Healing of muscle trauma after intramuscular injection of antibiotics in sheep: correlations between clinical, macroscopic and microscopic scores.

: The present study aimed to predict the resultant healing from early lesions (found days 3 and 10 post injection) caused by the intramuscular injection of veterinary antibiotic formulations. Nineteen marketed drugs were selected in order to screen a wide range of irritation conditions at the injection site. Nineteen ewes were each injected intramuscularly with one of the formulations. Each injection was at a different site, 3 and 10 days prior to slaughter. Fourteen of these ewes also received intramuscular injections at two other sites 21 and 32 days prior to slaughter. The tolerance was monitored by clinical examination of the injection site and by gross and microscopic pathology. Myodegeneration and fibre necrosis were determined histologically. The clinical scores did not correlate with the other findings. Myodegeneration correlated with the size of the lesion on day 3 post-injection and was not found thereafter. Although occasionally found alone, it was generally associated with and surrounded by fibre necrosis. When myodegeneration was the only lesion, regeneration was complete by day 21 and the fibrosis was minimal or absent. Necrosis at day 10 post-injection correlated with necrosis at days 3, 21 and 32 post-injection. Fibrosis became prominent around the necrotic muscles from day 10 post-injection. Healing from necrosis was slow with, in some instances, encapsulated debris still persisting at day 32 post-injection. The tissue irritation index correlated well with myodegeneration and necrose (acute lesions) and fibrose and necrose (older lesions). Thus, after considering a large sample of antibiotic formulations, this study indicated that healing could be predicted from the muscle fibre histopathology at days 3 and 10 post-injection. If myodegeneration was found alone, full recovery within 21 days could be predicted. If fibre necrosis was extensive, the healing involved encapsulating the necrotic tissues and thus resulted in extensive scar formation. The tissue changes explained why the irritation index of the lesions at days 21 and 32 post-injection could be predicted from their irritation index at days 3 and 10 post-injection. Likewise, the size of the lesion at days 21 and 32 post-injection could not be predicted from its size at day 3 post injection.