1.3 Influenza infection in other species

Influenza viruses infect a wide range of species, including pigs, horses, ducks, chickens and seals. In most of these other species the virus produces an acute infection.

For example, in most of the mammals the symptoms are very similar to those in humans: an acute infection of the respiratory tract, which is controlled by the immune response although fatal infections occur in some species. However, in wild ducks and other aquatic birds the virus primarily infects the gut and the birds do not appear to have any physical symptoms.

Despite this, ducks may remain infected for 2–4 weeks and during this time they shed virus in their faeces. Potentially this is a very important reservoir of infection; although flu viruses do not often cross the species barrier, the pool of viruses present in other species is an important genetic reservoir for the generation of new flu viruses that do infect humans.

This reservoir becomes particularly important in certain farming communities or in crowded conditions where animals (especially pigs and ducks) are continuously in close proximity with humans (Branswell, 2010). Although such conditions occur in many agricultural communities throughout the world, they are typically observed in South-East and East Asia thereby contributing to these geographical areas often being the source of radically new ‘hybrid’ strains of influenza that incorporate genes from different species-specific strains. (The genetics of influenza are discussed in Section 2.3.)

When strategies for controlling a disease are considered, awareness of the possible presence of an animal reservoir of infection is very important. For example, an immunisation programme against flu would substantially reduce the incidence of the current strain in humans but, because there is always a reservoir of these viruses in other animals, and these viruses are constantly mutating, another strain would inevitably emerge and be unaffected by immunisation. It is useful to distinguish diseases such as rabies, which primarily affect other vertebrates and occasionally infect humans (zoonoses), from diseases such as flu where different strains of the virus can affect several species including humans.