The Lancashire avian flu outbreak was probably caused by wild birds present on ponds near the infected premises, the Animal and Plant Health Agency has concluded.
About 170,000 birds were slaughtered when highly pathogenic H7N7 avian flu as detected at Staveleys Eggs, at Goosnargh, near Preston, on July 13.
The family-run business owns seven linked premises, two of which rear pullets supplying other five commercial laying premises, one of which became infected.
An epidemiological report by APHA concluded the virus probably resulted from an incursion of a low pathogenic strain of avian flu that subsequently mutated into a highly pathogenic strain.
The low pathogenic infection is estimated to have entered the infected premises between the end of May and mid-June, up to six or seven weeks before the virus was detected.
The ‘mutation event’ to the highly pathogenic strain is likely to have taken place at the end of June, with the virus continuing to spread between the birds until the completion of preliminary cleansing and disinfection of the infected premises on July 16, after the birds had been slaughtered.
The report acknowledges there is uncertainty as to the most likely source of infection.
It said: “However, the available evidence suggests that the source was the wildfowl present on the ponds on the premises, followed by an incursion into one group of free-range birds as a result of indirect contact, with subsequent spread to other epidemiological groups.
“In the case of the HPAI infection the evidence strongly suggests an initial mutation event in one of the sheds.”
The report said the conclusion was based on various pieces of evidence:
The H7N7 virus was closely related to strains circulating in wild birds and poultry in Northern Europe but was probably derived from ‘genetic reassortment in nature of two or more progenitor strains’.
Following extensive investigations, no evidence of avian flu infection has been found in other domestic poultry premises in the country.