Scientists suspect some people have a “genetic disposition” for infection with bird flu, which may explain why some get it and others don’t, and why it remains relatively rare, the World Health Organization said Thursday.
Evidence, mainly from a family cluster of cases last May in North Sumatra, Indonesia — when seven people in an extended family died — showed genetic factors might influence human susceptibility to the H5N1 virus, it said.
Only blood relatives were infected in the Karo district of North Sumatra, the largest cluster known to date worldwide, “despite multiple opportunities for the virus to spread to spouses or into the general community,” it added.
The theory — which it said merited further study — was contained in WHO’s report issued on Thursday, on a closed-door meeting of 35 scientific experts held in late September.
“A genetic predisposition for infection is suspected based on data from rare instances of human-to-human transmission in genetically-related persons,” the WHO said.
“This possibility, if more fully explored, might help explain why human cases are comparatively rare and why the virus is not spreading easily from animals to humans or from human to human,” it added.
Bird flu remains mainly an animal disease, but has infected 256 people since late 2003, killing 152 of them, according to the United Nations agency. Experts fear the virus could mutate and spark a human influenza pandemic, which could kill millions.
Overall, the H5N1 virus continues to show “inefficient spread,” both from animals to humans and among humans, it said.
Much about the disease remains poorly understood, but the present situation is serious and “the risk that a pandemic virus might emerge is not likely to diminish in the near future.”
Results from clinical trials of candidate pandemic vaccines had “not been promising” and it may be premature for countries to choose one to stockpile so as to protect their populations.
Mallard ducks have been identified as the “champion” spreaders of bird flu, and appear to shed the virus increasingly from the respiratory tract rather than via feces, the WHO said.
This finding will require modifying disease surveillance strategies so that samples are also taken from birds’ pharynx, as well as feces, it said.
“In terms of geographical spread of the virus, mallard ducks are now regarded as the 'champion' vectors; mute swans are highly susceptible birds that are thought to serve as sentinels, but probably not as vectors of virus transmission,” it said in the report, posted on its Web site.
Recent studies had shown that the virus is now moving both ways in “relay transmission,” from poultry to migratory birds and back again, it added.
Culling birds remains the control strategy of first choice, and had proven successful in Japan and South Korea, if costly.
High-quality vaccination of poultry was recommended in resource-strapped countries, yet ducks react differently to poultry vaccines which are designed for chickens, it said.