Are migratory birds accelerating the spread of avian influenza around the world? Paddy Woodworth looks at the evidence.
Last autumn was an anxious time for Irish birders. The arrival of hundreds of thousands of wildfowl, one of the great wonders of the annual cycle of nature, was tainted with fear.
Would these graceful harbingers of winter bring with them a threat to human lives and livelihoods, in the form of bird flu? And whether or not this actually happened, would the fear itself bring panic measures, like massive culls of birds and the draining of our few remaining wetlands? As the spring cycle of migration begins, it is natural for birders to feel relieved. There have been no recorded instances of bird flu here. There were bad moments, like the discovery of dozens of dead whooper swans in and around Wexford Wildfowl Reserve, but these were due to other causes. And panic measures have been ruled out, as national and international health organisations have insisted that culling birds and draining marshes would spread the disease, not contain it.
But just as the anxiety among birders begins to subside, disturbing new information is coming in. These reports strengthen the case that bird migration can play a role, albeit a very minor one so far, in carrying the potentially catastrophic virus around the world.
In particular, over the last two weeks, mute swans have been found dead from the lethal H5N1 strain of the virus, in southern Europe, while an infected pochard (a wild duck) was found as far west as France.
"We are surprised at these outbreaks," says Dr Richard Thomas of BirdLife International (BLI), an organisation which has closely monitored the disease in relation to wild birds. "They are entirely consistent with a migration pattern in response to bad weather around the Black Sea." This is significant because, until now, BLI had argued that infected wild birds died very quickly, and were therefore most unlikely to carry the disease very far. It is not known whether the mute swans in Southern Europe were infected by other fowl which had come from east of the Black Sea, or by agricultural poultry in that area, which is known to be infected. Either way, it seems they were able to fly hundreds of kilometres while infected, the first time this has been demonstrated.
THE RISK TO Ireland from this new development remains negligible, in the view of Dr Richard Collins, who has studied Irish swans over a long period, and is a familiar contributor to the RTÉ programme Mooney Goes Wild.
"Our mute swans are mainly resident," he says, "and only a very small number from mainland Europe ever reach this country." The Department of Agriculture and Food still appears unconvinced that the news from southern Europe does change the big picture.
"The jury is still out," a spokesperson told The Irish Times. "There is no proof yet that healthy birds can carry the H5N1 [ virus], and sick birds should not be able to fly these distances."
BirdWatch Ireland (BWI), the national birding organisation, which advises the Department of Agriculture and Food on the issue, has stressed since last summer that many of our winter wildfowl - whooper swans, white-fronted geese, brent geese - come from uninfected western areas like Iceland, Greenland and Canada. This view has been reflected in statements from the Minister for Agriculture and Food, Mary Coughlan.
However, BWI statements have also made it clear that many other winter migrants - several duck, more waders, and the rare Bewick's swan - do come from the east, though BWI spokesman, Niall Hatch, argues that most of their breeding grounds are well to the north of any infected zones.
Nor can we be sure that summer migrants soon to come from the south - common sandpiper, cuckoos, swallows - might not carry the disease.
"Any bird could bring it in, in theory," says Hatch, "but if any of these birds were infected it is most unlikely that they could make the journey." It is also true that there is no record to date of infection of wild birds in Africa, the source of many of our summer visitors. The only African infection so far was poultry in Nigeria, almost certainly due to illegal imports from China. But lack of monitoring in much of Africa weakens the argument that it is not a potential source of risk. "Absence of evidence is not evidence of absence," as Richard Collins warned in the context of mute swans.
Bird Life International has made a powerful case that wild birds are unlikely culprits for the spread of bird flu, because unsanitary poultry farming, the use of poultry faeces as fertiliser, and the trade in captive birds are much more probable routes of transmission.
NEVERTHELESS, RICHARD THOMAS agrees that there are "still a great number of unknown elements" in this complex situation. And he cites an article in the latest edition of the Proceedings of the National Academy of Sciences, published in the US this week, which appears to substantiate the most malign scenario regarding wild bird transmission.
This article suggested that some species, or individuals within a species, can carry the disease without falling victim to it, though Thomas points out that the numbers involved were very small.
And the authors still stress that "transmission within poultry is the major mechanism" for sustaining the H5NI virus in south China.
But they continue: "The findings of this study demonstrate that H5N1 viruses can be transmitted over long distances by migratory birds." That seems to dismiss the comforting theory that infected birds die, more or less, on the spot.
Another source of growing concern is the variety of bird species known to be infected. The focus to date has been on waterfowl, partly because of the association between wild duck and domestic duck in southeast Asia, and partly because a dead swan or goose is hard to miss, whereas a dead finch easily passes unnoticed.
But the list of infected species (see panel) now also includes buzzards, herons, an oriental robin and the tree sparrow, a very common bird in China and present, though scarce, in Ireland. The idea that a seed-eater like the sparrow could carry the disease is particularly worrying, because such birds regularly move among poultry in large numbers, even in enclosed areas, scrounging free food.
Our sparrows hardly migrate outside the country at all, but it is now clear that cross-species infection takes place, so incoming migrants could infect one or more of our seed-eating species, which could then infect poultry.
While the consequences for our poultry industry would be very severe, human infection need not follow if precautions are observed. The same applies to wild birds. The only documented infection from the wild is of two Turkish children who handled gloves used in disposing of two infected duck. Both the children survived.
At times of medical crisis, we look to scientists to provide us with black-and-white answers to urgent questions. Such answers rarely, if ever, exist, though as more data becomes available the patterns of bird flu transmission will become clearer.
In the meantime, it would seem prudent to prevent, as far as possible, all contact between poultry and wild birds. And continue to take some comfort in the fact that Ireland's geographical location, relative to migratory patterns, makes transmission from the wild a good deal less likely than in most other places in Europe.
The spread of bird flu
Infected Species
Wild birds infected with H5N1 virus
Mute Swan (Europe); Whooper Swan (Germany at least, Mongolia); Red-breasted Goose (Greece); Mallard (Turkey); Pintail (Turkey); Pochard (France); Smew (Slovakia); Bar-headed Goose (China, Mongolia); Asian Openbill (Thailand); Little Egret (Hong Kong); Grey Heron (Romania); Great Black-headed Gull (China); Great Cormorant (China); Peregrine Falcon (Slovakia, Hong Kong); Goshawk (Germany); Buzzard (Germany); Mountain Hawk-Eagles (smuggled from Thailand to Belgium in hand luggage); Japanese White-eye (Hong Kong); Oriental Magpie-Robin (Hong Kong); Tree Sparrow (China)
Captive birds infected
Poultry (Asia, Middle East, Turkey, Africa); Silver-eared Mesia (UK birds, allegedly imported from Taiwan where Silver-eared Mesia doesn't occur; possible origin mainland China); Blue-headed Parrot (UK, quarantined with Mesias)
Helplines
Anyone wishing to report dead birds should contact the Department of Agriculture and Food helpline 1890-252283
BirdWatch Ireland: www.birdwatchireland