Virologists have been especially worried about the virus making its way into pigs, because these animals are notorious viral incubators. “They can become infected with swine strains, bird strains and human strains,” says Brinkley Bellotti, an infectious disease epidemiologist at Wake Forest University in North Carolina. These strains can swap genes and give rise to new, potentially more infectious or harmful strains.
Thankfully, we haven’t seen any other cases in pig farms, and there’s no evidence that the virus can spread between pigs. And while it has been spreading pretty rapidly between cattle, the virus doesn’t seem to have evolved much, says Seema Lakdawala, a virologist at the Emory University School of Medicine in Atlanta, Georgia. That suggests that the virus made the leap into cattle, probably from birds, only once. And it has been spreading through herds since.
Unfortunately, we still don’t really know how it is spreading. There is some evidence to suggest the virus can be spread from cow to cow through shared milking equipment. But it is unclear how the virus is spreading between farms. “It’s hard to form an effective control strategy when you don’t know exactly how it’s spreading,” says Bellotti.
But it is in cows. And it’s in their milk. When scientists analyzed 297 samples of Grade A pasteurized retail milk products, including milk, cream and cheese, they found viral RNA from H5N1 in 20% of them. Those samples were collected from 17 states across the US. And the study was conducted in April, just weeks after the virus was first detected in cattle. “It’s surprising to me that we are totally fine with … our pasteurized milk products containing viral DNA,” says Lakdawala.
Research suggests that, as long as the milk is pasteurized, the virus is not infectious. But Lakdawala is concerned that pasteurization may not inactivate all of the virus, all the time. “We don’t know how much virus we need to ingest [to become infected], and whether any is going to slip through pasteurization,” she says.
And no reassurances can be made for unpasteurized raw milk. When cows are infected with H5N1, their milk can turn thick, yellow and “chunky.” But research has shown that, even when the milk starts to look normal again, it can still contain potentially infectious virus.
The most concerning development, though, is the rise in human cases. So far, 55 such cases of H5N1 bird flu have been reported in the US, according to the US Centers for Disease Control and Prevention (CDC). Twenty-nine of those cases have been detected in California. In almost all those cases, the infected person is thought to have caught the virus from cattle or poultry on farms. But in two of those cases, the source of the infection is unknown.