Four poultry workers in Colorado recently fell ill with avian flu, bringing the total number of cases in the United States to at least nine.

Though the numbers are small, researchers say the commonalities between the cases — all but one of which were reported in the past four months — are enough to get a sense of how the virus affects people.

Cases in the US have been relatively mild and limited to farm workers who have handled infected animals, suggesting 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, and runny nose, while a few have also developed conjunctivitis and measles.

“One thing we can say is that the current strain of the virus is not adapted to infect humans and may not be adapted to infect the lower respiratory tract,” said Matthew Binnicker, director of the Clinical Virology Laboratory at Mayo Clinic.

The cases have been attributed to the global outbreak of H5N1, a specific strain of avian influenza that swept through America’s poultry and dairy farms in 2020.

The first case in the country was reported in April 2022 in an inmate working on a farm in Colorado who was culling birds and whose only symptom was fatigue. Texas reported a second case in April, followed by two in Michigan and five in Colorado, the latest four of which were confirmed over the weekend.

The mild nature of these cases contrasts with the effect of influenza on birds and some mammals, such as seals, sea lions, foxes, skunks, and cats, which have died from the virus. There are over 99 million wild waterfowl, commercial poultry, and household chickens in the United States. Infected animals either died from the virus or were culled to prevent further infection. 160 dairy cows Since the virus was first detected in cattle in March, many cattle have become infected.

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

“It’s really scary to hear about such viruses, but the term is actually the USDA term for what happens to poultry,” said John Lednicky, professor of environmental and global health studies at the University of Florida. “Just because it’s highly pathogenic for birds doesn’t mean it’s highly pathogenic for mammals or for humans.”

Lednicky added that some H5N1 strains are deadly to humans, but others are not.

Since 1997, more than 900 total cases of H5N1 have been reported worldwide, about half of which were fatal. But the global mortality rate has fallen to about 27% over the past two years. Still, that figure largely reflects only those whose illness was severe enough to cause them to seek medical treatment.

Dr. Peter Parese, a professor of microbiology at the Icahn School of Medicine at Mount Sinai, said the figures include patients “who were hospitalized and, in retrospect, came into contact with a lot of the virus.”

Palese’s 2012 StudyBlood samples from 12,500 people with no recorded history of avian flu infection were examined and found that 1% to 2% of them may have had a previous infection with H5N1.

But experts worry the virus could one day mutate and cause more severe illness or become more capable of spreading from person to person (all transmission so far has been from animals to people).

“The concern is that as it infects more animals and then infects more humans, the virus will change,” Binnicker said.

Why is conjunctivitis associated with bird flu?

At least four of nine people with avian flu in the United States reported having conjunctivitis.

This was the case in at least one recent case in Colorado, linked to an outbreak at a commercial farm in Weld County where workers were slaughtering poultry.

The state announced earlier this month that another patient Dairy workers People who came into contact with the infected cows also developed conjunctivitis.

Texas 1 The patient had conjunctivitis and no other symptoms. The patient worked with dairy cows and developed redness and discomfort in her right eye in March. New England Journal of Medicine Case StudyThe individual reported wearing gloves while working but no eye protection.

Conjunctivitis is not the most common human symptom of avian influenza, but It has been recorded by some people Examples of infection with different strains H7N7 virus that emerged in the Netherlands in 2003.

Scientists say there could be a few factors that explain the recent spread of the condition. One is that farm workers don’t always cover their eyes when handling sick animals, which could result in dairy farmers ingesting raw milk, which can lead to infection. Carrying the virus — That’s how it seemed to them.

Perhaps it is Michigan Dairy Workers He developed mild conjunctivitis in May and was confirmed to have been infected with avian influenza.

The virus can also get into a person’s eye through respiratory droplets or aerosols (tiny droplets in the air), or by workers who touch the eye after handling infected animals or contaminated raw milk.

“The receptors on cells that the virus needs to bind to are quite abundant on cells in the eye, which may be one reason why people with avian flu have conjunctivitis,” Binnicker said.

The Centers for Disease Control and Prevention recommends treating people infected with avian flu with antiviral drugs, and some U.S. patients have been given Tamiflu, a drug also used to treat seasonal flu.

“Studies conducted to date have shown that Tamiflu is effective in treating currently circulating strains of avian flu,” Binnicker said, “and to be most effective, it usually needs to be administered within 48 hours after symptoms begin.”

Infection spreads, tests increase, number of infected people increases

Scientists say the reason why all but one of the U.S. cases have been reported since April could come down to two factors: First, the virus is spreading quickly among birds; Sporadically infect other animalsPeople who have been in contact with infected animals, such as pet cats, are more likely to become infected. Second, health officials have begun monitoring and testing people who have been in contact with infected animals if they develop symptoms.

The CDC estimates: At least 10,600 people under surveillance for bird flu At least 375 people have been tested since the outbreak began in commercial poultry in 2022.

“There’s probably a much higher amount of virus out there now than there was a year ago, but we’re also seeing more cases because we’re doing more testing,” Binnicker said.

Dr. Natasha Bagdasarian, Michigan’s chief medical officer, said local health departments are conducting tests for people with even the most minor symptoms.

“I think that’s why we’re seeing milder cases,” she said, “because of the aggressive symptom surveillance that we’re doing.”

For example, a Michigan worker who had conjunctivitis did not even see a doctor before being tested for avian flu. Other cases Farm workers who cared for the infected cows reported sore throats, coughs, and stuffy noses to local health authorities.

Baghdasarian said the fact that Michigan has tested about 60 people but has only found two cases shows it takes a lot of contact for a person to get sick, and that the workers who tested positive were not wearing full personal protective equipment and were working on tasks like milking and hydrating cows, he said.

“We’re not talking about people who have had momentary contact with these animals, who pass by a barn or a pen,” Bagdasarian said. “We’re not talking about people who only touch a cow once.”