two birds flying

H5N1 Avian Influenza

The H5N1 strain of  Avian Influenza was detected in North America earlier this year and has spread to commercial flocks. What are the short term and long term impacts of this outbreak?


H5N1 Avian Influenza  has wiped out more than 20 million egg-laying chickens on commercial U.S. farms – about 6% of the country’s flock. When influenza is found on a farm, all birds are destroyed to prevent spread. Europe particularly France is suffering its worst outbreak from H5N1 exacerbated by the invasion of Ukraine, the largest exporter of eggs to the EU. This strain is HPAI (highly pathogenic avian influenza) for birds but infections of  humans are very rare. Vaccines may help mitigate the impact with laying hens but because of mutation they are not precise in targeting strains of AI.


Low pathogenic varieties of H5N1 Avian Influenza are endemic in birds in North America. In 2008 an (HPAI), emergent strain of H5N1 developed which spread rapidly through Asia and Europe. The H5N1 strain is a fast-mutating and enzootic, that is, found in many bird populations. It is epizootic (an epidemic in nonhumans) and panzootic because it effect animal species, not just birds. It has killed tens of millions of birds and led to the destruction of hundreds of millions more by meat and egg producers trying to stop the spread. The emergence here, while inevitable, means strain and uncertainty  for the poultry industry. 

There was a (so far) worse outbreak of Avian Influenza In 2015. The H5N2 variety led to 50 million birds being destroyed. The outbreak was highly concentrated in the Upper Midwest with more than half the laying chickens culled from Iowa farms. Most of the outbreaks were in April and by June, the crisis was over. This  coincides with the spring  migratory patterns of birds and that the virus does not tolerate warm conditions. However there was strong evidence that humans and equipment helped spread the virus from farm to farm. Following the outbreak, new biosecurity and depopulation, disposal, and virus elimination protocols were put in place by the USDA. 

There was considerable concern that the variety of AI might be transmissible to humans. About 60% of humans known to have been infected with the Asian strain of HPAI A(H5N1) have died from it. Human cases are rare thus far. But AI varieties like H5N1 can mutate into strains capable of efficient human-to-human transmission. Another strain, H1N1 was responsible for the Spanish Flu in 1918 that killed 25 million people worldwide. So it is a constant concern for public health authorities.


In the short term, warm weather and the end of migrations should mean a seasonal end of the spread. Migration in the fall has been less of a problem, due to birds being in warm weather all summer.  

But H5N1 Avian Influenza has now been present in Europe for many years. Since December, farmers in Europe have had to cull more than 17 million birds. In North America, many bird species that carry it remain healthy and so can spread it over considerable distances. It has appeared in 40 wild bird species from crows to pelicans to bald eagles. Wild fowl spread the virus to domestic flocks by sharing foods, through excrement and, probably, via aerosol.  Future H5N1 outbreaks are likely.  

However, the fact that the  incidence of infected flocks is not as geographically clustered  indicates that the new rules and biosecurity have mitigated the spread. There is more that can be done to protect flocks from contact with wild birds, up to and including isolation during periods of outbreak. This will present challenges for some housing schemes, particularly free range and pasture-raised. There is some indication that proliferation of  backyard flocks may have accelerated the spread of the virus. More regulation may help there.

Finally, while commercial inactivated vaccines are available, they are problematic. First they require individual injection of birds. This is a difficult and costly process for commercial operations.  A second disadvantage is that the process may end up spreading other strains and diseases to the flock. No vaccine that covers all of the different  strains of the disease. Lastly, a vaccine regime has historically made exporting more difficult as the vaccines are never 100%  effective and so carry the risk of further spread. 


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