Section E. The diseases of dairy cattle

Abstract

I. MASTITIS BACTERIOLOGY PERHAPS the two most significant results of extensive research into bovine mastitis during the past few years are first, the recognition of a fairly well-defined species of streptococcus as the principal cause of chronic bovine mastitis, and secondly, general agreement in the biochemical and serological characteristics of the causative agent. The classification of the streptococci has been reviewed recently by Sherman(l). The attempted division of the streptococci into species is based primarily on the Lancefield serological method of grouping supplemented by cultural and biochemical tests. Eight main serological groups are distinguished, and Group B includes mastitis streptococci to which the name Str. agalactiae is commonly assigned. Within this group Stableforth (2) formerly recognized three serological types (o, b, c), but as a result of examining some 2000 further strains (3) isolated in Great Britain, he has redesignated these original types as 1 a, 2 a, 3 a, and two further subtypes 1 b and 36 have been distinguished. These strains were examined by direct agglutination, agglutination-absorption and precipitation tests. The characters of thirty-three strains of Str. agalactiae isolated in other European countries conformed in the main to the British subtypes l a and 2a. Stableforth(4) has also surveyed the incidence of these various serological types in fifty-two dairy herds. Streptococci were isolated from 1546 quarters of 782 cows. In nearly all cows having one or more quarters of the udder infected, the characters of the streptococcus conformed to one subtype. About 10 % of these pure infections were due to type 2, the remainder being equally distributed between type 1 (usually la) and type 3 (usually 3a). In thirty-eight herds out of a total of forty-five, 78 % of the individual infections were alike and in fifteen herds all were alike. Of these thirty-eight herds, twenty-three were infected with type 1, three with type 2 and twelve with type 3. In view of the tendency towards type specificity of infection within herds Stableforth considers that this lends support to the view that mastitis due to Str. agalactiae is a purely contagious disease. Plastridge & Hartsell(5) have examined 157 strains of weakly haemolytic streptococci recovered from fresh milk samples. Of these 122 possessed the biochemical characters of Str. agalactiae, and precipitin tests showed that they belonged to Lancefield's Group B. Most of these strains were isolated from animals with a history of permanent infection, but thirty strains, although biochemically resembling Str. agalactiae, differed from this organism serologically and by producing infection of relatively short duration, with or without evidence of mastitis. These cultures were tentatively designated as Str. pseudo-agalactiae.