of chickens: the changing face of an old foe.

are globally enzootic parasites that can cause coccidiosis in chickens. Until recently, remarkably little had changed over the last 40 years in the fundamental biology that underpins detection and control of . Tools such as microscopy and lesion scoring remain central to diagnosis, and control still relies on routine supplementation of diets with anticoccidial drugs or application of live vaccines. However, refocusing on aspects of economics, molecular biology, and bacteriology that relate to coccidiosis has prompted considerable change in dogma. The cost of coccidiosis in chickens has been difficult to define, but updating models created in the 1990s suggested an annual cost to the global poultry industry of £10.4 billion in 2016, rising to a peak of £12.9 billion in 2022 under the influence of the COVID-19 pandemic and regional wars. Surveillance using genomic sequence-based diagnostics has suggested the presence of three new species, supported by subsequent biological characterization of each line. Use of microbiome sequencing pipelines has revealed the breadth of impact infection exerts on enteric microbiota, contributing to dysbiosis and deteriorating litter conditions. Enhanced understanding of and the consequences of infection can be used to improve control and diagnosis with relevance to productivity and welfare, creating opportunities to optimize anticoccidial drug use.The cost of coccidiosis in chickens fluctuates considerably, peaking in 2022.Three new species can infect chickens and escape current vaccines. infection exerts wide-ranging effects on enteric microbiota.