Ecofactsheet: migratory birds & bird flu

Een groep trekvogels in V-formatie in de lucht
© "Migrating birds" by Timmy_L is licensed under CC BY 2.0.

Ecofactsheet: migratory birds & bird flu


Which virus?

There are many types of avian influenza ('bird flu'). They are categorized by different combinations of proteins on the surface of the virus. So far, 16 variations of 'H' (hemagglutinin proteins) have been identified and 9 variations of 'N' (neuraminidase). In addition, some types of bird flu are classified as 'low pathogenic' (mild) while other types are 'highly pathogenic': causing symptoms of serious illness - in poultry, in particular.

The strain of bird flu responsible for the recent outbreak in the Netherlands is a highly pathogenic form of H5N8.

Where does it come from?

H5N8 was first identified in China in 2010. It probably originated there, in free range farmed ducks. The same strain of the virus was later found in South-Korea and Japan, in both farmed poultry and wild birds (Eurasian teals, Baikal teals, spot-billed ducks, mallards, taiga bean geese, white-fronted geese and Bewick's swans). In early 2014, there was a major outbreak on South-Korean poultry farms.

The strain of the virus first found in the Netherlands in November 2014 so far appears to have infected five poultry farms - closed, not free range. Genetic testing has shown the European strain to be virtually identical to the East-Asian strain, suggesting it must have made its way from there. Farms in Germany, Britain, Italy, Hungary and the United States have also been infected. In Canada H5N2 was found in farms, with an H5 part very similar to the H from H5N8.

Wild birds and poultry

Wild birds have much greater natural resistance to bird flu than poultry. Waterbirds in particular - ducks, swans, geese, waders, gulls - often carry mild strains of the bird flu virus without any visible symptoms of illness, except from reduced activity. Chickens, on the other hand, are extremely vulnerable. This vulnerability can cause certain mild forms of bird flu (types H5 and H7) to mutate in poultry into highly pathogenic forms that are potentially lethal to them.

Once such a highly pathogenic form of the disease has developed, poultry may in turn infect wild birds with it ('spill-back transmission'). But some wild birds have such strong resistance that they can even carry these highly pathogenic forms of the virus without showing any visible signs of infection, and they can therefore further spread it.

How did the virus reach The Netherlands?

The answer to this and related questions remains unclear. Bird flu can be spread through the transport of poultry, poultry products and fertilizer, as well as by wild birds  - both migratory and resident. The evidence so far does not point to any single route the virus might have travelled.

Without more certainty on this point, it is extremely difficult to take effective measures both now and in the longer term.

Teal in Germany

On the German island of Rügen, six wild ducks were tested for bird flu that had been shot some 50 kilometres away from an outbreak on a turkey farm. Three turned out to have been healthy, two carried mild strains of the virus and one - a teal - was infected with H5N8. This German teal was the first wild bird in Western Europe confirmed to be a carrier, although the fact that it was flying around normally seems to suggest it wasn't very ill.

The German discovery didnot settle the question of how H5N8 spread to Europe. It is worth noting that no traces whatsoever of H5N8 bird flu had been found in samples from duck decoys as part of regular Dutch and European monitoring programmes.

Migration routes

Wild birds from East Asia do not migrate to Western Europe. Their main point of contact with 'our' migratory birds is when both groups stop over in their breeding and moulting grounds in Siberia. It is possible that H5N8 was transmitted there, and that it reached Europe through some kind of relay involving more than one species of bird.

Samples from waterbirds in the Arctic have so far not produced any evidence of widespread infection, past or present. But monitoring in this area would need to be stepped up significantly before any conclusion can be reached.


Research into infection among wild birds

If wild birds are indeed responsible for spreading H5N8, it should not be too hard to find traces of the virus in nature. That is why research teams from the Erasmus MC, NIOO-KNAW, the Centre for Avian Migration and the Dutch Centre for Field Ornithology set to work right away after the outbreak, taking samples from the surrounding area and analysing them.

In November 2014 NIOO-researchers have mainly collected fecal samples from mute swans, Bewick's swans and various species of ducks. They have also taken throat and cloacal swabs from the swans, as well as some blood samples.

On 1 December, deputy Economic Affairs minister Sharon Dijksma announced that feces of two wigeons near Kamerik (province of Utrecht) were found to contain traces of H5N8 bird flu. The genetic resemblance of the virus found in these wild ducks and the virus that has been surfacing in Eastern Asia since 2010 is high. This suggests they have the same source. In a wigeon shot in Siberia in September 2014 H5N8 was also confirmed, and to be precise it was a virus genetically intermediate between the European and the Asian forms. This find appears to support the theory of an indirect connection via a stop-over site where ducks from both areas can meet. More details can be found in the scientific paper about this research.

Latest developments

Even though no clear evidence has emerged as to how the disease could have spread in the Netherlands, most of the Dutch measures to contain bird flu have by now been withdrawn.

That does not mean there are no new cases of this bird flu in Europe. Early 2015 researchers found a sick black-baked gull on the German side of the border near the Dutch city of Delfzijl. It was later confirmed that the gull had been infected with H5N8 bird flu.

More recently, a sick chicken has been found in the holdings of a private holder in Anklam, in northeastern Germany. Also in Hungary poultry tested positive for this virus. In the Netherlands only a couple of mild not-related types of birdflu have been found on farms lately.

The latest news of 13 April 2015: in a second extensive sample collection effort the NIOO has found one widgeon dropping that contained H5N8 again. The sample was collected at the end of February and afterwards analysed by the Erasmus MC. The virus was at least until recently still present in the Netherlands. By now, the widgeons have left the country to breed elsewhere.

(Latest update: 13 April 2015)