In a recent study published in the journal Nature, a group of researchers characterized the pathogenicity and transmissibility of a highly pathogenic avian influenza (HPAI H5N1) (a severe, contagious bird flu that can infect humans and mammals) virus from cow milk in mammals.
Study: Pathogenicity and transmissibility of bovine H5N1 influenza virus. Image Credit: Toa55 / Shutterstock
Background
After unexplained symptoms and reduced milk production in dairy cattle in Texas, HPAI H5N1 was detected in cow milk and nasal wash samples on March 25, 2024. By May 30, 2024, the United States Department of Agriculture (USDA) confirmed 69 infected herds in nine states, with spread linked to cattle movement and potential transmission through contaminated milking equipment. This outbreak, alongside cases in farm workers and cats, underscores the public health risk. The bovine H5N1 virus is closely related to North American wild bird strains. Further research is necessary to understand its replication, pathogenicity, and transmission in mammals.
About the study
In the present study, animals were acclimated to the facilities before the start of the experiments, maintained on a 12-hour light cycle, and provided with food, water, and enrichment. Humane endpoint criteria included ≥35% body weight loss or inability to remain upright. In the United States (US), HPAI viruses are classified as 'Select Agents' under federal regulations, requiring immediate reporting to the Federal Select Agent Program. All experiments were conducted in Biosafety Level 3 (BSL-3) laboratories at the Influenza Research Institute at the University of Wisconsin-Madison, approved for studies with these viruses. The research received funding from the National Institute of Allergy and Infectious Diseases (NIAID) Centers of Excellence for Influenza Research and Response (CEIRR). All animal experiments and procedures were approved by the Institutional Care and Use Committees of the University of Wisconsin-Madison School of Veterinary Medicine.
Madin-Darby Canine Kidney (MDCK) cells were used for virus isolation and amplification, and no mutations emerged during passage in these cells. The isolated virus, A/dairy cattle/New Mexico/A240920343-93/2024 (H5N1), was fully sequenced and found to be part of the same clade as other publicly available cow H5N1 virus sequences. This virus and control viruses were used for all studies. Mice and ferrets were used to test replication, pathogenicity, and respiratory droplet transmission. Lactating mice were also used to study vertical transmission. Tissue samples were collected for virus titration and plaque assays, while receptor specificity was compared among different virus strains. The studies were reviewed and approved by relevant institutional committees, and the NIAID grant for the research was determined not to meet the criteria of Dual Use Research of Concern (DURC).
Study results
To evaluate the public health risk of H5N1 virus-containing milk, researchers demonstrated that oral consumption of milk from an HPAI H5N1-infected cow led to rapid disease symptoms and virus dissemination in Bagg Albino Laboratory-Bred Mouse/C strain J (BALB/cJ) mice. Repeating this experiment with smaller volumes of milk, they observed substantial weight loss and virus titers in the lung, nasal turbinate, and brain in mice inoculated with 25 μl or 10 μl of infected milk. In contrast, mice inoculated with 25 μl of milk from a healthy cow showed no symptoms. Mice inoculated with smaller volumes (5 μl and 1 μl) exhibited less apparent disease and sporadic virus replication. No seroconversion was observed in surviving mice.
To assess pathogenicity after respiratory exposure, female BALB/cJ mice were inoculated with serial dilutions of Cow-H5N1, monitoring body weight and survival. The mouse lethal dose 50 (MLD50) was 31.6 PFU, comparable to clade 2.3.4.4b HPAI H5N1 mink viruses from Spain, but higher than that of A/Vietnam/1203/2004 (VN1203)-H5N1, an avian H5N1 virus. Tissue tropism studies revealed systemic infections with Cow-H5N1 and VN1203-H5N1, with high virus titers in respiratory and non-respiratory organs, including mammary glands, teats, and muscle tissues. The Isumi-H1N1 virus was detected only in respiratory tissues. Infected ferrets exhibited similar virus replication patterns, with high titers in respiratory and non-respiratory organs, but no virus was detected in blood or muscle tissues.
Next, lactating mice were tested for vertical transmission of Cow-H5N1 to pups or adult contacts. Lactating females were inoculated and either reunited with their pups or placed with non-lactating adults. Virus replication was observed in lactating females, and some pups became infected, but no virus was detected in adult contacts. The virus was detected in the mammary glands and milk of some lactating females, suggesting vertical transmission via milk.
Finally, a respiratory droplet transmission experiment in ferrets showed that while Isumi-H1N1 transmitted efficiently, Cow-H5N1 did not. However, one exposed ferret had a low positive hemagglutination inhibition (HI) titer, indicating possible inefficient transmission. Receptor binding specificity studies revealed that Cow-H5N1 bound to both α2,3- and α2,6-linked sialic acids, unlike the avian VN1203-H5N1 virus, suggesting the potential for Cow-H5N1 to bind to cells in the human upper respiratory tract.
Conclusions
To summarize, HPAI H5N1 influenza viruses do not typically transmit efficiently among mammals, and influenza A viruses are rarely detected in cattle. However, the current HPAI H5N1 outbreak in dairy cows and spillover into other mammals poses significant risks to public health and the dairy industry. Despite over 850 human infections, sustained mammal-to-mammal transmission has not been reported, though recent outbreaks in mink and sea mammals suggest it is possible. In this study, a bovine HPAI H5N1 virus exhibited limited respiratory droplet transmission in ferrets, supporting findings of potential mammal-to-mammal transmission. The virus also displayed dual human/avian-type receptor-binding specificity, highlighting the pandemic potential of these viruses.
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