Penicillin G and penicillin G-tinidazole treatment of experimentally induced summer mastitis — effect on elimination rates of bacteria and outcome of the disease

Two udder hind quarters of ten pregnant heifers were inoculated experimentally with a combination of Actinomyces pyogenes, Peptostreptococcus indolicus and Fusobacterium necrophorum at a dose of 9×10 8 colony forming units (CFU) of each species of bacteria. The development of aerobic-anaerobic mastitis was followed-up clinically and through analysis of udder secretion samples. All heifers developed acute clinical mastitis. Thick, purulent secretions with foul odour were already present 24 h after infusion. At 32 h post inoculation, 19 of 20 quarters harboured all infused bacteria, the viable counts of bacteria varying from 10 4 to 10 5 CFU/ml. No bacteria were isolated from blood samples taken simultaneously. NAGase values of udder secretions, indicating tissue damage, were high. The heifers were treated systemically with penicillin G alone, or in combination with tinidazole. Treatment started 32 h after inoculation and was continued for three days. The anaerobic bacteria ( F. necrophorum and P. indolicus ) were usually eliminated within four days, while A. pyogenes infections often persisted in the quarters. Penicillin G with tinidazole was not more effective than penicillin G alone: of the four heifers which recovered from infection, two were treated with penicillin G alone and two with the drug combination. The most important factor affecting the recovery seemed to be the host response of individual heifers. The six heifers which did not recover from the infection were continuously infected after calving, harbouring A. pyogenes in ten quarters, and P. indolicus and F. necrophorum each in three quarters.