Identifying the major bacteria causing intramammary infections in individual milk samples of sheep and goats using traditional bacteria culturing and Real-time Polymerase Chain Reaction

Abstract Text: Milk provides the major source of income in dairy farms while one of the main causes of milk production losses is intramammary infection (IMI). Depending on the bacteria type involved, some bacteria cause clinical infection, while the majority of the bacteria cause subclinical with no visible signs of infection. Consequently, in order to identify infected animals, as well as the bacteria, milk sampling and laboratory diagnosis are needed. Use of DNA based methods such as real-time PCR has increased sensitivity and shortened time for bacteria identification, compared to traditional bacteriology; however, results should be regarded with caution. One-hundred and eight lactating dairy ewes (Manchega, 56; Lacaune, 52) and 24 Murciano-Granadina dairy goats were used for identifying the main bacteria causing IMI using traditional bacterial culturing and real-time PCR and their effects on milk performances. Milk samples were taken aseptically from each udder-half for bacterial culture and somatic cell count (SCC) 3 times throughout lactation. The IMI was assessed based on bacteria isolated in ≥2 samplings accompanied by increased SCC. Mammary gland infection was caused mainly by S. aureus and various CNS species, and resulted in lowering milk yield and decrease of its quality as indicated by coagulation. Prevalence of subclinical IMI was 42.9% in Manchega, 50.0% in Lacaune and 41.7% in goats, estimated milk yield loss being 13.1, 17.9 and 18.0%, respectively. According to bacteriology results, 87% of the identified single bacteria colonies or culture-negative growths were repeatable throughout samplings, and bacteriology and PCR had 100% agreement. Nevertheless, the study emphasized that one sampling may not be sufficient to determine IMI and therefore, other inflammatory responses such as increased SCC should be monitored to identify true infections. Moreover, when PCR methodology is used, aseptic and precise milk sampling procedure is the key for avoiding false positive amplifications. In conclusion, both PCR and bacterial culture methods proved to have similar accuracy for identifying infective bacteria in sheep and goats. Final methods choice will depend on their diagnosis time and analysis cost according to the farm management strategy (treatment and/or prevention of new infection). 

Keywords: Intramammary infection prevalence; Small ruminant; Real-time polymerase chain reaction