"Aaargh Plop", it turns out. Declan Butler has a story in tomorrow's Nature about the apparent spread of H5N1 among felines. Evidently, until recently, the "WHO argued that cats are not naturally susceptible to flu, and thateven if infected they would not shed large quantities of virus."
But as I observed a few days ago, it is odd that cats are dying in Europe so soon after the virus arrived there. Dr. Butler notes that H5N1 is not behaving as expected in cats:
...With bird flu it may be different. Later in 2004, Albert Osterhaus's team from Erasmus University in Rotterdam showed experimentally that domestic cats do die from H5N1 and do transmit it to other cats (T. Kuiken et al. Science 306, 241; 2004). And in January this year, the virus was found not only in sputum but also in faeces of experimentally infected cats, suggesting that infected animals may shed the virus extensively (G. F. Rimmelzwaan et al. Am. J. Pathol.168, 176–183; 2006).
It is unclear how these findings relate to cats in their natural environment. But in next month's issue of Emerging Infectious Diseases, Thai researchers describe a cat that died of H5N1 after eating a pigeon carcass. It showed similar pathology to cats experimentally infected with the virus.
It gets worse:
Andrew Jeremijenko, head of influenza surveillance at the US Naval Medical Research Unit 2 in Jakarta, Indonesia, detected H5N1 in a kitten he found near a poultry outbreak in Cipedang, West Java, and tested out of curiosity on 22 January. The virus from the kitten is closely related to recent H5N1 strains isolated from humans in Indonesia: it shares genetic changes found in human strains that are not present in samples from birds.
Interesting. The standard explanation for the sequence variation would be that the virus propagated at low levels in a cat after it consumed the bird. But I wonder about another possibility. Speculation Warning: The flu is very error prone during reproduction, which means that during infection in birds a great many sequences are produced that don't actually survive/reproduce in avians. But it is possible that a bird can carry a small number of sequences better adapted to mammals that do not proliferate in the bird. When consumed by a cat, the new sequences might be ready to go in the new host. That might -- might! -- account for the speed with which the virus started killing mammals in Europe.
Dr. Butler concludes his article with an anecdote about how prevalent feline H5N1 deaths may be in the wild. The confusion over cat deaths in Europe may simply be another example of ignorance about how H5N1 is actually behaving in nature:
Scientists may just be learning what is already common knowledge among Indonesian villagers. Peter Roeder, a consultant for the UN Food and Agriculture Organization, says locals have an onomatopoeic name for bird flu "that sounds like 'plop', the sound of a chicken hitting the ground when it falls out of a tree. They also have a name for the cat form of avian flu — 'aaargh plop' — because cats make a screaming noise before they fall out of the tree."