SUBCLINICAL MASTITIS ASSESSED BY DEVIATIONS IN MILK-YIELD AND ELECTRICAL-RESISTANCE

Concurrent falls in milk production and electrical resistance of compo site milk were examined in Israeli Holstein cows. The cows were milked three times a day by a system that recorded yield and the lowest elec trical resistance in the composite milk from the four glands. The stud y included two groups: cows that experienced on day 0 a decline in res istance and milk production greater than or equal to 20% from the mean of the previous 9 d (62 cows, case group) and cows that experienced n o such episodes over 9 d before and after a fixed day (118 cows, contr ol group). Bacteriological status and somatic cell count (SCC) or Cali fornia mastitis test scores were assessed on the fixed day in the cont rol group, and on days 0, 1 and 2 in the case group. California mastit is test scores greater than 2 and SCC thresholds of 5 x 10(5) cells/ml were used to create two classes of leucocytosis. There was no statist ically significant difference between the two groups in frequency dist ributions of pathogens and their types: in 30 % of cows infection was not detected, 33 % were infected by major pathogens (95 % of which wer e Staphylococcus aureus), and 53.5 % by minor pathogens (80 % Micrococ cus spp.). Cows in the case group had lower mills. production during t he 8 d following day 0. Mean electrical resistance was lower in infect ed cows and particularly in cows infected by Staph. aureus. High leuco cytosis was associated with reduced electrical resistance in both grou ps, and was found in 93 % of cows in the case group v. 25 % in the con trol group. The results suggest that falls in electrical resistance of milli and in milk production were not linked to a specific pathogen, and were followed by 3-8 d of reduced milk production and electrical r esistance. The study suggests that there are episodic aggravations in mammary health that do not evolve into clinical mastitis but may induc e significant losses in milk yield and quality.