More than three months since the first case of highly pathogenic avian influenza (H5N1) in cattle was confirmed in Texas, we’re starting to understand a little bit more about the disease. And that’s critical as H5N1 in dairy cattle lingers through the summer.

Cows in their second and higher lactation and those 100 or more days in milk continue to be the most likely to be affected with the disease. Anecdotally, herds with higher milk production appear to be experiencing a larger number of clinical cases and more severe clinical disease. Using rumination, activity, and milk production data, individual cow effects can easily be observed.

Rumination time, activity time, and milk production for an infected cow are shown below. Rumination time fell from over 600 minutes per day to bottom out at five minutes. Milk production dropped from 120 pounds per day to 20 pounds per day. (Missing milk production values represent time in the hospital pen.) This cow’s activity didn’t slip as much as the other parameters measured; it returned to normal rumination time over the course of about 12 days.

Reports from an infected Michigan herd are that many of these cows experience a fever of four to five degrees above normal then a drop in rumination time six hours later. So, the drop in milk production likely results from the dramatic reduction in feed and water intake. By the time the decline in rumination time or milk production is noticed, many of these cows may have returned to normal temperatures. Still, many will look depressed, be dehydrated, and have less activity.

A dairy recently reported moving infected cows into a pen that was using straw bales as a wind break. Although the cows were delivered fresh total mixed ration (TMR) twice a day, the cows preferred the straw bales. Other veterinarians have reported dairies placing alfalfa or wheat hay in the sick pen and the cows devouring most of it. In addition to modifying the diet fed, cows are frequently given anti-inflammatories, such as aspirin, as well as electrolytes.

Cows with clinical signs of pneumonia, severe mastitis, or other secondary disease are commonly placed on antibiotic therapy. These changes in management and treatment have led to most cows recovering; however, many cows do not return to levels of milk production that justify them remaining in herd. Some cows do not recover and either die or are euthanized, with some farms reporting mortality rates from 3% to 8%, though one herd reported 10%.

At the herd level, the changes in milk production and bulk tank somatic cell count (BTSCC) are also evident, as presented in the graph below. This herd experienced a 23% reduction in daily milk production per cow (with sick cows removed from the milk per cow per day calculation) and a jump of more than 200% in their average BTSCC. It took about a month for the herd to get back to its baseline BTSCC, though it still hasn’t returned to its pre-disease milk production per cow.

The dairy industry is less than six months into this challenge. With continued collaboration among dairy producers and researchers, our knowledge of how this disease spreads and, more importantly, how we can control and ultimately prevent cows and herds from becoming infected can be achieved.

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(c) Hoard's Dairyman Intel 2024
July 8, 2024
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