EQUINE VETERINARY JOURNAL Equine vet. J. (2008) 40 (7) 709-711 doi: 10.2746/042516408X363297 709 Background Equine infectious anaemia (EIA) was confirmed in Ireland on 15th June 2006. During the following 6 months, until 10th December 2006, a total of 38 EIA cases were detected, including a single case in Northern Ireland. These cases were linked within 2 clusters, centred in counties Meath and Kildare. The Meath cluster was primarily related to ‘Veterinary Practice X and its associated clinic’ and the Kildare cluster to ‘Veterinary Hospital Y’ (subsequently termed ‘the hospital’). Aspects of this outbreak have been presented elsewhere, including an overview of the outbreak (Brangan et al. 2008), information about the initial source of infection, aspects of diagnosis and clinical presentation during the outbreak, and of the modes of transmission and spread within the Meath cluster (More et al. 2008). Objectives This paper presents the findings of an epidemiological investigation of the Kildare cluster, with emphasis on the mode(s) of transmission and spread of infection. Methods The Kildare cluster was investigated using standard epidemiological methods of outbreak investigation, which included establishing the existence of an outbreak, verifying the diagnosis, defining and identifying cases, describing and analysing the data in terms of time, space and animal factors, developing and evaluating hypotheses, refining hypotheses and carrying out additional studies, implementing (or modifying) control and prevention measures, and communicating the findings. Confirmed cases included all horses with serological and/or virological evidence of infection with EIA virus, based on one or more of the following tests: the agar gel immunodiffusion (AGID, Coggins) test, one or more commercially-available ELISA tests, immunoblot and/or quantitative PCR and RT-PCR. Unconfirmed cases included all horses with clinical and/or epidemiological (but no corroborating serological and/or virological) evidence consistent with EIA infection. Information about the hospital (including physical infrastructure, routine work practices and defined events) was obtained through observation and staff interview at several on-site visits. As relevant, photographs were taken and maps were created. Daily admission sheets and hospital treatment records were examined to obtain information about cases and associated animals. A range of hypotheses concerning the transmission of infection was evaluated, including iatrogenic transmission, transmission as a result of close contact and vector-borne transmission, using criteria described previously (More et al. 2008). In order to assess the relative plausibility of alternative modes of transmission, each case was first assigned an ‘infection status’ (primary case, probable primary case, secondary case, possible tertiary case, tertiary case), after reviewing all relevant data and identifying all plausible point(s)/window(s) and place(s) of infection. A case-control study was also conducted to elucidate risk factors associated with being a case. The study period covered the period between 6th May (the date of entry to the hospital of the first primary case) and 14th June 2006 (the date of exit from the hospital of the third primary case). The reference population included all horses, both in-patients and accompanying animals that were hospital residents for at least one night during the study period, except the 3 primary and 2 probable primary cases. All confirmed EIA cases in the reference population were enrolled as cases in the study. All other horses in the reference population were eligible for selection as controls. Horse-level data were collected from a range of sources, including daily charts, billing records and discussions with hospital staff. A multivariable exact logistic regression model was developed based on a forward stepwise selection procedure. Results In total, 22 confirmed EIA cases were linked to the Kildare cluster, including 21 horses that also been residents of the hospital located An outbreak of equine infectious anaemia in Ireland during 2006: The modes of transmission and spread in the Kildare cluster Full peer reviewed paper available electronically at www.evj.co.uk/suppinfo S. J. MORE*, I. AZNAR, T. MYERS † , D. P. LEADON ‡ and T. A. CLEGG Centre for Veterinary Epidemiology and Risk Analysis, UCD Agriculture, Food Science and Veterinary Medicine, University College Dublin, Belfield, Dublin 4; † Department of Agriculture, Fisheries and Food, District Veterinary Office, Poplar House, Poplar Square, Naas, Co.Kildare; and ‡ Irish Equine Centre, Johnstown, Naas, Co. Kildare, Ireland. Keywords: horse; equine infectious anaemia; EIA; Ireland; epidemiology; transmission; spread; outbreak investigation; veterinary hospital *Author to whom correspondence should be addressed. [Paper received for publication 22.04.08; Accepted 24.08.08]