On March 25, the U.S. Department of Agriculture (USDA), Food and Drug Administration (FDA), and Centers for Disease Control and Prevention (CDC) announced the detection of highly pathogenic avian influenza (HPAI), specifically the H5N1 strain, in U.S. dairy cattle for the first time.
The H5N1 avian influenza virus has been confirmed in dairy cattle across 14 states. It has been identified in 64 herds in Colorado, 41 in California, 31 in Idaho, 29 in Michigan, 26 in Texas, 13 in Iowa, and nine each in Minnesota and New Mexico. Additional cases are found in South Dakota, 4 in Kansas, 2 Oklahoma, and one each in North Carolina, Ohio, and Wyoming.
Tests show that the strain belongs to the H5N1 Eurasian lineage goose/Guangdong clade 2.3.4.4b. This clade has been affecting wild birds and commercial poultry flocks. It has also caused infections in various wild and domestic mammals in the U.S.
Common signs in affected cows include decreased appetite, lower milk production, and abnormal milk appearance. Lactating cows are primarily affected, with less than 10% of cows in a herd showing symptoms.
The H5N1 virus appears to spread between cattle, indicating bovine-to-bovine transmission. Animal movement is a significant risk factor for spreading the disease. The virus may also transfer from dairy cattle premises to nearby poultry farms. Shared personnel, uncleaned vehicles, and frequent visitors increase local transmission risk.
Currently, there is no evidence that wild birds are transmitting H5N1 to cattle, but this possibility cannot be dismissed.
Since April 29, a federal order mandates testing of lactating dairy cattle for HPAI before interstate movement. Positive test results must be reported to the USDA’s Animal and Plant Health Inspection Service (APHIS). Initially, these restrictions apply to lactating dairy cows, but adjustments may occur as scientific understanding evolves.
On May 1, Michigan issued an emergency order requiring poultry and livestock producers to implement specific biosecurity measures. As of July 22, Colorado mandated that all licensed dairy farms submit weekly bulk-tank samples for HPAI testing.
Canada has tightened import requirements for U.S. dairy cattle.
Federal and state agencies are testing clinical samples, including unpasteurized milk, nasal swabs, and tissue samples. They are also conducting viral genome sequencing. The USDA and state health officials encourage producers to collaborate with veterinarians on testing efforts.
So far, testing has not shown changes in the virus that would increase its transmissibility to humans. The CDC recommends monitoring individuals exposed to HPAI-infected animals. Anyone developing symptoms should be tested for the H5N1 virus at their local or state public health department.
Impact on Other Animals and Public Health
So far, at least 35 cats have tested positive for the H5N1 avian influenza virus in states where the virus has also been detected in dairy cattle. In these cats, the illness has been severe, exhibiting neurological symptoms, significant oculonasal discharge, and a high mortality rate. This highlights the importance of keeping pets away from wild birds, rodents, and raw milk or colostrum.
The National Veterinary Services Laboratories (NVSL) confirmed the presence of H5N1 in alpacas at a facility where HPAI-infected poultry were depopulated in May 2024. While this detection was anticipated due to prior HPAI findings on the site, it is notable as the first confirmed case of HPAI in alpacas.
Since April 2022, 15 people in the U.S. have been confirmed to have infections with avian influenza virus type A (H5). The initial case involved a Colorado individual engaged in poultry culling. Subsequent cases included dairy farm workers exposed to infected cows in Michigan, Texas, and Colorado. Additionally, nine cases were linked to poultry disposal efforts in Colorado. The symptoms reported by dairy farm workers mainly included eye redness (conjunctivitis) and, in one instance, a cough without fever. The latest case occurred in Missouri, involving someone with no known exposure to sick animals. All affected individuals have recovered.
The CDC is actively monitoring these cases and confirms that they are unrelated, with no signs of person-to-person transmission. Currently, the CDC assesses the risk of HPAI to the general public as low.
Nonetheless, individuals who have close or prolonged unprotected contact with infected animals or their environments face a higher risk of infection.
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