Tularemia without lesions in grey tree squirrels: A diagnostic challenge

Authors: NELSON, DANIELLE - Washington State University; HALDORSON, GARY - Washington State University; STANTON, JAMES - Washington State University; Noh, Susan; BRADWAY, DANIEL - Washington State University; MANSFIELD, KRISTIN - Washington Department Of Fish & Wildlife; BASLER, TIMOTHY - Washington State University

Submitted to: Journal of Veterinary Diagnostic Investigation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/21/2013
Publication Date: 3/26/2014
Citation: Nelson, D.D., Haldorson, G.J., Stanton, J.B., Noh, S.M., Bradway, D.S., Mansfield, K.G., Basler, T.V. 2014. Tularemia without lesions in grey tree squirrels: A diagnostic challenge. Journal of Veterinary Diagnostic Investigation. 26(2):312-315.

Interpretive Summary: Tularemia, which is caused by Francisella tularensis, is an important bacterial disease which affects many species, including wildlife, domestic cats, sheep, and humans. This bacterium can be transmitted by mosquitoes and ticks, by direct inoculation into cuts or scrapes, and by aerosol. Due to this latter property and the fact that only a few bacteria (10 to 50) are required to cause disease, F. tularensis has the potential for use as a biological weapon. Francisella tularensis is endemic in wildlife populations, particularly small mammals, in many regions of the United States. In veterinary diagnostic laboratory settings, a presumptive diagnosis of tularemia can be made from the history and clinical signs in species known to be commonly affected. Additionally, lesions typical of tularemia, such as necrotizing hepatitis and lymphadenitis help to alert laboratory staff of the presence of this zoonotic pathogen so appropriate biosafety measures can be implemented. However, there are a few reports of subclinical infections in small mammals. Such infections present a particular risk to people handling tissues from these animals. In this paper, findings from 29 western grey and eastern grey squirrels submitted to the Washington Animal Disease Diagnostic Laboratory as part of an ecology study are presented. Fifteen of these squirrels had evidence of infection with F. tularensis. Nine of the squirrels had lesions typical of tularemia, while the remainder of squirrels lacked such lesions. These findings indicate that subclinical tularemia may be more common that has been previously recognized in squirrels. People which handle squirrels, and particularly tissues from squirrels, should take appropriate precaution to safeguard against exposure to F. tularensis.

Technical Abstract: Fifteen cases of Francisella tularenesis infection (tularemia) were identified in western grey (Sciurus griseus) and eastern grey (Sciurus carolinesis) 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 fifteen squirrels with F. tularenesis 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 or microscopic lesions typical of tularemia. All 15 squirrels with F. tularensis infection were identified by PCR from spleen, liver or lymph node (including all 6 squirrels without typical tularemia lesions), 7/15 squirrels were positive by direct fluorescent antibody (FA) assay of tissues and 5/15 squirrels were positive by culture of tissues. These findings underscore the importance of considering tularemia as a possible cause of death when no lesions of tularemia can be identified at necropsy. Furthermore, our findings suggest the possibility of subclinical infections in grey squirrels, and the importance of molecular diagnostics for definitive diagnosis of F. tularensis infection in wild squirrels.