Submitted to: Journal of Virology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/1998
Publication Date: N/A
Interpretive Summary: Influenza virus can cause serious disease, resulting in death in a variety of different animals including humans, chickens, pigs, and horses. Although the viruses are similar, normally the viruses that infect a chicken do not cause disease in humans and vice versa. However, several instances of a bird virus directly infecting and causing disease in pigs, horses, and seals had been observed. Until recently, no bird viruses had been known to cause infection and serious disease in humans. In May of 1997 a 3 year old child became infected with and probably died from an influenza infection. This influenza virus was determined to be a bird virus by examining its viral genes. One important gene, the surface protein hemagglutinin, was of a subtype that had previously only been found in birds. This subtype was called the H5 subtype, which distinguishes it from human influenza viruses that have a H1, H2 and H3 subtype. In November and December of 1997, 17 more human cases of infection with this H5 subtype occurred. Also, an outbreak of disease in chickens with a H5 subtype was also occurring in Hong Kong in 1997. The viruses from humans and chickens were compared, and they were found to be almost identical, which suggests that humans became infected from birds that had been infected with this virus. Further work as to why this chicken virus could cause disease in people is being investigated.
Technical Abstract: A recent outbreak of a H5N1 influenza virus in humans and chickens occurred In Hong Kong in 1997. The gene sequences of a virus from both humans and chickens were compared, and all eight RNA segments from the two viruses show greater than 99% sequence identity between them. However, none of the gene sequences were closely (>95% sequence identity) related to any other gene sequences found in the GenBank database. Phylogenetic analysis demonstrated that the nucleotide sequences of at least 4 of the 8 RNA segments clustered with Eurasian origin avian influenza viruses. The hemagglutinin gene phylogenetic analysis also included the sequence from an additional 3 human and 2 chicken H5N1 virus isolates from Hong Kong, and the isolates separated into two closely related groups. However, no single amino acid change separated the chicken origin and human origin isolates, but they all contained multiple basic amino acids at the hemagglutinin cleavage site, which is associated with a highly pathogenic phenotype in poultry. In experimental intravenous inoculation studies in chickens, all seven viruses were highly pathogenic, killing most birds within 24 hours. All infected chickens had similar pathologic lesions, including moderate- to-severe diffuse edema and interstitial pneumonitis. Asphyxiation from pulmonary edema and generalized cardiovascular collapse were the most likely pathogenic mechanism responsible for illness and death.