Relationship Between Broiler Body Weights, Eimeria maxima Gross Lesion Scores, and Microscores in Three Anticoccidial Sensitivity Tests.

Anticoccidial sensitivity tests (ASTs) serve to determine the efficacy of anticoccidial drugs against Eimeria field isolates in a controlled laboratory setting. The most commonly measured parameters are body weight gain, feed conversion ratio, gross intestinal lesion scores, and mortality. Due to the difficulty in reliably scoring gross lesion scores of Eimeria maxima , microscopic analysis of intestinal scrapings (microscores) can be used in the field to indicate the presence of this particular Eimeria. The goal of this study was to determine the relationship between E. maxima microscores and broiler body weights and gross E. maxima lesion scores in three ASTs. Day-old broiler chicks were raised for 12 days on a standard corn-soy diet. On Day 12, chicks were placed in Petersime batteries and treatment diets were provided. There were six birds per pen, four pens per treatment, and 12 treatments, for a total of 288 chicks per AST. The treatments were as follows: 1) nonmedicated, noninfected; 2) nonmedicated, infected; 3) lasalocid, infected; 4) salinomycin, infected; 5) diclazuril, infected; 6) monensin, infected; 7) decoquinate, infected; 8) narasin + nicarbazin, infected; 9) narasin, infected; 10) nicarbazin, infected; 11) robenidine, infected; and 12) zoalene, infected. On Day 14, chicks were challenged with an Eimeria field isolate by oral gavage. On Day 20, broilers were weighed, and gross lesion scores and microscores were classified from 0 to 4 depending on the severity of the gross lesion scores and E. maxima microscores. Data from three trials using different field isolates were statistically analyzed using a logarithmic regression model. There was no relationship (P = 0.1224) between microscores and body weight gain. There was a positive relationship between microscores and gross lesion scores (P = 0.004). However, there was also an interaction between isolate and treatment (P < 0.0001). Lastly, the interaction between isolate and gross lesion scores (P = 0.0041) demonstrates that the significance of the relationship between microscores and gross lesion scores may be dependent on pathogenicity of the challenge Eimeria or the amount of E. maxima in the inoculum.