RESPONSES OF THE ALLIGATOR 



697 



Fig. 3 Wright stained peripheral smear of anticoagulated blood of 

 healthy, uninfected Alligator mississippiensis. A, nucleated red blood 

 cells; E, eosinophil; F, basophU; G, immature nucleated red blood 

 cell. Note the loosely packed nuclear material, compared with the 

 compact darker staining nucleus of the mature red blood cell. 



an absence of elevated serum proteins in the alpha 2 region (Fig. 4). 

 Alligators held at 25 to 30°C maintained the best body condition 

 and generated the most striking immune response. Young animals 

 (0.5 to 1.0 years old) died within 3 weeks when placed in tanks at 

 35°C. This age group also was more susceptible to infection by any 

 route at the higher temperatures. This may represent limited host 

 defense responses of juveniles, increased sensitivity to thermal stress, 

 or some combination of these factors. 



Thermal stress seems to have a major effect on the host response 

 of A. mississippiensis. Animals kept at various temperatures (20, 25, 

 30, and 35°C) had similar base-line levels of white blood cell counts 

 (Fig. 6). After infection their white blood cells and neutrophilic 

 macrophages were maximum at 30°C. This temperature corre- 

 sponded to optimum recovery of infected animals. Exposure in the 

 35°C tank resulted in slightly lower neutrophilic macrophage 

 numbers and a uniformly fatal outcome (four of four infected 

 animals died). The ineffective host response at 35°C may be the 

 result of complex host agent factors, such as better growth for 

 Aeromonas or decreased phagocytosis by neutrophilic macrophages. 



