The Impact of a Monthly Rest Day on Avian Influenza Virus Isolation Rates in Retail Live Poultry Markets in Hong Kong N. Y. Kung, AB Y. Guan, A N. R. Perkins, B L. Bissett, C T. Ellis, C L. Sims, C R. S. Morris, B K. F. Shortridge, A and J. S. M. Peiris A A Department of Microbiology, The University of Hong Kong, University Pathology Building, Queen Mary Hospital, Hong Kong SAR, China B Epicentre, IVABS, Massey University, Palmerston North, New Zealand C Agriculture, Fisheries and Conservation Department, Hong Kong SAR, China Received April 14, 2002 SUMMARY. Retail live poultry markets (LPMs) may act as a reservoir of avian influenza viruses (AIV). In this study we test the hypothesis that a rest day in the LPMs where the stalls are completely emptied of poultry, cleansed, and restocked will reduce the isolation rates of avian influenza viruses. The isolation rate of H9N2 subtype viruses from chicken was significantly lower after the rest day than prior to it, indicating its impact in reducing transmission. In contrast, Newcastle disease virus (NDV) isolation rates appear unaffected by this intervention, possibly reflecting differences in herd immunity or virus transmission dynamics. RESUMEN. Impacto de la implementacio ´n de un ‘‘Dı ´a de Descanso’’ en los mercados de aves vivas en Hong Kong sobre los rangos de aislamiento del virus de la influenza aviar. Los mercados de aves vivas pueden actuar como reservorios del virus de la influenza aviar. En este estudio se prueba la hipo ´tesis de que un ‘‘dı ´a de descanso’’ en estos mercados, utilizado para el vaciado, desinfeccio ´n y reintroduccio ´n de aves para la venta reducirı ´a los rangos de aislamiento del virus de la influenza aviar. Los rangos de aislamiento del virus de la influenza aviar del tipo H9N2 a partir de las aves en estos mercados fueron significativamente menores despue ´s de la implementacio ´n del dı ´a de descanso, lo cual es un buen indicador de que esta medida sirve para reducir la transmisio ´n de la enfermedad. En contraste, los rangos de aislamiento del virus de Newcastle no se vieron afectados por la implementacio ´n del descanso, lo cual refleja las posibles diferencias en los niveles de inmunidad de las parvadas o la dina ´mica de transmisio ´n viral. Key words: poultry, chicken, avian influenza virus, Newcastle disease virus, live poultry markets, disease control Abbreviations: AIV ¼ avian influenza virus; NDV ¼ Newcastle disease virus; LPMs ¼ live poultry markets Live poultry markets (LPMs) are recognized as a reservoir of avian influenza viruses (AIV) and a possible source of infection for domestic poultry (9). These markets may also facilitate avian in- fluenza virus undergoing reassortment as different types of birds (chicken, pigeon, quail, pheasant, Guinea fowl, chukar partridge, etc.) are placed in cages at high density and close proximity to each other within markets. During the bird flu incident in Hong Kong in 1997, 18 people were infected with avian influenza virus H5N1 and hospitalized. Exposure to live poultry at such markets was the major risk factor of infection for humans (7). There also have been several human infections with avian H9N2 reported in Hong Kong (5,8) and the Guangdong province of China (2,3). The H9N2 viruses isolated from humans in Hong Kong were similar to avian H9N2 virus isolated from quail (A/ Quail/Hong Kong/G1/97-like) (5). In May 2001, H5N1 virus re-emerged in Hong Kong’s LPMs and the markets were depopulated in This proceedings manuscript documents an oral pre- sentation given in the Session on Field Experiences in Control and Eradication at the Fifth International Symposium on Avian Influenza, April 14–17, 2002, The University of Georgia, Athens, GA. AVIAN DISEASES 47:1037–1041, 2003 1037