By Natalie Willis, Reporter, Valley Ag Voice
The recent outbreak of H5N1— more commonly referred to as the avian or bird flu — in Central Valley dairies has led to significant concern and uncertainty among farmers. According to the U.S. Department of Agriculture, the first detection of avian flu in dairy cattle was found in March 2024 — the first detection in California came in late August.
Since then, the virus has been confirmed in dairy cattle in 14 states including 82 dairies in California, according to the American Veterinary Medical Association. The tests indicate that the virus in dairy cattle is H5N1 – Eurasian lineage goose/Guangdong clade — the same clade affecting wild birds and commercial poultry flocks, AVMA explained.
On Sept. 26, a webinar hosted by the California Dairy Quality Assurance Program, University of California Cooperative Extension, and California Milk Advisory Board, brought together disease control experts and dairy practitioners to discuss prevention strategies and ways to minimize losses if the virus reaches a dairy herd.
HOW IT SPREADS
According to a newsletter by Dr. Michael Payne, researcher at the UC Davis School of Veterinary Medicine and Director at CDQAP, the way the virus spreads through herds is unclear, but small challenge studies have shown that it can be transmitted through oral contact and contamination of the teat surface.
In an email, Payne explained that he suspects H5N1 is spread primarily during milking, though some experts disagree. Payne cross-referenced Kansas State University research with a challenge study by the USDA Agricultural Research Service. Both studies found that oral and respiratory exposure showed mild outcomes.
USDA researchers exposed four yearling heifers to aerosol mist on their nose and mouth, with all becoming infected but showing minimal symptoms, while Kansas State exposed nine calves via oral and nasal inoculation, leading to mild illness and no transmission to co-housed calves.
Mammary exposure, however, had more severe clinical effects. The USDA tested two lactating cows with the U.S. strain on their teats and udders, resulting in severe symptoms. Kansas State also found that inoculating three cows with a European train led to a severe mammary infection but no nasal shedding.
According to Payne’s newsletter, research on how avian flu spreads is ongoing to determine if the primary method of transmission is through nose-to-nose contact or during milking, or both. Additionally, CDQAP is partnering with dairy trade organizations to identify potential collaborator dairies for further research.
HERD HEALTH OUTCOMES
In birds, H5N1 is associated with high mortality rates, but this has not been the case for dairy cattle. Most affected cattle reportedly recover with treatment and the culling rate is low at 2% or less.
“Unlike the disease in cattle where most of them recover, tens of thousands or even millions of chickens or turkeys can die of [Avian Influenza] in a matter of days,” Payne said in an email. “Avian Influenza and Newcastle Disease can be thought of as the Ebola of poultry flocks. Fortunately, it’s not that bad in cows.”
Common clinical signs in affected cows include low appetite, reduced milk production, and abnormal appearance of milk, according to the AVMA. Lactating cows have been most highly affected.
The CDQAP webinar explained that electronically monitored herds reported fevers about 12 to 48 hours prior to cows going off feed and having decreased rumination.
“The “colostrum looking” milk often seen in the disease may be due to udder inflammation and severe dehydration. A clear nasal discharge may progress to purulent or bloody discharge, which in some cows may foreshadow a later bacterial pneumonia that requires treatment or culling,” the newsletter stated.
Younger cows are also more likely to show clinical signs. For instance, in one herd, 74% of all reported clinical cases were from 1st and 2nd lactation cows. Still, the percent of clinical cases within a herd varies — one study ranged from 7% to 22% of the herd.
Additionally, the webinar discussed that the Central Valley heat may be increasing the severity of clinical symptoms and the percentage of cases. Mortalities may also be higher than previously reported, with 10% to 15% of clinical cases reported euthanized or culled on certain herds.
Avian flu takes several weeks to move through a herd and one dairy reported cattle shedding the virus up to 14 days before showing symptoms. Peak shedding occurred five days after the first signs. According to the webinar, symptoms usually peak between seven and 14 days before gradually decreasing — this can last from two weeks to over a month.
After recovering, milk production often drops and may not fully return to previous levels. For example, a cow producing 100 lbs. of milk per day may drop to 20 lbs. but recover to 70-80 lbs., the newsletter explained.
“Whether this approximate 20% to 30% decrease in production represents a permanent change due to scaring of the udder and will persist in the next lactation is unknown,” the newsletter stated.
As H5N1 in dairy cattle is a recent development, it is unclear if reinfection is occurring. According to the newsletter, some herds thought to have recovered experienced new infections. It is unclear if the new cases represent cattle that initially escaped infection or are actual relapses of cows previously infected.
TREATMENT AND BIO-SECURITY
Effective treatment for infected dairy cattle focuses on oral fluids as keeping cows hydrated whilst re-establishing rumination quickens recovery and minimizes secondary pneumonia, abortions, mortalities, and culls.
Oral drenching with five to 6 gallons using electrolyte solution is effective, according to the newsletter. Cows should be treated one to two times per day depending on the number of affected cattle.
“A variety of commercial drench powders (TechMix, MB Restore, Drench-Mate) or homemade electrolyte recipes are being used effectively. Oral Fluid Therapy – During the peak of the outbreak producers may be treating 30-40% of the herd every day for a week or more, so an efficient system to deliver large volumes of drench is essential,” the newsletter stated.
CDQAP suggests that producers begin ordering or constructing their own drenching systems now. Additionally, anti-inflammatory drugs are used to treat affected cattle, but flunixin or aspirin to a dehydrated cow can result in death.
For more treatment options, farmers should confer with their local veterinarian or reach out to CDQAP.
With support from CDQAP, the California Department of Food and Agriculture shared a list of effective actions producers can follow to prevent the spread of avian flu in their herds. Practices include the proper testing and permitting of cattle from out-of-state as well as isolating new or returning cattle for 30 days.
Producers are also urged to train employees to report suspicious symptoms and follow strict biosecurity protocols as early detection can prevent the spread of disease and minimize the financial impact.
Proper sanitation such as power-washing and disinfecting trailers is recommended, especially if vehicles are shared with other farms. CDFA also recommends limiting non-essential visitors and coordinating with milk processors to ensure proper hygiene practices during milk pick-ups.
Additional prevention techniques can be found on the CDFA’s website.
