Authorities will treat the dead migratory seabird found at a beach in Perth’s northern suburbs as a presumed positive case of H5 bird flu, even though disease testing was unable to verify the strain.

CSIRO’s Australian Centre for Disease Preparedness in Victoria confirmed on Monday that the giant petrel found between Mullaloo and Whitfords had some type of an influenza virus of the H5 subtype, but was not able to confirm bird flu.

The inability to obtain a sequence may have been due to the level of decomposition of the carcass.

WA acting chief veterinary officer Dr Katie Webb said it was considered highly likely the petrel was infected with H5 bird flu, but this could not be definitively proven.

“This case will be considered as presumed positive based on the available test results, species involved, coastal location and the broader epidemiological picture,” Dr Webb said.

“This does not change our response in WA. We are treating this the same as a positive case based on the available epidemiological and laboratory evidence.”

The petrel is WA’s fifth believed bird flu case and first in the metropolitan area.

Its discovery last Tuesday, close to a popular off-leash dog beach in Hillarys, sparked a warning to Perth pet owners and a step-up in surveillance at city beaches.