by Four poultry workers in Colorado recently contracted bird flu, bringing the total number of human cases in the US to at least nine.
Although that number is small, disease researchers say the commonalities between the cases – all but one of which have been reported in the past four months – are enough to begin to piece together a picture of how the virus might play out. on people.
The US cases have been relatively mild and limited to farm workers who handled infected animals – a sign that the virus in its state does not pose a serious threat to humans.
Some patients reported typical flu symptoms such as fever, chills, cough, sore throat or stuffy nose. Some 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 infecting the lower respiratory tract,” said Matthew Binnicker, director of the Clinical Virology Laboratory at the Mayo Clinic.
The cases stem from the global outbreak of H5N1, a particular strain of bird flu that emerged in 2020 and hit poultry and dairy farms in the US.
The first human case in the country was reported in April 2022, in an inmate who slaughtered birds on a farm in Colorado and whose only symptom was fatigue. Texas reported its second case in April, followed by a pair in Michigan and five in Colorado – the latest four confirmed over the weekend.
The mild nature of the cases stands in contrast to the flu’s effect on birds and some mammals – including seals, sea lions, foxes, skunks and cats – have died from the virus. As of January 2022, more than 99 million wild waterfowl, commercial poultry and backyard flocks in the US have been affected, meaning they either died from the virus or were killed to prevent further transmission. And around 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 that, when used in the context of bird flu, means it has a high potential to kill chickens.
Hearing about such a virus “makes people very scared, 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 fatal in humans, while others are not.
Of the more than 900 total cases of H5N1 strains in humans reported worldwide since 1997, about half have been fatal. But in the last two years, the global mortality rate has been lower: around 27%. And even then, those numbers only reflect those who were sick enough to seek medical treatment.
Dr. Peter Palese, a professor of microbiology at the Icahn School of Medicine at Mount Sinai, said the patients in that tally were those “who were in the hospital and, in retrospect, were exposed to a large number of viruses.”
Palese’s 2012 research, which examined blood samples from 12,500 people without a documented bird flu infection, found that 1% to 2% of them may have been previously infected with H5N1.
Experts remain concerned, however, that the virus could one day mutate into a version that causes more serious disease or spreads from person to person. (So far, all transmissions have been from animals to humans.)
“The concern is that as more animals are infected, and more people are infected, the virus will change,” Binnicker said.
Why is pink eye associated with bird flu?
Of the nine US bird flu patients, at least four reported pink eye.
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. The workers were killing poultry.
The state reported earlier this month that another patient there, a dairy worker exposed to infected cattle, also developed pink eye.
One Texas case involved conjunctivitis without other symptoms. That 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 on the job.
Conjunctivitis is not the most common symptom of bird flu in humans, but it has been documented in some people infected with different strains, such as the H7N7 outbreak in the Netherlands in 2003.
Scientists said several factors could explain the recent prevalence of symptoms. One is that farm workers do not consistently cover their eyes when dealing with sick animals. As a result, dairy workers could get raw milk – which has been shown to carry the virus – in their eyes.
That’s likely what happened to a dairy worker in Michigan who developed mild conjunctivitis and was diagnosed with bird flu in May.
The virus can also enter people’s eyes through respiratory droplets or aerosols (small 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 attach to is quite common in cells in the eye, and that may be one explanation for why we see conjunctivitis in individuals infected with bird flu,” said Binnicker.
The Centers for Disease Control and Prevention recommends that people with bird flu be treated with antivirals. Some of the US patients have received Tamiflu, a drug also used to treat seasonal flu.
“The studies that have been done so far have shown that Tamiflu is effective in treating the strain of poultry that is currently circulating,” Binnicker said. “It usually needs to be administered within 48 hours of the onset of symptoms to be most effective.”
More circulation, more testing, more cases
The reason that all but one of the US cases have been reported since April, scientists say, may come down to two factors. First, the virus is spreading rapidly among birds and occasionally 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 bird flu and at least 375 have been tested since the outbreak in commercial poultry began in 2022.
“There’s probably a much higher number of viruses out there today compared to a year ago, but we’re also picking up more cases because we’re testing more,” Binnicker said.
Dr. Natasha Bagdasarian, chief medical officer for the state of Michigan, said local health departments have screened for even the slightest symptoms.
“I think that’s why we’re seeing the mild cases,” she said. “It’s because of this active monitoring of symptoms that we’re doing it.”
The Michigan worker with conjunctivitis, for example, did not even seek medical attention before being tested for bird flu. Another Michigan case involved 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 indicates that it takes a lot of exposure for people to get sick. The workers who tested positive were also not wearing full personal protective equipment and were involved in tasks such as milking cows or giving them fluids, she said.
“We’re not talking about people who had a fleeting encounter with these animals, who walked over a barn or a pen,” Bagdasarian said. “We’re not talking about people who touched a cow once.”
This article was originally published on NBCNews.com
