Relationships among intramammary health, udder and teat characteristics, and productivity of extensively managed ewes

Authors: KNUTH, RYAN - Montana State University; STEWART, WHITNEY - University Of Wyoming; Taylor, Joshua - Bret; BISHA, BLEDAR - University Of Wyoming; YEOMAN, CARL - Montana State University; VAN EMON, MEGAN - Montana State University; Murphy, Thomas - Tom

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2021
Publication Date: 4/1/2021
Citation: Knuth, R.M., Stewart, W.C., Taylor, J.B., Bisha, B., Yeoman, C.J., Van Emon, M.L., Murphy, T.W. 2021. Relationships among intramammary health, udder and teat characteristics, and productivity of extensively managed ewes. Journal of Animal Science. 99(4):1-10. https://doi.org/10.1093/jas/skab059.
DOI: https://doi.org/10.1093/jas/skab059

Interpretive Summary: Mastitis is an inflammation of the mammary tissue typically in response to a bacterial infection. Signs of clinical mastitis include immense udder swelling, fever, and secretion of abnormal milk. In sheep, clinical infection is associated with increased risk of ewe and lamb mortality and is estimated to account for 7% of all ewes culled each year in the U.S. However, ewes with subclinical mastitis (SCM) have no observable signs of infection and, because of this, the prevalence and economic impact of SCM are not well understood. Milk was sampled from clinically healthy ewes at Montana State University and the USDA U.S. Sheep Experiment Station. Bacteria which have the potential to cause mastitis were present in 36% of milk samples. Morphometric traits of the udder were not found useful for predicting SCM. Somatic cell count (SCC) in milk, an indication of immune response, was negatively associated with weight of lamb weaned. Ewes with elevated SCC were expected to wean 5 to 8 kg less lamb than healthy ewes, which equates to a lost revenue of $19 to $32 per ewe. This research confirms that SCM is common in lactating ewes and is an economically important disease. Future research will focus on how to mitigate the negative impact of SCM on U.S. sheep production.

Technical Abstract: Mastitis is an economically important disease and its subclinical state is difficult to diagnose, which makes mitigation more challenging. The objectives of this study were to screen clinically healthy ewes in order to: (1) identify cultivable microbial species in milk, (2) evaluate somatic cell count (SCC) thresholds associated with intramammary infection, and (3) estimate relationships between udder and teat morphometric traits, SCC, and ewe productivity. Milk was collected from two flocks in early (< 5 d) and peak (30 to 45 d) lactation to quantify SCC (n = 530) and numerate cultivable microbial species by culture-based isolation followed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS; n = 243) identification. Within flock and lactation stage, 11% to 74% (mean = 36%) of samples were culture positive. More than 50 unique identifications were classified by MALDI-TOF MS analysis, and Bacillus licheniformis (18% to 27%), Micrococcus flavus (25%), B. amyloliquefaciens (7% to 18%), and Staphylococcus epidermidis (26%) were among the most common within flock and across lactation stage. Optimum SCC thresholds to identify culture-positive samples ranged from 175 × 10*3* to 1,675 × 10*3* cells/mL. Ewe productivity was assessed as total 120-d adjusted litter weight (LW120) and analyzed within flock with breed, parity, year, and the linear covariate of log10 SCC (LSCC) at early or peak lactation. Although dependent on lactation stage and year, each 1-unit increase in LSCC (e.g., an increase in SCC from 100 × 10*3* to 1,000 × 10*3* cells/mL) was predicted to decrease LW120 between 9.5 and 16.1 kg when significant. Udder and teat traits included udder circumference, teat length, teat placement, and degree of separation of the udder halves. Correlations between traits were generally low to moderate within and across lactation stage and most were not consistently predictive of ewe LSCC. Overall, the frequencies of bacteria-positive milk samples indicated that subclinical mastitis (SCM) is common in these flocks and can impact ewe productivity. Therefore, future research is warranted to investigate pathways and timing of microbial invasion, genomic regions associated with susceptibility and husbandry to mitigate the impact of SCM in extensively managed ewes.