When dairy farmers in the Midwest and Mid-Atlantic learned that highly pathogenic avian influenza (HPAI, also known as H5N1) was affecting dairy cattle in Texas, most weren’t concerned.

However, the virus has since been identified in dairy cattle in several other states including Idaho, Kansas, New Mexico, South Dakota, Michigan, Ohio and North Carolina.

Initial dairy cattle infections were traced to wild birds, but subsequent infections in other isolated states were likely due to animal movement. When the illness was first discovered in Texas, researchers didn’t find any links between infected farms. Feed suppliers, veterinarians, milk routes and co-ops were not a common thread.

In the Midwest, sequencing of viral DNA showed that the strain found in dairy cattle matched the circulating strains of HPAI in the flyways of migrating wild birds.

Dr. Alex Hamberg, Pennsylvania state veterinarian, explained that avian influenza virus is adapted to birds, and the HPAI circulating throughout the nation is adapted specifically to waterfowl as well as gallinaceous birds such as chickens and turkeys.

“Originally it was reported in second lactation and older animals; now they’re finding it in first calf heifers and some younger animals as well,” said Hamberg. “Any dairy cattle are susceptible.”

Clinical signs of HPAI in dairy cattle include decreased appetite, decreased milk production, milk that is thick, involuntary dry-off, stools that are either tacky and thick or loose and some animals have a fever.

“Cattle recover over several weeks,” said Hamberg. “They aren’t dying from it, and cattle that dry off eventually go back to lactation.”

While that’s good news, the mode of transmission remains a concern. Current thoughts based on data show that HPAI is being transmitted directly from wild birds to cattle. Hamberg said the supposition is that cattle are becoming infected directly from waterfowl.

“There has been some evidence suggesting there is cow-to-cow transmission,” said Hamberg. “There’s still a lot we don’t know. The USDA is working to try to figure out how it’s working and spreading. As we learn more about the epidemiology – how it moves through a population – we can build better response and biosecurity protocols and do more to protect cattle.

“Some farms reported bird die-offs at about the same time cattle became ill,” Hamberg added. “That’s what made them think cattle were infected with HPAI and started testing down that route.”

According to AVMA News, the affected dairy farm in Ohio had received cows from a Texas dairy farm that later reported confirmed HPAI. In addition, three sick cats on one of the affected Texas dairy farms tested positive for HPAI.

Hamberg recommended strict on-farm biosecurity, especially on farms where both poultry and dairy cattle are present. Those working with livestock on such farms should use appropriate biosecurity measures when moving to and from cattle and bird housing areas. Pastured cattle that have access to streams may be at increased risk if waterfowl are present in or around those streams.

Although the virus has been detected in milk from infected cattle, pasteurized milk is safe to drink. “Viral loads are highest in mammary tissue,” said Hamberg. “We find virus in lesser quantities in nasal passages and other tissue. Cattle that are clinically ill with [HPAI] should be removed from the milk supply and segregated until they recover. Milk from sick animals should not go in the bulk tank. Sometimes sick cows dry off completely or have thick, colostrum-like milk so it should be obvious which cows are affected. They can be isolated and the remainder of milk from healthy cattle can go in the tank.”

On farms where calves are fed milk, heat-treating that milk is recommended.

Managing HPAI in dairy cattle

In quoting an FDA statement, Hamberg said, “The FDA recognizes that this is an evolving situation, and we still have limited data on asymptomatic or pre-symptomatic shedding in cattle. FDA’s current best recommendations are that raw milk, raw milk cheese and other raw dairy products should not be manufactured from asymptomatic cattle that have been exposed. ‘Exposed cattle’ generally means cattle located on the same premises with cattle with suspected or confirmed H5N1.”

Dr. Erin Luley, assistant director, Animal Health & Diagnostic Services, PA Department of Agriculture, addressed additional points. Luley said at this point, it’s unknown whether asymptomatic heifers or beef cattle spread HPAI; however, USDA is initiating research on this aspect.

“At this time, it appears that the virus replicates in mammary tissue,” she said. “Animals that aren’t lactating are presumed to not be a source of high viral load that would be high risk for transmitting the virus.”

Animals from affected premises cannot be moved other than to slaughter. “Based on what we know about the virus, it has the potential to spread through milking equipment,” said Luley. “If cows are in the slaughter channel and slaughtered within 72 hours, based on what we know of the virus at this time, that does not appear to be a source of likely spread into another milking herd.”

Luley emphasized that biosecurity on farms is essential to protect dairy herds. “While we can impose a quarantine order to prevent the risk from animal movement, unfortunately, we can’t apply a quarantine order to migratory waterfowl so that risk remains,” she said. “That’s where biosecurity is our best tool.”

Kyle Van Why, USDA-APHIS Wildlife Services, explained that while migrating ducks and geese are the primary carriers of HPAI, other species such as raptors are also problematic because they consume mortalities.

Many other bird species carry HPAI, as well as some mammals including foxes and rodents. Low pathogenic avian influenza is carried by smaller mammals including rabbits, skunks and cats but Van Why said there’s good reason to believe mammals could potentially carry and transmit HPAI.

While birds such as starlings, house sparrows and pigeons are not good carriers of HPAI, such species are often present in large numbers so when they show up at a dairy farm, there’s a good chance they are transmitting HPAI simply because there are so many birds.

Van Why said these species often interact with waterfowl and can introduce HPAI to dairy cattle via feathers, feet and nesting material. Initiate good vector management and manage farm cats because they could be consuming songbird species that carry the disease.

For assistance with bird issues on the farm, call the USDA Hotline number at 866.487.3297.

Visit interstatelivestock.com to find out where and how cattle can be moved.

For current information on HPAI infections in livestock, visit aphis.usda.gov/livestock-poultry-disease/avian/avian-influenza/hpai-detections/livestock.

by Sally Colby