Response to somatic cell count-based selection for mastitis resistance in a divergent selection experiment in sheep

Abstract

A divergent selection experiment in sheep was implemented to study the consequences of log-transformed somatic cell score (SCS)-based selection on resistance to natural intramammary infections. Using dams and progeny-tested rams selected for extreme breeding values for SCS, we created 2 groups of ewes with a strong divergence in SCS of approximately 3 genetic standard deviations. A survey of 84 first-lactation ewes of both the High and Low SCS lines indicated favorable responses to SCS-based selection on resistance to both clinical and subclinical mastitis. All clinical cases (n = 5) occurred in the High SCS line. Additionally, the frequency of chronic clinical mastitis, as detected by the presence of parenchymal abscesses, was much greater in the High SCS line (n = 21) than in the Low SCS line (n = 1). According to monthly milk bacteriological examinations of udder halves, the prevalence of infection was significantly greater (odds ratio = 3.1) in the High SCS line than in the Low SCS line, with predicted probabilities of 37 and 16%, respectively. The most frequently isolated bacteria responsible for mastitis were staphylococci: Staphylococcus auricularis (42.6% of positive samples), Staphylococcus simulans, Staphylococcus haemoliticus, Staphylococcus xylosus, Staphylococcus chromogenes, Staphylococcus lentus, Staphylococcus warneri, and Staphylococcus aureus. The incidence of positive bacteriology was greater in the High SCS line (39%) than in the Low SCS line (12%) at lambing, indicating that High SCS line ewes were especially susceptible to postpartum subclinical mastitis. Negativation of bacteriological results from one sampling time point to the next was markedly different between lines after weaning (e.g., 41 and 84% in the High and Low SCS lines, respectively). This result was consistent with differences in the duration of infection, which was much greater in the High SCS line compared with the Low SCS line. Finally, ewes from the High SCS line consistently had greater SCS in positive milk samples than did ewes from the Low SCS line (+2.04 SCS, on average), with an especially large difference between lines during the suckling period (+3.42 SCS). Altogether, the preliminary results suggest that the better resistance of Low SCS line ewes, compared with High SCS line ewes, was principally characterized by a better ability to limit infections during the peripartum period, to eliminate infections during lactation, and quantitatively to limit the inflammation process and its clinical consequences.

Genetic selection for reduced somatic cell counts in sheep milk: A review

“…These results, therefore, indicate that selection for reduced SCS can help to reduce mastitis incidence. In this regard, results by Rupp et al (2009) from a firstlactation survey in dairy sheep have provided evidence that selection based on SCS estimated breeding values (EBVs) may help to improve resistance to clinical and subclinical mastitis. Low SCS line animals showed a lower incidence of clinical mastitis, a lower prevalence of mammary abscesses and subclinical intramammary infections, especially at parturition.…”

Review: Towards the agroecological management of ruminants, pigs and poultry through the development of sustainable breeding programmes: I-selection goals and criteria

“…This has mainly focussed on diseases with a high incidence and major economic impact, such as mastitis in dairy ruminants, infection by helminths during grazing in small ruminants and in outdoor poultry systems. Selection against infection by a given pathogen or group of pathogens has been successful during several experiments, despite low to moderate heritability values in poultry , pigs (Kadowaki et al, 2012), and ruminants (Morris, 2007;Rupp et al, 2009). However, this type of selection may impair the ability of animals to resist pathogens other than those targeted by the selection process.…”

Association of <i>TLR4</i> and <i>CARD15/NOD2</i> polymorphisms with SCC in Holstein–Friesian cattle

“…Unsurprisingly, quantitative trait loci (QTL) for mastitis and/or SCC have been identified in nearly all chromosomes of the bovine genome (Rupp et al, 2009). The genes that are strong potential markers for resis-tance/susceptibility to udder inflammation include, inter alia, genes encoding PAMP-recognition receptors (PRRs) (Sharma et al, 2006).…”

“…Mastitis can be classified as subclinical or clinical, depending on whether clinical signs are present or not. In dairy practice, this inflammatory process in the dairy cow's mammary gland is generally detected by measuring somatic cell count, SCC (Schukken et al, 2003). SCC has a higher heritability than clinical mastitis (CM), allowing more effective genetic progress (Heringstad et al, 2003).…”

“…Due to the involvement of multiple etiological agents, it has always been a challenge to veterinarians throughout the world (Vashney et al, 2012). Mastitis is the consequence of the adverse effect on the mammary gland and the cow's immune system of many factors -environmental (both external and internal), genetic and the interactions between them (Rinaldi et al, 2010;Schukken et al, 2011).…”