June 1, 2006

Little is known about the highly pathogenic H5N1 avian flu virus that has made recent headlines—or about forms of the virus that are more widespread but pose no threat to birds or people. An article in Science (April 21, 2006) reviews what is known about the ecology of avian flu—how the behavior of birds, for example, may influence how the disease is spread and where it persists.

Low incidence in wild birds, even near outbreaks in poultry

The authors, led by Björn Olsen from Umeå University in Sweden, note that the prevalence of the highly pathogenic H5N1 virus in wild birds is low, even in areas with outbreaks in poultry. They conclude that problems with the deadly avian flu viruses originated from poultry outbreaks, and that control measures must be implemented in the global poultry industry—not through culling wild birds.
     

Wildfowl affected more than other birds

     
Most avian flu viruses are low pathogenicity forms that cause mild, if any, symptoms, in birds. Low pathogenicity avian flu viruses have been found in at least 105 wild bird species from 26 different families. Wildfowl appear to carry these viruses more readily than other birds. Ducks, geese, and swans are among the species primarily affected and, to a lesser extent, gulls, terns, and waders.

Previous studies found that the low pathogenicity avian flu viruses can remain infectious in lake water for up to 4 days at 22 degrees Celsius, and for more than 30 days at 0 degrees. The authors suggest that aquatic birds may become sick after ingesting water contaminated with feces from infected birds. The virus appears to be especially prevalent in dabbling ducks, which feed in surface water (as opposed to diving ducks, which dive deeper for food).

Gulls and terns tend to be infected by different strains of avian flu viruses than those readily infecting ducks. In gulls, the highest virus prevalence is reported in late summer and early fall, possibly because they breed in large colonies, where the virus can spread through repeated, close contact among colony members.

In ducks, researchers have found that infection occurs most frequently in juveniles, presumably because they have less effective immune responses than adults. In North America, researchers have found that the prevalence of low pathogenicity viruses in ducks is about 60 percent near their Canadian breeding areas in fall. The incidence drops to 0.4–2 percent on the wintering grounds in the southern United States. In general, surveillance studies have found that low pathogenicity viruses show up in cycles, depending on the species, time, and place.

A geographic look at avian flu viruses

The authors remark that little is known about the potential movement of avian flu viruses between hemispheres. The researchers found different genetic lineages of avian influenza viruses in Europe and North America. This implies that the exchange of viruses is generally limited within these regions. The authors note, however, that many wading birds are intercontinental migrants that have the potential to carry low pathogenicity viruses around the world.

The researchers say that factors affecting transmission may differ between the highly pathogenic H5N1 virus and the better-studied low pathogenicity viruses. They note that recent efforts to monitor birds for the highly pathogenic H5N1 virus provide a unique opportunity to learn more about low pathogenicity viruses in wild birds too. 

Citation:

Olsen, B., V.  J. Munster, A. Wallensten, J. Waldenström, A. D. M. E. Osterhaus, and R. A. M. Fouchier. 2006. Global patterns of influenza A virus in wild birds. Science 312: 384–388.