Avian influenza in dairy cows has continued to spread since it was first detected in a Texas dairy herd on March 25.

Three months later the count is up to 139 confirmed herds in 12 states. In the last 30 days, there’s been 57 new cases in seven states. Most intense right now are infections in Colorado, with 23 new cases since June 10.

This is an unexpected turn of highly pathogenic H5N1, being in lactating dairy cattle, said Jamie Jonker, chief scientist for National Milk Producers Federation.

“One of the interesting things about H5N1 in dairy cows is that it is going into the mammary gland rather than manifesting itself as a respiratory issue,” he said during an update webinar on the virus hosted by Council for Agricultural Science and Technology.

Lactating cows

Infection occurs in lactating dairy cows of all ages and stages of lactation, although they are more likely to occur in multiparous cows in second lactation or older and in mid-lactation, being around 150 days or so, he said.

“On average, while this does vary from farm to farm, 10 to 15% of lactating cows do have clinical symptoms. And unlike in poultry where it’s a high mortality event, it’s a low-mortality event in dairy cows so far, being in the 1-to-2% range,” he said.

Symptoms include a drop in feed intake, which decrease rumination, and a drop in water intake, which leads to dehydration.

Milk production

“These symptoms really come about because of what happens in the mammary gland. We have significant levels of virus that are in the milk, and milk yield in the clinically infected animals drops precipitously,” he said.

In some severely affected cows, all four quarters of the mammary gland are infected. Less severely affected cows might only have one or two quarters infected.

The virus peaks in the dairy herd about one week after first seeing clinically infected animals and tapers off at about two weeks, he said.

“We find that milk production recovers in about 30 to 45 days later, but not all cows that were clinically infected recover milk production — some do not recover milk production at all,” he said.

Disease transmission

The virus came into dairy cattle from an unknown host, probably in December. It circulated for about another six weeks or so. In late January/early February, the industry started seeing a mystery disease in the Texas panhandle. Because no one was expecting H5N1 in dairy cattle, it took until March 25 to diagnose it, he said.

“Subsequently within a herd, because it’s in the mammary gland, there is some spread that happens during the milking process. We have also seen spread from one herd to another herd due to the movement of infected animals, animals that may be infected but do not yet have clinical symptoms,” he said.

There has also been lateral flow from dairy farms to poultry operations. The likely routes of those transmission might include shared workers on dairy farms and poultry farms and mechanical conveyances such as vehicles. There has been lateral spread to at least 21 commercial poultry flocks in at least four states, he said.

Jonker and other presenters also talked about the critical role of biosecurity in containing the virus.