An outbreak of highly pathogenic H5N1 avian influenza in Korea, 2008

Abstract

In spite of intensive surveillance programs for the control of HPAI, an outbreak of highly pathogenic avian influenza (HPAI) H5N1 in Korea in April 2008 caused serious damage to poultry farms, as did previous outbreaks in 2003/2004 and 2006/2007. Six viruses were selected from the Korean 2008 isolates for genetic analysis, and all eight gene segments from each of the influenza viruses were sequenced. A phylogenetic analysis showed that all of the viruses were of the same virus type and that the hemagglutinin (HA) gene was clustered with that of clade 2.3.2 viruses. However, the internal and neuraminidase (NA) genes were closely related to those of the clade 2.3.4 viruses (recent human and bird isolates from Southeast Asia).

Introduction

Highly pathogenic H5N1 influenza viruses have repeatedly caused serious outbreaks of disease on poultry farms in Asia since 2003. Wild aquatic birds, including ducks, are the natural reservoir of influenza type A viruses, and they play an important role in the mutation and replication of the viruses without displaying any sign of disease (Guan et al., 2002, Webster et al., 1992). However, during the Asian H5N1 outbreak of 2003–2005, domestic ducks were among the species affected by the epidemic, even though they were not as fatally affected as chickens (Sturm-Ramirez et al., 2005). Similarly, a large outbreak of H5N1 virus in Qinghai Lake, China in 2005 showed that various migratory bird species were affected or died, which allowed the spread of the virus across continents through air travel routes (Chen et al., 2005, Liu et al., 2005, OIE, 2005).

In South Korea, an outbreak of HPAI caused by H5N1 was first reported on December 10, 2003. A total of 19 cases (10 in chickens and 9 in ducks) in 7 provinces were reported over the course of the outbreak (Kwon et al., 2005). In breeder ducks infected with the virus (CK/KR/ES/03(H5N1)), there were very mild clinical signs of infection, such as a decrease in egg production and feed consumption, and no mortality (Lee et al., 2005). On November 22, 2006, a secondary outbreak of H5N1 resulted in seven cases (four chicken farms, two duck farms, one quail farm) in three provinces. The affected chickens showed a sudden increase in mortality rate with severe clinical signs, whereas ducks only showed decreased egg production with no other clinical signs and no deaths (Lee et al., 2008). On April 1, 2008, there was a third outbreak of the highly pathogenic H5N1 virus. The avian influenza virus isolated from the chickens at a poultry farm in Gimje City, which is located in the southwest region of South Korea, was designated A/Chicken/Korea/Gimje/08 (H5N1) and affected around 150,000 layer chickens with a 1.3% mortality rate at the time of identification. This index case was detected at the early stage by the National Veterinary Research and Quarantine Service (NVRQS) after a prompt report by a farmer. After 2 days, the domestic duck (34 days old) carcasses were received at the Animal Disease Diagnostic Center of NVRQS. The morbidity of this domestic duck farm was 60%, with severe clinical signs (neurological signs, diarrhea) and a mortality rate of up to 50% (ratio of dead individuals per infected house) until the time of slaughter. Unlike the previous 2 outbreaks, this outbreak spread to 11 provinces of the country in a short time (42 days) and resulted in 33 infected premises (21 chicken farms, 6 duck farms, 6 farms raising various birds). Moreover, this virus affected many bird species in the live bird markets (LPMs), including domestic geese and pheasants; it was lethal to these birds as well as to ducks. Therefore, this study was conducted to characterize the source of the HPAI H5N1 virus that induced severe symptoms in ducks and caused an epidemic in the spring season through an analysis of the full viral genomes of the six isolates of the Korean 2008 viruses.