A longitudinal observational study on the incidence of mastitis in smallholder dairy cows in Sodo town and its surroundings, Ethiopia was conducted during the period from October 2015 to March 2016 with the aim of estimating incidence of mastitis in smallholder dairy cows, investigating potential risk factors associated with mastitis, and isolating mastitis-causing bacteria in milk of smallholder dairy cows. All the sixty-seven lactating cows from the nine smallholder dairy farms were registered for the follow up study. The incidence of mastitis at cow and quarter levels was followed up for six months. On top of that, a questionnaire survey on smallholder dairy cow management and milking procedure was performed at the farms where the study animals resided. The results of this study revealed 50.7 % (n = 34/67) prevalence of mastitis at cow and 29.1 % (n = 78/234) at quarter levels. The total incidence risk was found to be 60.6 % (n = 20/33). Based on bacteriological examination, 90.8 % of the collected samples (n = 59/65) were found to be mastitis bacteria positive. Pathogenic bacteria belonging to five genera were involved in causing mastitis. Among these isolates, Staphylococcus aureus was the predominant mastitis pathogen (n = 23/65, 39 %) found in the study area followed by Streptococci species (n = 12/65, 20.3 %), Coagulase-negative Staphylococci species (n = 11/65, 18.6 %), Escherichia coli (n = 8/65, 13.6 %), and Bacillus species (n = 3/65, 5.0 %). The least mastitis pathogen isolated was Corynebacterium spp. (n = 2/65, 3.4 %). The influence of 22 potential risk factors on the incidence of mastitis was also investigated. Among others, late lactation stage, low daily milk yield, male milkers, dry cow therapy only at last milking of lactation were elicited to be highly risky for mastitis. On the other hand, milkers’ work experience, herd size, teat distance from the ground, and milkers’ skill of finding milk clots appeared to be not risky for the incidence of mastitis whereas, age greater than or equal to eight year, parity above or equal to six, and milk yield less than three liters per day were significantly associated with the prevalence of mastitis (p < 0.01. p < 0.05, p < 0.05, respectively). Although not significant, owners as milkers and late lactation stage had higher influence on the prevalence of mastitis (p = 0.058 and p = 0.147, respectively). In conclusion, the relatively high incidence of mastitis in the study area can be responsible for serious impact on the economy of smallholder dairy farmers mainly by reducing the quantity and quality of milk yield and undermining fertility of the dairy cows. Thus, continuous education of the smallholder dairy farmers is needed for better mastitis control programs.
