Although it is well known that highly pathogenic avian influenza (HPAI) viruses spread very rapidly among chickens within sheds, the spread was slower than expected for the initial 10 days at an index farm in Japan during 2004. To understand the field conditions for the slow virus spread, we examined contact and airborne transmission of the H5N1 virus to chickens in a negative-pressure isolator using various numbers of infected chickens and separate compartments. We found that contact transmission did occur inefficiently when one or two chickens were infected, whereas the transmission was efficient when four chickens were infected. Also, since the H5N1 HPAI viruses and virus antigens were detected in the trachea, duodenums, and other internal organs of large-bill crows found dead around the outbreak houses in 2004, it is possible that the H5N1 HPAI viruses can be transmitted by wild crows.
(Research Team for Zoonotic Diseases, TEL +81-29-838-7708)
