In 2005, Chinese poultry farmers were reported to have used amantadine to protect birds against avian influenza .  In Western countries and according to international livestock regulations, amantadine is approved only for use in humans. Chickens in China have received an estimated billion doses of amantadine.  Avian flu ( H5N1 ) strains in China and southeast Asia are now resistant to amantadine, although strains circulating elsewhere still seem to be sensitive. If amantadine-resistant strains of the virus spread, the drugs of choice in an avian flu outbreak will probably be restricted to the scarcer and costlier oseltamivir and zanamivir , which work by a different mechanism and are less likely to trigger resistance.