Fatal H5 Infections on Canadian Turkey Farm Raise Concerns

Commentary

Recombinomics Commentary 16:55
January 24, 2009

The initial test, which was conducted at a lab in Abbotsford, was prompted when a worker noticed the birds seemed to be suffering from some sort of respiratory distress.

Mortality among the birds in the two barns at the farm has been normal to “perhaps slightly elevated,” Stephens said.

The above comments on the H5 outbreak in British Columbia increase concerns that the outbreak is due to H5N1. Although H5N1 frequently causes rapid and dramatic increases in bird mortality, some birds, such as waterfowl, can have asymptomatic H5N1 infections.

H5N1 pathogenicity is linked to a polybasic HA cleavage site and early mortality in experimental chickens. HPAI failures to lead to more symptoms in waterfowl are frequently linked to prior exposure of the waterfowl to low path H5, which creates cross reactive immunity. Media reports have indicated the turkeys have H5 antibodies.

H5 has been widespread in North America in recent years, so terrestrial poultry, such as the turkeys at the above farm could have immunity that would produce mortalities that were “slightly elevated”. Most would be asymptomatic or have some respiratory distress. The clade 2.2 (Qinghai) strain of H5N1 is largely limited to the upper respiratory in waterfowl, but produces fatal infections in naïve hosts, including humans.

More information on the delayed H5 analysis, such as the sequence of the HA cleavage site, or size of the HA or NA insert, would be useful.