Abstract
Eimeria species parasites infect the gastrointestinal tract of chickens, causing disease and impacting on production. The poultry industry relies on anticoccidial drugs and live vaccines to control Eimeria and there is a need for novel, scalable alternatives. Understanding the outcomes of experimental infection in commercial chickens is valuable for assessment of novel interventions. We examined the impact of different infectious doses of Eimeria tenella (one low dose, three high doses) in three commercial layer chicken lines, evaluating lesion score, parasite replication and cytokine response in the caeca. Groups of eight to ten chickens were housed together and infected with 250, 4,000, 8,000 or 12,000 sporulated oocysts at 21 days of age. Five days post-infection caeca were assessed for lesions and to quantify parasite replication by qPCR and cytokine transcription by RT-qPCR. Comparison of the three high doses revealed no significant variation between them in observed lesions or parasite replication with all being significantly higher than the low dose infection. Transcription of IFN-γ and IL-10 increased in all infected chickens relative to unchallenged controls, with no significant differences associated with dose magnitude (p > 0.05). No significant differences were detected in lesion score, parasite replication or caecal cytokine expression between the three lines of chickens. We therefore propose 4,000 E. tenella oocysts is a sufficient dose to reliably induce lesions in commercial layer chickens, and that estimates of parasite replication can be derived by qPCR from these same birds. However, more accurate quantification of Eimeria replication requires a separate low dose challenge group. Optimisation of challenge dose in an appropriate chicken line is essential to maximize the value of in vivo efficacy studies. For coccidiosis, this approach can reduce the numbers of chickens required for statistically significant studies and reduce experimental severity.
Highlights
Coccidiosis, caused by protozoan parasites of the genus Eimeria, is a gastrointestinal disease commonly associated with chickens that has implications for chicken welfare and economic consequences for producers [1, 2]
A low challenge dose widely used in experimental studies of Eimeria replication was included for comparison [13, 14]
Both parasite replication and lesion score were higher in chickens dosed with 4,000 E. tenella oocysts compared to those challenged with 250 oocysts
Summary
Coccidiosis, caused by protozoan parasites of the genus Eimeria, is a gastrointestinal disease commonly associated with chickens that has implications for chicken welfare and economic consequences for producers [1, 2]. Ingestion of a larger number of sporulated oocysts results in higher parasite loads in the gut and higher oocyst outputs, fecundity reduces as dose size increases in a response known as the crowding effect [6]. Other factors such as the host immune response, which is impacted by host nutrition, genetics and previous or concurrent infection, contribute to variation in parasite replication and disease severity [7,8,9]. Calculating the parasite dose required to induce pathology within an ethical framework to minimize unnecessary suffering to experimental animals can be challenging
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