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This also makes the efforts of Vietnam importing new populations of chickens pretty pointless, in that wild birds are the main carriers of H5N1. Unless of course they are importing birds who are carrying some type of immunity to this strain, as some wild asian mallards seem to have.
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I'm not sure just what measures Vietnam took apart from the cull. (There were no chickens or ducks when I visited the country at the end of 2005, but according to Hanoi newspaper reports at the time, chicken would soon be available again.) The campaign did seem to have positive effects: After 61 cases were recorded in Vietnam in 2005, there were none at all in 2006. Since then, however, there have been a few cases. It would seem to me that Vietnam has a special problem in that many rural people, especially women, spend much of their working day in open water.
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As far as rabies (and I am not sure its a good example), well theres all kinds of strains out there which have become specialized for one type of host (canine rabies) but is transmittable to a whole host of animals, man included. Is it really a new jump or an old genetic code that rears its head during mutations, hence the bat transmission being something that should be expected, rather than declared a new specie jump? Does anyone know where these particular bats obtained their original infection from? Would be kinda ironic if it was from some domestic animal, say a dog brought over on one of the prison transports to australia so many years ago.
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I mentioned rabies specifically because it is one of the conditions that are problematic as far as the definition of a species jump is concerned. (Its unique and aggressive mode of transmission is responsible for its widespread presence in many mammals, rather than its transmissibility.) Australian bat lyssavirus (which is present in most states, but not outside Australia) is not a typical rabies virus, and appears to be quite different from the canine rabies virus - but that is a very interesting question! The original host of rabies viruses has never been found, but is most likely to be an African bat. There are several rabies strains that do not spread all that readily between species: If a rabid dog bites another dog and a human, for instance, the dog is far more likely than the human to get rabies.
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What tide was turned? Theres like 65,000 flu related deaths in the USA a year and not one of them related to H5N1. We have what? Less than 400 H5N1 (or is it 800?) deaths worldwide with the potential for a handful of them being person to person transmissions. Does the potential exist for H5N1 making a transition into something like the spanish flu? Sure. But it also has the potential of making that 'true jump' without being more than a typical flu season with all its new strains and variations of the same old, same old.
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I was referring to avian flu specifically, but perhaps it is too early to speak of the tide turning: Around 1975 the WHO thought it had malaria licked. There has been a decline in the number of cases of avian flu, but vigilance should be maintained. Worryingly, recent cases appear to have an increased mortality rate - although it could be that at the height of the outbreak more people with milder symptoms reported to doctors, or that doctors were more likely to have patients with flu symptoms tested.
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If I remember right, they have already seen mutation in the ebola virus which allows more of its hosts to survive. Its really not in a virus best interest to kill its host, well not kill them fast anyways. I suppose this is part of the evolutionary reason why viri and bacteria can mutate so fast. Whoops, I didnt mean to burn down the house Honest!
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Probably the most important reason why emerging diseases seem to decrease in virulence - and certainly a more plausible explanation than that people acquire immunity en masse to a particularly virulent strain.
