2024-04-28 06:54 来源：得道网
禽流感在奶牛之间传播了几个月的消息，甚至在美国杂货店货架上的牛奶中出现了病毒碎片，这引发了人们对该病毒对人类构成风险的新担忧。这些问题包括:这种被称为高致病性H5N1禽流感病毒是否适应于更好地感染哺乳动物?人们会因为喝那种牛奶而得禽流感吗?
《科学新闻》找到了专家来寻找这些问题的答案。简短的回答是，由于牛奶的巴氏消毒和禽流感病毒的传播方式，对人类的风险仍然很低。下面是你应该知道的更深入的研究。
Probably not in pasteurized milk.
On April 23, the U.S. Food and Drug Administration announced that fragments of the bird flu’s genetic material had turned up in grocery store milk. Initial results indicate that about 1 in 5 samples contain bits of the virus’s RNA, the agency announced April 25. Samples taken from areas where infected dairy cows have been found were more likely to test positive for the bird flu virus than those with no infected herds.
That doesn’t mean that whole infectious viruses are present, says Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota in Minneapolis.
“At first blush, it would surely seem as if it should be a great concern in terms of human health. However, the good news is that while the virus is in the milk … the pasteurization process actually is very effective in killing those virus particles so that in fact, we don’t have to be concerned about ‘Are we ingesting infective material?’”
Pasteurization raises milk to a high enough temperature to kill bacteria and viruses. Milk has always had bits of dead bacteria such as E. coli and Listeria that aren’t filtered out, Osterholm says. So it’s not surprising to find genetic remains of the flu virus, he says, “but that by itself does not at all suggest a public health concern.”
H5N1 is an envelope virus. And envelope viruses — ones that wrap themselves in a blanket borrowed from a host cell’s membrane — are “just a little bit wimpier than non-envelope viruses and a little bit easier to inactivate,” says Meghan Davis, an environmental epidemiologist at the Johns Hopkins Bloomberg School of Public Health. “That means that there’s some reassurance that pasteurization ought to be working.”
Because H5N1 has only recently been found in cattle, no studies have directly tested milk pasteurization’s ability to kill the virus, the FDA said in a statement April 23. But studies have shown that egg pasteurization, which is done at lower temperatures than milk pasteurization, inactivates the virus.
For those reasons, government officials stress that pasteurized milk is safe to drink. But the FDA recommends that people don’t drink raw milk, which has not been pasteurized.
Davis says there are many reasons to avoid raw milk and products made from it. “We know that raw milk [can contain] other infectious diseases and there have been outbreaks metaed to raw milk consumption. So categorically I don’t recommend it.”
Some goats have also been infected with H5N1, so Davis suggests avoiding raw goat and sheep milk products, too.
Decades of evidence suggest that’s not likely, Osterholm says.“We have no evidence that humans have become infected from influenza A virus via ingestion.”
Some scavenger mammals have become infected with H5N1 from eating dead birds. But to get into cells, influenza viruses need to grab onto receptors, cell surface proteins studded with certain sugars. In humans, those sugars are different than the versions in scavengers. People carry the entry portals in their upper respiratory tract and the eyes. The one person in the United States who recently caught bird flu worked with cows at a farm in Texas and was diagnosed with conjunctivitis, an eye infection.
It might even be difficult for people to catch the bird flu from infected cows, Osterholm says. “If you look at the experience we’ve had in the past, even with all of the human contact that occurred with infected flocks [of] turkeys and chickens over the course of the past several years, we’ve just seen an absence of infection in humans.” Both the U.S. Centers for Disease Control and Prevention and World Health Organization have designated the virus as low risk for humans.
The U.S. Department of Agriculture first announced finding a strain of bird flu in dairy cattle in Kansas and Texas on March 25 (SN: 4/3/24). Since then, the virus has been found in dairy cows in eight states. The same version of the virus infecting cows — called clade 2.3.4.4b — has also been found in poultry in five states, the USDA said in a federal order. That order announced mandatory testing for all dairy cows before they can be moved to another state, and also requires tracing the previous movements of cows from infected herds. The order will go into effect on April 29.
