Also in this series:
Part 1 - Two Viruses
Part 3 - Can I Catch Bird Flu From My Chickens?

You know about flu. You’ve probably been up close and personal with it a few times. Maybe you called in sick, then stayed on the couch, flat on your back, feeling feverish and wrung out. Perhaps you had a headache, stuffy sinuses, a sore throat, and just felt miserable.

You also probably know about bird flu, the disease that afflicts both wild and domestic birds. If you have a flock, you’ve probably been paying attention as this year’s bird flu epidemic has been working its way across the country. Human flu can be deadly, especially in the very young, the very old, and those with certain chronic medical conditions. But for chickens, bird flu is always extremely deadly. Mortality approaches 100%, and because the disease is so infectious, flock owners are legally required to euthanize any surviving birds in an infected flock.

Maybe you’re wondering why these two diseases, the one in birds and the human one, share the name “flu.” Here’s why: The viruses that cause these two diseases are closely related. Bird flu, human flu, as well as flu in pigs, horses, dogs, cats, bats, foxes, skunks, or you name the animal, are all related. “Flu,” of course, refers to influenza, and all influenza viruses are part of the same family of viruses, Orthomyxoviridae.

Life would so much easier for virologists, and all of us for that matter, if each flu strain confined itself to one animal species. But nope. Some strains that cause disease in a particular animal species can also make other animal species sick. For example, the current epidemic bird flu virus, in addition to sickening and killing millions of birds, has caused lethal infections in baby foxes in at least seven US states, and has also been found in bobcats in Wisconsin, a coyote pup in Michigan and Canadian skunks. All of these animals probably acquired flu by eating birds that were infected with the virus.

And there have been two humans who have tested positive for the current bird flu virus—one in the US and one in the UK. Both people had close contact with sick chickens. One reported feeling fatigued and the other reported no symptoms at all.

To complicate the situation ever further, sometimes flu viruses mutate and acquire the ability to grow, reproduce, and spread in a new animal species. When a flu strain jumps to a new species, it can be very deadly, and because the newly infected species has no immunity to the new virus, the virus can spread far and wide—a pandemic. That’s what happened in the 1918 “Spanish,” the 1957 “Asian,” and the 1968 “Hong Kong” flu pandemics. In these cases, a flu virus that originated in birds infected pigs and then learned how to infect humans.

This year, as we continue to deal with the human Covid pandemic, we also worry that the bird flu epidemic might find its way to our flocks. Should we also worry that bird flu might find its way to people? Are we on the cusp of another human pandemic? It’s scary to think about. It’s also complicated. So, let’s start with the basics.

What’s a virus?

To get a handle on how viruses infect humans, chickens, and other living things it’s important to understand what they are and how they work. So, in a few paragraphs, here are the basics.

Viruses are pretty simple little guys. You can think of them as tiny infectious specks that cause disease in living things—plants, insects, bacteria, chickens, people, hippopotamuses, and so on. Viruses infect living things by finding their way into cells. Unlike plants, insects, bacteria, chickens, people, hippopotamuses and all living things, viruses are not cells themselves, nor are they made up of cells. And they don’t have most of the necessary equipment to live or reproduce on their own. They have to hijack the machinery of the living cells in order to live and reproduce.

What equipment do viruses have? They only have, in total, two or three parts. They are, as I said, simple little guys. First, all viruses have a container. It’s made out of protein and it’s called the capsid. Inside the capsid is the genetic code—the RNA or DNA that tell the virus what to do and how to do it. That’s the only thing that’s inside a virus and that’s the second part. The third part, that some but not all viruses have, is a fatty protective envelope around their capsid. And that’s it. Simple.

How do viruses attack a living cell? Imagine a bunch of tiny viruses trying to get into a cell in a chicken’s lung. The viruses are “locked out” of the cell unless they have a specific “key.” The cell’s “lock” is called the receptor molecule. The viral key is called a fusion protein. If the lock doesn’t fit the key, the viruses just bounce off the cells and can’t get in.

