August 2018

SIGNALMENT (AFIP Accession #1953401):  Beaver (Castor canadensis)

HISTORY:  Multiple beavers died in one stream without showing prior clinical signs.

HISTOPATHOLOGIC DESCRIPTION:  Liver: Multifocally and randomly replacing approximately 40% of the hepatic parenchyma, there are numerous, variably sized (up to 1mm in diameter) areas of lytic necrosis characterized by abundant eosinophilic cellular and karyorrhectic debris admixed with moderate numbers of degenerate neutrophils and fibrin.  Heptocytes at the margins of the areas of lytic necrosis often are dissociated from hepatic cord and have hypereosinophilic cytoplasm and pyknotic nuclei (necrosis).  Hepatocytes often contain granular, yellow-brown or green-brown cytoplasmic pigment (hemosiderin, lipofuscin or bile).  Within multiple central veins and portal vessels, when areas of lytic necrosis are directly adjacent to vessels, vessel walls, including all vascular tunics, are effaced and replaced by necrotic debris, fibrin, hemorrhage and degenerate neutrophils (vasculitis and vascular necrosis). Occasionally, fibrin thrombi occlude vessels.  Multifocally, portal areas are minimally expanded by low to moderate numbers of lymphocytes, plasma cells, neutrophils, and macrophages.

MORPHOLOGIC DIAGNOSIS:  Liver:  Hepatitis, necrotizing, acute, multifocal and random, moderate, with necrotizing vasculitis and fibrin thrombi, beaver (Castor canadensis), rodent.

ETIOLOGIC DIAGNOSIS:  Hepatic francisellosis

CAUSE:  Francisella tularensis

SYNONYMS:  Tularemia, Rabbit Fever, Deerfly Fever, O’Hara’s Disease (Japan)


  1. tularensis ssp. tularensis: Jellison type A; most virulent, especially for laboratory rabbits; North America; wildlife reservoir; tick-rabbit infection cycle
  2. tularensis ssp. holarctica, formerly ssp. palaearctica: Jellison type B; less virulent; Europe, Asia and North America; waterborne disease of rodents
  3. tularensis ssp. mediaasiatica: Central Asia
  4. tularensis ssp. novicida, formerly Francisella novicida: North America







For hepatic necrosis/necrotizing hepatitis in rodents and lagomorphs:



  1. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: John Wiley and Sons, Inc; 2016: 185-186, 278.
  2. Brown VR, Adney DR, Bielefeldt-Ohmann H, et al. Pathogenesis and immune responses of Francisella tularensis strains in wild-caught cottontail rabbits (Sylvilagus spp.). J Wildl Dis. 2015;51(3):564-575.
  3. Feldman AF. Tularemia. J Am Vet Med Assoc. 2003;222:725-730.
  4. Gyuranecz M, Fodor L, Makrai L et al. Generalized tularemia in a vervet monkey (Chlorocebus aethiops) and a patas monkey (Erythrocebus patas) in a zoo. J Vet Diagn Invest. 2009;21:384-387.
  5. Gyuranecz M, Szeredi L, Makrai L, et al. Tularemia of European Brown Hare (Lepus europaeus): A Pathological, Histopathological, and Immunohistochemical Study. Vet Pathol. 2010;47(5):958-963.
  6. Hestvik G, Uhlhorn H, Södersten F,et al. Tularaemia in European Brown Hares (Lepus europaeus) and Mountain Hares (Lepus timidus) Characterized by Histopathology and Immunohistochemistry: Organ Lesions and Suggestions of Routes of Infection and Shedding. J Comp Pathol. 2017;157(2-3):103-114.
  7. Hutt JA, Lovchik JA, Dekonenko A, Hahn AC, Wu TH. The Natural History of Pneumonic Tularemia in Female Fischer 344 Rats after Inhalational Exposure to Aerosolized Francisella tularensis Subspecies tularensis Strain SCHU S4. Am J Pathol. 2017 Feb;187(2):252-267.
  8. Ketz-Riley CJ, Kennedy GA, Carpenter JW et al. Tularemia type A in captive bornean orangutans (Pongo pygmaeus pygmaeus). J Zoo Wildl Med. 2009;40:257-262.
  9. Nelson DD, Haldorson GJ, Stanton JB, et al. Francisella tularensis infection without lesions in gray tree squirrels (Sciurus griseus): a diagnostic challenge. J Vet Diagn Invest. 2014;26(2):312-315.
  10. O’Toole D, Williams ES, Woods WL et al. Tularemia in range sheep: an overlooked syndrome? J Vet Diagn Invest. 2008;20:508-513.
  11. Origgi FC, Pilo P. Francisella Tularensis Clades B.FTN002-00 and B.13 Are Associated With Distinct Pathology in the European Brown Hare (Lepus europaeus). Vet pathol. 2016;53(6):1220-1232.
  12. Padeshki PI, Ivanov IN, Popov B, et al. The role of birds in dissemination of Francisella tularensis: first direct molecular evidence for bird-to-human transmission. Epidemiol Infect. 2010;138(3):376-379.
  13. Raymond CR, Conlan JW. Differential susceptibility of Sprague-Dawley and Fischer 344 rats to infection by Francisella tularensis. Microb Pathog. 2008;46:231-234.
  14. Russo BC, Brown MJ, Nau GJ. MyD88-dependent signaling prolongs survival and reduces bacterial burden during pulmonary infection with virulent Francisella tularensis. Am J Pathol. 2013;183(4):1223-1232.
  15. Slight SR, Monin L, Gopal R, et al. IL-10 restrains IL-17 to limit lung pathology characteristics following pulmonary infection with Francisella tularensis live vaccine strain. Am J Pathol. 2013;183(5):1397-1404.
  16. Valli VEO, Kiupel M, Bienzle D. Hematopoietic system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer"s Pathology of Domestic Animals. Vol 3. 6th ed. Philadelphia, PA: Elsevier Saunders; 2016: 184-186.

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