Ionophore susceptibility of Eimeria zaria: First characterisation in a cryptic Eimeria species of chickens

Seven Eimeria species have long been recognised in chickens, but molecular analyses over the past two decades have revealed unexpected diversity within the genus, identifying three candidate new species: Eimeria lata, E. nagambie, and E. zaria. Despite increasing reports of these species, their biology and epidemiology remain poorly understood. Current commercial anticoccidial vaccines do not effectively control them, and their drug sensitivities have not been characterised. Here, the efficacy of three anticoccidial drugs, monensin + nicarbazin ("Monimax"), narasin ("Monteban"), and salinomycin ("Sacox"), has been evaluated against an E. zaria type specimen in vivo. All anticoccidial treatments reduced lost body weight gain (BWG) over the 12 days immediately post-infection with 5 × 10⁴ sporulated E. zaria oocysts, losing 4.1% (P < 0.05), 1.2% (P < 0.05), and 7.3% (P > 0.05), respectively, compared with 15.8% in untreated challenged controls. New species-specific quantitative PCR assays targeting the putative single-copy E. zaria Apical Membrane Antigen 1 (EzAMA1) locus and the multicopy Internal Transcribed Spacer 2 (EzITS-2) sequence showed that monensin + nicarbazin and salinomycin suppressed parasite replication in the duodenum by 99%, whereas narasin did not. Descriptive counts of oocyst excretion per gram litter (OPG) confirmed this prophylactic efficacy against parasite replication. These results indicate that E. zaria is broadly susceptible to ionophore anticoccidials but, as for other Eimeria species, reduced sensitivity can emerge under field conditions. The new qPCR assays provide sensitive, specific tools for improved monitoring and assessment of E. zaria infections, supporting improved understanding of occurrence and epidemiology.