WASHINGTON — When traces of H5N1 bird flu showed up in cow’s milk last year, it raised fears that the virus could become more infectious to humans. So far, that hasn’t happened, virologist Richard Webby reported April 23 at the World Vaccine Congress Washington.
Cows were surprise hosts for the virus. Influenza viruses latch on to sialic acid attached to sugar molecules that decorate the outside of cells. It turns out that the way sialic acid is attached to some sugar molecules on cow mammary gland cells resembles attachments, or receptors, in birds. This arrangement allows the H5N1 virus to infect birds and cows, said Webby, of St. Jude Children’s Research Hospital in Memphis, Tenn.
Cattle also have sialic acid attachments like those that flu viruses use to grab and infect human cells. Researchers feared that having both humanlike and birdlike receptors in the same mammary glands might make it easier for bird flu viruses to adapt, making transmission between people easier.
In cow mammary glands, the virus reaches levels that are about as high as scientists can achieve in a lab. “This is ‘kid in a candy store’ if you’re a virus in the udders of these cows,” said Webby, who also directs a World Health Organization center for monitoring influenza in birds and animals. That giddy growth might work to humans’ advantage. “Within the mammary gland of the cow, these H5N1 [viruses] grow so well in there, there’s no real pressure on them to change” to grab the human receptor. “But that could change tomorrow.”
H5N1 in cattle also presents another danger, Webby said. The more people who catch H5N1 bird flu after contact with infected cattle or milk, the higher the chance of the virus adapting to spread easily from person to person. To date, 70 people in the United States have been infected, with 41 cases linked to exposure to dairy herds. One person died after getting one of the variants found in cattle, though he caught it after contact with backyard chickens and wild birds.
Two versions of H5N1 avian influenza have infected dairy cattle in the United States. Many other mammals, including dolphins and porpoises, have also been infected with related variants. Those marine mammals “were never on the list” of animals that scientists thought bird flu could infect, Webby said.
But the wide variety of mammals infected may not be caused by a new property of the virus, he said. “It’s just that there’s so much more virus around,” because it replicates to “ridiculous” levels in the birds and animals it infects.