Epidemiological and economic study of bovine subclinical mastitis caused by Staphylococcus aureus, non-aureus Staphylococcus, and Mammaliicoccus spp. in Pernambuco state, Brazil.
Mastitis caused by Staphylococcus aureus and non-aureus staphylococci and mammaliicocci (NASM) represents an increasing threat to the milk production chain, driven by the extensive use of antimicrobials and resulting economic losses in dairy herds. The aim of the thesis was to investigate the prevalence of S. aureus and NASM subclinical mastitis, with a specific focus on multidrug-resistant S. aureus (MDRSA), and to assess the economic impact of these infections on milk production in Pernambuco state, Northeastern Brazil.
The study included five dairy farms with Holstein-Gyr crossbred cattle housed in semi-confined systems with limited grazing access. The average herd size consisted of 100 lactating cows (range: 56-229 cows), with mean milk production of 15 kg milk/day/cow (range: 8-36.4 kg milk/day/cow).
Samples were collected from 2,120 mammary quarters (530 cows), 40 swabs from milk utensils, 5 bulk tank milk samples, and 22 swabs (11 from nostrils and 11 from hands) of milkers from five dairy farms. The identification of 191 S. aureus and 124 NASM isolates was performed using Matrix-Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry (MALDI-TOF MS), with antimicrobial resistance assessed using the disk diffusion method.
From an epidemiological perspective, we genetically and phenotypically characterized MDRSA isolates associated with subclinical mastitis and examined risk factors for MDRSA-induced infections. Of the 191 S. aureus isolates, 14 (7%) were identified as MDRSA, including 12 isolates from cows (4 primiparous and 8 multiparous), 1 from bulk tank milk, and 1 from a milking utensil. Interestingly, 36% (5 out of 14) of MDRSA isolates were resistant to cefoxitin, although none carried the mecA or mecC genes.
The genetic relatedness of the MDRSA isolates was evaluated using Pulsed-field Gel Electrophoresis (PFGE), which revealed a higher likelihood of herd-specific strains. Multilocus Sequence Typing (MLST) of cefoxitin-resistant MDRSA isolates identified clonal complexes CC97 (ST126, typically livestock-associated) and CC1 (ST7440, typically community-associated). To our knowledge, this is the first report of S. aureus ST7440 isolated from bovine mastitis in Brazil.
Risk factors for subclinical mastitis caused by MDRSA were assessed using logistic regression. Factors such as parity, stage of lactation, somatic cell count (SCC), milk production, and herd size were highlighted as key influences on infection risk. These findings underscore the importance of monitoring antimicrobial resistance and MDRSA distribution to minimize the spread of infection and reduce subclinical mastitis cases.
From an economic standpoint, the impact of mammary quarter infection caused by S. aureus and NASM on milk production was estimated using a linear mixed model to analyze milk payment criteria for healthy vs. infected cows. Data were collected over three consecutive months, with 155 healthy mammary quarters classified as the control group and 257 infected quarters categorized as the infected group. Our results indicated that milk yield and milk price were critical factors influencing economic return. For cows with one or more infected quarters, NASM infection led to a reduction in economic return of $0.41 to $0.65 per cow per day, and S. aureus infection caused a reduction of $0.25 to $0.36 per cow per day. Although no significant differences were observed in economic return between healthy cows and those infected with NASM or S. aureus, further research is needed to refine these estimates and develop effective prevention and control strategies.
To our understanding, no previous study has examined the effect of subclinical mastitis caused by multidrug resistant (MDR) bacteria in economic return. Among the cows infected with MDRSA (n= 14), subclinical mastitis did not show significant differences in economic return compared to cases caused by susceptible strains. However, additional studies are necessary to determine whether MDR bacterial infections have further long-term economic implications for dairy herds.
Amanda Thaís Ferreira Silva recently completed her PhD in Animal Bioscience at Federal Rural University of Pernambuco (UFRPE) in Brazil, which included a Sandwich Doctoral Program at Michigan State University in the United States, where she was a visiting scholar at the TopMilk Laboratory. She also hold a BSc in Veterinary Medicine (DMV equivalent) from UFRPE, with an academic mobility program at the University of Wisconsin – River Falls in the United States (non-degree seeking), as well as a MSc in Animal Bioscience from UFRPE. Currently, she is a lecturer and the veterinary program coordinator at FACOL University Centre (UNIFACOL), a private veterinary school in Brazil. Her interests include dairy cow mastitis, milk quality, antimicrobial resistance, One Health, and scientific communication.
