Avian influenza: case number H5N1

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Avian influenza: case number H5N1
Avian influenza: case number H5N1

File number H5N1

Outbreaks of bird flu still frighten the country - or again and again: Veterinarians are finding dead wild animals infected with H5N1 in more and more new regions of Germany. New studies and data are increasingly pointing to certain bird species as vectors, but the ultimate certainty is still lacking.


In an initially relatively unnoticed article, the journal Science reported on March 2 that there were increasing signs of a transnational spread of the avian influenza pathogen H5N1 by migrating wild birds [1]. According to data that has not yet been published but is available to the World He alth Organization (WHO), the H5N1 strains detected in Russia, Kazakhstan, Turkey, Iraq and even Nigeria can be traced back to the same type of pathogen that infected 6,000 wild birds on Lake Qinghai last spring killed in China.

Because of this apparent stability of the virus, the WHO suspects that the highly pathogenic form of the bird flu pathogen has now adapted to at least certain waterfowl species and is no longer killing them. In this way, H5N1 could then migrate with the animals and reach new regions. This assessment is also shared by a publication by virologists led by Robert Webster of St. Jude Childern's Research Hospital in Memphis, Tennessee, who had examined more than 13,000 fecal and swab samples from wild birds - including 5,000 ducks - and actually in six evidently since 2003 he althy animals found the virus [2].

For Wolfgang Fiedler, head of the Max Planck Institute for Ornithology in Radolfzell, this does not come as a surprise, since it is also not advantageous for the bird flu trigger to kill its hosts too quickly - it inhibits or prevents it but thereby its spread. Rather, it is foreseeable that less pathogenic strains will soon prevail. However, this moderated virulence relates more to wild birds and does not yet say anything about the corresponding lethal effect in domestic poultry or in the rare cases of infection in humans.

Webster and his colleagues conclude from their investigations that H5N1 not only circulates among wild birds, but can also be transported by them over long distances. Consequently, an east-west spread from China to western Europe by migratory birds would be entirely conceivable. However, Wolfgang Fiedler discreetly contradicts this, because there is no bird species that migrates directly from the epicenter of avian influenza among wild birds - Lake Qinghai - to western Siberia: "A gradual transmission from east to west, i.e. perpendicular to the main migration directions, would be However, this would require several autumn and spring migration seasons, but there was less than a year between the Qinghai outbreaks and Rügen."

A disastrous interaction between human activities - illegal transport of poultry, feathers or chicken manure - and animal migration behavior, which could have carried the virus further west, cannot be ruled out either. And of course, based on Webster's findings, it's entirely possible that H5N1 has been circulating undetected in wild birds for a long time and made its way to Europe earlier without causing high mortality.

According to Fiedler, the current outbreak patterns in Central Europe can only be explained by "the fact that the virus must have been transported in last autumn at the latest". Because after that there was no further influx of migratory birds from known bird flu areas. The Max Planck researcher also considers the thesis that H5N1 was brought to Germany by tufted ducks and pochards from western Siberia to be plausible.

However, this transport must also have taken place last autumn, because they reached their resting and wintering areas in October and are found there in highest numbers in November. It was only when the long, hard winter put the wild birds under great pressure, which led to an increased risk of infection, for example due to very high bird densities in a few small free ice holes, that the animals died on a larger scale.

Fiedler is not yet certain whether bird flu was primarily responsible for this: "The mortality rates are not unusually high in any of these places. One-year-old mute swans suffer a mortality rate of up to 40 or even 50 in hard winters percent, and like every winter there were well over a thousand one-year-olds among the 10,000 mute swans in the Wittow Ferry area." However, Fiedler does not want to rule out that the hesitant removal of the carcasses increased the infection rate of other birds - especially the scavengers.

However, according to the Friedrich Löffler Institute – the federal research institute for animal diseases – pathogenic changes in the internal organs of some of the swans, which are clearly common to avian influenza, were found without a doubt. Contrary to Webster's work, however, evidence of H5N1 in live wild birds in Europe is still lacking, although countless samples have been examined in recent years, and especially since last autumn. This also applies to the two suspicious duck species. A possible evidence gap could be the low sampling density in Europe despite everything - or the results are not yet publicly available, which has already been criticized by various researchers.

Wolfgang Fiedler also thinks other ways of introduction are possible: illegal transport of wild and domestic poultry, poultry products or even chicken manure imported from China. Significantly, the disease broke out in large poultry farms in Nigeria, Turkey, India or Egypt, while infected wild animals were reported later or not at all.

However, these cases do not explain the occurrence of H5N1 in Bavaria, Brandenburg or on Rügen. Many questions remain unanswered; that's why Wolfgang Fiedler continues to call for increased research zeal. Should it actually turn out that avian influenza has been lingering (unnoticed) in Central Europe since last autumn, this could also mean a certain easing of the situation: the risk of the disease spreading from wild to domestic poultry would be lower than previously assumed.

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