Domestic cats in Thailand have also been infected with the H5N1 virus.11,12 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.
Influenza in greyhounds: A cautionary tale
A second outbreak of H5N1 avian influenza in nondomestic cats occurred in October 2004 in a tiger zoo in Thailand.13 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.
Experimental inoculation of domestic cats
In addition to these outbreaks, researchers have demonstrated the susceptibility of domestic cats to H5N1 avian influenza
virus.14 Unlike in previous studies with other influenza virus strains, cats developed severe respiratory disease after exposure to
H5N1 virus by ingestion, intratracheal inoculation, or contact with experimentally infected cats. Intratracheal inoculation
of three cats resulted in fever beginning Day 1 after exposure and in decreased activity, conjunctivitis, and labored breathing
by Day 2 after exposure. One cat died on Day 6. Pathologic findings included focal pulmonary consolidation and diffuse alveolar
damage. Two cats housed with these infected cats and three cats fed virus-infected chicks also developed similar clinical
signs and pathology. Infection with H5N1 avian influenza virus was confirmed in all these cats by virus isolation from pharyngeal
swabs and by immunohistochemistry on lung tissue.
THE VETERINARIAN'S ROLE IN INFLUENZA SURVEILLANCE
The H5N1 outbreaks in domestic and nondomestic cats in Thailand emphasize the potential of this virus as a feline pathogen.
Although poultry and wild ducks are the primary reservoir of the virus, several species of songbirds are also susceptible
to infection.15 These bird populations could serve as a conduit of human infection through feline intermediaries. With each genetic reassortment
of the influenza viruses, species barriers become less effective (Figure 2). Many influenza viruses can no longer legitimately be categorized as equine, avian, human, or swine viruses. It seems reasonable
for veterinarians to consider including influenza as a differential diagnosis in cats with respiratory and neurologic disease.
Prompt recognition of avian influenza in any susceptible population of animals will help control its spread and decrease the
chance of another devastating influenza pandemic.
Figure 2. Genetic reassortment of influenza viruses may lead to additional species being involved in transmission.