Aetiology and pathology of subclinical mastistis in dairy cows

PhD

Three investigations were carried out in this study. Two of them dealt with epidemiological aspects of subclinical mastitis in dairy cows. One of them was carried out in Danish herds while the other was carried out in Kenyan herds. The third investigation looked into the pathogenesis of bovine mastitis. Mastitis prevalence rate was measured by examining single quarter milk samples (QMS) for the presence of micro-organisms using cultural methods, and the cellular content utilizing the California Mastitis Test reactions. Other epidemiological aspects of mastitis were investigated by examination of the mammary quarters of each cow and by collecting information concerning mastitis from the owners of these herds. Mastitis pathogenesis was investigated using ' both gross and microscopic appearances of the teats and the mammary gland parenchyma. The health status of these quarters had previously been assessed by examining six daily milk samples for the presence of micro-organisms and the somatic cell content (SCC), the latter by using automatic electronic cell counting method. In addition, post mortem teat cistern puncture milk samples (TCPS) were examined for the presence of bacteria. In Denmark, the epidemiological investigation of mastitis involved 5 herds with a total of 122 cows. These cows were of various breeds, and were in different lactation numbers and stages. In all herds, the cows were zero-grazed (confinement-fed), milked by machines, and the owners applied mastitis control and preventive measures. Microbiological assessment of single quarter milk samples showed that 152 of 478 (31.8%) quarters were infected with one or more types of micro-organisms, and that the 74 of 122 (60.7%) cows were infected in one or more quarters. On the basis of the CMT reaction and taking a score of 3 as mastitis positive, the quarter prevalence rate was 40.5% (184 of 454 quarters). Using the quarter infection status together with CMT score as the diagnostic criteria, 94 of 454 (20.9%) quarters were positive on both tests while 226 of 454 (49.8%) quarters were mastitis negative. On the other hand, 44 of 454 (9.7%) quarters were bacteriologically positive but CMT negative, while 89 of 454 (19.6%) quarters were CMT positive but bacteriologically negative. Six types of micro-organisms were associated with mammary infections in these cows. Staphylococcus aureus was the most commonly encountered organism accounting for 66.7% (106 of 159) isolations. The others were isolated as follows: Streptococcus spp., 21.4% (34 of 159); micrococci, 6.9% (11 of 159); Coliforms, 2.5% (4 of 159); Corynebacterium pyogenes, 1.9% (3 of 159), and yeasts, 0.6% (1 of 159) isolations. The 68 Danish dairy cows (in one herd) involved in the study of mastitis pathogenesis were found to be of different breeds and ages. They were also in different phases of the lactation period. The owner of this herd used a machine for milking and practised mastitis control and preventive measures. Of the 272 quarters, 9 (3.3%) of them were dry while only 263 yielded milk for analysis. If milk from a quarter yielded a particular micro-organism on two or more of the six sampling days, the quarter was regarded as infected by that micro-organism. On this basis, the quarter infection rate was 30.4% (80 of 263 quarters). The number of infected quarters was found to vary on daily basis with an average of 70.8 of 263 (26.9%) quarters per day. The types of bacferia associated with the quarter infections were as follows: Staph, aureus, 67.5% (56 of 83); micrococci, 18.1% (15 of 83); Str. dysqalactiae, 10.8% (9 of 83); Str. agalactiae, 1.2% (1 of 83); C. pyogenes, 1.2 (1 of 83), and coliforms, 1.2% (1 of 83) isolations. Using the SCC as the diganostic criterion and considering a quarter as mastitic if it yielded milk with SCC of ^ 300,000/ml on two or more of the six sampling days, the quarter prevalence rate was 35.0% (92 of 263 quarters). On this basis the quarter prevalence rate varied from day to day with an average of 29.9% (78.7 of 263) quarters being positive every day. Analysis of post mortem teat cistern punctures samples gave a quarter infection rate of 42.2% (111 of 263). The types of bacteria isolated -from TOPS were as follows: Staph, aureus, 37.6% (44 of 117); C_;_ bovis, 29.9% (35 of 117); micrococci; 16.2% (19 of 117); Str. dysgalactiae, 12.8% (15 of 117); coliforms, 2.6% (3 of 117); and C. pyogenes, 0.9% (1 or 117) isolations. Morphologically, mastitis could only be detected in 15 quarters, nine of which were dry and the other six had yielded grossly abnormal milk. The other change which could be discerned grossly, was the increase in the amount of connective tissue. This was restricted to the ventral and peripheral areas of the mammary quarters. Since this change affected quarters showing microscopic evidence of inflammation as well as those that did not, it was thought to have resulted from more than one cause, viz. physiological disturbances and/or inflammatory process in the udder. By the same token, bacteria were isolated from milk samples of quarters showing gross and microscopic abnormalities as well as those that did not. Microscopically, the inflammatory reaction was more common in the areas of the teat cistern and/or Furstenberg's rosettes than either in the mammary parenchyma or the lining of the teat canals. This finding suggests that, upon entry of bacteria through the teat canal, the initial inflammatory reaction takes place in the teat cistern and/or Furstenberg's rosettes before it spreads to other areas of the mammary gland. In Kenya, mastitis epidemiology was investigated in 23 dairy herds with a total of 242 cows. The number of cows in each herd, their breeds, ages, lactational stages were variable. In 21 of 23 herds, the cows were milked by hand while in the other two herds, machines were used for milking. Apart from one herd in which disinfectant udder washing before milking was practised, in all other herds no mastitis control or preventive measures were applied. Analysis of QMS showed that 418 of 962 (43.5%) quarters were infected with one or more types of bacteria, and that 103 of 242 (71.5%) cows were infected in one or more quarters. Estimation of milk cellular content by means of CMT reaction gave a quarter prevalence rate of 33.4% (293 of 878). The types of bacteria recovered from the milk samples were as follows: Str. uberis, 22.0% (104 of 472); gtr. dvsqalactiae, 6.4% (30 of 472); Str. aqalactiae, 5.5% (26 of 472); Str. faecalis, 4.5% (21 of 472); micrococci, 34.8% (164 of 472); Staph. eureus, 7.8% (37 of 472); coliforms, 18.0% (85 of 472), and C. pyogenes, 1.1% (5 or 472) isolations.