Research Summary - 4
Evaluating the transmissibility of highly pathogenic avian influenza virus H5N1 following inoculation in dairy bulls
| Date/Time: |
9/13/2025
15:15
|
| Author: |
Zachary K Seekford |
| Clinic: |
Texas A&M University |
| City, State, ZIP: |
Bryan, TX 77807 |
Zachary K. Seekford, PhD
1
;
Ellen Ruth A. Morris, PhD
2
;
Damon J. Smith, BS
1
;
Melissa Kahl-McDonagh, PhD
3
;
Kurt Zuelke, DVM, PhD
3
;
Gabriel Gomez, DVM, PhD, DACVP
2
;
Kiril M. Dimitrov, DVM, PhD
2
;
Ky G. Pohler, PhD
2
;
1Animal Science, Texas A&M University, College Station, TX 77843
2Texas A&M Veterinary Medical Diagnostic Laboratory, College Station, TX 77843
3Global Health Research Complex, Texas A&M University Division of Research, College Station, TX 77843
Introduction:
In late 2021, a Eurasian strain of highly pathogenic avian influenza virus (HPAIV) H5N1 (clade 2.3.4.4b) was detected in North America and has since undergone major reassortment events, notably forming two new genotypes (B3.13 and D1.1) which spilled over into dairy cattle. As of July 2025, the virus has spread to 17 states and affected at least 1,070 farms. Consistent with field surveillance observations, experimental intramammary inoculation of dairy cows with B3.13 results in the development of clinical necrotizing mastitis, histopathological lesions within the mammary gland, dramatic reductions in milk production, and viral shedding via milk. Presently it remains unknown if dairy bulls are susceptible to HPAIV infection, or if infection would result in viral shedding via the reproductive tract.
Materials and methods:
To close this gap in knowledge, we inoculated sexually mature bulls intranasally (n = 2; 759 ± 25.7 kg) and intraprepucially (n = 2; 563.2 ± 193.5 kg) with 106 TCID50 of a virus isolated from an infected dairy cow (B3.13). Control bulls (n = 2; 569.3 ± 8.0 kg) were inoculated with Brain Heat Infusion (BHI) medium alone. Rectal temperatures, sera, nasal swabs, saliva, ocular swabs, prepuce swabs, semen, urine, and feces were collected at selected time points for 15 days post inoculation (dpi). At the conclusion of the 15-day sample collection period, bulls were sacrificed, and comprehensive necropsies were conducted collecting more than 50 tissues for histopathological and RT-PCR analysis. Viral detection was determined by RT-PCR and virus isolation. Influenza A nucleoprotein antibody identification was conducted using commercially available NP-ELISAs (IDEXX and IDVet).
Results:
No differences in rectal temperature were observed. Virus isolation performed on the collected samples indicated that dairy bulls did not shed infectious virus for the duration of the study. Traces of virus were detected by real-time RT-PCR in the nares ipsilateral to inoculation shortly after inoculation (1 and 2 dpi; Ct values >30). Bulls inoculated intranasally seroconverted by 14 dpi as shown by NP-ELISA testing, while neither intraprepucially nor control inoculated bulls had quantifiable antibodies by ELISA. Results from hemagglutination inhibition and virus neutralization assays and necropsy histopathology and tissue RT-PCR are pending.
Significance:
In conclusion, preliminary evidence suggests that neither intranasal nor intraprepucial inoculation results in the development of clinical symptoms or viral shedding in dairy bulls. Collectively, our findings suggest that the development of clinical symptoms observed in lactating dairy cows may be related to the unique viral tropism to dairy cows’ mammary gland.
