A study recently published in the journal Emerging infectious diseases investigated influenza infections in cattle on a dairy farm in Texas, United States of America (USA).

Since the avian-originated influenza virus of clade 2.3.4.4b (H5N1) arrived in the northern parts of the Americas in late 2021, there have been several outbreaks of mammals of various species, including humans. Nevertheless, the virus strains have not harmed dairy cattle. Cattle are tolerant but resistant to infection with influenza A (IAV), influenza B, and influenza C; however, they are susceptible to infection with influenza D virus. Influenza D viruses are believed to be transmitted between cows by direct contact or aerosol transmission over short distances via the respiratory tract, and potential influenza D virus transmission to humans is of concern. However, veterinarians have investigated several pathogens associated with respiratory episodes in livestock.

Study: Avian influenza virus A(H5N1) in dairy cattle, Texas, USA. Image credit: McKenzie Kizer / Shutterstock

About the study

In the present study, researchers examined dairy cattle samples from a Texas farm from March to April 2024 using various pathogen identification methods.

The researchers conducted molecular screening to identify the causative agent of the cattle diseases. In some cases, they performed cell culture experiments and next-generation sequencing (NGS) on bovine swab biosamples. They targeted six groups of viruses: coronaviruses, adenoviruses, influenza viruses, enteroviruses, pneumoviruses and paramyxoviruses.

On March 21, the dairy farm's care team collected nasal swab samples from 14 cows with symptoms of illness and six cows without symptoms of illness and shipped them in appropriate containers. The cattle exhibited symptoms such as lethargy, decreased appetite, increased respiratory secretions, elevated temperatures (up to 40.6 °C or 105 °F), abnormal stools and decreased milk production. There were no dead cats, birds or other dead wildlife. At the time of sample collection, cattle disease was declining.

The researchers analyzed the swab samples and filled out questionnaires the next day. On March 28, they requested more milk samples to determine whether the pathogen was enteric in nature. On April 1, they took rectal and nasal swabs from ten more symptomatic cows; the researchers received them two days later.

The researchers took 40 samples from 30 cows, of which 24 were sick and six were healthy. The age of the cattle examined ranged from 2.3 to 7.10 years. Farm staff began observing illnesses in the cattle on March 6. Between March 10 and 12, more than 4.8% of the cows showed clinical flu-like symptoms and were hospitalized for treatment.

Several workers also developed flu-like symptoms and were unable to work between March 4 and 6. However, there were no hospitalizations, conjunctivitis, or serious illnesses among workers. One maternity ward worker was treated on-site for flu-like symptoms; two milkmaids also developed flu-like symptoms and stayed home. The researchers conducted phylogenetic analyses to compare related viral infections using data from the Global Initiative on Sharing All Influenza Data (GISAID) and GenBank, as well as eight viral genetic segments.

Results

Several bovine swab samples showed molecular findings of H5 IAV infection. None of the first 20 bovine samples showed the presence of coronaviruses, adenoviruses, influenza D viruses or enteroviruses. Among the samples, three (one sick cow and two healthy cows) showed molecular findings suggestive of viral infection with Pneumoviridae or Paramyxoviridae.

Several swabs in Vero E6, MDCK and MDBK also showed molecular evidence of H5 IAV infection. Molecular analysis of the HA cleavage region indicated high virus pathogenicity. The results were confirmed by next-generation sequencing (NGS); a cell culture sample obtained from the nasal cavity of cattle yielded a complete (H5N1) genome, which was confirmed as HPAI of clade 2.3.4.4b.

Phylogenetic analysis showed that the virus's eight genetic segments were similar to 13 viruses from the Texas epizootic group. The H5N1 strain had several novel mutations compared to related viral pathogens. The PB2-M631L mutation enhances the ability of the H5N1 virus to replicate in humans by increasing polymerase enzyme activity. NGS findings showed that nasal swabs from the sick cow tested positive for Pneumoviridae or Paramyxoviridae containing bovine viral diarrhea virus (BVDV), a likely cause of disease.

Conclusions

The study identifies the H5N1 clade 2.3.4.4b virus as the etiologic agent of the Texas cattle epidemic. Pathogenicity studies in animal models could improve our understanding of such viruses. The researchers detected H5N1 in symptomatic cows without other viral co-infections. The higher HPAI-H5N1 prevalence in nasal swabs than in rectal swabs suggested respiratory transmission of H5N1 in cows.

The NGS results showed the presence of BVDV in nasal swabs from one sick cow and two healthy cows. BVDVs commonly cause mild respiratory infections on cattle farms; therefore, their presence cannot necessarily explain illness in dairy cows. The H5N1 genome detected in the nasal samples from one cow is similar to strains from cattle, a cattle worker, and deceased birds, suggesting a multispecies epidemic in Texas. Further research is needed to determine transmission routes, which requires interdisciplinary collaboration and a One Health strategy.