Somatic cell count and California Mastitis Test as a diagnostic tool for subclinical mastitis in ewes

ABSTRACT

Background: Infectious mastitis has been described as one of the main diseases affecting animals during lactation. The disease in sheep has been studied for many years in countries where mastitis has an economical importance. However, the interest in mastitis studies in animals raised for food production has increased, because the disease may cause a reduction in weight and an increased mortality in lambs. In this study, Somatic Cells Count (SCC) and California Mastitis Test (CMT) were related to bacterial isolation for mastitis diagnosis in Corriedale sheep. Material, Methods & Results: Twenty nine (29) ewes, varying from 2-5 years of age, with different numbers of lactation and born lambs and never before machine-milked were used. Milking was done from October to November, once a week in the morning period, with oxytocin application. Four milk collections were made, at biweekly intervals for bacteriologic, SCC and CMT test, which were analyzed considering each gland as a sampling unit. Low incidence of subclinical mastitis (9.66%) was observed, with the majority (15/17) caused by coagulase negative Staphylococcus. There was no change in milk production related to SCC or bacteria isolation. However changes in milk components occurred in the presence of subclinical mastitis. A poor relation (k = 0.115) was determined between the results obtained in the bacteriological test and the SCC, with low sensitivity (13.33%) and an increased number of false negative results (13%). Comparing the SCC and CMT results as the diagnostic method for subclinical mastitis, a low (r = 0.2319) but significant (P = 0.0209) correlation was observed, as well as a poor concordance (k = 0.152). Using bacterial isolation as the standard test, it was determined that the CMT has low sensitivity (28.57%) as the diagnostic method of mastitis in ewes. Discussion: Mastitis has been considered an economically important disease in the production of sheep for meat and wool. According to reports, the frequency of its clinical occurrence may range from zero to 50%. In meat-producing herds, a low weight gain in lambs has been associated with subclinical mastitis and the study of mastitis in Corriedale sheep is justified since this is considered a breed of meat sheep with the best milk production. This breed is being crossed with milkproducing breeds, such as Laucane, to form milk-producing herds. Bacterial isolation has been adopted as the diagnostic method of mastitis in all livestock breeds. Similarly to the observation made by this study, Staphylococcus and, in a few cases, Streptococcus, have been the microorganisms most frequently involved in subclinical mastitis in sheep. The milk from ewes with mastitis tends to have a lower fat and lactose content than that of healthy ewes, due to the affected secretory function of these animals. The SCC of milk ewes has not yet been established, but its count in a healthy udder may reach up to 1.5x106 cel.mL-¹. Similarly, the CMT score to be used in sheep is still controversial, but the maximum score (+++) is adopted to indicate mastitis. The high number of false-negative and false-positive reactions observed in SCC and CMT tests means that healthy and ill animals are incorrectly identified and that no preventive and curative measures are adopted. Since the utilization of only one diagnosis method in sheep mastitis, without confirmation by bacteriologic test is not conclusive, the SCC and CMT should be used cautiously in sheep mastitis diagnosis.