Although the above map of recent bird flu outbreaks around the world is lit up like a Christmas tree, it undoubtedly under-represents what has truly been a remarkable resurgence of avian flu around the world over the past three months.
Unlike the dramatic diaspora of H5N1 in 2005-2006 - when a single HPAI virus jumped from 15 Southeast Asian countries to more than 60 nations across Europe and the Middle East - we're watching more than a half dozen subtypes (H5N1, H5N8, H5N6, H7N9, H7Nx, and H5Nx) spreading around the world.
As they spread, these viruses continue to evolve into new genotypes, and increasingly, are producing new subtypes as well. Clade 18.104.22.168. H5 viruses (H5N8 in particular), has spun off multiple subtypes (H5N2, H5N1, H5N5, etc.) in Taiwan, North America, France, and Northern Europe (see OIE Notification: HPAI H5N5 In The Netherlands).
In addition to their growing diversity and greatly increased geographic range, some of these subtypes (H5Nx & H7N9) have demonstrated subtle changes in behavior, including greater virulence, increased host range, and/or enhanced transmission from birds to humans (see EID Journal: Morens & Taubenberger On The Evolution Of HPAI H5Nx and MMWR: Assessing The 4th Epidemic Wave Of H7N9 In China).
Tracking bird flu outbreaks has always been a game of incomplete information, and so even under the best of circumstances, we are likely only seeing the tip of the iceberg.
Some countries have very limited surveillance and testing capabilities, with large regions too remote - or not under full government control - to allow for reliable reporting. Countries like Libya, Nigeria, Myanmar, Cambodia, and North Korea come to mind, along with numerous other countries in Sub-Saharan Africa.
Other countries have a documented history of suppressing or delaying news of avian flu outbreaks (think: China, Indonesia, Egypt, etc.) - and while logistics of surveillance, testing, and reporting are likely partially to blame - are often calculated political decisions.
The day-to-day trickle of cases we get from China's individual provinces, and local media, are often `augmented' weeks (or sometimes months) later by NFHPC reports, or reports to the WHO (see HK CHP: China Reports An Additional 83 H7N9 Cases For December).
Egypt, which two winters ago saw the largest outbreak of human H5N1 infections (160+) ever recorded in a single year, has been absolutely silent this winter. Their Ministry of Health website is only rarely updated, and their MOH Bird Flu FAQ hasn't been updated since the summer of 2014, and still shows 180 cases since 2006 (WHO shows 356 as of Dec 2016).
What, if anything, is going on with H5 avian flu in Egypt this winter is unknown. Perhaps there is simply nothing to report, but that would represent a remarkable turnaround, given its recent history in that country.
Indonesia, Vietnam, and Cambodia are also countries that have been heavily impacted by avian flu in the past, but currently are reporting little or no activity in poultry, and no human cases. India, for all of its outbreaks over the past 10 years, has yet to report a human infection with H5N1.
We know that even small differences in HPAI viruses can change their behavior (see Differences In Virulence Between Closely Related H5N1 Strains), so it is possible that in these areas, the strains of H5N1 circulating either are, or have become, less infectious to humans. Some of the credit may go to public health messaging, aimed at reducing the risks of infection. There could be other factors we simply don't know.
Bird flu moves in mysterious ways, and just when we think we have a handle on it, it changes.
But despite these unusual blank spots on our bird flu radar, avian flu is clearly on the ascendant around the globe, and so no one should be too surprised if we start seeing reports from some of these `quiet' areas over the next few months.