The organisation collected data on the characteristics and biosecurity measures of infected farms using surveys and site visits, studied wind and airborne viruses as possible causes of viral spread, and conducted a genetic analysis of the viruses detected in the United States.

After conducting investigations on over 80 commercial poultry farms, APHIS analysis indicates that there are likely several ways the virus could be transmitted, including lapses in biosecurity practices and environmental factors.

APHIS said that they cannot currently associate HPAI transmission with one particular factor or group of factors in a statistically significant way, and will continue to update this report regularly as more analyses are completed.

Scientists believe wild birds were responsible for introducing HPAI into commercial poultry. Survey results in the report showed that 35 per cent of the turkey farm respondents had observed wild birds in the poultry houses.

While wild birds are the original pathway for the virus’ introduction into the United States, it appears the virus was spreading in other ways as well, given the number and proximity of farms affected by HPAI.

For instance, the report provides evidence that a certain cluster of farms was affected by identical viruses, pointing to possible transmission among those farms. In addition, genetic analyses of the HPAI viruses suggest that independent introductions as well as transmission between farms were occurring in several States concurrently.

Insufficient application of recommended biosecurity practices was identified as a likely cause of some virus transmission. For example, APHIS observed the following:

• sharing of equipment between an infected and noninfected farm;
• employees moving between infected and noninfected farms;
• lack of cleaning and disinfection of vehicles moving between farms;
• and reports of rodents or small wild birds inside the poultry houses.

Based on an analysis by APHIS, environmental factors may also play a part in transmitting HPAI. Air samples collected outside of infected poultry houses contain virus particles, indicating that the virus could be transmitted 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 5 days later. APHIS is conducting additional analyses to better characterise environmental factors that may contribute to virus spread.