Streptococcus Agalactiae in Dairy Herds. Its Incidence and Relationship to Cell Count and Inhibitory Substance Levels in Bulk Milk

Abstract

SUMMARY Composite milk samples from 379 dairy herds were taken at monthly intervals for one year and cultured by intensive methods for Str. agalactiae. Studies were made in parallel to determine the presence of inhibitory substance residues, and to assess the somatic cell count of each sample by electronic counting techniques. Str. agalactiae was found to be present in 146 (38 · 52 per cent) of the 379 herds, the sample incidence being 12 · 3 per cent (519 of 4221 samples) ; 34 per cent of the 146 herds were positive only once, 16 per cent twice, 8 per cent three times and so on, only one herd being culturally positive on twelve successive months. The significance of the results is discussed, particularly in relationship to the annual cell count means of the herds involved. A good correlation was observed, the more frequently Str. agalactiae isolation being made, the higher the cell count. Despite the possible influence of infections other than Str. agalactiae on herd cell counts, a correlation coefficient of 0 · 75 was obtained. The bracketing of infected herds into cell count ranges illustrated that only 20 · 7 per cent had annual means of On a sample basis it was found that 1 · 73 per cent of the 4221 samples were IHS positive at > 0 · 05 i.u./ml; 5 · 2 per cent of these were from Str. agalactiae herds, while 1 · 24 per cent were from herds culturally negative. The organism was isolated from nine samples which were IHS positive.