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

Abstract

a b s t r a c tMastitis is an inflammation of the udder, mainly caused by bacteria, and leads to economic loss, due to discarded milk, reduced milk production, reduced milk quality and increased health costs in both dairy sheep and cattle. Selecting for increased genetic resistance to mastitis can be done directly or indirectly, with the indirect selection corresponding to a prediction of the bacteriological status of the udder based on traits related to the infection. The most frequently used indirect method is currently milk somatic cell count (SCC) or somatic cell score (SCS). This review reports the state of the art relating to the genetic basis of mastitis resistance in sheep and explores the opportunities to use SCC as selection criterion in a breeding programme to improve resistance to mastitis in sheep, discussing the actual situation and prospects for improvement. It has been stressed, in particular, that although it is unlikely that selection for mastitis resistance by the farmers on their own will be successful, there is good prospect for genetic improvement if reliable pedigree and performance recording is implemented across flocks, combined with breeding value estimation. To achieve this, a strong and well-structured organization to implement and support the programme is essential.

Association between polymorphism of ABCG2 gene and somatic cell count in Czech dairy sheep breeds

“…Mastitis is becoming a major health problem in dairy sheep (8). It is an inflammatory disease of the mammary gland, which is manifested by an increased number of somatic cells in milk (18). Inflammation of the mammary gland is caused mainly by bacteria -the most prevalent are coagulase-negative staphylococci (22,24).…”

“…Subclinical mastitis negatively influences milk yield, and the low milk yield (especially with a reduced lactose concentration) is associated with a low cheese yield and cheese quality (22). The limit of somatic cell count (SCC) is not currently laid down for sheep -in cattle, this limit is 250 000-300 000 SCC/ml (18). Leitner et al (11) classified the quality of milk with respect to reduced milk production in sheep and goats with udder infection: infection of 25% of udders in a herd was associated with 4.1% milk loss and 5.2% curd loss; whereas infection of 75% of udders in a herd resulted in 12.2% milk loss and 15.5% curd loss.…”

Evaluation of somatic cells in milk of ewes as possible physiological level

“…SCC in the milk of individual dairy sheep can be used as an indicator of husbandry and milking hygiene, the welfare of animals, but especially the udder health and the presence of subclinical mastitis. In dairy sheep the mastitis leads to decrease in milk production and its quality, increase presence of pathogens inappropriate for consumption of dairy products and increase cost for treatment (Riggio and Portolano, 2015). The prevalence of subclinical forms of ewes mastitis ranges from 5 to 50% (Bergonier et al, 2003; Contreras et al, 2007Olechnowicz and Jaskowski, 2014).…”

Association study between β-defensin gene polymorphisms and mastitis resistance in Valle del Belice dairy sheep breed

“…Selection for genetic resistance to mastitis can be done directly or indirectly. Direct selection relates to the diagnosis of the infection, whereas indirect methods have been widely applied based on the evaluation of the degree of inflammation or of internal mammary lesions (Riggio and Portolano, 2015).…”