H5N1 influenza outbreak in Texas dairy cattle sparks multi-species transmission concerns

A recent study published in the journal Emerging Infectious Diseases examined influenza infections among cattle on a dairy farm in Texas, United States of America (USA).

Since the 2.3.4.4b clade bird-origin influenza A/cattle/Texas/56283/2024 (H5N1) virus arrived in the northern parts of America by late 2021, several spillover incidents among mammals of diverse species, including humans, have occurred. Nonetheless, the viral strains did not harm dairy livestock. Cattle are permissive but resistant to infections by influenza A (IAV), influenza B, and influenza C viruses; nevertheless, they are vulnerable to infections by the influenza D virus. Influenza D viruses are assumed to transfer between cows by direct touch or aerosol transmission over brief distances via the respiratory route, and potential influenza D viral spillovers to humans are alarming. However, veterinarians have investigated different pathogens associated with livestock respiratory epizootics.

Study: Avian Influenza A(H5N1) Virus among Dairy Cattle, Texas, USA. Image Credit: McKenzie Kizer / Shutterstock

About the study

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

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

On March 21, the dairy farm care team collected nasal swab biospecimens from 14 cows presenting with disease signs and six without signs and shipped them in suitable containers. Cattle presented with signs such as lethargy, reduced appetite, increased respiratory secretions, elevated temperatures (up to 40.6°C or 105°F), abnormal bowel motion, and reduced milk production. There were no deceased cats, birds, or other dead wildlife. Cattle diseases were on the decline at the time of collecting specimens.

The researchers analyzed the swab samples and filled out questionnaires the following day. On March 28, they requested additional dairy specimens to determine whether the etiological agent was enteric. On April 1, they obtained rectal and nasal swabs from 10 other symptomatic cows; the researchers received them two days later.

The researchers obtained 40 specimens from 30 cows, of which 24 were sick while six were healthy. The age range of the sampled cattle was 2.3 to 7.10 years. Farm staff observed cattle illnesses from March 6 onward. Between March 10 and 12, more than 4.8% of cows presented with clinical influenza-like symptoms and were hospitalized for treatment.

Several workers also presented with influenza-like signs and could not work between March 4 and 6. However, there were no hospitalizations, conjunctivitis, or severe illness among workers. A maternity care worker received local care for influenza-resembling signs; two milkers also developed influenza-like signs and remained at home. The researchers performed phylogenetic analyses to compare related viral infections using the Global Initiative on Sharing All Influenza Data (GISAID) and GenBank data and eight viral genetic segments.

Results

Multiple cattle swabs showed molecular findings of H5 IAV infection. None of the initial 20 cattle samples showed coronavirus, adenovirus, influenza D virus, or enterovirus presence. Among the specimens, three (one sick cow and two healthy cows) exhibited molecular findings indicative of a Pneumoviridae or Paramyxoviridae viral infection.

Multiple swabs in Vero E6, MDCK, and MDBK also showed molecular findings of H5 IAV infection. Molecular HA cleavage region analysis indicated high viral pathogenicity. Next-generation sequencing (NGS) supported the results; one cell-cultured specimen obtained from the nasal cavity of cattle provided a complete (H5N1) genome, confirmed as HPAI of the 2.3.4.4b clade.

Phylogenetic analyses showed that the eight genetic segments of the virus resembled 13 viruses of the epizootic clade in Texas. The H5N1 strain possessed multiple novel mutations compared to related viral pathogens. The PB2-M631L mutation enhances the H5N1 viral ability to proliferate in humans through increased polymerase enzyme activity. NGS findings indicated that the diseased cow with nasal swabs tested positive for Pneumoviridae or Paramyxoviridae, which contained bovine viral diarrhea virus (BVDV), a probable illness cause.

Conclusions

The study identifies the H5N1 2.3.4.4b clade virus as the etiological agent for the Texas cattle epidemic. Pathogenicity investigations 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 indicated respiratory transmission of H5N1 among cows.

The NGS results revealed the presence of BVDV in nasal swabs from one ill cow and two healthy cows. BVDVs frequently cause minor respiratory infections in cattle ranches; hence, their presence may not explain dairy cow sickness. The H5N1 genome discovered in a cow’s nasal samples is similar to strains from cattle, a livestock worker, and deceased birds, indicating a multi-species epidemic in Texas. Further research is required to establish transmission pathways, requiring interdisciplinary collaboration and a One Health strategy.