The latest outbreak of avian influenza in Western Europe has many people pointing the finger at migratory birds as spreaders of the disease. In reality, the link has not been conclusively proven. “Getting our facts straight is in everyone’s interest”, says Henk van der Jeugd, head of the Centre for Avian Migration and Demography at NIOO-KNAW. “Without more certainty, we cannot take effective measures to contain the disease.”

It’s tempting to blame migratory birds. The fact that they move across large distances seems to put them in an ideal position to become infected and in turn infect other birds. But “much about the way in which the disease is spread and how it reaches poultry farms is as yet unclear”, says Henk van der Jeugd. So what do we actually know? And more importantly: what do we still need to find out? If migrating birds are involved, how does that work.

Among wild birds, it’s mostly waterbirds that are potential carriers of the avian influenza virus: swans, geese, ducks, waders and gulls. A number of species are migratory. So migratory birds could indeed play a role in spreading the disease. But that does not mean they are always the source of outbreaks.

Migratory mallards

In October, a NIOO-study was published that underlines the need for further, thorough research. Mallards are one of the species most often found to be infected with avian influenza. But the NIOO-research concluded that instead of importing the virus into the Netherlands, most migratory mallards probably contract it here.

The annual bird flu peak in autumn does coincide with the arrival of large numbers of migrants, but it is unlikely that migratory mallards already carry the virus with them at that stage. Rather, they appear to be more susceptible to strains of the virus found in the Netherlands than Dutch mallards would be at that time. This vulnerability is what causes the peak. So we have to be careful not to jump to conclusions.

Different strains

There are many strains of bird flu, identified by different combinations of Hs and Ns. Some strains are quite common, others more rare. But they all have their own, unique dynamics. In addition to seasonal patterns, the incidence of the virus among wild birds is characterized by longer cycles as well. This makes it difficult to reach any single, definitive conclusion.

The strain of avian influenza that was featured in the NIOO-study was not the same as the one identified in the latest outbreak in the Netherlands, at a farm in the village of Hekendorp. This latest strain had not been found in Europe before now. So it is unlikely to have spread from a local source.

Prime suspects?

On 3 November, an avian influenza virus similar to the H5N8-strain identified during outbreaks in South-Korea earlier this year, was isolated from droppings of two Bewick’s Swans in Japan. Soon after, there were outbreaks of H5N8 in Germany, the Netherlands and the UK. Bewick’s swans are also found in the Netherlands. However, these swans do not migrate to the Netherlands from Southeast Asia – in fact, no such migration route exists even for other birds.

The only way in which Bewick’s swans could have imported Asian bird flu into Western Europe is indirectly: through contact with other migratory birds in areas where large numbers of them stop over, moult or breed before going on to their various destinations. This could have happened in the Russian arctic, where populations of migrating waterfowl from East and West potentially meet.

The arctic route seems a possible scenario, but one that is likely to involve other species such as arctic-breeding ducks or geese and not Bewick’s swans. However, the results of what little monitoring has been done of bird flu in wild birds in the Russian arctic have so far been largely negative.

Research and monitoring

So if migratory birds were involved in the latest outbreak, it’s likely to have been through some kind of complicated relay rather than via any direct route. Also, if migratory birds were involved, we should be able to find evidence in wild birds in the area around the affected poultry farms and elsewhere in Western Europe right now. And we should find it in arctic areas where large numbers of migrating birds come together during summer.

The NIOO’s Centre for Avian Migration and Demography will continue to look into the possible connection between bird flu and migratory birds. This involves monitoring the incidence of the virus and investigating the migration patterns of wild birds in detail. For these efforts to be effective, ecologists and virologists will need to work closely together.

A proposal to track the movements of migratory ducks that breed in arctic areas has already been submitted for funding as a possible next step.

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The NIOO is one of the largest research institutes of the Royal Netherlands Academy of Arts and Sciences (KNAW), with around 300 employees and students. It is specialised in fundamental and strategic ecological research. As of early 2011, the NIOO is based in a sustainably-built research laboratory in Wageningen, the Netherlands.

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