Abstract:
The milk sector in Rwanda can be made competitive through improved udder health resulting in higher milk yields. This thesis investigated prevalence and aetiology of subclinical mastitis (SCM) in dairy cows, antimicrobial resistance and molecular epidemiology of udder pathogens. Screening for SCM with California Mastitis Test (CMT) was done in 828 cows in 429 herds from five regions in Rwanda. Milk was sampled from udder quarters with CMT score ≥3. Herd bulk milk quality and safety was investigated to generate knowledge for quality control. Overall SCM prevalence was 70.4% on herd level, 66.3% on cow level and 39% on quarter level. Overall 73.9% of all cultured milk samples were bacteriologically positive. Non-aureus staphylococci (NAS) followed by Staphylococcus (S.) aureus were the predominant pathogens. Staphylococcus chromogenes, epidermidis and sciuri were the most prevalent NAS. There was a high diversity of S. aureus sequence types, with both humans and cows as possible sources. Penicillin resistance exceeded 60% in all staphylococci. Among S. aureus isolates, 83.3% were resistant to penicillin, 100% to clindamycin and 20% to tetracycline. Main risk factors for SCM with implications on management routines included housing of cows in individual cattle kraal and on earthen floor, poor hygiene (hands, cows and milking area), absence of foremilk stripping, increasing stage of lactation, Holstein breed, lack of calf suckling and of feeding after milking. Total bacterial count and somatic cell count was high in milk from farms and milk collection centers, which indicate poor udder health and hygiene and contamination along the transport chain. Presence of Escherichia coli, Salmonella spp. and brucella antibodies in milk was common. Antimicrobial residues in milk was uncommon. In conclusion, SCM is common in dairy herds in Rwanda and the majority of causative pathogens exhibited penicillin resistance. The high microbial load has implications for milk quality, processability and public health. The high genetic diversity of S. aureus should be considered in future studies of disease spread. A mastitis control plan is recommended.
