Francisella tularensis infection without lesions in gray tree squirrels (Sciurus griseus): a diagnostic challenge

Fifteen cases of Francisella tularensis infection (tularemia) were identified in western gray (Sciurus griseus) and eastern gray (Sciurus carolinensis) squirrels submitted to the Washington Animal Disease Diagnostic Laboratory between 2008 and 2011. All of the squirrels originated in Washington State, a geographical area with endemic tularemia in wildlife. Nine of the 15 squirrels with F. tularensis infection had gross (2/15) or microscopic (9/15) multifocal necrotizing lesions in the spleen, liver, or lymph nodes, typical of tularemia. Special stains did not reliably identify intralesional bacteria microscopically. Six of the 15 squirrels infected with F. tularensis lacked gross and microscopic lesions typical of tularemia. All 15 squirrels with F. tularensis infection were identified by polymerase chain reaction tests on the spleen, liver, or lymph node (including all 6 squirrels without typical tularemia lesions); 8 out of 9 squirrels were positive by direct fluorescent antibody test of tissues, and 5 out of 15 squirrels were positive by culture of tissues. The findings underscore the importance of considering tularemia as a possible cause of death when no lesions of tularemia can be identified at necropsy. Furthermore, the findings suggest the possibility of subclinical infections in gray squirrels, and the importance of molecular diagnostics for definitive diagnosis of F. tularensis infection in wild squirrels.