H5N1’s Summer Lull: Getting Shorter?

 
Chart generated by Gemini using USDA Data

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As the above chart illustrates, for the past few years we've witnessed a similar avian flu pattern; a summer lull followed by a sharp increase in outbreaks starting in either September or October, and running through late spring. 

While the amplitude of poultry outbreaks during the winter has fallen (possibly due to better biosecurity measures) a bit, the number of months with elevated activity has increased. 

Avian flu outbreaks in September 2025 and 2026 have far exceeded outbreaks for the same month in 2023 and 2024, while last spring's avian flu activity extended well into May. 

Our summer lull remains, but appears to be shrinking.  

• In 2023-2024, the U.S. saw 3 months of strong activity (> 25 outbreaks/mo). 
• In 2024-2025 that number jumped to 5 months
• And in 2025-2026 we saw 6 months with > 25 outbreak/mo. 

And while September isn't completely in the books - and there are often delays in reporting - over the past 30 days the United States has already reported 20 poultry outbreaks (18 in September). 

After reporting zero outbreaks during June, July, and August - Canada has reported 6 outbreaks in Manitoba over the past 2 weeks (Sept 10th-24th).  

In 2024, Canada's summer break in avian flu reports ran from April 10th to October 21st (194 days). In 2025, the lull lasted from May 15th to September 9th (117 days).  This year, from May 16th to September 10th (also 117 days).

As we saw yesterday in ECDC/EFSA Quarterly Avian Influenza Overview June-August 2026, Europe saw unprecedented levels of HPAI in wild birds over the 2025-2026 season.

And South Korea reported both increased diversity and infectivity among avian flu viruses which sparked a difficult and lengthy 2025-2026 avian flu season, which began 47 days earlier than the 2024-2025 season. 

These shifts aren't necessarily permanent, nor are they reliable indicators of what this fall and winter avian flu season will look like. But we shouldn't ignore them, either. 

By now most commercial poultry producers are presumably well practiced in implementing biosecurity measures to protect their flocks (despite which, outbreaks still happen). But the APPA estimates `Eleven (11) million U.S. households own backyard chickens (a 28% increase from 2023)'.

A recent survey (see MMWR: Knowledge, Attitudes, and Practices Regarding Avian Influenza Among Owners of Backyard Flocks), found significant gaps in their knowledge of dealing with avian flu.

Although the risks of infection are believed low, we've already seen at least two fatalities (here, and here) in the United States from H5Nx following `. . . exposure to a combination of a non-commercial backyard flock and wild birds.'

In an attempt to avoid future incidents, last year the University of Florida's Extension office (UF/IFAS) released two H5N1 related publications; one for backyard poultry owners, and another for consumers of poultry products and milk.

Given what may be on the horizon, both are recommended reading.