Ducks are the ideal reservoir for bird flu because increasingly they seem to be able to carry the infection without showing signs of illness. As water fowl they shed the virus into ponds and lakes (through feces, mostly), and other birds can acquire the virus.
Over the past couple of years we've heard of more and more of these asymptomatic ducks, from places like Vietnam, Thailand, and yes, even Europe.
This from CIDRAP last October.
Oct 26, 2007 (CIDRAP News) – Apparently healthy domestic geese and ducks in Europe may be harboring the H5N1 avian influenza virus, posing a risk to other poultry and to humans who have contact with them, the United Nations Food and Agriculture Organization (FAO) warned in a statement yesterday.
In some parts of Europe the disease could become endemic in poultry populations, much as it has in some Asian countries, as well as Egypt and Nigeria, the FAO said. The agency said its warning was prompted by the detection of H5N1 in diseased domestic ducks by German scientists. The researchers also found that asymptomatic ducks on two farms had antibodies to the H5N1 virus.
"It seems that a new chapter in the evolution of avian influenza may be unfolding silently in the heart of Europe," said Joseph Domenech, the FAO's chief veterinary officer, in the statement.
Birds that acquire the virus, sicken, and die are less likely to transfer the virus across long distances. Unfortunately, ducks are often immune to its effects, creating a durable host for the virus.
Vietnam, particularly in the Mekong Delta region, has millions of free range ducks. No one really knows how many. Too many to count, anyway. And vaccinating all of these free range birds is probably impossible.
An animal health worker spraying sterilizer in a duck enclosure to prevent bird flu in the Mekong Delta
The Mekong Delta provinces risk a massive bird flu recurrence because of free-range duck farming and the inadequate control of poultry transport and trade.
Dong Thap Province’s Department of Agriculture and Rural Development said large numbers of unvaccinated ducks roamed free around local rice farms.
“It is hard to control the free range ducks in the fields,” the department’s vice manager Duong Nghia Quoc said.
Ducks account for 80 percent of the province’s poultry, he added.
The situation was the same in An Giang and Kien Giang provinces.
“There is no quarantine documentation for ducks roaming from one place to another, so we can only drive them away and ask their owners to retrieve them back to their home,” the head of Hau Giang Province’s Animal Health Department Nguyen Hien Trung said.
The number of unvaccinated ducks in the delta provinces was unknown.
Another problem was the provinces’ loose control on fowl trade and transportation.
All the provinces have set up epidemic control stations on the roads but poultry are mainly transported on the extensive river networks which are hard to police.
The trade of unquarantined poultry was rampant at suburban markets because of local authorities’ loose management of communes and hamlets.
It was impossible for animal health taskforces to check the markets daily, Quoc said.
“The provincial administration has asked districts and communes to establish a central place to sell poultry to help control the epidemic but they haven’t complied yet,” he said.
Ten provinces, including the Mekong Delta’s Vinh Long Province, have reported bird flu this year and five people out of every six who report H5N1 infections have died.
Viruses require a host in order to survive. Outside of a living cell, they cannot reproduce, and unlike bacteria, are very shortlived and fragile. Deny them a host, and you eradicate the virus.
That is how smallpox was eradicated in the 1970's, doctors used ring-vaccination techniques, where they inoculated everyone in contact with an existing smallpox patient. Over time, the virus had no host to jump to, and died out.
With avian flu, the combination of culling and vaccination were hoped to eradicate the virus, and in some countries, they've had some success.
But in many places - like Vietnam, Indonesia, and Egypt -, the virus continues to be endemic, and very possibly, carried by hosts we are not aware of. We know that in addition to birds, H5N1 can infect humans, dogs, cats, civets, and other small mammals.
Unfortunately, what we don't know could fill volumes.
Which will make the ultimate eradication of this virus very difficult.