Immunohistochemical detection of Francisella tularensis in formalin-fixed paraffin-embedded tissue

Tularemia is a zoonotic disease caused by Francisella tularensis. Histopath ologic lesions of lymph nodes in cases of ulceroglandular tularemia include stellate microabscesses similar to those seen in cat-scratch disease or ly mphogranuloma venereum. When the lung is involved, the pathology is that of an acute necrotizing pneumonia. Traditionally, diagnosis is based on serol ogy, culture, and fluorescent antigen detection, but each of these methods presents problems. We studied formalin-fixed tissues from four tularemia ca ses as well as control tissues by using immunohistochemistry (IHC) for F, t ularensis. Positive immunostaining was seen in all the tularemia cases exam ined. Three cases were fatal, and positive IHC staining could be seen in th e lungs (bacteria in cellular debris, and intracellularly in alveolar macro phages, endothelia, and polymorphonuclear leukocytes), spleen tin necrotic areas), lymph nodes tin stellate abscesses), and liver tin Kupffer cells). The fourth patient had ulceroglandular tularemia, and the lymph node biopsy demonstrated the typical stellate microabscesses. By IHC, there was positi ve staining of bacteria in the abscesses and occasional macrophages and end othelial cells in nonnecrotic areas. In conclusion, F. tularensis can be de tected by IHC in formalin-fixed tissue, which, if available as biopsy mater ial, can be useful for diagnosis. MC can be helpful in our understanding of tularemia pathogenesis because bacteria are visualized in the context of t he surrounding tissue morphology.