Outbreaks of bovine mastitis caused by specific Mycoplasma bovis strains recurring at multi-year intervals

Mycoplasma bovis is a major cause of bovine mastitis. Intermittent shedding of the organism for many months is a feature of cows with intramammary infection. A dairy farm in Japan experienced a mastitis outbreak caused by M. bovis in 2016, as well as 2 additional outbreaks and 1 case in 2020-2021. The causative strains in the 3 outbreaks shared a common and identical genetic feature, the insertion of a transposase gene at the same site within the phosphate acetyltransferase-2 gene. Additionally, all isolates were genotyped to closely related sequence types (ST21 and ST141) by multilocus sequence typing, and had similar pulsopatterns by pulsed-field gel electrophoresis. Our results indicate that infection with the same causative strain remained in this herd and environment for 4 y. Treatment with fluoroquinolones, guided by antimicrobial susceptibility test results, eliminated M. bovis from 16 of 20 M. bovis-infected cows, as confirmed by culture and somatic cell counts. However, mastitis caused by other bacteria occurred in 9 M. bovis-free cows within 2 mo of the last treatment.