Comparison of serological assays for detecting antibodies in ducks exposed to H5 subtype avian influenza virus

In the experimental study, the proportion of HI antibody-positive ducks increased slightly, from 0.57 when using chicken RBCs to 0.60 when using horse RBCs. The HI tests indicated almost perfect agreement (kappa？=？0.86) when results were dichotomised (titre？≥？4 log2), and substantial agreement (weighted kappa？=？0.80) for log titres. Overall agreements between the two HI tests were greater than between either of the HI tests and the VN test. The use of horse RBCs also identified a higher proportion of antibody positives in field duck sera (0.08, compared to chicken RBCs 0.02), with also almost perfect agreements for dichotomized results (Prevalence and bias adjusted Kappa (PABAK)？=？0.88) and for log titres (weighted PABAK？=？0.93), respectively. Factors that might explain observed differences in the proportion of antibody-positive ducks and in the agreements between HI tests are discussed.In conclusion, we identified a good agreement between HI tests. However, when horse RBCs were used, a higher proportion of sera was positive (titre？≥？4 log2) than using chicken RBCs, especially during the early response against H5N1 virus. The HRBC-HI might be more responsive in identifying early H5N1 HPAI serological response and could be a recommended assay for avian influenza sero-surveillance in both wild and domestic birds.The spread of the Eurasian lineage of H5N1 highly pathogenic avian influenza (HPAI) from China to other countries across Asia, Europe, the Middle East, and Africa is an unprecedented epizootic event. Although the initial outbreaks of H5N1 HPAI virus in Hong Kong, China, were successfully eradicated in late 1997 [1], the virus re-emerged in 2001 and 2002 causing HPAI outbreaks with high mortalities of chickens on commercial farms [1] and deaths of migratory birds and waterfowl, including ducks, in two local parks in Hong Kong [2]. At least three waves of H5N1 HPAI spread then occurred [3]: firstly, to East Asia and Southeast Asia between 2003 and 2004 [4,5]; se