Scientists at the University of Texas Medical Branch (UTMB) in the United States have detected the highly pathogenic H5N1 avian influenza virus (HPAIV) in cattle and milk samples collected from two Texas dairy farms.

The study, currently available on the medRxiv preprint server*, highlights the need to develop effective interventions against H5N1 HPAIV to prevent the spread of the pathogen.

Crédit photo : McKenzie Kizer / Shutterstock

Background

*Important Notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Highly pathogenic avian influenza A viruses (HPAIV) H5N1 have caused millions of deaths in wild birds and poultry worldwide. Recently, these viruses have been identified in many animal species, with HPAIV H5N1 HA clade 2.3.3.4b being the most predominant type in these spillover events.

The U.S. Department of Agriculture recently detected H5N1 HPAIV infection on 172 cattle farms in 13 states. Four dairy farm workers and nine poultry farm workers are also suspected of being infected with the virus.

Given the seriousness of the situation, the current study was designed to investigate the presence of novel respiratory viruses on two Texas dairy farms.

Study design

The study was conducted on two Texas dairy farms, where H5N1 HPAIV infection had recently been detected in cattle. The farm owners invited the scientists to conduct the investigation because they had a research project to study new respiratory viruses on livestock farms in the United States and Mexico.

A total of 17 farm workers were recruited for the study. They provided nasopharyngeal and serum samples for analysis. Five of the 17 workers reported suffering from respiratory diseases and using different medications to treat this condition.

Across both farms, a total of 39 bovine nasal swab samples and 14 milk samples were collected for molecular analyses, cell and egg cultures, and Sanger and next-generation sequencing to isolate and characterize respiratory viruses.

Three-hour bioaerosol samples were also collected from different agricultural regions to study the presence of novel viruses in the environment.

Observations importantes

The study detected H5N1 HPAIV in nine of 14 milk samples and one of 39 nasal swab samples from cattle. However, none of the nasopharyngeal swabs collected from farm workers tested positive for influenza A viruses and coronaviruses.

A nasal swab taken from a sick cow showed the presence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Among 14 serum samples collected from farm workers, two showed high levels of neutralizing antibodies against a recombinant influenza A H5N1 virus.

Among the 24 bioaerosol samples collected from the two farms, none showed molecular evidence of influenza A and coronavirus.

H5N1-positive milk samples were further analyzed using Sanger sequencing and next-generation sequencing for viral characterization. Results revealed that the viruses are closely related to other recent epizootic H5N1 strains from Texas belonging to clade 2.3.4.4b.

Mutational analysis revealed the presence of multiple common mutations in the isolated viral genomes. These mutations are associated with viral virulence and spread, host specificity switching, and drug resistance.

Importance of the study

The study found evidence of H5N1 HPAIV infections in cattle from two Texas dairy farms. Although the virus was detected in only one nasal swab sample from cattle, approximately 64% of milk samples tested positive for H5N1 HPAIV.

These results indicate that even though the virus is rapidly cleared from nasal tissues, infected cattle can shed H5N1 HPAIV into milk for an extended period.

More importantly, the detection of neutralizing antibodies to HPAIV in farmworkers highlights the possibility of previous infections that remained undetected during the acute infection phase.

This highlights the possibility of underreporting of HPAIV infections among farmworkers, which can then lead to large outbreaks even outside farmworker communities.

“I’m convinced there are more people infected than we know,” lead author Gregory Gray, MD, MPH, an infectious disease researcher at UTMB, told NPR. “That’s largely because our surveillance is very poor.”

The U.S. Centers for Disease Control and Prevention (CDC) decides whether to vaccinate agricultural workers against HPAIV based on surveillance data, which indicate a low prevalence of HPAIV infection.

This low prevalence is due to low screening rates among farm workers. Many infections may remain undiagnosed in people who have been exposed to sick dairy cows.

“What we’re seeing may not be exactly the tip of the iceberg, but it’s certainly not the whole story,” said Richard Webby, director of the World Health Organization Collaborating Center for Influenza at St. Jude Children’s Research Hospital in Memphis, Tennessee.

Studies to identify new respiratory viruses are rarely conducted on dairy farms due to fear of business losses. As Gregory Gray noted, without collaborative efforts between government and the livestock industry, the risks of this virus in the United States will remain unknown.

This highlights the need to collect more comprehensive epidemiological data, which are necessary for designing future interventions against H5N1 HPAIV in cattle herds.

*Important Notice: medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.