By Mary Van Beusekom
Pigs are moderately susceptible to infection with a bovine-derived H5N1 avian influenza virus but don't spread it to other pigs, a non–peer-reviewed study published on the preprint server bioRxiv suggests.
The investigators inoculated nine 4-week-old Yorkshire piglets through the trachea, nose, and mouth with the H5N1 B3.13 virus grown on bovine uterine surface cells. Three other uninfected piglets housed in the same pen served as sentinels.
The researchers collected nasal swabs daily until the end of the study, oropharyngeal swabs on days 1, 3, 5, 7, 10, and 14, and serum for antibody evaluation. After euthanizing the infected piglets on different days, they sampled tissues and fluid in the lungs for virologic and pathologic analysis.
"Pigs are an important reservoir in influenza ecology because they serve as a mixing vessel in which novel reassortant viruses with pandemic potential can be generated," the Kansas State University-led research team wrote.
They noted influenza viruses' ability to cross species barriers: "Since HPAI H5 clade 2.3.4.4b became dominant in wild bird populations, spillovers to mammalian hosts have been frequently reported worldwide, indicating that viruses possessing the 2.3.4.4b HA [hemagglutinin] may have improved ability to replicate in mammals compared to other HPAI lineages."
Little evidence for extensive additional mammalian adaptation
All piglets appeared healthy and remained active throughout the study. Infectious H5N1 virus was isolated from the nasal swabs of one infected pig on day 2 post-inoculation and another pig on days 4 and 5. The oropharyngeal swabs of three pigs were positive for H5N1 on days 1 and 3, but viral concentrations were very low.
Quantitative polymerase chain reaction (PCR) revealed that the nasal or oropharyngeal swabs of seven piglets were positive for viral RNA at least once from days 1 to 5, which the authors said suggests low, intermittent viral shedding. The pulmonary fluid of two pigs showed low levels of infectious virus on day 3, and another two had live virus on day 5. On days 3 and 5, lung fluid samples from the pigs who tested negative were positive for H5N1 and suspected-positive on PCR.
Low concentrations of infectious virus were also isolated from the lungs of four infected pigs on days 3 and 5. One throat sample tested positive for viral RNA in multiple respiratory samples and lymph nodes on days 3 and 5, and one central nervous system sample was positive on day 5. At 14 days, no infectious virus was isolated from the infected pigs.
All swabs and fluid and tissue samples from sentinel pigs were negative for virus, but nasal and oral swabs of one sentinel were suspected positive on PCR. Viral genome sequencing revealed several mutations, including one in H5N1 hemagglutinin previously tied to a likely increased ability to infect mammals in one pig, but the mutation didn't reach consensus levels.
"Taken together these observations suggest that, although some potentially mammalian-adapting mutations arose in some animals, there is little evidence for extensive additional mammalian adaptation of the bovine-derived 349 HPAI H5N1 B3.13 virus in pigs," the researchers wrote.
Sentinel pigs never infected
Macroscopic pulmonary lesions in the infected pigs had sporadic dark red and depressed lobules representing regions of lobular collapse typical of influenza virus A infections on day 3 and, less often, at 5 days.
Microscopic pulmonary lesions were characterized by mild to moderate multifocal non–pus-forming interstitial pneumonia with rare necrotizing bronchitis and/or bronchiolitis affecting multiple lung segments.
Viral antigen was most abundant on day 3 and was present in all remaining infected animals by day 5. By day 7, no neutralizing antibodies could be detected in three infected pigs. Two infected pigs were seropositive by 10 days. At the end of the study, neutralizing antibodies were undetectable in one pig's serum, but the other two remaining infected pigs had neutralizing antibodies against H5N1. No sentinel piglet had detectable antibodies.
"Here, we show that oro-respiratory infection of pigs resulted in productive replication of a bovine-derived HPAI H5N1 B3.13 virus," the study authors wrote. "Infectious viruses were mainly identified in the lower respiratory tract of principal infected pigs, and sero-conversion was observed in most of the principal pigs at later time points."
The sentinel pigs' seronegativity throughout the study indicates a lack of viral transmission, the researchers said. "The results support that pigs are susceptible to a bovine-derived HPAI H5N1 B3.13 virus, but this virus did not replicate as robustly in pigs as mink-derived HPAI H5N1 and swine-adapted influenza viruses," they wrote.
"Given the important role of pigs in IAV [influenza virus] ecology as a mixing vessel for generating the novel reassortant viruses with pandemic potential, enhanced surveillance of pigs is warranted," they concluded.
Source : umn.edu