The fact that the same people along with the same equipment were working simultaneously across multiple dairy farms are among the main risk factors for the continued spread of highly pathogenic H5N1 avian influenza in dairy cattle, the United States Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) stated on 13 June 2024 in a pair of new epidemiological reports.
One of the reports is an overview based on findings from questionnaires completed by affected dairy herds, and the other is a deep dive into dairy cattle and poultry outbreaks in Michigan, the state most affected by dairy cattle outbreaks, which now total at least 94.
At a press conference held on 13 June, Kammy Johnson, DVM, PhD, veterinary epidemiologist at APHIS, said that the multistate epidemiological report provides a national clinical overview of the disease in cattle and the routes of spread, while the Michigan report is an early snapshot showing what is happening in the field.
Both suggest three key factors for between-farm transmission: shared equipment and vehicles, shared personnel who may inadvertently carry the virus between farms on their clothing or boots, and animal movements. “The big picture is that improving biosecurity is truly critical,” she said.
To date, genomic evidence continues to suggest a single introduction from wild birds, with further spread between dairy farms, such as from Texas to Michigan in the early weeks of the outbreak. The virus is now spreading between farms due to multiple direct and indirect factors, according to the APHIS reports.
Multiple factors driving between-farm spread
The questionnaires revealed that more than 20% of farms received cattle within 30 days of clinical signs appearing, and that 60% of farms continued to move cattle off the premises after animals showed signs of disease.
The majority of farms had cats on site, and more than 50% of these had sick or dead cats. In addition, more than 20% of dairy farms also kept chickens or poultry, nearly all of which had sick or dead birds. USDA officials stated that cats are the canary in the coal mine, but that cats and other animals, such as wild birds, are likely not playing a major role in viral spread, although they could act as fomites.
Clinically, lactating cows are the most severely affected, with clinical signs observed in fewer than 10% of herd animals and an average mortality and culling rate of less than 2%.
Meanwhile, the Michigan report is based on data collected by an APHIS strike team that the Michigan Department of Agriculture and Rural Development invited to the state to investigate links between infected dairy farms and the indirect effects on poultry facilities. It includes findings from 15 dairy farms and 8 poultry producers.
Approximately 20% of dairy workers, and some of their family members, worked across multiple dairy farms. Around 7% of workers on affected dairy farms also worked on poultry farms. Regarding equipment, approximately 62% of farms shared vehicles for transporting cattle, and only 12% cleaned vehicles prior to use.
The farms also received frequent visitors, including veterinarians, feed consultants, and contracted hauliers. Nearly all of the affected farms are part of the same dairy cooperative, and all affected farms used the same dead stock transport service.
To date, the outbreak strain B3.13 has not been detected in migratory waterfowl in Michigan.
The report’s authors commended the willingness of Michigan dairy producers to participate in the investigation, emphasising that the findings have greatly expanded the body of knowledge on B3.13, both in Michigan and at the national level. “This report could not have been completed without them.”
Lingering questions about newly affected states and the full extent of the outbreak
At the briefing, Johnson said it is too early to assess how H5N1 spread to the three most recently affected states: Iowa, Minnesota, and Wyoming. She added that the report paints a national picture of what is known to date about the outbreaks and adds useful context for understanding developments.
She said the findings serve as a sobering reminder that farms already have the tools to manage viral spread. “Biosecurity is the key to prevention.”
Mark Lyons, DVM, who heads the Ruminant Health Centre within APHIS Veterinary Services, said it remains unclear whether the outbreaks are still gaining momentum, and that animal health officials are still working to understand the full extent of the situation, although they expect to find additional cases.
As of yesterday, 12 June 2024, the APHIS total stood at 94 outbreaks on dairy farms across 12 states. Yesterday, the Iowa Department of Agriculture and Land Stewardship reported the state’s third outbreak in a dairy herd, affecting a second location in Sioux County.
Updates on human investigations and vaccine production
In response to questions about the second Michigan case — a dairy industry worker who, unlike the other previous patients, presented with respiratory symptoms — Nirav Shah, JD, MD, Principal Deputy Director of the Centers for Disease Control and Prevention (CDC), said that the polymerase chain reaction (PCR) cycle threshold (Ct) value for the patient’s sample was high, suggesting a lower quantity of viral RNA in the sample.
The situation made it difficult for CDC scientists to generate a complete genomic sequence, although they were able to reconstruct a large portion of it, he said.
To date, as of June 2024, the findings are reassuring, as there are no changes suggesting that the virus that infected the patient has enhanced transmission capacity, and there are no changes in the neuraminidase suggesting reduced susceptibility to H5 vaccines or existing therapeutic agents, Shah said.
Regarding vaccines, David Boucher, PhD, Director of Infectious Disease Preparedness and Response at the Department of Health and Human Services’ Administration for Strategic Preparedness, said that federal health officials remain on track to produce 4.8 million vaccine doses from bulk antigens within a 3- to 4-month timeframe, with manufacturing set to begin in mid-July.