Wild birds were responsible for introducing highly pathogenic avian influenza into commercial poultry operations, but the virus spread through other pathways, the Animal and Plant Health Inspection Service of the U.S. Department of Agriculture said in its “Epidemiological and Other Analyses of HPAI-Affected Poultry Flocks: June 15, 2015 Report.”

The number and proximity of commercial poultry farms in addition to biosecurity lapses also played a role in spreading the virus, APHIS said.

“For example, APHIS has observed the following: sharing of equipment between an infected and non-infected farm; employees moving between infected and non-infected farms; lack of cleaning and disinfection of vehicles moving between farms; and reports of rodents or small wild birds inside the poultry houses,” the agency said. “APHIS is compiling these practices and will present these findings in a subsequent update of this report.”

APHIS also found evidence that a specific cluster of farms was affected by identical viruses. Genetic analyses of the viruses suggest independent introductions and transmission between farms were occurring in several states concurrently.

Environmental factors also played a role in the spread of avian influenza, APHIS reported. Air samples collected outside infected poultry houses contained virus particles which suggest the virus is transmissible by air.

“In addition, preliminary analysis of wind data shows a relationship between sustained high winds and an increase in the number of infected farms approximately five days later,” the agency said in its report. “APHIS is conducting additional analyses to better characterize environmental factors that may contribute to virus spread.”

APHIS said the agency has increased its collaboration with individual states and the poultry industry to promote biosecurity best practices, and the agency plans to continue meeting with industry stakeholders and federal, state and local governments throughout the outbreak.

“Moving forward, APHIS plans to continue sharing what it learns with state and industry partners through regular conversations and meetings, including an Industry/U.S.D.A./State Animal Health Meeting in July where the agency will focus specifically on biosecurity,” APHIS said.