Spread of Influenza Virus A (H5N1) Clade 2.3.2.1 to Bulgaria in Common Buzzards

Atanaska Marinova-Petkova, Georgi Georgiev, Patrick Seiler, Daniel Darnell, John Franks, Scott Krauss, Richard J. Webby, and Robert G. Webster

Author affiliations: Regional Diagnostic Laboratory on Avian Influenza and Newcastle Disease in Birds, Varna, Bulgaria (A. Marinova-Petkova); National Diagnostic and Research Veterinary Medical Institute, Sofia, Bulgaria (A. Marinova-Petkova, G. Georgiev); and St. Jude Children’s Research Hospital, Memphis, TN, USA (P. Seiler, D. Darnell, J. Franks, S. Krauss. R.J. Webby, R.G. Webster)

Abstract

On March 15, 2010, a highly pathogenic avian influenza virus was isolated from the carcass of a common buzzard (Buteo buteo) in Bulgaria. Phylogenetic analyses of the virus showed a close genetic relationship with influenza virus A (H5N1) clade 2.3.2.1 viruses isolated from wild birds in the Tyva Republic and Mongolia during 2009–2010. Designated A/common buzzard/Bulgaria/38WB/2010, this strain was highly pathogenic in chickens but had low pathogenicity in mice and ferrets and no molecular markers of increased pathogenicity in mammals. The establishment of clade 2.3.2.1 highly pathogenic avian influenza viruses of the H5N1 subtype in wild birds in Europe would increase the likelihood of health threats to humans and poultry in the region.

Wild aquatic birds are considered natural reservoirs of all known influenza virus subtypes (1). Highly pathogenic avian influenza viruses (HPAIVs) usually cause asymptomatic infections in waterfowl. Compared with that for poultry, the number of reported outbreaks of HPAIVs in wild birds (aquatic or terrestrial) before 2002 was low; 1 influenza A (H5N3) outbreak occurred in wild common terns (Sterna hirundo) in South Africa in 1961 (2), and H7 subtype HPAIV was isolated from a Saker falcon (Falco cherrug) in Italy in 2000 (3). In December 2002, a die-off of aquatic waterfowl caused by an HPAIV (H5N1) occurred in Penfold Park in Hong Kong. That event was followed by a second outbreak a week later in Kowloon Park, also in Hong Kong (4). Since then, cases of HPAIVs (H5 and H7 subtypes) in wild birds have been reported often.
In May 2005, a massive HPAIV (H5N1) outbreak occurred in wild aquatic birds in Qinghai Lake, western People’s Republic of China; 6,184 gulls, geese, great cormorants, and ruddy shelducks died (5). The lake is a staging area for migratory waterfowl, and the scientific community’s fear that the virus would spread during migration (6) was realized when the so-called Qinghai-like influenza virus A (H5N1) clade 2.2 spread to western Siberia in Russia and then to many countries in Asia, the Middle East, Europe, and Africa at the end of 2005 and during 2006, killing poultry flocks and wild birds.
The 2008 classification system used to describe the evolution and diversification of the HPAIVs (H5N1) that emerged from the A/goose/Guangdong/96 lineage (7) was updated in 2011. Phylogenetic analysis of all isolated influenza (H5N1) viruses showed that some of the 10 first-order clades (0–9) had stopped circulating in 2008 or earlier (clades 0, 3, 4, 5, 6, 8, 9), as had some second- and third-order groups of clade 2. Meanwhile, clades 1, 2.1.3, 2.2, 2.2.1, 2.3.2, 2.3.4, and 7 continued to evolve rapidly (8). Clade 2.3.2 is widely distributed in Asia, particularly in China, Hong Kong, Korea, Vietnam, Laos, Bangladesh, Nepal, Mongolia, and the Tyva Republic; it is also distributed in eastern Europe, mainly in Romania and Bulgaria (8,9). Tyva is part of the Siberian Federal District of Russia, which is located north of Mongolia. The remaining circulating influenza (H5N1) clades have specific geographic locations: clade 1 circulates in southern Vietnam and Cambodia; clade 2.1.3 in Indonesia; clade 2.2 in India and Bangladesh; clade 2.2.1 in Egypt; clade 2.3.4 in China, Hong Kong, Vietnam, Thailand, and Laos; and clade 7 in China and Vietnam.
Before 2006, no avian influenza outbreaks in poultry had been reported in Bulgaria; 4 cases of HPAIV (H5N1) clade 2.2 were confirmed in dead swans found in 4 regions of the country early that year (10). On March 15, 2010, the carcass of a common buzzard (Buteo buteo) containing HPAIV (H5N1) was found at St. Konstantin and Helena Black Sea Resort in Bulgaria and submitted to the Regional Diagnostic Laboratory on Avian Influenza (Varna, Bulgaria). The virus was characterized as clade 2.3.2.1.