STUDY OF MASTITIS IN DAIRY SHEEP USING SURVIVAL ANALYSIS

Risultato della ricerca: Other

Abstract

Mastitis is the most prevalent disease present in livestock species leading to economic loss. In dairy sheep,it caused mainly from bacterial infections. The aim of this work was to investigate the risk of having mastitisin Valle del Belice dairy ewes during the first lactation, due to environmental or contagious pathogens,using a survival analysis approach.All test‐day records from primiparous ewes were collected from five flocks. All test‐day were grouped intwo data sets, one with mastitis due to environmental pathogens (ENV) and the other with mastitis due tocontagious pathogens (CON). In this analysis the follow up period of a ewe was the lactation, consequentlyall the record began with lambing. Ewes without infection at the end of the first lactation were treated asright censored. The analysis was performed with the Survival kit v.6.1 (Ducrocq et al., 2010). Cox model wasused: h(t; x_m,z_m )=h_0 (t)exp{x_m^' (t)┤β+z_m^i u where: h_0 (t) is the baseline hazard; β contains thetime‐independent covariates: litter size (LS, 2 classes), age at first lambing (AFL, 4 classes); and timedependentcovariates: milk production within flock (MK, 3 classes); somatic cell count within flock (SCC, 3classes); EP: environmental pathogens, that was only considered when analyzing contagious pathogens (2classes) and CP: contagious pathogens, that was considered only when analyzing environmental pathogens(2 classes); u contains the time‐independent random effect of FYS: flock‐year‐season (46 classes).For ENV and CON about 48% and 85% of the records were right censored, respectively. Chi‐square testapproximations based on likelihood ratio test were calculated for all the effects in the model. For the ENV,the time‐dependent fixed effects: MK, SCC and CP were significant. A higher risk of being infected (hazardratio, HR, 1.41) was observed for ewes with low MK compared to reference class (i.e. MK=2). Higher HRswere found for animals that had a medium and high level of SCC (1.80 and 2.23, respectively) compared tothe reference class of low level of SCC. A lower HR was observed for animals that have been infected withcontagious pathogens (0.08). For CON, SCC and EP were significant:. A higher HR was found for animals thathad a medium (1.79) and high (3.96) level of SCC compared to the reference class of low level of SCC. Alower HR was observed for animals that have been infected with environmental pathogens (0.09).Results suggest that selection for decreased SCC may be effective to reduce mastitis incidence and thebreeding goal should favor ewes with lowest observed SCC.Ducrocq, V., Solkner, J. and Meszaros G. (2010). Survival Kit v6 a Software Package for Survival Analysis. In:9th World Cong. Genet. Appl. Livest. Prod., Leipzig, Germany.
Lingua originaleEnglish
Numero di pagine1
Stato di pubblicazionePublished - 2013

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