Antigenic drift refers to point mutations that occur during virus replication that result in changes in the encoded protein sequences. These
changes can result in an inability of an animal's immune system to recognize the protein, even if the animal was previously
exposed to the parent virus. In this case, the progeny virus is still considered to have the same subtype (e.g. H3N8), but it has changed enough to infect previously immune animals or even animal species that were not susceptible to infection
with the parent virus. Antigenic drift within an avian influenza virus was likely responsible for the 1918 human influenza
Antigenic shift refers to gene segments from one influenza virus recombining with gene segments from another influenza virus. For this to
occur, a single host cell in an animal must be infected by two different strains of virus. After dual infection of the cell
occurs, both viruses begin to replicate, and gene segments from both viruses are packaged in various combinations in virus
particles. This process is called genetic reassortment.2,6 Pigs are susceptible to both avian and human influenza virus infections, so they can serve as intermediate hosts for the
formation of new viruses containing both avian and human influenza genes (Figure 1). Reassortment of avian and human influenza virus genes, and the resulting antigenic shift, accounted for the 1957 and 1968
human influenza pandemics.
Figure 1. Genetic reassortment of human and avian influenza viruses has occurred in pigs and is thought to have caused influenza
pandemics in 1957 and 1968. It is unclear whether cats might also serve as a vessel for reassortment.
A recent news report in Nature stated that a large proportion of the pig population in Indonesia is infected with avian influenza virus H5N1 without signs
of disease.7 Concurrent introduction of a human influenza virus into Indonesian swine could set up the ideal situation for genetic reassortment.
This type of reassortment can occur anywhere in the world. In the United States, recent outbreaks of respiratory disease in
pigs have been caused by influenza viruses formed through genetic reassortments of avian, swine, and human influenza viruses.8
WHAT DOES THIS MEAN TO FELINE PRACTITIONERS?
Historically, cats have been considered to be resistant to influenza virus infections. Cats can be experimentally infected
with human H3N2 virus, but the infection is usually subclinical.9 Although cats live in close contact with people, feline influenza outbreaks have not been recognized. Whether human-to-cat
transmission is possible is unknown, but it clearly has not been associated with disease in the past. During 2003 and 2004,
however, several events occurred that suggested that cats can—and do—become infected with the H5N1 strain of avian influenza
Outbreaks in nondomestic and domestic cats in Thailand
In December 2003, two tigers and two leopards in a zoo in Thailand died after exhibiting signs of fever and respiratory distress.10 The animals had been fed fresh poultry carcasses from a local slaughterhouse. Although the diagnosis had not been made at
the time, chickens in the area were dying of H5N1 influenza virus infections. Postmortem examination of tissues from the tigers
and leopards revealed fibrinous pneumonia, multifocal hemorrhage in internal organs, and encephalitis. Immunohistochemistry,
reverse transcriptase-polymerase chain reaction testing, and virus isolation confirmed a diagnosis of H5N1 avian influenza
virus infection. The virus was virtually identical to the strain that was circulating in chickens at the time of infection.
All the animals had been vaccinated with an attenuated feline panleukopenia virus vaccine two weeks before their illness.
Although there was no evidence of panleukopenia virus-induced disease, immunosuppression associated with the vaccine may have
contributed to the severity of the influenza in these cats.10