The tonsillar carriage of Pasteurella haemolytica in lambs

Abstract

To study colonization of the tonsils by Pasteurella haemolytica, attempts were made to isolate the organism from the tonsils of lambs which died or were killed in the course of unrelated experiments. In addition, repeated tonsillar swabs were taken from 37 lambs and their 28 dams over a 3-month period. From lambs under 3 days old, only A serotypes and untypable strains of P. haemolytica were recovered, while T serotypes were first detected in 3-week-old lambs. As the age of the lambs increased, the fall in the isolation rate of A serotypes and of untypable strains was accompanied by an increase in T serotype isolation. In lambs 9 to 12 weeks old, over 80 per cent of isolates were T serotypes. Of the 222 swabs from live lambs, 88 (40 per cent) yielded P. haemolytica, with more than one serotype being recovered from 24 of the 88 positive swabs. Multiple isolations, mostly of 2 serotypes, were made from lambs of all ages but were commonest at 12 weeks of age. Of the 117 isolates, 70 (60 per cent) were of biotype T, 31 (26 per cent) were of biotype A and 16 (14 per cent) were untypable. P. haemolytica was isolated from 25 of the 28 ewes on 1 to 5 of the 6 sampling occasions. The 65 positive swabs yielded 81 T serotypes (78 per cent), 21 A serotypes (20 per cent) and one untypable strain as 32 single and 33 multiple isolations. The typed isolates comprised 7 A serotypes and 3 T serotypes. Only one serotype (A6) isolated from lambs was not recovered from ewes. It was concluded that the tonsils of lambs can be colonized by P. haemolytica acquired from their dams very soon after birth. Early and perhaps preferential establishment of A serotypes and untypable strains is succeeded by increasing dominance of T serotypes.