Subclinical mastitis in sheep: etiology and association with milk somatic cell count and ewe productivity in three research flocks in the Western U.S.

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

Submitted to: Translational Animal Science
Publication Type: Proceedings
Publication Acceptance Date: 5/21/2019
Publication Date: 12/16/2019
Citation: Knuth, R., Stewart, W., Taylor, J.B., Yeoman, C., Bisha, B., Page, C., Rowlye, C., Lindsey, B., Van Emon, M., Murphy Jr, T.W. 2019. Subclinical mastitis in sheep: etiology and association with milk somatic cell count and ewe productivity in three research flocks in the Western U.S. Translational Animal Science. 3(1):1749-1753. https://doi.org/10.1093/tas/txz078.
DOI: https://doi.org/10.1093/tas/txz078

Interpretive Summary: In its most basic definition, mastitis is an inflammation of the mammary tissue typically in response to a bacterial infection. Though the majority of mastitis research has focused on dairy-type animals, it can affect lactating female mammals of all breeds/species. Symptoms of clinical mastitis include immense udder swelling, fever, and secretion of abnormal milk. In sheep, clinical infection has animal welfare implications and is associated with increased veterinary costs and enhanced ewe and lamb mortality risk. Recently, it was estimated that clinical mastitis was the culprit of ~7% of all ewes culled each year in the U.S. Ewes infected with subclinical mastitis (SCM) have no observable symptoms and their milk appears visually normal despite harboring an infection. Because of this, ewes with SCM are not diagnosed or treated and, therefore, its prevalence and economic impact are not well understood. It was hypothesized that the incidence of SCM would be high within three research flocks and that it would be associated with reduced lamb growth. Milk was sampled from a total of 110 clinically healthy ewes at Montana State University, the University of Wyoming, and the U.S. Sheep Experiment Station in early and mid-lactation. Across flocks, 26 – 54% of milk samples tested positive for one or more bacteria. The most common species identified were Escherichia coli, Staphylococcus spp., and Bacillus licheniformis. Somatic cell count (SCC), an indication of the number of white blood cells in milk, could predict ewe infection status but this methodology is labor intense and expensive to employ in most production scenarios. Ewes that tested positive were expected to wean litters that 5.2 kg lighter than healthy ewes. With current lamb prices, SCM is expected to reduce returns by $26 per ewe per year. Therefore, subclinical mastitis is a common and economically important disease in sheep production and future research to mitigate its impact is warranted.

Technical Abstract: Clinical mastitis is an economically devastating disease for sheep producers, resulting in losses attributed to ewe culling, lamb morbidity and mortality, and veterinary costs. However, since subclinical mastitis (SCM) has no visually observable symptoms, its diagnosis is difficult and economic impact unknown. The objectives of this study were to identify bacterial species present in ewe milk, evaluate somatic cell count (SCC) thresholds to predict intramammary infection (IMI), and estimate the effect of SCM on ewe productivity in 3 research flocks. Milk was collected from ewes twice in lactation and was used to quantify SCC and identify taxa by both PCR and culture followed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). In total, 54% of samples were culture-positive for any bacteria and 26% were positive within the PCR test screen. Sixty unique classifications at either the species or genera level were identified by MALDI-TOF-MS; the most common were Bacillus licheniformis (22%), B. altitudinis (11%), Staphylococcus auricularis (9%), S. epidermidis (9%), and B. amyloliquefaciens (8%). Multiplex PCR (16-plex) allowed for identification of 13 different species with Escherichia coli (51%), Staphylococcus spp. (including S. aureus; 32%), Klebsiella spp. (18%), Enterococcus spp. (14%), and Trueperella pyogenes and/or Peptoniphilus indolicus (12%) being the most common. Somatic cell count thresholds were evaluated as diagnostic tools to infer ewe IMI status. Over the range of 100 to 2000 x 103 cells/mL, Youden’s Index was maximized at 1375 and 400 x 103 cells/mL for ewe culture and PCR status, respectively. At these thresholds, sensitivity and specificity values were 29% and 82% for culture growth classification and 74% and 47% for PCR status. The effect of ewe culture status on total litter weaning weight was examined in one flock. On average, culture-positive ewes weaned 5.2 kg lighter (P = 0.04) litters than healthy ewes and represent an economic loss of $26 per ewe. Subclinical mastitis is a common and costly disease in U.S. flocks and future research aimed at mitigating its effects is warranted.