The spread of highly pathogenic avian influenza (subtype H5N1) clades in Bangladesh, 2010 and 2011.

Since the global spread of highly pathogenic avian influenza H5N1 during 2005-2006, control programs have been successfully implemented in most affected countries. HPAI H5N1 was first reported in Bangladesh in 2007, and since then 546 outbreaks have been reported to the OIE. The disease has apparently become endemic in Bangladesh. Spatio-temporal information on 177 outbreaks of HPAI H5N1 occurring between February 2010 and April 2011 in Bangladesh, and 37 of these outbreaks in which isolated H5N1 viruses were phylogenetically characterized to clade, were analyzed. Three clades were identified, 2.2 (21 cases), 2.3.4 (2 cases) and 2.3.2.1 (14 cases). Clade 2.2 was identified throughout the time period and was widely distributed in a southeast-northwest orientation. Clade 2.3.2.1 appeared later and was generally confined to central Bangladesh in a north-south orientation. Based on a direction test, clade 2.2 viruses spread in a southeast-to-northwest direction, whereas clade 2.3.2.1 spread west-to-east. The magnitude of spread of clade 2.3.2.1 was greater relative to clade 2.2 (angular concentration 0.2765 versus 0.1860). In both cases, the first outbreak(s) were identified as early outliers, but in addition, early outbreaks (one each) of clade 2.2 were also identified in central Bangladesh and in northwest Bangladesh, a considerable distance apart. The spread of highly pathogenic avian influenza H5N1 in Bangladesh is characterized by reported long-distance translocation events. This poses a challenge to disease control efforts. Increased enforcement of biosecurity and stronger control of movements between affected farms and susceptible farms, and better surveillance and reporting, is needed. Although the movement of poultry and equipment appears to be a more likely explanation for the patterns identified, the relative contribution of trade and the market chain versus wild birds in spreading the disease needs further investigation.