Ionophores are a commonly used feed additive for animals and when used properly are safe. When feed mis-mixing occurs and an elevated dose of ionophore is given, a toxicosis can develop. Myocardial and skeletal muscles are the targets of a toxicosis. In many species there is a delay from the time of ingestion of a toxic dose in feed to when clinical signs occur. This makes it difficult to collect the feed in question that was at an elevated concentration. Cardiac troponins in serum can be used to make a diagnosis of an ionophore toxicosis.

Mycoplasma bovisis associated with bovine respiratory disease (BRD) and chronic pneumonia and polyarthritis syndrome (CPPS) in feedlot cattle. No efficacious vaccines forM. bovisexist; hence, macrolides are commonly used to control mycoplasmosis. Whole genome sequences of 126M. bovisisolates, derived from 96 feedlot cattle over 12 production years, were determined. Antimicrobial susceptibility testing (AST) of five macrolides (gamithromycin, tildipirosin, tilmicosin, tulathromycin, tylosin) was conducted using a microbroth dilution method. The AST phenotypes were compared to the genotypes generated for 23S rRNA and the L4 and L22 ribosomal proteins. Mutations in domains II (nucleotide 748;E. colinumbering) and V (nucleotide 2059 and 2060) of the 23S rRNA (rrl) gene alleles were associated with resistance. All isolates with a single mutation at Delta 748 were susceptible to tulathromycin, but resistant to tilmicosin and tildipirosin. Isolates with mutations in both domain II and V (Delta 748 Delta 2059 or Delta 748 Delta 2060) were resistant to all five macrolides. However, >99% of isolates were resistant to tildipirosin and tilmicosin, regardless of the number and positions of the mutations. Isolates with a Delta 748 mutation in the 23S rRNA gene and mutations in L4 and L22 were resistant to all macrolides except for tulathromycin.