Bird flu in humans: What scientists know about the symptoms of the 9 US cases so far

Four Colorado poultry workers who recently contracted bird flu bring the total of human cases in the United States to at least nine.

While that number is small, researchers say the commonalities among the cases — all but one of which were reported in the past four months — are enough to begin to get a sense of how the virus may affect people.

In the United States, cases have been relatively mild and limited to farm workers who handled infected animals, a sign that the virus in its current state does not pose a major threat to humans.

Some patients reported typical flu symptoms such as fever, chills, cough, sore throat or runny nose. Several had conjunctivitis or pink eye.

“One thing we can conclude is that the current strain of the virus is not well adapted to human infection, and may not even be well adapted to lower respiratory tract infection,” said Matthew Binnicker, director of the Mayo Clinic’s clinical virology laboratory.

The cases stem from a global outbreak of H5N1, a particular strain of bird flu that began in 2020 and has affected poultry and dairy farms in the United States.

The country’s first human case was reported in April 2022, in an inmate who slaughtered poultry at a Colorado farm and whose only symptom was fatigue. Texas reported the second case in April, followed by two in Michigan and five in Colorado, the most recent four of which were confirmed over the weekend.

The mild nature of the cases contrasts with the flu’s effect on birds and some mammals, including seals, sea lions, foxes, skunks and cats, which have died from the virus. Since January 2022, more than 99 million wild waterfowl, commercial poultry and backyard flocks in the United States have been affected, meaning they have died from the virus or been culled to prevent further transmission. And about 160 dairy herds have been affected since the virus was first detected in cows in March.

This H5N1 strain is considered highly pathogenic, a term which, when used in the context of avian flu, means that it has a high potential to kill chickens.

“It really scares people to hear about a virus like this, but it’s actually a term that the USDA uses to refer to what’s happening in poultry,” said John Lednicky, a research professor of environmental and global health at the University of Florida. “Just because it’s highly pathogenic in birds doesn’t mean it’s highly pathogenic in mammals or humans.”

Lednicky added that some strains of the H5N1 virus are deadly to humans, while others are not.

Of the more than 900 cases of H5N1 flu reported worldwide since 1997, about half have been fatal. But in the last two years, the global death rate has been lower: about 27%. And even then, those numbers only include people who are sick enough to need treatment.

Dr. Peter Palese, a professor of microbiology at the Icahn School of Medicine at Mount Sinai, said the patients included in the count are those who “were hospitalized and, in retrospect, had contact with large amounts of virus.”

Palese’s 2012 research, which examined blood samples from 12,500 people without known bird flu infection, found that 1 to 2 percent of them may have already been infected with the H5N1 virus.

Experts, however, worry that the virus could one day mutate into a version that causes more severe illness or spreads from person to person. (So far, all transmissions have been from animals to humans.)

“The problem is that as more animals and people get infected, the virus will evolve,” Binnicker said.

Why is conjunctivitis associated with bird flu?

Among the nine U.S. patients with bird flu, at least four reported conjunctivitis.

That’s what happened in at least one of the recent cases in Colorado, linked to an outbreak at a commercial farm in Weld County. Workers were slaughtering poultry.

The state reported earlier this month that another patient, a dairy worker who had been exposed to infected cattle, also developed conjunctivitis.

In Texas, one case involved conjunctivitis without other symptoms. This person worked with dairy cows and developed redness and discomfort in his right eye in March. The person reported wearing gloves but no eye protection at work, according to a case study published in the New England Journal of Medicine.

Conjunctivitis is not the most common symptom of bird flu in humans, but it has been reported in some people infected with different strains, such as during an H7N7 outbreak in the Netherlands in 2003.

Scientists said several factors could explain the recent prevalence of the symptom. One is that farm workers do not routinely cover their eyes when caring for sick animals. As a result, dairy farm workers could get raw milk, which is thought to carry the virus, in their eyes.

That’s likely what happened to a Michigan dairy worker who developed mild conjunctivitis and was confirmed to have bird flu in May.

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The virus can also enter the eyes through respiratory droplets or aerosols (tiny airborne droplets). Some workers may also have touched their eyes after handling infected animals or contaminated raw milk.

“The receptor on the cells that the virus has to bind to is quite common in cells in the eye, and that could be one explanation for why we see conjunctivitis in people infected with bird flu,” Binnicker said.

The Centers for Disease Control and Prevention recommends treating people with bird flu with antivirals. Some U.S. patients have been given Tamiflu, a drug also used to treat seasonal flu.

“Studies to date have shown that Tamiflu is effective in treating the currently circulating strain of avian influenza,” Binnicker said. “It generally needs to be given within 48 hours of the onset of symptoms to be most effective.”

More spread, more tests, more cases

Scientists say the reason all but one U.S. case has been reported since April may be due to two factors. First, the virus is spreading rapidly among birds and sporadically infecting other animals, such as domestic cats, increasing the risk of human exposure. Second, health departments have begun monitoring and testing people exposed to infected animals if they develop symptoms.

The CDC estimates that at least 10,600 people have been monitored for avian influenza and at least 375 have been tested since the outbreak began in commercial poultry in 2022.

“There is probably a much higher amount of virus today than there was a year ago, but we are also detecting more cases because we are doing more testing,” Binnicker said.

Dr. Natasha Bagdasarian, Michigan’s state medical director, said local health departments are looking for even the mildest symptoms.

“I think that’s why we’re seeing mild cases,” she said. “It’s because of the active monitoring of symptoms that we’re doing.”

For example, a Michigan worker who had conjunctivitis did not even see a doctor before being tested for bird flu. The other Michigan case involved a farm worker who worked with infected cows and reported a sore throat, cough and congestion to local health officials.

Bagdasarian said the fact that Michigan has recorded only two cases after testing about 60 people suggests that humans need significant exposure to get sick. The workers who tested positive also weren’t wearing full personal protective equipment and had been involved in tasks such as milking cows or giving them fluids, she said.

“We’re not talking about people who had casual contact with these animals, who walked past a barn or a paddock,” Bagdasarian said. “We’re not talking about people who just touched a cow once.”