Public health experts still don鈥檛 know how a bird flu virus has been spreading for months among dairy cows in the US 鈥� though early evidence points to contaminated milking equipment.
Since late March, have tested positive for H5N1, a bird flu virus that has killed millions of birds and thousands of mammals worldwide. Genetic analysis of samples from infected cattle suggests the outbreak began when cows on a Texas farm contracted the virus from wild birds. It then spread to other herds as infected cows moved between farms. But how the virus jumps from cow to cow is unclear.
鈥淭his is a critical question that is still under investigation,鈥� said at Johns Hopkins University in Maryland during a press briefing on 15 May. 鈥淭here are most likely two ways the virus is spreading,鈥� he said.
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The first is respiratory transmission. Infected cows may be expelling the virus in small droplets of mucous or saliva that other cows then inhale or ingest when sharing food and water, says at Emory University in Atlanta, Georgia.
However, infected cows show , and their nasal swabs contain low concentrations of the virus, suggesting another mode of transmission.
鈥淚 think it鈥檚 the milking equipment,鈥� says Lakdawala.
Raw milk from sick cows contains and can thus transmit the virus. Several cats contracted bird flu after drinking unpasteurised milk, for instance. This is why it is crucial that humans avoid raw milk products.
Although adult cows don鈥檛 drink raw milk, they do share milking equipment. Dairy workers normally wash a cow鈥檚 udder before hooking it up to the milker and again after removing it. 鈥淏ut some dairies do and some dairies don鈥檛 disinfect the milking equipment between each cow,鈥� says at the Texas Department of Agriculture. Consequently, rubber inside the milking equipment can contain residual raw milk.
In an unpublished study, Lakdawala and her colleagues found that H5N1 in milk survives on rubber surfaces for at least an hour at concentrations high enough to be infectious. Another preliminary study from at the University of Copenhagen in Denmark and her colleagues showed that mammary glands from a lactating dairy cow contain multiple .
鈥淲hether there is some infection through the skin or some other route, I don鈥檛 know, but that [milking equipment] definitely has contamination potential,鈥� says Lakdawala.
Miller says the Texas Department of Agriculture is recommending that dairy farmers disinfect milking equipment between each cow. The most common way to do so is by installing a backflush system, which flushes disinfectant through the milker after each use, says at the Idaho Dairymen鈥檚 Association. 鈥淭hat鈥檚 been something our dairymen have been sharing readily, is 鈥榟ey, if you don鈥檛 have backflush in your system, get it, because it helps prevent the spread鈥�,鈥� he says.
But all of this evidence is far from conclusive, and poor surveillance has made it nearly impossible to pinpoint or rule out other potential pathways of transmission 鈥� which must be done if we are to contain the outbreak, says Lakdawala. To incentivise farmers to boost surveillance, the (USDA) announced on 10 May that it would compensate dairy farmers for lost milk production and H5N1 testing costs.
