Monday, May 22, 2006

Poultry and human bird flu

One of the most repeated "facts" about the human cases of bird flu is that virtually all cases to date come from intimate contact with sick poultry. But the evidence does not show this.

One third of the Vietnamese cases are said to be without an adequate history of poultry contact, at least a third (if not more) of the Indonesian cases, and many of the Chinese cases. WHO continually repeats the necessity of the poultry connection but knows better. The Vietnamese figure is from WHO epidemiologist Peter Horby (personal communication reported in EFSA Monograph; see earlier post here). WHO is quite familiar and distressed about the situation in Indonesia. And in an article today, Helen Branswell writes:
The WHO has been extremely concerned that none of China's 18 confirmed human cases to date have occurred in areas where outbreaks in poultry were previously reported. In most of the cases thorough investigations after the fact have pointed to some possible exposure to poultry. But the lack of obvious links disturbs international public health authorities. (Helen Branswell, Canadian Press; my emphasis)
Yet WHO continues to say publicly all evidence so far shows the principal connection is from close contact with sick or dead birds.

WHO should not say the evidence shows "close contact" with sick or dead birds is required. We don't know that and it probably isn't true. If it were it wouldn't be so hard to elicit histories of poultry exposure in so many cases. The other possibilities are less close contact through poultry dust, feces, feathers, etc. If WHO wants to call this "close contact with sick or dead poultry" they are free to do so but they are using a private language. And it is largely an inference based on the fixed idea that almost all human cases can be traced back more or less directly to birds.

Given the evidence, we should keep our minds open to other possibilities, namely, contaminated food or water, an as yet unidentified animal reservoir or vector, and of course person to person transmission. At this point I believe WHO is probably right in substance: most cases probably are of proximate bird origin. But they don't have the evidence that they claim and I find that bothersome.

This is the way obvious facts are missed: we don't see what is staring us in the face until hindsight dramatically improves our vision.