End of March, The US Department of Agriculture (USDA) announced that it had discovered cases of bird flu in dairy cows. The disease was originally discovered on dairy farms in Texas, Kansas and New Mexico, but there are now 36 confirmed outbreaks in dairy herds in nine states.
Although the H5N1 virus is widespread in wild birds, it is now also circulating in dairy cows in the United States. The USDA has confirmed transmission between cows in the same herd, from cows to birds, and between different dairy herds.
But the reported outbreaks likely represent a significant underestimate of the true spread of the virus, says James Wood, head of veterinary medicine at the University of Cambridge. “It is likely that there will be significant underreporting and underdiagnosis,” he says.
Food and Drug Administration (FDA) testing of retail milk samples could provide clues about how widespread the virus is. The agency found virus fragments in one in five samples of commercial milk, even though this virus had been deactivated by pasteurization and was therefore not infectious.
There is only one confirmed human infection in the outbreak so far: someone in Texas who had close contact with dairy cattle. Their only reported symptom was conjunctivitis, and the person was told to isolate and take an antiviral drug for flu. However, anecdotal reports of illness on dairy farms suggest that human infections may be more widespread than official data suggests. Although human infections are rare, the virus is dangerous – just over half of the human cases recorded by the World Health Organization over the past two decades have been fatal.
Dairy workers are most at risk of possible infections in the current outbreak, but understanding the extent of any infections is extremely difficult, says James Lawler, a professor of infectious diseases at the University of Nebraska Medical Center. More than half of workers in the U.S. dairy industry are immigrants, and many of them are undocumented.
It’s unlikely that these undocumented workers want to put themselves at risk by coming in for testing, Lawler says. “Many of the workers are deterred by not raising their hands because of their status as illegal immigrants.” The result, Lawler said, is that it is difficult for scientists to track possible human spread of the virus.
Another problem is giving dairy farm owners incentives to report when their animals appear sick. The USDA Animal and Plant Health Inspection Service provides payments specifically for poultry farmers who must kill their livestock due to avian influenza infections. Dairy farmers are not compensated for reporting infections, giving producers an incentive to remain silent, increasing the risk that outbreaks spiral out of control and spread to other cattle or farm workers.
This poses a major problem for tracking the spread of the disease. “How will it be beneficial from a producer’s perspective to share or even test and understand if there is a virus circulating in their herd?” Lawler says .