Petersson, Katherine [faculty advisor, Department of Animal & Veterinary Science]




somatic cells; somatic cell count; sheep; milk; pyronin Y-methyl green; methylene blue; stain; mastitis


Somatic cell count is a key method used to evaluate the quality of milk in today’s growing dairy sheep industry. Somatic cells are body cells, primarily the white blood cells, found in a milk sample. If an infection such as mastitis is present, the number of somatic cells in the milk increases (Gonzalo, et al. 1992). Producers routinely perform somatic cell counts on cow and goat milk. For dairy cattle in the United States, the somatic cell count can not exceed 750,000 cells per milliliter, for commercial milk. For dairy goats the accepted limit is 1,000,000 cells per mL of milk. Somatic cell count procedures on sheep milk however, have not been widely tested and limits are highly debatable, ranging from 250,000 cells per mL to 1,000,000 per mL (Pengov 2001). This project will focus on obtaining valuable information about somatic cell counts in lactating ewes. There are several methods for determining somatic cell count. Currently sheep milk is evaluated by cow milk standards using the methylene blue stain. However, cows and sheep have different secretory systems. Sheep shed more cellular debris in their milk than cows do, and the blue stain can sometimes falsely mark these cell particles as somatic cells (Gonzalo et al 2003). Goats have the same secretory system as sheep, and the stain used to evaluate their milk’s somatic cell count is better at differentiating between cells and cellular debris. The green stain is the only officially acceptable method to directly measure somatic cell count in goat milk (Haenlein 1997). This points to the conclusion that comparisons using dyes with higher tint specificity, such as pyronin Y-methyl green, would be of great use in sheep milk (Gonzalo et al. 2003). For this project milk samples were collected from lactating ewes at the University of Rhode Island’s Peckham farm. Several samples were taken from each ewe and collected in a plastic tube. The milk was then refrigerated and later pipetted in small aliquots onto slides. The slides were then dried and stained with either methylene blue or pyronin Y-methyl green stain. Once the slides were stained, they were examined under the oil immersion objective lens of the microscope and a direct microscopic somatic cell count was taken. Our preliminary results have shown that milk stained with pyronin Y-methyl green has a lower somatic cell count than the same sample stained with methylene blue. This indicates that the blue stain does not only stain somatic cells but extracellular components as well, making it less suitable for use in sheep milk. For this reason, we recommend that the green stain be used for somatic cell count in sheep rather than the blue stain.