While it is known that influenza viruses can frequently infect a variety of animal species, including humans (for a review, see ref., it is relatively uncommon for companion animals (e.g. pet cats and dogs) to be infected by the highly pathogenic avian influenza (HPAI) H5N1 virus. This article summarizes our current understanding of HPAI H5N1 infections in pets with the aim of raising awareness about these emerging and re-emerging infections in domestic animals.

Influenza viruses belong to the Orthomyxoviridae family and consist of four major types: influenza A, B, C, and D. Influenza A virus (IAV) can infect a wide range of avian and mammalian species, including some avian species (e.g. birds, ducks, chickens, turkeys), swine (pigs), equine (horses), and canine (dogs) species, besides humans. Influenza B virus (IBV) is known to infect both humans and seals, while influenza C virus (ICV) can infect humans and pigs. Additionally, recent evidence suggests that swine, cattle, and potentially humans can also be infected by influenza D virus (IDV). However, the most significant human IAVs can cause annual (seasonal) epidemics and occasional outbreaks. These are new human IAV strains that can arise from an animal origin [via zoonotic transmission event(s)] and then spread rapidly among human populations with no pre-existing immunity against these new strains, which can result in excessive mortality and morbidity. There are four influenza pandemics in modern times, including the 1918–1919 Spanish flu, 1957 Asia flu, 1968 Hong Kong flu, and 2009 Swine flu (Figure 1). For example, the 1918–1919 Spanish flu caused the most damage, with an estimated 500 million infections and 50–100 million deaths worldwide.