But if the viral fusion protein fits the cell receptor molecule, the virus attaches, gains entry into the cell, inserts its genetic code into the genetic code of the invaded cell, and forces the invaded cell to manufacture a bazillion baby viruses—just like a hacker inserts a computer virus into a computer and forces the computer to run disreputable programs. (OH! Wait! That’s why they call computer viruses computer viruses!)

Remember the lock and key analogy. Later, I’ll say more about how some strains of flu have learned how to make new keys, thus making themselves into super-viruses that cause pandemics.

And that’s pretty much it. Now you know all there is to know about viruses. Send me five bucks and I’ll send you your diploma. (Um…I’m kidding. You know that, right?)

Bird Flu and All Its Cousins – the Orthomyxoviridae

The orthomyxoviruses are divided into seven groups (genera) and four of those genera cause flu in a variety of animals. Influenza D infects cows and pigs. No human cases have ever been documented. Influenza C has been found in pigs and dogs. It can also cause mild cases in humans, but has never caused epidemics or pandemics. Influenza B is mostly found in humans. Influenza B viruses can cause human epidemics but have never caused pandemics. Because flu B constantly circulates in humans, two strains of influenza B are always included in the yearly flu vaccine. For many years influenza B was thought to occur exclusively in people, but recently it has been found in seals, oddly enough.

Influenza A: Decimated Flocks, Ailing Humans, Pandemics and Things That Go Bump in the Night

And then we get to the bad boy. Influenza A is the only flu virus that causes bird flu. Most influenza A strains have been found in wild waterbirds like ducks and geese as well as domestic ducks and geese and other domestic fowl, like chickens and turkeys. Most of these strains only cause mild disease, or are completely asymptomatic. A few, though, are very lethal.

Influenza A viruses also infect a whole host of mammals, such as pigs, dogs, horses, and whales. And humans! Since any number of strains of flu A are constantly circulating in humans, two influenza A strains are always included in the yearly flu vaccine. Rarely, but significantly, an influenza A strain living in animals mutates, and makes a new “key” (remember the key?) that can unlock the lock on human cells. And it develops the ability to grow and spread in humans, leading to pandemics. Reports of influenza pandemics go back at least 500 years, and on average a flu pandemic occurs every 40 years.

Influenza A viruses are classified into subtypes and each subtype is further divided into strains. Scientists have named subtypes of flu A based on the unique surface proteins on each one. Remember the viral protein coat; the capsid? There are protein “spikes” sticking out from the surface of the capsid.

One spike is made of a protein called haemagglutinin. Another is made of another protein called neuraminidase. There are 18 different haemagglutinin proteins that scientists have numbered (H1, H2, H3, etc.). And there are 11 neuraminidase proteins (you guessed it—they are numbered N1, N2, N3, etc. Creative folks, these scientists!) The subtypes, then, are named according to the N and H combination present, e.g. A(H3N8), A(H5N1), A(H5N6), A(H5N8), and so on.

You Take the Low Flu and I’ll Take the High Flu

Scientists classify all bird flu strains into two groups depending on their ability to cause disease. The word for the ability to cause disease is “pathogenicity.” Thus, the two groups are LPAI (low pathogenicity avian influenza) and HPAI (high pathogenicity avian influenza).

There are strains of LPAI constantly circulating in wild birds, and one may show up in your coop unnoticed. It may cause some ruffled feathers and a drop in egg production, or may be completely asymptomatic.

HPAI is the bad one. It causes severe disease, is extremely contagious, and can kill your entire flock very quickly. The strain of bird flu that has been marching across the US this year is HPAI, and that’s why everybody is so concerned.

And so….

Wow! It’s gotten very sciencey in here. Let me check—is this still a chicken blog? Yep. We’re still talking about avian flu. Here’s one final nugget: The HPAI strain that has been circulating in wild birds and domestic poultry in the US this year belongs to the subtype H5N1. And because you’ve just read this article you know exactly what that means! Remember that sentence in bold type and then casually drop it into conversation at your next cocktail party. Everybody will be so impressed! They’ll all want you to mentor their kids when they do their next science project!

And stay tuned for the next article in this series, where I’ll talk about the chance of your chickens getting bird flu, the chance of you getting bird flu from your chickens, and how to reduce those chances. Also, I’ll talk a bit about the unlikely but frightening possibility of a flu pandemic.