Margaret C. Barr, DVM, PhD
College of Veterinary Medicine
Western University of Health Sciences
Pomona, CA 91766

Editors' note: This is an excerpt of a peer-reviewed article that originally ran in Veterinary Medicine's October 2005 issue. It has been updated to include information about the continued spread of avian influenza in cats.

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.¹ 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. However, we now know that cats can—and do—become infected with the H5N1 strain of avian influenza virus.

In December 2003, two tigers and two leopards in a zoo in Thailand died after exhibiting signs of fever and respiratory distress.² 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.²

Domestic cats in Thailand have also been infected with the H5N1 virus.^3,4 In February 2004, a group of 15 cats living near an affected poultry farm became ill. According to news reports, three of the cats had been tested for avian influenza by researchers at Thailand's Kasetsart University, with two cats having positive results at that time. Fourteen of the 15 cats had died, and the last cat was very ill. The method of influenza transmission in these cats was unknown. During the same month, another cat in Thailand became ill five days after eating a pigeon carcass.⁵ The cat was initially febrile and depressed and then developed convulsions and ataxia before it died two days after the onset of illness. H5N1 influenza virus antigen was detected by immunohistochemistry in brain, heart, lung, liver, kidney, and spleen samples from this cat. Virus isolates from the cat were almost identical to isolates from infected pigeons and chickens in the same region.

A second outbreak of H5N1 avian influenza in nondomestic cats occurred in October 2004 in a tiger zoo in Thailand.⁶ Initially, the cats likely became infected by ingesting raw infected bird carcasses. Tigers infected later in the outbreak were probably infected by cat-to-cat transmission because they were fed cooked poultry beginning a few days after the first tigers became ill. Clinical signs in affected tigers included respiratory distress, a serosanguineous nasal discharge, neurologic signs, and high fever. Leukopenia, thrombocytopenia, and elevated liver enzyme activities were common laboratory findings. Most of the animals had severe lung congestion and hemorrhage. Infection with H5N1 avian influenza virus was confirmed in several animals by using immunohistochemistry, virus isolation, or both techniques. Twenty-nine tigers died during the first week of the outbreak, and a total of 147 of the zoo's 441 tigers either died or were euthanized during the three weeks after the first tigers became ill.

In February 2006, a domestic cat in Germany was infected with H5N1 avian influenza virus, probably by ingesting an infected wild bird. This case was quickly followed by the detection of three H5N1-positive cats in Austria.⁷ Several H5N1-infected swans were found in the same regions as the infected cats. In response, the European Standing Committee on the Food Chain and Animal Health (SCFCAH) issued recommendations for veterinarians and pet owners. The statement recommended inspecting and testing sick or dead cats that might have had contact with wild birds, reporting suspected cases to appropriate authorities, and preventing contact between pet cats and wild birds. It also recommended the same procedures for dogs. The SCFCAH report emphasized that the risk of avian influenza virus infections in cats remains low, and the risk of cat-to-human transmission is even lower at this time.⁸