Epidemiology and economics of mastitis in the smallholder dairy sector of Kiambu district, Kenya
This thesis describes a study of the epidemiology and economics of mastitis on smallholder dairy farms in Kiambu District. The study was carried out as part of a larger integrated study on health and production constraints on smallholder dairies in central Kenya. The objectives were to: 1) estimate the prevalence/ incidence of clinical and subclinical mastitis and identify factors which influence the occurrence of mastitis on these farms; 2) evaluate any existing control measures on the farms and to design alternative mastitis control strategies; and 3) conduct clinical trial of different mastitis intervention programmes on farms with a view to: a) estimating the effect of mastitis on productivity, b) estimating the costs and benefits of alternate mastitis intervention strategies at the farm level, and c) identifying the economically optimal or most efficient intervention strategy or combination of strategies. The study was conducted in two phases. The first phase was a one year observational study that focussed on several aspects of health and production conducted in 1991/92. Farms were selected in a two stage stratified random sampling strategy from among farmers belonging to the dairy cooperative societies. The first stage was the random selection of six out of 14 dairy societies. The second stage was the random selection of 15 farmers from a list of active members from each of these six dairy societies for a total of 90 farms. These farms were visited monthly for 12 months. The observational study was followed by an intervention trial on mastitis carried out over a one year period (May 1993 to April 1994) on a different set of 100 randomly selected herds from two dairy cooperative societies namely Kikuyu and Nderi. The trial farms were visited once every two months. During the two study phases, survey and production data were collected and quarter-milk samples divided into two equal parts: one part for somatic cell counting and the other part for aerobic bacteriological culture. The response rate was high, with over 95 % voluntary participation rate, for both studies). Breeds kept were mostly exotic genetically-heretogenous B. taurus (66%) and a few high proportion B. taurus crosses with B. indicus (34%). Overall, productivity in these herds was low. Average milk yield was only 5.8 kg/day (median = 5 kg/day) and lactation length was long (16 months). Nonmastitis factors associated with changes in milk yield (and lactation curves) were: season, grazing system and breed. The feeding of grain/concentrate was not associated with changes in milk yield. Clinical mastitis risks were low (1 % per month). Somatic cell counts (SCC) were high (median = 620 x 1<Y); the suggested threshold of 300,000 cells/ml would classify 71 % of quarters as positive for subclinical mastitis. Bacteria were commonly isolated, with S. aureus the most-common pathogen isolated from 22.1 % of all samples. Infections with mastitis pathogens, cow-age and milk yield were associated with increases in SCC. However, S. aureus was the only mastitis pathogen associated with decreased milk yield. Few specific mastitis-control measures were applied and farm-to-farm variation of SCC was low (9%). Profit margin per carrying capacity unit (CCU) on these farms was estimated at 42 % per year. Potentially recoverable costs from sub-clinical mastitis was estimated at 109 kg (5.7 %) equivalent of milk yield per average cow per year. xv Three intervention measures were applied namely, improved management practices, therapy of suclinical cases and a combination of both improved management practices and therapy of subclinical cases. Milk yield, SCC, CMT and results from bacterial culture from these intervention groups were compared to those from a control group. However, the intervention measures employed did not explain any variation in milk yield and all the other responses measured.