Infectious Diseases 2023

A recent study explored the pathobiology of early U.S. H5N1 Highly Pathogenic Avian Influenza Viruses (HPAIV) belonging to clade 2.3.4.4b in 2021, along with two H5N8 HPAIVs associated with prior outbreaks in the U.S. (clade 2.3.4.4c, 2014) and Europe (clade 2.3.4.4b, 2016) in both chickens and turkeys. Variations in clinical manifestations, mean death times (MDTs), and the transmissibility of the virus were identified between chickens and turkeys. Turkeys, as a general trend, exhibit greater susceptibility to infection from various low pathogenicity and HPAIV isolates sourced from wild waterfowl and poultry, and they tend to transmit the virus more effectively to direct contacts. The study revealed that the 2014 H5N8 virus displayed heightened infectivity and transmissibility among turkeys compared to chickens. While the infectivity of the 2016 H5N8 and 2021 H5N1 viruses was more comparable between chickens and turkeys (refer to the table below), the transmission to contacts remained more efficient in turkeys.

Turkeys infected with the viruses took longer to die than chickens (mean death times of 2.6 to 8.2 days for turkeys and 1 to 4 days for chickens), which prolonged the period during which they shed the virus and increased the likelihood of transmission to other birds.
One of the key differences in how the disease manifested between chickens and turkeys in our study was the longer mean death times observed in infected turkeys compared to chickens. This difference may be due to less severe damage to the vascular endothelium in turkeys than in chickens.
Exceptionally long mean death times have also been reported in turkeys infected with other H5 clade 2.3.4.4 viruses. Similarly to the new study, a shorter mean death time was associated with a higher infecting dose. The delayed death of turkeys while they continue to shed the virus through both the oropharyngeal and cloacal routes increases the probability of direct virus transmission or indirect transmission through the accumulation of environmental contamination. The high susceptibility of turkeys to HPAIV infection, coupled with high viral titers and a prolonged shedding period, would favor the spread of H5N1 HPAIVs in turkeys. Future research will continue to evaluate this lineage of virus in poultry to gain a deeper understanding of the pathobiology of evolving genotypes and their role in virus spread.