Time-to-event analysis of mastitis at first-lactation in Valle del Belice ewes B. Portolano a, ⁎ , R. Finocchiaro a , J.B.C.H.M. van Kaam b , V. Riggio a , D.O. Maizon a a Dipartimento S.En.Fi.Mi.Zo.-Sez. Produzioni Animali, Facoltà di Agraria, Università degli Studi di Palermo, Italy b Istituto Zooprofilattico Sperimentale della Sicilia “A. Mirri”, Italy Received 7 June 2006; received in revised form 27 September 2006; accepted 13 November 2006 Abstract A time-to-event study for mastitis at first-lactation in Valle del Belice ewes was conducted, using survival analysis with an animal model. The goals were to evaluate the effect of lambing season and level of milk production on the time from lambing to the day when a ewe experienced a test-day with a recorded SCC greater than or equal to 750,000 cells/ml, and to estimate, for this trait, its heritability and the percentage of variation explained by the flock-year of lambing effect. A dataset with 2468 first- lactation records, collected from 1998 to 2003 in Valle del Belice ewes allocated in 17 flocks, was used. The Cox model used included lambing season and total milk yield adjusted for lactation length as fixed effects and flock-year of lambing effect and individual additive genetic effect as random effects. In total 40.5% of the records were censored. Results indicated that ewes lambing from April to July were at a higher risk of mastitis than those lambing from August to November (conventional season), and that ewes in the highest class of milk production were at a higher risk of mastitis than those in the lowest level. The heritability for the time interval between lambing and first test-day with mastitis was 3% on the logarithmic scale and 4% on the real scale. The proportion of variation, in the time interval between lambing and first test-day with mastitis, explained by the flock-year of lambing effect was 19% on the logarithmic scale and 27% on the real scale; this seems to stress the importance of flock management. © 2006 Elsevier B.V. All rights reserved. Keywords: Mastitis; Somatic cell count; Dairy sheep; Survival analysis; Milk production 1. Introduction Clinical and subclinical mastitis are some of the most frequent diseases affecting small dairy ruminants. Mastitis is important for economic, hygienic (con- sumption of dairy products) and legal reasons (such as EU directives 46/92 and 71/94 defining the bacterio- logical quality of milk). Although management could be the most effective way to prevent mastitis, selection for mastitis resistance has been shown to be a valid alternative (Barillet et al., 2001; Heringstad et al., 2003). Direct selection against clinical or subclinical mastitis, however, is difficult because events of any Livestock Science 110 (2007) 273 – 279 www.elsevier.com/locate/livsci ⁎ Corresponding author. Dipartimento S.En.Fi.Mi.Zo.-Sez. Produ- zioni Animali, Facoltà di Agraria, Viale delle Scienze-Parco d'Orleans, Italy. Tel.: +39 0917028868; fax: +39 0917028873. E-mail address: baldop@unipa.it (B. Portolano). 1871-1413/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.livsci.2006.11.010