A woman is measuring the level of milk in a bulk, stainless steel tank.

Reassess Biosecurity to Minimize Dairy Herd Risk of H5N1 Infection

Dr. Julie SmithAnimal Health, Livestock diseases, Risk assessment

With dairy herds in three new states confirmed positive for highly pathogenic avian influenza (HPAI)Opens a new window in the past week, it is time to reassess your biosecurity to minimize the risk to your herd.

The first introduction of HPAI into a dairy herd in 2024Opens a new window is considered a spillover event from birds. Subsequent infection of herds appears to be from dairy cows or fomites and not new introductions from birds. So the risk to dairy cattle appears to be much higher from exposure to dairy cattle than to birds. However, poultry remain at risk of infection from wild birds and now are at risk from dairy cattle, too.

Studies are underway to learn more about the specific pathways of spread between dairy farms. That additional information will inform more specific biosecurity guidance, but we can work with what we know already.

What is Known

We know that cows test positive for the HPAI virus before exhibiting fevers or other clinical signs (such as reduced feed intake, reduced milk production, and abnormally thick consistency of milk). The virus can be detected in bulk tank milk prior to seeing sick cows, but will become undetectable a month or so later. The specific technologies or approaches used to monitor cows for signs of illness will affect the lag time between actual and apparent infection. 

Not all herds will have the same experience, so extrapolate from case studies like the following with caution. One case study showed virus in bulk milk was detectable by PCR slightly over two weeks before a spike in sick cows was experienced. PCR is the abbreviation for polymerase chain reaction, a type of testing to detect genetic material from specific organisms, such as a virus. The bulk milk reverted to PCR test negative about three weeks after seeing the significant spike in sick cows. Becoming PCR test negative appears to correspond with the development of what is called neutralizing immunity. After an animal’s immune system develops specific proteins called antibodies that can bind to the virus, the virus cannot replicate.

Herd Testing

The herd test status program being piloted in selected states designates several categories of herds relative to infection status. The important things to know are that:

  1. Finding virus by PCR means virus is actively circulating in a herd.
  2. Finding only antibody to the nucleoprotein of the virus (by NP-ELISA) means the herd has recently experienced an outbreak of any strain of avian influenza.

In herds known to have been infected, not all lactating animals become infected or develop antibodies. Virus stops circulating in a herd when a protective level of herd immunity has developed. We don’t know what percentage of a herd needs antibody titers in order to achieve herd immunity. However, we do know, as heifers calve in or new animals are introduced, the proportion of animals with protective antibodies will fall, and the herd will become susceptible to new or re-infection over time.

There are other important things that are yet unknown. We do not know:

  1. How long the antibody response lasts.
  2. How non-lactating animals respond to the virus.

Some will take their chances. To err on the side of caution, assume the antibody response does not last forever and that non-lactating animals may be infected but not visibly ill.

The Bottom Line

Asking for assurance that a herd has not been exhibiting clinical signs prior to bringing an animal from that herd into yours is not enough to be sure you won’t be bringing in an infected animal. Testing will increase the chance of making the right decision whether you are moving a cow from next door or several miles down the road. In many cases, testing is required before moving lactating animals across state lines. Not testing is like closing your eyes before crossing a busy road. Keep your eyes open and keep your herd healthy.

Share this Post

About the Author

Dr. Julie Smith

Dr. Julie Smith is a research professor at the University of Vermont. Julie received her B.S. in biological sciences, D.V.M., and Ph.D. in animal nutrition at Cornell University. Since 2002, she has applied her veterinary background to programs in the areas of herd health, calf and heifer management, and agricultural emergency management. She has taught courses in animal welfare, calf biology and management, and ABCs of biosecurity to undergraduates, mostly in the Department of Animal and Veterinary Sciences. As a veterinarian and spouse of a dairy farmer, Julie is well aware of the animal health and well-being concerns of dairy animals. Julie has conducted trainings for Extension educators, livestock producers, and community members on the risks posed by a range of animal diseases, whether they already exist in the United States, exist outside of the United States, or pose a risk to both animal and human health. In all cases, she emphasizes the importance of awareness and prevention. In addition, she has led a number of projects on biosecurity and emergency disease preparedness.