The past and present threat of avian influenza in Thailand

Abstract

Avian influenza H5N1 infection was first identified in Thailand in January 2004. Since then, there have been three major outbreaks in the cold season of 2003-2004 and in the rainy and cold seasons of 2004-2005 and 2005-2006. More than 62 million birds died or were culled. The burden shifted from large industrial farming in the first outbreak to small farms, backyard chickens, and free-grazing ducks. Up to November 2005, there were 20 confirmed cases of human H5N1 infection. Thirteen of these died. Most of the confirmed cases were solitary ones except for three persons in a single family, and epidemiological evidence indicated that person-to-person transmission may have been involved in this cluster. However, sequence analysis of the virus in the cluster did not suggest any changes that might enhance the viral ability to get transmitted among humans. H5N1 viruses in Thailand and Vietnam belong to a single lineage genetically and are antigenically distinguishable from the viruses of the same genotype Z from southern China and Indonesia. Despite the seemingly subsiding epidemic in Thailand, the problem is far from resolved. H5N1 viruses are still sporadically isolated from domestic poultry as well as from wildlife. More important, isolates were also found in asymptomatic animals. Natural selection may have adapted the virus to a less aggressive form. This would make the virus more elusive and difficult to control. A threat of a pandemic strain emerging from the H5N1 virus is still imminent. A national strategic plan for avian influenza control and influenza pandemic preparedness has been implemented. The plan aims at effective control of avian influenza spread in animals as well as in humans for a three-year period and at efficient pandemic preparedness within one year. Nevertheless, more regional and international collaboration is needed. With proper collective preparedness, there is a hope that the threatening influenza pandemic can be prevented by confining and eliminating a potential pandemic strain at its origin. In December 2003, poultry farms in the eastern, central, and northern regions of Thailand experienced large-scale die-offs. The outbreak started from the eastern region of the country. The disease caused rapid death, with a very high attack rate. At that time, H5N1 outbreaks had been reported in South Korea, Vietnam, and Japan (OIE, 2005). A few humans with pneumonia were suspected to originate from contact with sick or dead poultry. Final diagnosis in these patients was not done as clinical samples were not available at the time when proper diagnostic testing became available. On 23 January 2004, the first case of human H5N1 infection in Thailand was reported. It was a boy from Kanchanaburi, a province about 100 km west of Bangkok. He was admitted to Siriraj Hospital in Bangkok and was diagnosed to have severe progressive pneumonia. The patient was initially treated with broad spectrum antibiotics, and respiratory samples were tested for influenza virus. The laboratory result showed that the patient harbored influenza virus, and sequencing of the viral RNA indicated that the virus belonged to the H5 subtype (Chokephaibulkit et al., 2005; Puthavathana et al., 2005). When this result was reported to the Ministry of Public Health, the government announced that there was a highly pathogenic avian influenza (AI) outbreak in Thailand. The Department of Livestock Development (DLD) confirmed the presence of H5N1 viruses in poultry on the same day. Subsequent analysis of the virus from patients and animals confirmed that it was H5N1 AI virus of genotype Z and was closely related to the virus from Vietnam (Viseshakul et al., 2004; Puthavathana et al., 2005). © 2008 Springer US.