Interdependence and distribution of subclinical mastitis and intramammary infection among udder quarters in dairy cattle Donagh P. Berry * , William J. Meaney Dairy Production Department, Teagasc, Moorepark Production Research Centre, Fermoy, Co. Cork, Ireland Received 19 September 2004; received in revised form 16 December 2005; accepted 7 February 2006 Abstract The objective of the present study was to determine the distribution of subclincial mastitis and intramammary infection (IMI) across udder quarters and to quantify, if any, the level of inter- dependence between quarters. Subclinical mastitis was defined as an udder quarter somatic cell count of 250,000. Intramammary infection was defined as the presence of pathogens in an udder quarter. The data used in the analyses consisted 6577 test-day records from 2034 lactations with observations on subclinical mastitis and 8428 test-day records from 2575 lactations with observations on IMI; each test-day record consisted data on all four udder quarters. Records were from animals on three research herds in southern Ireland. Observed distributions of IMI were compared to expected distributions, estimated using a binomial probability distribution, assuming independence among quarters. Generally, the observed frequencies deviated significantly from the expected frequencies, indicating some level of interdependence between quarters. Intraclass correlations were estimated from a mixed model including cow and test-day record nested within cow as random effects; fixed effects in the model included herd, year of calving, month of calving, parity, days in milk at sampling and interactions. Intraclass correlations within test-day record ranged from 0.14 to 0.50 for subclinical mastitis and from 0.02 to 0.40 for IMI. Thus, substantial interdependence between quarters exists which should be accounted for when analysing data from experiments across udder quarters. Failure to do so may result in incorrect inferences from statistical tests. # 2006 Elsevier B.V. All rights reserved. Keywords: Udder; Quarters; Interdependence; Distribution; Mastitis www.elsevier.com/locate/prevetmed Preventive Veterinary Medicine 75 (2006) 81–91 * Corresponding author. Tel.: +353 254 2386; fax: +353 254 2340. E-mail address: donagh.berry@teagasc.ie (D.P. Berry). 0167-5877/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.prevetmed.2006.02.001