Ken Baker: How did avian flu cause the death of so many chickens?

The federal Centers for Disease Control and Prevention (CDC) reports that between Jan. 14, 2022 and March 1, 2023, the U.S. has experienced 58,525,971 cases of poultry affected with avian influenza (bird flu) … where the word "cases" implies deaths.

Worthy of note here is the CDC’s use of the word “affected” rather than infected. Although I could find no official statement on this word choice, it seems to reflect the fact that when bird flu is found in a flock of domestic birds, federal rules require culling — killing — of the entire flock to slow the spread of the disease.

We are (hopefully) currently experiencing the tail end of the worst outbreak of bird flu in the nation’s history, the primary reason why egg prices have soared over the past year from a mean price for a dozen eggs of $1.78 in December 2021 to $4.25 in December 2022. To be sure, inflation, Putin’s war on Ukraine, and shortages in the labor market have each played a role, but the sudden and drastic reduction in the nation’s population of egg-laying hens during 2022 has been the principal bad actor in this sorry tale.

There are four main categories of influenza (flu) viruses, which virologists refer to as Types A, B, C, and D. Type A viruses cause bird flu, the great majority of which have low pathogenicity (don’t cause serious illness). However, some Type A viruses are highly pathogenic to birds and are labelled HPAI A — High Pathogenicity Avian Influenza Type-A viruses.

Another way virologists categorize the many types of viruses is by proteins found on their surfaces. Of these, Hemagglutinin and Neuraminidase proteins, (mercifully abbreviated as H and N proteins) are commonly used for distinguishing one type of virus from another. There are 16 different forms of H proteins and nine forms of N proteins that have been observed on the surfaces of avian (that is, Type A) viruses.

Recent avian flu is one of most virulent

One of the most virulent of the bird flu viruses, and the principal one that’s been causing most of the trouble recently, has H protein No. 5 and N protein No. 1 on its surface, or H5N1 for short. Putting it all together, the main (but not the only) type of virus that’s been ravaging the nation’s population of egg-laying hens over the past year is the HPAI A(H5N1) virus.

Since 2005, the world has seen three major waves of HPAI-caused bird flu. The first of these, confined largely to Asia, ran from 2005 to 2010 and led to the death of 55.2 million poultry. The second and third waves impacted poultry across the globe with the 2011 to 2019 wave killing 139.9 million, and the 2020-present wave having killed 193.9 million by December 2022.

Of these 389 million poultry deaths, 52.4 % were caused by the H5N1 virus, 28.5 % by the H5N8 virus, and 19 % by several other forms of H5 virus.

There’s one more layer of complexity. Geneticists have been tracking evolutionary changes in the H5N1 virus over the past several decades. As mutations pop up or disappear, they can materially change how the virus operates. By the end of 2021, the most common H5N1 version (or “clade” in genetics-speak) in the world had become a form labeled 2.3.4.4b.

Current avian flu causes mortality in 48 hours

Unfortunately, this HPAI A(H5N1) 2.3.4.4b virus is especially pathogenic, typically attacking multiple internal organs within chickens and domestic turkeys and causing 90% to 100% mortality within 48 hours.

The current epidemic began in the U.S. with the announcement by the USDA on Jan. 14, 2022 that a wild duck (an American widgeon) that had been taken by a hunter in South Carolina had tested positive for the H5N1 virus. Since then, the CDC reports, 777 outbreaks of influenza in 47 states have severely impacted both commercial poultry operations and backyard hobbyist flocks.

The rapid spread of the disease has been facilitated by the fact that domestic fowl can contract it from wild ducks, geese, and shorebirds who shed the virus in their nasal secretions, saliva, and feces.

Interestingly, chickens raised for meat have been much less affected than those raised for egg production, probably due to their much shorter life cycle. While a broiler’s life runs about 6 to 9 weeks from hatching to slaughter, it typically takes 5 to 6 months for an egg-laying hen to get to full productivity, and with a life span of some 100 weeks, they simply have more time to encounter the virus.

In the 27 years since the H5N1 virus was first detected in a domestic waterfowl in China in 1996, there’ve been very few cases of humans contracting the disease. But of the 868 cases (1 in the U.S.) reported by the World Health Organization — all of which involved persons working in close proximity with live chickens — 53% have proven fatal.

Not surprisingly, the USDA has strong surveillance and inspection programs overseeing poultry production and processing facilities to ensure that products from infected birds do not enter the food chain. And the proper handling and cooking of poultry and eggs to keep raw products away from other foods — just as we protect ourselves from infection by Salmonella or E. coli bacteria — adds a further, common-sense layer of protection for our families.

Ken Baker is a retired professor of biology and environmental studies. If you have a natural history topic you would like Dr. Baker to consider for an upcoming column, please email your idea to fre-newsdesk@gannett.com.

This article originally appeared on Fremont News-Messenger: A detailed look at the flu that decimated egg-laying chickens