Outbreak control and viral evolution of the highly pathogenic H5N1 avian influenzain Thailand

Abstract

Thailand is known for its success in controlling the highly pathogenic H5N1 avian influenza epidemic. Despite the explosive outbreak in 2003-2004 similar to other countries in the East and Southeast Asian region, the epidemic was bought down under control in 2006 with the elimination of infectious sources by "stamping out", strengthened biosecurity measures, and periodic active surveillance as the main control strategies. The initial epidemic in 2003-2005 involved mainly medium to large scale farming, resulting in massive economic loss. After 2006, sporadic cases occurred seasonally in backyard poultry in certain repeated outbreak areas involving limited number of poultry. The last animal outbreak was reported in 2008, and the last indigenous human case was detected in 2006. The reduction of viral population size was also evidenced by the reduced viral sequence diversity after 2006. The viral sequences showed little changes without evidences of positive selection during this low level endemic period and no known human-adapted mutations were observed. Similarity of viral sequences among outbreak seasons indicated that the virus was maintained in a local reservoir between outbreak seasons. Virus of similar lineage was occasionally isolated from local wild birds and migratory birds. Although some of these birds migrate in a route covering Southeast Asia to the epidemic hot spots in Central Asia, the similarity of viral sequences in these birds to the local virus suggested that the birds acquired the virus locally rather than carrying new viruses into the country. The small reservoir size in a limited area suggested that the virus can be eradicated from the country. While the current situation in the country is well under control, new kindle from undetected local reservoirs and import of new viral strains through human or wildlife activities are still a threat. © 2013 by Nova Science Publishers, Inc. All Rights Reserved.