Poultry producers are being advised to be selective when using Antibiotic Sensitivity Testing (AST) as a tool for assessing the efficacy of anticoccidial programmes.

Dr Tom Dutton, technical consultant for poultry at Elanco Animal Health, says while AST is a valuable tool for determining the relative activity of chemical coccidiostats, its relevance is much more limited when applied to certain anticoccidials, particularly ionophores.

“AST works well for comparing the efficacy of strong chemical treatments designed to control coccidia, but it’s far less helpful when used to assess other types of anticoccidials such as ionophores,” he explains.

“This is because of how ionophores work. They reduce the coccidia challenge, but don’t eliminate it completely, leaving enough coccidia behind for birds to develop immunity.

“With less selection pressure compared to strong chemicals, they maintain an ionophore-susceptible population of coccidia allowing continuous use of ionophores over extended periods.”

Because ionophores are designed to allow low level coccidia cycling, Dr Dutton explains that lesions may still be present during AST, which can lead to conclusions that don’t necessarily correlate with field performance.

“AST protocols typically involve exposing birds to a high and concentrated coccidia challenge over a short period of time, which doesn’t reflect how coccidiosis normally develops in commercial poultry houses, where infection pressure builds gradually through the production cycle,” he explains.

“In these short-term challenge models, the intensity and timing of exposure can amplify lesion expression, meaning AST may indicate reduced efficacy for ionophores even when gut health and productivity outcomes remain strong in the field.”

Dr Dutton therefore warns against using AST to inform changes to established coccidiosis control programmes, particularly given the potential impact on flock health stability.

“It can take a couple of crops of birds to start seeing the consequences of changing coccidiosis control programmes but by that point the stability has been lost, and it can take several crops to recover from that,” he says.

“Stable and continuous coccidiosis control will support bird performance crop after crop, so any potential changes need to be carefully weighed up and guided by reliable information.”

He adds: “AST can be helpful for testing multiple chemical anticoccidials against each other, but its value depends heavily on the type of anticoccidials being tested.”