15.06.2015

For the past several months, the USDA’s Animal and Plant Health Inspection Service (APHIS) has conducted epidemiological investigations and other studies with the goal of identifying transmission pathways of highly pathogenic avian influenza (HPAI). This report includes the results to date of investigations spanning more than 80 commercial poultry facilities, as well as other in-depth studies and analyses performed with the assistance of academic, Federal, State, and industry partners.
APHIS will update this report regularly as more analyses are completed.
APHIS concludes that at present, there is not substantial or significant enough evidence to point to a specific pathway or pathways for the current spread of the virus. APHIS have collected data on the characteristics and biosecurity measures of infected farms and studied wind and airborne viruses as possible causes of viral spread, and conducted a genetic analysis of the viruses detected in the United States.
APHIS scientists believe wild birds were responsible for introducing HPAI into commercial poultry. However, given the number and proximity of farms affected by HPAI, it appears the virus is spreading in other ways as well. For instance, one analysis 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 are occurring in several States concurrently.
Although APHIS cannot at present point to a single statistically significant pathway for the current spread of HPAI, a likely cause of some virus transmission is insufficient application of recommended biosecurity practices. For example, APHIS has observed 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 poultry houses.  
Environmental factors may also play a part in transmitting HPAI. APHIS found that genetic material from the HPAI virus could be detected in air samples taken inside and outside infected poultry houses, supporting the idea that the virus can be transmitted through air. Further reinforcing this concept is preliminary analysis of wind data that shows a relationship between sustained high winds (25 mph or greater for 2 days or longer) and an increase in the number of infected farms 5 to 7 days later.