H5N1 may have jumped into cattle only once and then spread from cow-to-cow, evolutionary biologist Michael Worobey of the University of Arizona in Tucson posted on X (formerly Twitter) after he and colleagues analyzed 239 bird flu viruses from cattle and other species. The jump may have happened in late December 2023, months earlier than thought.
These findings are still preliminary and haven’t been independently verified by other scientists, Davis cautions. But they do suggest that bird flu in cattle is “probably more widespread than what we’ve identified based on the reported cases on the USDA dashboard. What we’re looking at is probably circulation, potentially cow-to-cow, but … we don’t yet know the extent.”
“At the moment we have both concern and reassurance,” she says. Despite the low risk, “we have the concern that this particular H5N1 has had staying power and has been around globally since 2020. It has been in the U.S. since 2022.” The virus continues to cause major outbreaks in birds and has jumped into many species of mammals. That pattern is different from what was seen with earlier iterations of H5N1 avian influenza. “It would flash up, we’d have cases and then it would die off by the next year. But that’s not what we’re seeing here.”
Yes, but cases tend to be fairly mild. H5N1 infections in cows can cause a drop in appetite and milk production, abnormal feces, tiredness, fever and other symptoms.
Davis, a former dairy veterinarian, says that vets often get calls that cows are “off feed,” meaning the animals aren’t eating well or giving as much milk as usual. Those symptoms are not specific to any particular disease, so cases of H5N1 influenza may easily have been overlooked.
Some cows may not have any symptoms. Lung tissue taken from one cow with no symptoms tested positive for the virus, the USDA announced April 24.
The virus has also been found in cats on dairy farms in Texas and in Poland, South Korea and France. At least one cat died, according to the World Organization for Animal Health.
For poultry such as chickens and turkeys, the virus can be deadly, destroying flocks in a matter of days.
Possibly, Osterholm says. “In our business, we almost sleep with one eye open, just because on any one given day there could be a change in the virus from mutation or reassortment.”
Reassortment happens when flu strains swap part of their genetic instructions. Reassortment generated the H1N1 flu strain that caused a pandemic in 2009. In a pig, bits from the 1918 pandemic flu strain combined with pieces from a virus that was already a mish-mash of a swine flu virus and an avian flu virus. The resulting virus eventually infected people (SN: 5/22/09; SN: 2/12/10). The current H5N1 resulted from the reassortment of avian influenza viruses from poultry and wild birds.
“What we are concerned about is adaptation of the virus to better suit some of the receptors that humans might have, which is why any transmission out of bird populations into any mammal is initial cause for concern,” Davis says. People working on dairy farms, those in the dairy production chain and anyone else who may come into contact with raw milk, cows or poultry should wear protective equipment to avoid possible infection. She also advocates for testing farm workers, their families and others in their communities.
Both Davis and Osterholm are concerned about the virus infecting other food animals.
“The challenge that I see right now on U.S. farms is a virus getting into hogs,” Osterholm says. Pigs carry receptors similar to the ones found in both humans and birds, making swine a hog-heaven for bird flus that have potential to become a pandemic. Meanwhile, Davis says, “if this virus can infect both cows and goats, sheep are on my suspicion list for another potential species of concern.”
H5N1 avian influenza viruses have been found in birds since 1996 and have infected more than 880 people globally since 1997. In the United States, a poultry worker tested positive for H5N1 in 2022. The dairy farmworker with conjunctivitis was only the second person in the United States with a newlineed bird flu infection.
So far, H5N1 has not developed the ability to spread easily from person-to-person, which would make it a pandemic-potential virus. “Lightning could strike tomorrow,” Osterholm says, “but at least based on the track record so far, there’s been very little evidence to support that this is going to infect humans [and] then be transmitted by people to other people.”
