CDFA’s most recent data show that of the roughly 770 California dairies that have been infected with Avian Influenza. Some 630, more than 80%, have been released from quarantine. Of the approximately 140 infected herds remaining, most are making steady progress towards quarantine release.
For a small number of producers, however, the pathway back to normal has remained prolonged and frustrating. CDFA estimates some 10 to 15 herds have experienced unusually long quarantines, in one case over 10 months. These farms might achieve one or two negative weekly bulk tank tests, only to have a weak positive again, resetting the quarantine release clock. There are reports from dairies in other states experiencing similar problems.
Just as frustrating for several California producers is to have been actually released from quarantine only to have a positive bulk tank some weeks or months later. These cases have been reported as typically involving a limited number of clinical cows, most often springing heifers.
What’s causing persistent or reoccurring positive bulk tanks?
CDFA’s Dr. Natalie Ward says it’s useful to remember that the PCR assay used for HPAI detection is extraordinarily sensitive. “Just one cow actively shedding virus can trip a bulk tank positive,” Ward says. What might be allowing the virus to remain circulating on some farms is an area of intense research. In California alone, some $2 million of state and federal research is currently being conducted. Investigations into potential causes of persistent or reoccurring positive bulk tanks include:
Chronic Shedders?
Dr. Jason Lombard of Colorado State’s vet school has directed some of the largest H5N1 dairy surveys in the country. He explains that one possible explanation could be non-clinical cows shedding for prolonged periods. “We’ve detected cows that were continuing to shed virus out to at least 90 days, long after they recovered clinically.” Dr. Lombard says. The role of persistent shedders remains speculative for now, however, at least until whole-herd surveys can be performed to identify potential chronically infected cows.
Carried by Birds?
Certainly, migratory waterfowl were determined to be the original source of Texas spillover event in 2024. Federal testing of birds in the vicinity of dairies however, has revealed that local, non-migratory birds, such as blackbirds, starlings, doves and pigeons have been positive for H5N1 genetic material. This raises the worrisome scenario that HPAI is circulating within local bird populations potentially shedding the virus into cows’ feed and water sources. Whether the virus is reproducing within these nuisance birds and being shed in numbers sufficient to infect cows remains unknown.
Carried by Flies?
Researchers with the USDA’s Agricultural Research Service (ARS) are collaborating with industry partners in California to survey both biting flies, such as Stable Flies, and non-biting flies, such as House Flies. The goal is to determine if these insects can carry the disease-causing virus and understand their potential role in transmitting the disease from cow-to-cow or from herd-to-herd.
Carried by Dust?
In yet another industry-academic collaboration, researchers from Emory University are collecting aerosol samples on quarantined California dairies from a variety of sites including in the parlor, pens and over lagoons. Air from these sites is run through filters to collect particulate matter which is assayed for the presence of both HPAI genetic material and intact, infectious virus.
New Herd Additions?
One more potential cause of persistent or reoccurring positive bulk tanks is newly purchased or returning cattle. Infected but non-clinical cattle, or youngstock returning from calf ranches, could be harboring the virus, serving as a new nidus of herd infection. Because much of the herd might be immune due to previous infection, the amount of clinical disease in the herd could remain limited. Alternatively, new or returning animals who have never experienced HPAI infection might be introduced to the virus which is persisting in low levels in the dairy environment. Such “immunologically naïve” animals, having no protective antibodies, could develop full-blown clinical disease and shed sufficient virus to trip a positive bulk tank test.
Decaying Cow Immunity?
Complicating producers’ biosecurity efforts is a lack of data on how long immunity persists following natural infection. This is true regardless of whether the virus is being re-introduced by outside sources (i.e. local birds, asymptomatic cattle) or is simply circulating at low levels within the herd. Following experimental infection protective antibodies have persisted for at least one month, but unpublished bulk tank studies suggest such protection might wane after 3 to 6 months.
Guidance for producers?
The research described above as well as other investigations will eventually describe how HPAI circulates among domestic animals and wildlife. Producers however understandably want to know what then can do now. State and federal animal health officials advise producers not become complacent about biosecurity once released from quarantine. CDQAP’s Bird Flu homepage contains a biosecurity section where the best references to help protect your herd have been collected. Ultimately HPAI cattle vaccines, which are in the final stages of approval, should provide producers permanent relief. — By Dr. Michael Payne, UC Davis, School of Vet. Medicine, Director, California Dairy Quality Assurance Program
