Four poultry workers in Colorado recently contracted bird flu, bringing the total number of human cases in the U.S. to at least nine.
While that number is small, disease researchers say the similarities between the cases — all but one of which have been reported in the past four months — are enough to paint a picture of how the virus might affect people.
In the US, cases have been relatively mild and limited to farm workers who handled infected animals, a sign that the virus in its current form does not pose a significant threat to humans.
Some patients have reported typical flu symptoms such as fever, chills, cough, sore throat or runny nose. Several patients have had conjunctivitis or pink eye.
“One thing we can conclude is that the current strain of the virus is not well adapted for human infection, and may not even be well adapted for lower respiratory tract infection,” said Matthew Binnicker, director of the Mayo Clinic’s Clinical Virology Laboratory.
The cases come amid a global outbreak of H5N1, a specific strain of bird flu that emerged in 2020 and hit poultry and dairy farms in the US.
The nation’s first human case was reported in April 2022, in an inmate who shot birds at a Colorado farm and whose only symptom was fatigue. Texas reported its second case in April, followed by two in Michigan and five in Colorado — the four most recent 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. As of January 2022, more than 99 million wild waterfowl, commercial poultry and backyard flocks in the U.S. have been affected, meaning they have either died from the virus or been culled to prevent further transmission. And about 160 herds of dairy cows have been affected since the virus was first detected in cows in March.
This H5N1 strain is considered highly pathogenic. In the context of bird flu, this means that the disease has a high risk of killing chickens.
Hearing about a virus like this “really scares people, but that term is actually a USDA term for what happens 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 H5N1 are deadly to humans, while others are not.
Of the more than 900 total human cases of H5N1 strains reported worldwide since 1997, about half have been fatal. But over the past two years, the global fatality rate has been lower: about 27 percent. And even then, those numbers largely reflect only those people who were sick enough to seek treatment.
Dr. Peter Palese, a professor of microbiology at the Icahn School of Medicine at Mount Sinai, said the patients in that count are those who “were hospitalized and, in retrospect, were exposed to large amounts of virus.”
Palese’s 2012 study, which examined blood samples from 12,500 people without documented bird flu infection, found that 1% to 2% of them may have been previously infected with H5N1.
Experts still worry, however, that the virus could one day mutate into a version that causes more severe illness or that can spread from person to person. (So far, transmission has occurred only from animals to people.)
“The concern is that as more animals and then more people become infected, the virus will change,” Binnicker said.
Why is pink eye linked to bird flu?
Of the nine American bird flu patients, at least four reported conjunctivitis.
That was the case in at least one of the recent cases in Colorado, which was linked to an outbreak at a commercial farm in Weld County where workers were culling poultry.
The state reported earlier this month that another patient, a dairy farmer who had been in contact with infected cattle, also developed pink eye.
In Texas’ case, it was conjunctivitis with no other symptoms. The person worked with dairy cows and developed redness and discomfort in his right eye in March. According to a case study in the New England Journal of Medicine, the person reported wearing gloves but no eye protection at work.
Conjunctivitis is not the most common symptom of bird flu in humans, but it has been reported in people infected with other strains, such as in the 2003 H7N7 outbreak in the Netherlands.
Scientists said a few factors could explain the recent prevalence of the symptom. One is that farm workers don’t consistently cover their eyes when handling sick animals. As a result, dairy workers could be getting raw milk — which has been shown to carry the virus — in their eyes.
That’s likely what happened in May to a Michigan dairy farmer who developed a mild case of conjunctivitis and turned out to have bird flu.
The virus can also reach people’s eyes through respiratory droplets or aerosols (small droplets in the air). Also, some workers may have touched their eyes after handling infected animals or contaminated raw milk.
“The receptor on the cells that the virus needs to bind to is quite widespread in cells in the eye, and that could explain why we see conjunctivitis in individuals infected with bird flu,” Binnicker said.
The Centers for Disease Control and Prevention recommends treating people with bird flu with antiviral drugs. Some U.S. patients have been given Tamiflu, a drug also used to treat seasonal flu.
“The studies done to date have shown that Tamiflu is effective in treating the currently circulating strain of bird flu,” Binnicker said. “It usually needs to be administered within 48 hours of the onset of symptoms to be most effective.”
More spread, more testing, more cases
The reason all but one case has been reported in the U.S. since April could be due to two factors, scientists say. First, the virus spreads rapidly among birds and sporadically infects other animals, such as domestic cats, increasing the chance of human exposure. Second, health officials have begun monitoring and testing people who have been exposed to infected animals if they develop symptoms.
The CDC estimates that since the outbreak began in commercial poultry farming in 2022, at least 10,600 people have been screened for bird flu and at least 375 have been tested.
“There is probably a lot more virus circulating today than there was a year ago, but we are also seeing more cases because we are testing more,” Binnicker said.
Dr. Natasha Bagdasarian, Michigan’s chief medical officer, said local health officials there have been screening for even the mildest symptoms.
“I think that’s why we’re seeing the mild cases,” she said. “It’s because of this active symptom monitoring that we’re doing.”
For example, the Michigan worker who had conjunctivitis didn’t even seek medical attention before being tested for bird flu. The other case in Michigan was a farm worker working with infected cows who reported a sore throat, cough and congestion to local health officials.
Bagdasarian said the fact that Michigan has seen only two cases after testing about 60 people suggests that people need a lot of exposure to get sick. The workers who tested positive also weren’t wearing full personal protective equipment and were doing tasks like milking cows or giving them fluids, she said.
“We’re not talking about people who had brief contact with these animals, walking past a barn or a pen,” Bagdasarian said. “We’re not talking about people who just touched a cow once.”
This article was originally published on NBCNews.com