Avian influenza H5N1 outbreak in a flock of mute swans in the city of Torun, Poland, in 2006

The outbreak of highly pathogenic avian influenza (HPAI) infection caused by H5N1 virus in a flock of mute swans wintering in the city of Toruń in western Poland in 2006 was described. Following the confirmation of H5N1 infection in 3 dead swans, 113 apparently healthy swans were placed in an aviary. After 5 d, another dead swan was found in the aviary and was later proved positive for HPAI/H5N1. A lack of regulations concerning the control of HPAI in wild birds, lead to the prolonged indecision regarding the measures that were to be applied to the flock. Finally, due to the quickly approaching flood wave on the Vistula River, and a danger of the aviary being flooded, samples of tracheal and cloacal swabs as well as blood samples were collected from 112 live swans and submitted for testing. Under the pressure of time, initially RT-PCR/H5 and haemagglutination inhibition (HI) test with H5N2 antigen were performed and based on the results, 32 PCR-positive swans were euthanised while the rest of the flock (regardless of serological status) was released. Thereafter, secondary tests were carried out comprising of real time RT-PCR/M, virus isolation, HI with antigens H1N1, H3N2, H5N1, H7N1 and H9N2, agar gel immunodiffussion test, and competitive ELISA. The additional serological tests confirmed that more than 70% of swans were infected with H5 virus, but a group of the swans were also positive for other subtypes, mainly H1. The low mortality rate in the flock might indicate the previous contact with the AIV virus and a partial protection against HPAI/H5N1, or that innate resistance to AIV infection is higher than commonly believed.