Foxes scavenging carcasses of infected poultry caused the spread of bird flu in Norfolk, a report published by Defra has said.
35,000 chickens were culled on one North Tuddenham farm
It also blamed the poultry virus outbreak on free range flocks coming into contact with infected wild birds.
In March two free range flocks and a broiler breeding unit were affected in Norfolk with the H7N3 virus.
The report on the outbreak follows an investigation by the Animal Health and Welfare Directorate.
Almost 100,000 chickens from the three flocks in Norfolk had to be slaughtered.
Inspectors assessed 206 holdings, visited 169 farms where chickens were kept and imposed restrictions on 45 of them while further investigations were carried out.
The report criticised inadequate measures at many farms to protect themselves against biological threats like viruses.
It said that poultry keepers should prevent scavenging of dead birds by foxes and other "free-living species".
The practice of deliberately feeding foxes with poultry carcasses should also stop.
The report said the virus was probably introduced into other flocks from one free range egg laying flock after foxes brought back carcasses of infected chickens to their dens.
The virus was then introduced into poultry houses on the contaminated footwear of husbandry management staff and egg collectors.
Unsupervised contractors and delivery drivers who visited different sites, with low levels of biosecurity awareness, were also blamed for spreading the condition.
The report called for more awareness of hygiene and better security of sites against bio-threats such viruses and diseases carried by carcasses and scavenging animals like foxes.