Potential First Known Case of Mammal-to-Human Bird Flu Transmission Detailed

Strong genetic evidence makes it highly likely that a dairy farm worker in Texas contracted avian influenza A (H5N1) from cattle, according to a detailed case report from the CDC and Texas Department of State Health Services.

Genome sequencing from a conjunctival swab taken from the worker revealed that all gene segments of the virus were closely related to the H5N1 viruses detected in dairy cattle and wild birds in Texas in March, reported Timothy Uyeki, MD, MPH, of the CDC, and colleagues in a brief correspondence in the New England Journal of Medicine.

"Our findings are suggestive of human infection occurring in the dairy farm setting and cow-to-human transmission of HPAI [highly pathogenic avian influenza] A (H5N1) virus from presumptively infected cows to an exposed dairy farm worker," the authors wrote in an accompanying supplementary report.

The case of H5N1 in the dairy farm worker, whose only symptom was conjunctivitis, was first announced on April 1.

"If cow-to-human transmission occurred, to our knowledge, this would be the first reported case of mammal-to-human HPAI A (H5N1) virus transmission reported worldwide," Uyeki and colleagues noted.

"However, we cannot exclude fomite transmission because no specimens from cows or environmental samples were available from the worker's farm for testing," they cautioned.

In addition to the lack of specimens from cows and the worker's farm environment, the researchers were not able to perform assessments of other potential worker exposures at the dairy farm, nor were they able to collect serum samples to assess for seroconversion in the worker or household contacts. Additional conjunctival and nasopharyngeal swab specimens from the worker to assess viral levels or duration of viral shedding were also not available, the authors pointed out.

Since 2020, H5N1 clade 2.3.4.4b subtype viruses have spread widely among wild birds worldwide, resulting in outbreaks in poultry and other animals. Recently, the clade 2.3.4.4b viruses were identified in dairy cows and in unpasteurized milk in multiple U.S. states, as well as in cats.

The adult farm worker initially presented only with mild symptoms of conjunctivitis in the right eye. The worker had no changes in vision or systemic signs or symptoms, and also reported no contact with any animals, including birds, except dairy cows. Some of those cows were showing signs of illness similar to other cows in the same region of Texas infected with H5N1. Of note, conjunctivitis in humans from H5N1 infection has rarely been documented.

The worker reported wearing gloves but no other protective equipment when working with the cows. The worker could have contracted conjunctivitis through direct contact with hands contaminated with cow respiratory secretions, milk, or fomites or via respiratory droplets or aerosols from sick cows or the environment, the authors speculated.

Conjunctival and nasopharyngeal swabs were taken from the worker's right eye, and initial testing of the samples were performed at a CDC laboratory within the Texas Tech Bioterrorism Response Laboratory. Real-time reverse transcription polymerase chain reaction (RT-PCR) testing suggested infection with influenza A and A(H5). Samples were then sent to the CDC for confirmatory testing.

The worker was told to isolate at home and was treated with oral oseltamivir (Tamiflu) 75 mg twice a day for 5 days. Members of the worker's household were also treated with oseltamivir for postexposure prophylaxis. The worker's symptoms resolved rapidly over a period of several days.

While acute conjunctivitis is a clinically mild illness, H5N1 viruses, including those belonging to clade 2.3.4.4b, pose pandemic potential and have caused severe respiratory disease in humans.

The H5N1 virus isolated from the worker's specimen had a change in the genome associated with adaption that is known to occur in mammals infected with avian influenza A viruses, including H5N1. However, the viral sequences from the worker and from cattle in Texas "maintained primarily avian genetic characteristics," the authors wrote. No changes were observed in the hemagglutinin gene that would increase receptor-binding specificity in the human upper respiratory tract and increase risk of transmission to humans, Uyeki and colleagues noted.

There were much higher levels of viral RNA detected in the worker's conjunctival swab than the nasal swab, they wrote, which "could reflect differences in viral replication in the conjunctivae and nasopharynx."

Additionally, the researchers found no genetic evidence of reduced susceptibility to currently available influenza antiviral drugs.

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