Virologists are particularly concerned that the virus will enter pigs, as these animals are notorious virus incubators. “Pig strains, bird strains and human strains can become infected,” says Brinkley Bellotti, an infectious disease epidemiologist at Wake Forest University in North Carolina. These strains can exchange genes and give rise to new, potentially more infectious or harmful strains.
Fortunately, we have not had any further cases on pig farms and there is no evidence that the virus can spread between pigs. And while the virus spreads fairly quickly among cattle, the virus doesn’t seem to have evolved much, says Seema Lakdawala, a virologist at Emory University School of Medicine in Atlanta, Georgia. This suggests that the virus made the jump to livestock, probably from birds, only once. And the herds have been spreading ever since.
Unfortunately, we don’t really know yet as it spreads. There is some evidence to suggest that the virus can be spread from cow to cow through shared milking equipment. But it is not clear how the virus spreads between farms. “It’s hard to create an effective control strategy when you don’t know exactly how it’s spreading,” says Bellotti.
But it is with cows. And it’s in their milk. When researchers analyzed 297 samples of pasteurized Class A retail dairy products, including milk, cream and cheese, they found viral RNA from H5N1 in 20% of them. These samples were taken from 17 states in the US. And the study was conducted in April, just weeks after the virus was first detected in cattle. “I’m surprised that we’re perfectly fine with … our pasteurized dairy products containing viral DNA,” says Lakdawala.
Research suggests that if the milk is pasteurized, the virus is not infectious. But Lakdawala worries that pasteurization may not inactivate the entire virus all the time. “We don’t know how many viruses we have to ingest (to become infected) and whether any will pass pasteurization,” he says.
And no assurances can be made for unpasteurized raw milk. When cows are infected with the H5N1 virus, their milk can thicken, turn yellow and “curd”. However, research has shown that even when the milk starts to look normal again, it can still contain potentially infectious viruses.
The most worrying development, however, is the increase in human cases. According to the US Centers for Disease Control and Prevention (CDC), 55 such cases of H5N1 bird flu have been reported in the US so far. Twenty-nine of those cases were found in California. In almost all of these cases, the infected person is believed to have caught the virus from livestock or poultry on farms. But in two of these cases, the source of infection is unknown.
