Study suggests the zoonotic-like avian H3N8 virus has limited efficiency for human-to-human transmission and is unlikely to cause severe disease in humans

In a recent study published in the United States (U.S.) Centers for Disease Control and Prevention’s (CDC) Emerging Infectious Disease researchers demonstrated that the novel and avian hemagglutinin 3 neuraminidase 8 (H3N8) virus showed limited replication in explant cultures of lung and bronchial tissue.

Background

Reassortments between avian influenza viruses from wild birds and domesticated poultry result in viruses that can cross species boundaries sporadically, causing zoonotic infections. The zoonotic influenza viruses H7N9, H5N1, and H10N8 are thought to be viruses with the internal genes of H9N2 with acquired neuraminidase and hemagglutinin from wild birds.

Two cases of H3N8 infections among humans were recently reported from Hunan province and Henan, China, in 2022 — the first was acute respiratory distress syndrome in a four-year-old boy, and the second was mild infection in a five-year-old boy.

Additionally, phylogenetic analyses indicated that novel H3N8 viruses resulted from reassortments comprising the European avian H3 gene, the North American avian N8 gene originating from wild birds and wild-bird avian influenza virus, respectively, and the H9N2 internal genes from avian influenza viruses from poultry in China.

Compared to the human influenza A viruses (H3N2), the antigenic profiles of novel avian H3N8 viruses are significantly different. The human population exhibits very little cross-reactive immunity to the H3N8 viruses.

About the study

In the present study, the researchers isolated the novel H3N8 virus from chickens since the virus had a 99.1% and 98.7% similarity in the hemagglutinin and neuraminidase genes, respectively, with the viruses causing zoonotic infections among humans in China. The avian H3N8 virus was isolated from droppings of wild birds and found to be genetically dissimilar to the zoonotic virus. A few other avian and human influenza viruses were also used in the study.

The viruses were propagated in eggs or Madin-Darby canine kidney (MDCK) cells. The viruses were cultured in ex vivo human lung and bronchial cultures. The viral replication was assessed using real-time quantitative reverse transcription polymerase chain reaction (qRT-PCR) to measure the viral matrix protein segment ribonucleic acid (RNA). Infectious titers of the viruses were measured using the 50% tissue culture infectious dose (TCID₅₀) assay. Influenza A nucleoprotein immunohistochemical staining was also performed on the infected tissue cultures after 48 hours of infection.

Results

The results indicated that in the bronchial tissue cultures, the levels of viral RNA were similar for the novel H3N8, avian H3N8, and H5N1 viruses. The pH1N1 virus had the highest viral RNA levels, while H9N2 had higher viral RNA levels than the avian and novel H3N8 and H5N1 viral strains. The H5N1 virus had the highest viral RNA levels in the human lung tissue culture, followed by the H9N2 strain. The avian H3N8 virus had the lowest viral RNA levels in lung tissue.

The TCID₅₀ assay results indicated similar patterns with the highest infectious viral titers for pH1N1 in bronchial tissue cultures and H5N1 in lung tissue cultures. For H9N2, the infectious viral titers, and viral RNA copies showed a discrepant trend, with high viral RNA copies but low viral titers in bronchial tissue cultures. The avian H3N8 virus had the lowest viral titers in lung tissue.

Furthermore, novel H3N8 virus did not replicate well in the MDCK cells but replicated well in the avian cells, indicating that the virus has not yet successfully adapted to mammalian hosts. This finding was also confirmed by the low replication titers of the novel H3N8 virus in human lung and bronchial tissue cultures.

The immunohistochemical analyses reported that the most extensive levels of antigen distribution were observed for bronchial tissue infected with pH1N1, while bronchial tissue infected with H9N2, H5N1, and novel H3N8 viral strains showed moderate levels of antigen distribution. Tissue cultured infected with avian H3N8 showed no antigen staining.

Similar to the qRT-PCR and TCID₅₀ assay results, the lung tissue immunohistochemistry results showed extensive antigen staining for cells infected with H5N1, followed by lung tissue infected with pH1N1, novel H3N8, and avian H3N8 viral strains.

Conclusions

Overall, the findings indicated that the novel and avian H3N8 viral strains showed low replication in human lung and bronchial tissue cultures compared to other human and avian influenza viruses. Therefore, the ability of these viruses to transmit between humans is limited, and these viral strains are unlikely to cause severe diseases. However, the reassortment between avian influenza viruses among domesticated and wild birds continues to present the risk of potential zoonotic infections.