ANALYSIS OF IMMUNOLOGICAL CROSS-PROTECTION AND SENSITIVITIES TO ANTICOCCIDIAL DRUGS AMONG 5 GEOGRAPHICAL AND TEMPORAL STRAINS OF EIMERIA-MAXIMA

Two laboratory strains (USDA strain No. 68 isolated from the eastern s hore of Maryland 15 Sears ago and a University of Guelph strain isolat ed from an Ontario broiler house 23 years ago) and 3 recent field stra ins of Eimeria maxima [isolated in Maryland (MD), North Carolina (NC) and Florida (FL)] mere tested for their ability to induce cross-protec tive immunity and their sensitivities to a variety of anticoccidial co mpounds, To assess immunological cross-protection, 1-day-old chicks me re inoculated and subsequently challenged at 10 days of age, testing a ll possible combinations of initial inoculating (immunizing) and subse quent challenge strain, Six days post-challenge, chicks were killed an d weight gains and lesion scores were determined and compared to sham inoculated and challenged, and sham challenged age-matched controls. T he 2 laboratory strains and the NC strain were fully cross-protective against each other by both these measures. In contrast, the MD and FL strains induced complete protection only against the homologous strain . Reciprocally, no other strains protected chicks completely against t he FL and MD strains. Drug sensitivity studies using 10 different anti coccidial formulations at prescribed drug levels showed significant di fferences between the 2 laboratory strains and the 3 recently isolated field strains; more recent isolates from commercial broiler houses de monstrated complete or partial resistance to a wider range of anticocc idial compounds. No correlation was seen between cross-protection and sensitivities to anticoccidials. Published by Elsevier Science Ltd.