Letting the H5N1 cat out of the bag

A little noticed paper in CDC's Emerging Infectious Diseases takes us back to a year ago when dead and dying birds infected with H5N1 were first found near water in Germany, Slovenia and Austria. In mid February 2006 a sick swan picked up from a river in Austria was taken to an animal shelter in Graz. It died a day later. While there it infected 13 of 38 other birds (swans, ducks and chickens), detected 3 days later. That day the poultry area of the shelter was disinfected after the birds were removed.

But the animal shelter had more than birds:

In the same shelter were 194 cats; most had access to an outdoor enclosure near the poultry area and were separated from the birds by a wire-mesh fence. On several occasions, 1 or 2 unidentified cats were observed climbing the fence and entering the poultry area. Ingestion of birds by cats was not observed. Austrian authorities ordered random sampling of the cat population at the shelter because of spatial proximity of poultry and cats and the possible exposure of cats to infective debris of the birds. The bird area was left unoccupied while the cats were under observation. The purpose of this study was to monitor health status and possible transmission within a large cat population with proven natural exposure to H5N1 influenza virus. (Leschnik et al., Emerging Infectious Diseases)

Eight days after the swan arrived at the shelter throat swabs were taken on 40 cats. Three were positive for H5N1 by PCR (which detects the presence of H5N1 specific genetic sequences). A week later, none of the 40 cats had positive throat swabs. Three cats that died in the interim were autopsied and no evidence of respiratory disease detected (cat deaths in the shelter were ot unusual at any time). All three were PCR negative. A week after that (three weeks after the index case's arrival) 167 cats were still available for examination, but 24 cats had been adopted and sent to private households. They had been examined on discharge and within a week afterwards. On those two exams, all appeared healthy. The remaining cats were sent to a quarantine area a day's travel from the shelter and observed, in two separate groups, until day 50.

Two cats developed antibodies to H5N1. One was a virus positive cat, the other two were PCR negative cats. Thus four cats of the forty tested for virus showed some signs of infection but no signs of any illness. Two of these cats were euthanized and autopsied along with 12 others that died in the observation period. No signs of influenza infection or virus in any of the internal organs were found. All the personnel at the shelter and the quarantine area were "clinically monitoried for any influenzalike symptoms," but "results of this monitoring were unremarkable and virus excretion by the cats was not detected. . . ."

What to make of this paper? As the authors note, it is the first description of asymptomatic infection with high path H5N1 in domestic cats. Other studies (here and here) under laboratory conditions, have showed cat to cat transmission and rapidly developing fatal disease. While the rationale for not doing serology on the staff was that there was no evidence of viral excretion, there was evidence of viral shedding in the first two weeks as shown by positive PCR throat swabs in three cats. The lack of antibody development in other cats was interpreted as lack of horizontal (cat to cat) transmission, although two cats did develop antibodies that had negative PCR initially. The cat with positive virus and no seroconversion was possibly a false positive from a contaminated sample, at least according to the authors, but we know little about seroconversion in cats after infection.

The difference between this naturally acquired infection and the laboratory studies remains to be explained. The authors suggest either strain differences or the size of the infecting viral load may be the reason. It should be noted these cats were on average less healthy than the laboratory studied cats, with many in the original group of 194 suffering from immunodeficiency states and most with concurrent infections with other viruses. The authors suggest this would make them more vulnerable, but we can also see this as a difference that might work against H5N1 infection. This is all speculation, however.

How did the cats (whether 3 or 4) become infected? The authors believe it was from contaminated fecal material from the bird pen deposited on the cats' fur and ingested through cat self-grooming, but they could not rule out aerosolization of the virus carried to the cats via the air. Differences between Europe and Asia were also noted:

Until recently, the avian flu situations in Asia and Europe appeared to differ. In Asia, large numbers of poultry have been infected and culled. Human and feline cases are mainly associated with close contact with infected poultry or ingestion of contaminated meat that was not sufficiently cooked. In Europe, mainly wild aquatic birds were infected, and only a few turkey farms were affected by H5N1 infection. Because direct contact with poultry is more limited in Europe than in Asian countries and the main source of food for cats in Europe is either commercial cat food or wild rodents and small birds, virus uptake during hunting and ingestion of poultry and aquatic birds is unlikely. Large aquatic birds are normally not a major source of food for cats, although infected birds may have caused the deaths of 3 cats found on the island of Ruegen, Germany.

This paper provides us with more data points although not more clarity. Sometimes cats acquire fatal disease with high viral shedding. Sometimes, as here, they are infected without clinical symptoms and apparently little viral shedding. Whatever the true current situation with cats and H5N1 infection, this species is not different than any other in one important respect (including us). Things can change. As the virus changes the role of infection in domestic cats can change along with it. We know they can become infected and they are mammals. If the disease becomes more transmissible in humans, it would be wise to keep an eye on the role of companion animals as reservoirs or vectors. Since the principal vector would still be other people, this may not be a significant concern. But it's something to keep in the back of our minds.

Finally, from the outbreak perspective we see both alert authorities who took advantage of a unique event to acquire valuable information, but also a case where an infected animal introduced into an animal shelter infects other animals, some of whom are then sent out into the human population as adopted cats. This time we seem to have dodged a bullet. But this kind of event is bound to occur. Once the virus attains the ability to transmit from animal to animal, it can hardly be contained.

Something else to keep in mind.