Seasonality of H5N1 in poultry
While North American poultry farmers, the USDA, and public health agencies anxiously await the expected return of HPAI H5 bird flu this fall and winter, for many other parts of the world bird flu hasn’t taken its usual seasonal sabbatical. From Asia, to Africa, and even from parts of Europe – bird flu reports have continued to stream in to the FAO and OIE over the summer.
Like human flu, bird flu is largely seasonal, and spreads better in cooler, less humid environments.
While summer outbreaks are not unheard of – particularly in countries where the virus is heavily entrenched – the past three months has seen an unusual number of outbreaks. A mapped comparison of HPAI reports to the OIE over the past four summers (June 1st-Sept 1st) illustrates the point nicely.
Reports made in June likely reflect outbreaks in May, but the difference in both volume and geographic range between 2015 and previous years is striking. Missing from these maps are outbreaks from Egypt and Indonesia, neither of which routinely file OIE reports.
Although we’ve seen sporadic outbreaks in the UK, Germany, and Mexico - Asia (primarily China, Taiwan, Korea, and Vietnam) and Africa (primarily Nigeria, Ghana, Côte d'Ivoire, and Egypt)- have been the hardest hit regions this summer.
There are, of course, areas of the world where surveillance and reporting is either nonexistent or unreliable.
After the great H5N1 Diaspora of 2006, avian flu activity began to gradually recede around the world, and by 2010 outbreaks were mostly confined to a handful of countries (China, Egypt, Indonesia, Bangladesh, India, etc.)
The emergence of H7N9 in China in the spring of 2013, followed quickly by the arrival of H10N8, H5N6, H5N8, has helped to reverse this trend. Of these, H7N9 and H5N8 (and its reassorted progeny) have had the biggest impact so far, but other subtypes continue emerge, evolve, and threaten.
Last February, in response to this unprecedented emergence of new flu subtypes, the remarkable and rapid spread of HPAI H5 viruses to Europe and North America, and Egypt reporting the worst human H5N1 outbreak in history, the World Health Organization released a blunt assessment called:
The current global influenza situation is characterized by a number of trends that must be closely monitored. These include: an increase in the variety of animal influenza viruses co-circulating and exchanging genetic material, giving rise to novel strains; continuing cases of human H7N9 infections in China; and a recent spurt of human H5N1 cases in Egypt. Changes in the H3N2 seasonal influenza viruses, which have affected the protection conferred by the current vaccine, are also of particular concern.
As we’ve seen in the past, avian flu is extremely unpredictable. In 2008, when it appeared as if H5N1 would sweep the world, it began an unexpected retreat. Instead of the bird flu threat, we were blindsided the next year by a swine origin H1N1 pandemic. Proving that past performance is no guarantee of future results.
The difference in 2015 is that we have multiple HPAI subtypes making major inroads around the globe.
H7N9 is expected to return for a 4th winter appearance this fall, and while it has only impacted China so far – like with the HPAI H5 viruses – it too could hitch a ride on migratory birds and reach other nations. H7N9 is also continually evolving and reassorting (see EID Journal: H7N9’s Evolution During China’s Third Wave – Guangdong Province), meaning the virus we see this fall and winter may not behave the same as the H7N9 of the past.
H5N1 has shown new vigor in 2015, showing up in countries that have not reported sightings in years. A new clade has recently emerged in Egypt (see Eurosurveillance: Emergence Of A Novel Cluster of H5N1 Clade 22.214.171.124), spreading quickly among poultry, and is apparently the driving force behind the biggest human outbreak of H5N1 to date.
With its unexpected arrival in Taiwan, Russia, Europe, and North America, in just over a year H5N8 and its reassortant progeny (H5N2, H5N3, H5N1) have become better travelled than H5N1 had managed in more than a decade of trying.
Where it turns up this fall and winter – and whether it produces any new reassortants – are the $64 questions.
While the summer’s level of bird flu activity doesn’t necessarily portend what is to come this fall and winter, it is only prudent to take note of recent trends . . . and plan accordingly.