Transmission of Mannheimia haemolytica from the tonsils of lambs to the teat of ewes during sucking.

Objective of the work was to study whether Mannheimia haemolytica may be transmitted from the mouth of the lambs into the teat of the dam during sucking. We compared bacterial populations within the teat duct and milk of ewes immediately before and immediately after sucking by the lambs. Tonsils of lambs of the ewes were swabbed. M. haemolytica strain DAG21T recovered from a teat duct of a ewe was compared to strain DAG21R recovered from the tonsils of her lamb by using 16s rRNA sequencing. We used those two isolates and another one of known pathogenicity, for challenging ewes: (i) 2 mm deep into healthy teats, (ii) 2 mm deep into teats with chapping lesions or (iii) into the cistern of healthy mammary glands. Of samples collected before suckling, 20/792 were bacteriologically positive, and of those after, 50/792 were bacteriologically positive (P<0.001); in 37 cases, a negative sample became positive. One M. haemolytica (DAG21T) was recovered after suckling from a teat duct of a ewe. The organism was isolated from 57/90 tonsillar swabs from lambs. Risk of infection of ewe' teats was 0.004 throughout lactation, being greatest (0.021) during the 3rd week of lactation. The 16s rRNA sequences of strains DAG21T and DAG21R were identical over 1450 nucleotides. Phylogenetic analysis showed that the two isolates clustered together with isolates of M. haemolytica. Organism deposition into healthy teats caused subclinical mastitis; deposition into teats with lesions or directly into mammary gland caused clinical mastitis. When results of inoculation of the three strains were compared between them, statistical significance was always P>0.9. Results provide clear evidence that suckling by lambs can lead to transmission of M. haemolytica into the teats of the ewes; the bacteria have the potential to cause mastitis if circumstances are favourable.