Studies on Influenza Virus Transmission between Ferrets: the Public Health Risks Revisited

Fouchier, Ron A. M.

The higher bound of the range of case fatality rates of 60% stems from the number of deaths recorded among laboratoryconfirmed cases of A/H5N1 influenza reported to the WHO (31). Since mild cases of infection-those individuals that do not consult a physician or remain untested-are not recorded, the true case fatality rate of A/H5N1 virus infections in humans is unknown. Due to intrinsic difficulties associated with serology data to estimate the numbers of previously infected individuals, there is no consensus on the incidence of A/H5N1 infections in Southeast Asia (32, 33) , but case fatality rates orders of magnitude lower than 60% have been inferred (27) . In addition, it is important to note that fatalities in ferrets infected with A/H5N1 virus via respiratory droplets or aerosols have not occurred, contrary to when ferrets received large dosages of A/H5N1 virus directly in the (lower) airways (12, 13, 16) .