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The objective was to describe the physicochemical changes during the early phase of subclinical mastitis and to associate them with pathological findings. A Mannheimia haemolytica strain was deposited into one teat duct of 25 ewes and the clinical, bacteriological, cytological, physicochemical (pH, milk composition), gross-pathological and histological findings were subsequently recorded. The organism was consistently isolated from samples of teat duct material (140/150) but not from mammary secretion (50/150). California Mastitis Test (CMT) scores increased (>1) and remained high (143/150 samples) after challenge; polymorphonuclear neutrophils (PMN) predominated in milk films, but the proportion of lymphocytes and macrophages progressively increased. Increased pH values (>7·0) were recorded in the mammary secretion from the challenged side. Furthermore, content of fat, total proteins and lactose therein decreased markedly. Histological changes (leucocytic infiltration, destruction of epithelial cells) were observed in the mammary parenchyma of the ewes. The present results confirm that the reduction of milk constituents is the effect of cellular damage and can occur soon after infection.
We used a Mannheimia haemolytica isolate to study differences in susceptibility to experimental mastitis between two breeds of dairy sheep. The isolate was deposited into the teat duct of Karagouniko (K, n=8) or Frisarta (F, n=8) ewes. The animals were monitored by means of clinical, bacteriological, cytological and pathological methods. K ewes did not develop any systemic or mammary clinical signs, whilst F ewes became ill and developed acute clinical mastitis 12 h later (P<0·001). Bacteria were isolated from 34/48 samples from K ewes and from 46/46 samples from F ewes. Positive California mastitis test (CMT) results were 17/24 samples from K ewes and 23/23 samples from F ewes; leucocytes were seen in Giemsa-stained films. Total pathology score summed over all group K ewes was 41 (maximum possible: 128); Man. haemolytica was isolated from 12/24 tissue samples. Total pathology score summed over all group F ewes was 93; Man. haemolytica was isolated from 24/24 tissue samples. Hyperplastic lymphoid nodules consisting of lymphocytes and plasma cells with germinal activity were characteristically present at the border between teat duct–teat cistern of group K ewes; no such structures were observed in teats of group F ewes. The results identified differences in susceptibility/resistance to a mastitis pathogen among animals of the two breeds. Defence mechanisms of the teat appeared to be inadequate against the invading organisms; as lymphoid nodules have been considered important defensive mechanisms of the ovine teat, their observed lack in Frisarta ewes might have predisposed them to development of mastitis.
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