Isolation and identification of Staphylococcus aureus from bovine mastitic milk and their drug resistance patterns in Adama town, Ethiopia

A cross-sectional study was carried out from November 2008 April 2009 to estimate the prevalence of mastitis caused by Staphylococcus aureus, to assess the associated risk factors and to determine the antimicrobial resistance pattern in Adama town, East Shoa, Ethiopia. From 102 markets oriented small holder dairy farms, a total of 300 lactating cows were tested for mastitis using the California Mastitis Test (CMT). One hundred and forty of the cows (46.7%) had mastitis, of which 10.0% (30/300) and 36.7% (110/300) showed clinical and sub clinical mastitis, respectively. The quarter level prevalence was 29.0% (348/1200); from which the clinical form was 5.4% (65/1200) and the subclinical was 25.6% (283/1200). Of the 65 quarters with clinical cases, 18 had blind teats while 47 had active mastitis. A total of 140 (30 from clinical and 110 from subclinical cases) milk samples were collected and cultured for S. aureus of which 59 resulted in growth of the bacterium (10 from clinical and 49 from subclinical cases). Mastitis prevalence showed significant variation among cows of different age groups (p = 0.005), different housing systems (p = 0.000) and at different lactation stages (p= 0.016). Thus, bovine mastitis was more likely to occur in cows above 6 years of age (OR = 3.4, 95% CI = 0.9, 13.7), that were kept in muddy houses (OR = 5.3, 95% CI = 3.2, 8.9) and were at a lactation stage of above 6 months (OR = 3.6, 95% CI = 1.44, 9.03). The results of antimicrobial susceptibility testing revealed that S. aureus was highly susceptible to chloramphenicol (100%) followed by gentamycin (91.7%), kanamycin (88.9%) and streptomycin (86.1%). In contrast, isolates were highly resistant to penicillin (94.4%), trimethoprimsulfamethoxazole (58.3%) and amoxicillin (36.1%). In conclusion, this study confirms the importance of S. aureus as a mastitis causing bacterium and identifies risk factors associated with the disease in the Ethiopian setting.