Divergent selection on milk somatic cell count in goats improves udder health and milk quality with no effect on nematode resistance

摘要

Milk somatic cell count (SCC) is commonly higher ingoats than in cattle and sheep. Furthermore, the abilityof milk SCC to predict mastitis is considered lower ingoats than in cattle and sheep, and the relevance ofsomatic cell score (SCS)-based selection in this specieshas been questioned. To address this issue, we created 2divergent lines of Alpine goats using artificially inseminatedbucks with extreme estimated breeding valuesfor SCS. A total of 287 goats, 158 in high- and 129 inlow-SCS lines, were scrutinized for mastitis infections.We subjected 2,688 milk samples to conventional bacteriologicalanalyses on agarose and bacterial counts wereestimated for positive samples. The SCS, milk yield,fat content, and protein content were recorded every3 wk. Clinical mastitis was systematically noted. Asubset of 40 goats (20 from each line) was subsequentlychallenged with Haemonchus contortus and monitoredfor anemia (blood packed cell volume) and fecal eggcounts to see if SCS-based selection had an indirecteffect on resistance to gastrointestinal nematodes. Milkproduction traits, including milk quantity, fat content,and protein content, were similar in both goat lines. Incontrast, the raw milk SCC almost doubled betweenthe lines, with 1,542,000 versus 855,000 cells/mL in thehigh- and low-SCS lines, respectively. The difference inbreeding value for SCS between lines was 1.65 geneticstandard deviation equivalents. The Staphylococcus spp.most frequently isolated from milk were S. xylosus, S.caprae, S. epidermidis, and S. aureus. The frequency ofpositive bacteriology samples was significantly higherin the high-SCS line (49%) than in the low-SCS line(33%). The highest odds ratio was 3.49 (95% confidenceinterval: 11.95-6.25) for S. aureus. The distribution ofbacterial species in positive samples between lines wascomparable. The average quantity of bacteria in positivesamples was also significantly higher in high-SCSgoats (69 ± 80 growing colonies) than in low-SCS goats(38 ± 62 growing colonies). Clinical cases were rare andequally distributed between high- (n = 4; 2.5%) andlow-SCS (n = 3; 2.3%) lines. Furthermore, the largerthe amounts of bacteria in milk the higher the SCS level.Conversely, goats with repeatedly culture-negativeudders exhibited the lowest SCC levels, with an averageof below 300,000 cells/mL. We therefore confirmed thatSCS is a relevant predictor of intramammary infectionand hygienic quality of milk in goats and can be usedfor prophylactic purposes. After challenge with H. contortus,goats were anemic with high fecal egg countsbut we found no difference between the genetic lines.This result provides initial evidence that resistance tomastitis or to gastrointestinal nematodes infections isunder independent genetic regulation. Altogether, thismonitoring of the goat lines indicated that SCS-basedselection helps to improve udder health by decreasingmilk cell counts and reducing the incidence of infectionand related bacterial shedding in milk. Selection forlow SCC should not affect a goat’s ability to cope withgastrointestinal nematodes.