Bovine mastitis is a very important production disease in cattle dairy herds in Kenya. The objectives of this thesis were to determine: 1) the types of bacterial infections in dairy cows using the California Mastitis Test (CMT) and culture at dry-off; 2) the effectiveness of different dry cow therapy options for treating existing mastitis infections; and 3) factors associated with Staphylococcus aureus infections at dry-off and post-calving in smallholder dairy farms in Kiambu County, Kenya. The study targeted cows at the point of dry-off, and farms with such cows were recruited through cooperative societies and the help of artificial insemination service providers. On twenty farms, 32 cows with CMT-positive quarters at dry-off were recruited into the study, including 75% Friesian, 19% Ayrshire and 6% Jersey, totaling 121 quarters. From all the CMT-positive quarters, milk samples were aseptically collected for bacterial culture. Additionally, the positive quarters were randomly allocated to receive either dry cow therapy (DCT) plus internal teat sealant (ITS) or ITS alone, and farm- and animal-level factors were captured through a questionnaire. The project was undertaken between the months of September 2019 to March 2020. The mean herd size of the farms was 11 cattle, with the composition of 4.2, 1.3, 2.9 and 2.3 milking cows, dry cows, heifers and calves respectively. Mean daily milk production for the dairy farms was 55 liters. On milking practices, 75% of the farms use hand-milking only, of which 47% and 53% squeeze and pull the teats, respectively, while 15% use machine milking and 10% use both machine and hand milking methods. All farms use a cloth for udder cleaning, with 90% using one cloth for all the cows in the farm. Additionally, 80% of the farmers were drying their cows gradually, but only 40% were sometimes using dry cow therapy. Subclinical S. aureus mastitis was among the most common type of infection at dry-off (54.6% of CMT-positive quarters and 81.3% of CMT-positive cows were infected with S. aureus). Dry cow therapy significantly reduced the proportion of quarters infected with S. aureus from 67.9% at dry-off to 44.0% post-calving (35% reduction), but did not significantly reduce other infections, although proportions of other isolates were low. The final multivariable logistic regression model found there were 0.35 times lower odds of S. aureus infection post-calving than at dry-off. The odds of S. aureus infection were 0.14 times lower when milk production was over 2.5 kg/day than when it was under 2.5 kg/day. There were 0.29 times lower odds of S. aureus infection when udders had scant dirtiness versus when they were clean. However, compared to “clean” lower leg cleanliness, scant and moderate dirtiness had 8.7 and 4.5 times higher odds of S. aureus infection, respectively. Finally, compared to no bedding, mattresses and sawdust/grass/leaves/crop waste had 0.14 and 0.06 times lower odds of S. aureus infection, respectively. This study points out the importance of subclinical mastitis at dry-off in smallholder dairy farms, and therefore provides information on how dry cow therapy can be used to address the problem. Preventive measures against subclinical mastitis (e.g. hygiene in the cow cubicle and use of post-milking teat dip) and strategic screening of milking cows with CMT, especially at dry-off so that cows with subclinical mastitis at dry-off can be treated with DCT, can go a long way in the control of the disease, and thus help reduce the cost associated with it.

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