| Summary: | For reasons unknown, <i>Eimeria maxima</i> is unique among <i>Eimeria</i> species infecting chickens in the immunovariability it displays among isolates from different geographical areas. <i>Eimeria maxima</i> oocysts (named EmaxAPU3) were isolated late in grow-out (6 weeks) from litter in a commercial broiler operation that was using <i>Eimeria</i> vaccination as the coccidiosis control program. Cross-protection studies (n = 4) were conducted in immunologically naïve chickens between EmaxAPU3 and two <i>E. maxima</i> lab strains (EmaxAPU1, EmaxAPU2) by immunizing with one <i>E. maxima</i> strain and challenging with either the homologous or heterologous <i>E. maxima</i>. As measured by oocyst output, immunization with EmaxAPU1 protected against homologous challenge (EmaxAPU1) and against heterologous challenge with EmaxAPU3, but not against EmaxAPU2. Similarly, immunization with EmaxAPU3 protected against homologous challenge (EmaxAPU3) and against heterologous challenge with EmaxAPU1, but not against EmaxAPU2. Immunization of chickens with EmaxAPU2 elicited a protective response against homologous challenge (EmaxAPU2), but not against EmaxAPU1 nor EmaxAPU3. The most plausible explanation for the appearance of this immunovariant late in grow-out is that <i>E. maxima</i> APU3 escaped immunity directed to <i>E. maxima</i> antigenic types in the commercial vaccine.
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