As most people have heard by now, H5N1 has reached Germany and is confirmed to have killed a cat (AP via the NY Times). I think the time scale is of interest here. The virus has only just reached Western Europe, evidently via migrating birds, and already it has jumped to mammals. In contrast, there appear to have been few cases in felines in Asia, despite the amount of exposure mammals have had there. From the AP report:
In addition to the large cats infected in Thailand, three house catsnear Bangkok were found to be infected with the virus in February 2004. In that case, officials said one cat ate a dead chicken on a farm where there was a bird flu outbreak, and the virus apparently spread to the others.
I suppose its possible cat deaths are going unreported throughout Asia, but if the AP report is correct then transmission to cats is very low probability, and I find it odd that the virus is already confirmed to have killed a cat in Germany. It makes one wonder how the sequence is changing.
Fortunately, there are as of yet no known cases of cat-to-human transmission. But human exposure to the virus has only just begun in Europe, and the coming months will increase this contact.
All news reports seem to agree that the virus arrived in France and Germany via migrating wild birds. In an article focusing on the effects of the virus on the French poultry industry, Craig Smith notes in the New York Times that:
...The real threat, many experts fear, may come in the weeks ahead as pintail, garganey and shoveler ducks begin arriving from their wintering grounds in Africa, where the virus has already spread among poultry. The annual migration toward northern breeding grounds is expected to last until the end of May.
Smith also describes how migration patterns have been somewhat unusual this year due to extremely cold weather. Thus the spread of the virus may be enhanced by changing weather patterns, increasing the likelihood of transportation into areas of the world densely populated by both humans and domesticated animals. This sort of thing is only going to happen more often.
The AP (via the NY Times) recently picked up this thread with an article entitled, "Scientists See Growing Animal - Disease Risk." The article begins, "Humans risk being overrun by diseases from the animal world, according to researchers who have documented 38 illnesses that have made that jump over the past 25 years," and winds up:
One explanation may be the recent and wide-scale changes in how people interact with the environment in a more densely populated world that is growing warmer and in which travel is faster and move extensive, Marano said. Those changes can ensure that pathogens no longer stay restricted to animals, she added. Examples from recent human history include HIV, Marburg, SARS and other viruses.
That prospect leaves open the question of what future threats await humans.
''It always surprises us. We think that avian flu will be the next emerging disease. My guess is something else might come out before that,'' said Alan Barrett, of the University of Texas Medical Branch at Galveston. ''It's very hard to anticipate what comes next.''
SARS, in particular, is an excellent example of surprise from nature. It is also an example of how ill prepared we are for emerging diseases. It is clear from recent work that if the SARS coronavirus had been just a little more virulent, and if it had spread just a little more before symptoms emerged, then the epidemic would likely not have been held in check by public health measures. Moreover, it is only because coronaviruses caught the attention of a talented virologist several years before that the community was able to get a handle on the virus as quickly as it did. More on this in an upcoming post.
(UPDATE, 5 March 06: Ralph Baric is the virologist mentioned above. Here's the story.)