(Bloomberg) -- For influenza viruses, imperfection is a strength. They constantly mutate, producing new strains that challenge immune systems primed to fight earlier varieties. That’s what makes flu a life-long threat to humans and the animal species — mainly birds — that are vulnerable to it.
Since 2020, a strain of highly pathogenic avian influenza called H5N1 has been decimating both wild and domestic birds. Now it’s spreading among US dairy cows, and has infected US farmworkers exposed to sick cattle and poultry. A man in Mexico died with a strain of bird flu, H5N2, that hasn’t been seen in humans before.
The US Centers for Disease Control and Prevention and the World Heath Organization say the overall risk to the general public remains low, but public-health officials remain on alert for any indication of the most feared outcome: human-to-human transmission that could trigger a pandemic.
1. When did H5N1 begin spreading?
The H5N1 strain was first discovered in 1996 in geese bred in southern China. It’s proved to be alarmingly adept at jumping continents and species and has rampaged through captive, commercial birds across the globe; governments ordered millions of birds slaughtered to limit the contagion.
A variant that emerged in 2020, clade 2.3.4.4b, led to infections in mammals like farmed mink in Spain and Peruvian sea lions. The same strain is ripping through wild birds and poultry in the US and has gained a foothold among dairy cows, with cases popping up among people who work with sick animals.
While those infections spurred heightened attention, the period from 2017-2024 has been the quietest period of H5N1 activity in humans since the virus first appeared, said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
2. What do we know about H5N2?
Not much yet. The strain is related to H5N1; both are in the influenza A virus family. But there’s been little research on H5N2. While the deceased patient in Mexico marks the first laboratory-confirmed case in humans, researchers have identified people with antibodies that could signal prior infections.
The death in Mexico was surprising because the 59-year-old patient, who had underlying health conditions, had no history of exposure to animals most likely to transmit the virus.
3. How bad is bird flu for humans?
Bird flu can be fatal. Since the start of 2020 through May 3, 32 confirmed human cases of H5N1 had been reported globally to the World Health Organization, resulting in eight deaths.
But the health implications among the more recent cases in the US this year have been limited. The patients diagnosed with avian influenza through July all had mild symptoms that were treatable with antiviral drugs. None died.
4. What are the symptoms?
There are 13 reported cases in the US this year, all farmworkers in close contact with dairy cows or poultry, many of whom reported irritated eyes. Health authorities are looking for conjunctivitis, also known as pink eye, as an indication that someone may be infected with bird flu. Eyes are the only part of the human body with “avian” receptors that the virus is adept at attaching to.
Recent patients in Colorado and Michigan also experienced upper respiratory infections, raising concerns about transmission. The CDC is monitoring the virus for genetic mutations that could make it more dangerous, with no sign yet of worrisome changes.
5. How are people infected with bird flu treated?
Anyone with a suspected or confirmed infection should be treated immediately with one of the antiviral drugs known as neuraminidase inhibitors, such as Roche Holding AG’s Tamiflu, according to the CDC. Patients should get the medicine regardless of the severity of the disease or the length of time since symptoms began, the agency said. It also issued, on an emergency basis, instructions to medical professionals on how to give drugs like Tamiflu preventatively to people who have been exposed to the viruses with pandemic potential, in an effort to block infection.
6. Is there a bird flu vaccine for humans?
Bird flu vaccines are being stockpiled, but aren’t currently available to the public. The US asked vaccine-maker CSL Seqirus to produce 4.8 million doses this summer, to add to hundreds of thousands already on hand. However, experts warn a future strain of the virus that transmits between humans would likely require a different shot. The CDC isn’t recommending immunization for farmworkers because infections have been relatively mild, Nirav Shah, principal deputy director of the CDC, said in July.
The US Department of Health and Human Services awarded Moderna Inc. $176 million for testing an mRNA vaccine that targets several influenza strains, including the current bird flu viruses. The hope is that the newer technology could be quickly updated if needed.
The US Department of Agriculture, meanwhile, is looking into a potential H5N1 vaccine for cows, said Eric Deeble, acting senior adviser for H5N1 response.
7. How many US cows and birds have been impacted?
As of July 26, 172 cow herds across 13 states have tested positive for bird flu; the CDC has previously said it believes the cows are transmitting it to each other via contact with infected milk. Commercial milking machines are often used on dozens of cows per day, and milk droplets can remain infectious on contaminated equipment for hours.
The number of reported cases is likely an underestimate due to testing challenges, Osterholm said. Wastewater surveillance tools used to detect Covid-19 are now being used to track bird flu in humans and cattle.
US officials haven’t recommended the destruction of infected cows, which often recover, while millions of birds have been culled. The virus has been detected among flocks in 48 US states, affecting more than 100 million birds.
While milk from infected cows, which may be discolored or thickened, is being diverted or destroyed, the losses shouldn’t have a major impact on supply, according to the USDA. They haven’t affected the price of milk or other dairy products, the agency said.
8. Are milk, cheese, ice cream and other dairy products safe?
Yes, so long as the products have been pasteurized, a process that kills viruses and most bacteria.
On May 10, the Food and Drug Administration said that it had tested about 300 samples of dairy and egg products — including pasteurized milk, sour cream, cottage cheese and baby formula — all of which proved to be safe. Among samples of commercial milk tested by the FDA, about 20% contained traces of the virus, which are thought to be harmless.
US health agencies suggest Americans avoid raw milk and unpasteurized products. Research shows H5N1 survives in refrigerated raw milk for at least five weeks and can infect mice. No human cases have been identified that relate to consumption of unpasteurized dairy products.
9. Are there implications for the meat supply?
Bird flu virus particles were detected in the beef tissue of only one dairy cow, which was destroyed so its meat wouldn’t enter the food supply. There haven’t been any reported cases of bird flu in beef cattle. Officials have said they are very confident in the safety of the meat supply based on their tests.
10. What are scientists looking for?
Scientists recently discovered that cow udders contain receptors that avian and human influenza viruses can latch onto. Having both kinds of receptors makes it possible for other animals such as pigs to transmit diseases between species, which is what happened with the deadly 2009 swine flu pandemic. The finding raises concern about what might happen if a cow gets simultaneously infected with bird flu and human influenza.
- Surveillance of sewage suggests the virus may be more widespread among dairy cows than reported.
- Alphabet Inc.’s Verily is mining your poop for bird flu.
- Read Bloomberg’s reporting on US testing gaps.
- The USDA maps bird flu infections in mammals in the US.
- Alerts about infections in dairy cows from the USDA.
--With assistance from Madison Muller, Jessica Nix, Gerson Freitas Jr. and John Lauerman.
(A previous version of this story corrected the number of infected cow herds in the seventh section.)
©2024 Bloomberg L.P.
