Back-calculation method shows that within-flock transmission of highly pathogenic avian influenza (H7N7) virus in the Netherlands is not influenced by housing risk factors Marian E.H. Bos a, *, Mirjam Nielen a , Guus Koch b , Annemarie Bouma a , Mart C.M. De Jong c , Arjan Stegeman a a Department of Farm Animal Health, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 7, 3584 CL Utrecht, The Netherlands b Avian Virology, Department of Virology, Central Veterinary Institute of Wageningen UR, P.O. Box 65, 8200 AB Lelystad, The Netherlands c Department of Quantitative Veterinary Epidemiology, Animal Sciences Group, P.O. Box 338, 6700 AH Wageningen, The Netherlands 1. Introduction During the last decades avian influenza viruses caused numerous outbreaks in poultry worldwide, causing vast economical and societal damage. Both in the developed and the developing world the eradication of highly pathogenic avian influenza (HPAI) viruses is extremely difficult in poultry dense regions, and can only be stopped by virtually killing all poultry in an affected region (Boender et al., 2007; Bowes et al., 2004; Capua et al., 2002; Stegeman et al., 2004). Besides the economical and societal damage, animal welfare is at stake, as well as human health because of exposure to the large amounts of virus produced. Clearly, more knowledge on the mechan- ism of transmission between flocks is essential to develop control programmes that stop virus transmission in a more efficient way. Yet, until now studies on the transmission between flocks have mainly had their emphasis on the (lack of) effectiveness of different control measures, such as (pre-emptive) culling, bio security measures and transportation bans (Garske et al., 2007; Mannelli et al., 2007; Stegeman et al., 2004; Stegeman et al., 2005). Moreover, risk factors for the introduction of HPAI virus into a flock were studied. Thomas et al. (2005) found an Preventive Veterinary Medicine 88 (2009) 278–285 ARTICLE INFO Article history: Received 7 December 2007 Received in revised form 24 October 2008 Accepted 11 December 2008 Keywords: Avian influenza H7N7 Transmission Risk factors Field data ABSTRACT To optimize control of an avian influenza outbreak knowledge of within-flock transmission is needed. This study used field data to estimate the transmission rate parameter (b) and the influence of risk factors on within-flock transmission of highly pathogenic avian influenza (HPAI) H7N7 virus in the 2003 epidemic in The Netherlands. The estimation is based on back-calculation of daily mortality data to fit a susceptible- infectious-dead format, and these data were analysed with a generalized linear model. This back-calculation method took into account the uncertainty of the length of the latent period, the survival of an infection by some birds and the influence of farm characteristics. After analysing the fit of the different databases created by back-calculation, it could be concluded that an absence of the latency period provided the best fit. The transmission rate parameter (b) from these field data was estimated at 4.50 per infectious chicken per day (95% CI: 2.68–7.57), which was lower than what was reported from experimental data. In contrast to general belief, none of the studied risk factors (housing system, flock size, species, age of the birds in weeks and date of depopulation) had significant influence on the estimated b. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +31 30 253 6487; fax: +31 30 252 1887. E-mail address: m.e.h.bos@uu.nl (Marian E.H. Bos). Contents lists available at ScienceDirect Preventive Veterinary Medicine journal homepage: www.elsevier.com/locate/prevetmed 0167-5877/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.prevetmed.2008.12.003