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 commonalities among cases, which have been reported almost entirely in the last four months, are enough to give a general picture of how the virus may affect people.
Cases in the U.S. have been relatively mild and limited to farm workers who care for infected animals, a sign that the virus in its current form does not pose a major threat to humans.
Some patients reported typical flu symptoms such as fever, chills, cough, sore throat or runny nose. A few had conjunctivitis or pink eye.
“As a result, we can conclude that the current strain of the virus is not well adapted to human infection and may not even be well adapted to infecting the lower respiratory tract,” said Matthew Binnicker, director of the Mayo Clinic Clinical Virology Laboratory.
The cases stem from a global outbreak of H5N1, a specific strain of bird flu that emerged in 2020 and affected poultry and dairy farms in the United States.
The country’s first human case was reported in April 2022, in a prison inmate who was culling birds on 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 latest four confirmed over the weekend.
The mild nature of the cases contrasts with the flu’s impact on birds and some mammals, such as 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 U.S. 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 to be highly pathogenic, with a high potential to kill chickens when used in the context of avian influenza.
Learning about such a virus “really scares people, but that term is actually a USDA term used to describe 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 in humans, while others are not.
Of the more than 900 cases of the H5N1 strain reported globally since 1997, about half have been fatal. But the global death toll has been lower in the past two years: about 27%. And even then, those numbers largely reflect only people sick enough to seek treatment.
Patients in that count are people who “were hospitalized and, in retrospect, came into contact with a large amount of the virus,” said Dr. Peter Palese, a professor of microbiology at the Icahn School of Medicine at Mount Sinai.
Palese’s 2012 study, which examined blood samples from 12,500 people without a documented bird flu infection, found that 1 to 2 percent of them may have previously been infected with H5N1.
But experts fear the virus could one day mutate into a version that causes more serious illness or spreads from person to person. (So far, all transmissions have been from animals to people.)
“The concern is that the virus will change as more animals and more people become infected,” Binnicker said.
Why is pink eye associated with bird flu?
At least four out of nine bird flu patients in the US have been reported to have pink eye.
That was true in at least one of the recent cases in Colorado, linked to an outbreak at a commercial farm in Weld County, where workers were sorting poultry.
The state reported earlier this month that a dairy worker exposed to infected cattle also contracted pink eye.
One case in Texas had conjunctivitis without other symptoms. The person worked with dairy cows and developed redness and discomfort in his right eye in March. The person reported wearing gloves at work but not eye protection, according to a case study in the New England Journal of Medicine.
Although conjunctivitis is not the most common symptom of bird flu in humans, it has been documented in some people infected with variant strains, such as during the H7N7 outbreak in the Netherlands in 2003.
Scientists said there are several factors that could explain the recent prevalence of the symptom. One of them is that farm workers do not always close their eyes when handling sick animals. As a result, raw milk, which has been shown to carry the virus, may get into dairy workers’ eyes.
The same thing likely happened to a dairy worker in Michigan who had mild pink eye and was confirmed to have bird flu in May.
The virus can also enter people’s eyes through respiratory droplets or aerosols (tiny airborne droplets). Or, 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 common on cells in the eye, and that may be one of the reasons 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 received Tamiflu, a drug also used to treat seasonal flu.
“Studies so far have shown that Tamiflu is effective in treating the currently circulating strain of avian influenza,” Binnicker said. “To be most effective, it generally needs to be administered within 48 hours of the onset of symptoms.”
More spread, more testing, more cases
Scientists say there are two possible reasons why nearly all of the U.S. cases have been reported since April. First, the virus spreads rapidly among birds and occasionally infects other animals, such as domestic cats, increasing the likelihood of human exposure. Second, health departments have begun tracking and testing people who have been exposed to infected animals if they develop symptoms.
The CDC estimates that at least 10,600 people have been monitored for bird flu and at least 375 people have been tested since the outbreak in commercial poultry began in 2022.
“There is probably a lot more virus today than there was a year ago, but we are detecting more cases because we are doing more testing,” Binnicker said.
Dr. Natasha Bagdasarian, Michigan’s chief medical executive, said local health departments are screening to detect even the mildest symptoms.
“I think that’s why we’re seeing mild cases,” he said. “Because of the active symptom monitoring that we’re doing.”
For example, the Michigan worker with conjunctivitis didn’t even go to the doctor before getting tested for bird flu, while the other case in Michigan 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 saw only two cases after testing about 60 people suggests that people need a lot of exposure to get sick. The workers who tested positive were also not wearing full personal protective equipment and were involved in tasks such as milking the cows or giving them fluids.
“We’re not talking about people who have casual contact with these animals, who walk past a barn or a pen,” Bagdasarian said. “We’re not talking about people who just touch the cows once.”
This article was originally published on NBCNews.com