — by mosquitoes. You might wonder how a mosquito can bite a feather covered bird, but there are small areas around the eye and elsewhere where the insect can and does gain access to the bird’s bloodstream. If that mosquito bites us, we can be infected, but this requires a “bridge species” of mosquito that bites both humans and birds and most mosquitoes don’t do that. Enough do, however, so 12,000 human cases of WNV have appeared in 44 of the 50 states and bird infections in every state except Alaska and Hawaii (map and 2006 case counts here). Human cases occur where there are infected birds because person to person transfer of the virus by a mosquito is thought not to happen because the level of virus in the human blood stream is not high enough.
But it is the effect on 20 common species of backyard birds that is the subject of the Nature paper. Disentangling the effects of the virus from the many other factors that affect bird populations is not an easy task:
To do so, they designed species-specific predictive models based on knowledge of the prevalence of the virus, exposure to mosquitoes and overall mortality for 20 different bird species, each species representing a specific combination of urban (human) association and susceptibility to the virus. The model was applied to 26 years of population data for six geographical regions to construct probability distributions for the expected abundance of each bird species in a given region before and after the arrival of the virus. (Carsten Rahbek News and Views, Nature)
The most affected birds, by their estimate, were the “peri-domestic” (backyard) species common to cities and suburban environments. One of the hardest hit was the American robin, but other species, including crows, chickadees, and bluebirds also experienced ten year population lows after the large outbreaks of 2002 – 2003. It was originally thought that crows and jays were the main victims of WNV, but that is probably because these birds are large and their corpses visible. We now know that many bird species, probably well over 100, are also infected, although many are apparently quite unaffected by the virus and may be a significant reservoir.
This is one of the first to gauge the effects of this bird infection on common species in North America, and even in the small number of species examined in the paper there are many uncertainties. WNV is a nasty human disease with a significant case fatality ratio and frequently debilitating after effects in survivors.
Maybe we don’t care that much about other species. But there are some good reasons why we should.
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