12-year-old male castrated domestic shorthaired cat (Felis cats).Presented to the referring veterinarian with a history of anorexia, pyrexia, lethargy, diarrhea and oral ulcers of seven days duration.
The mesenteric and mandibular lymph nodes were markedly enlarged, and the corticomedullary architecture was effaced by friable, pinpoint yellow-white raised nodules. The pulmonary parenchyma contained several slightly raised, white nodules which were miliary to 2 mm in diameter. 10 and 20 ml of clear, yellow-red fluid was found in the abdominal cavity. The livers were yellowish-brown and the hepatic parenchyma contained several white, slightly raised nodules which were miliary to 2 mm in diameter and evenly distributed through the parenchyma. The spleens contained multiple white, round, raised nodules and were approximately 1-2 mm in diameter.
Spleen: The parenchyma is disrupted by multiple nodules composed of degenerate neutrophils, and macrophages and varying amounts of amorphous to slightly fibrillar eosinophilic material (fibrin) and basophilic cellular and nuclear debris (liquefactive necrosis) with loss of lymphoid tissue from the areas surrounding these nodules. Overall, there is a moderate degree of depletion of the periarterial lymphoid sheaths. Erythrocyte and myeloid precursors are scattered throughout the splenic parenchyma with rare mega-karyocytes (extramedullary hematopoesis).
Moderate to severe multifocal acute necrosuppurative splenitis
Specimens of spleen, liver, and lung submitted for culture, PCR and DNA sequencing were positive for Francisella tularensis.
Francisella tularensis, the causative agent of tularemia, is an aerobic, gram-negative coccobacillus. The two most common biovars are F. tularensis biovar tularensis and F. tularensis biovar palaeartica. F. tularensis biovar tularensis biovar is more common in North America.(1, 10, 6, 2, 8) F. tularensis biovar tularensis has been the only biovar identified in fatal systemic infections in cats.(2) Infections have been reported in cats, rabbits, rodents, dogs, cattle, sheep, horses, non-human primates, and humans with cats, rabbits, rodents, and sheep appearing to be the most susceptible to natural infection.(1,6,2) Tularemia is transmitted via bites of infected arthropods (most often ticks or flies), ingestion of infected rabbits and or rodents, inhalation, and mucous membrane exposure.(1, 10, 6, 8) Clinical signs commonly associated with tularemia include fever, anorexia, lymphadenopathy, hepatosplen-omegaly, dehydration, and neutrophilia or neutropenia with toxic change. Common gross findings with tularemia include multifocal splenic, hepatic, lymphoid, and pulmonary necrosis, oral and gastrointestinal ulceration, and enterocolitis.(1, 10, 6, 2, 8) Microscopic findings typically include necrotizing inflammation in multiple organs (spleen, liver, lungs, lymph nodes, and gastrointestinal tract common) with varying numbers of associated neutrophils and macrophages. Colonies of intralesional gram negative coccobacilli may be visualized as well.(1,6,2) Definitive diagnosis is based on successful isolation of F. tularensis from specimens obtained from affected animals,(1, 10, 6, 2, 8) fluorescent antibody reaction and agglutination with F. tularensis anti-serum,(1, 10, 6) immunohistochemistry on formalin fixed paraffin embedded tissues,(2,8) or PCR and DNA sequencing.(8)
Spleen: Splenitis, necrotizing, multifocal, marked with lymphoid depletion, fibrin thrombi and extramedullary hematopoiesis.
Conference participants agreed both red and white pulp are affected by necrosis, but the lesions are centered on the white pulp, with severe lymphoid depletion. Participants described the abundant fibrin thrombi within the section, but noted they are non-occlusive with no evidence of infarction. Differential diagnoses which were discussed included Yersinia pseudotuberculosis, Yersinia enterocolitica and Salmonella spp. infection, which share many gross and histologic characteristics with tularemia, as well as Yersinia pestis. The history provided is typical for both plague and tularemia infection. However, Yersinia enterocolitica and pseudotuberculosis are generally associated with large colonies of gram-negative bacilli visible histologically, and Salmonella spp. infection is less commonly seen in domestic cats. Special stains that can be used to identify F. tularensis include Gram stains as well as both Giemsa and Warthin-Starry. In this case Gram stains did not help identify organisms in the section.
Tularemia has been reported in nearly all 50 states except Hawaii, and in 2015 is on the rise in North and South Dakota, Colorado, Wyoming, and New Mexico. More common modes of infection in humans include cleaning/skinning infected lagomorphs and arthropod bites, but also include infectious from contaminated water supplies as well as consumption of contaminated undercooked meat. Sporadic outbreaks of tularemia are known to occur in sheep resulting in late term abortion and may be associated with abundant tick populations.(5) In some cases of small mammal infection, classic lesions may not be present grossly or histologically. Additionally, in wildlife species, autolysis and freeze artifact may interfere with evaluation of gross lesions. Subclinical infections are known to occur, but the role of subclinically infected animals in disease transmission is unclear, which serves to highlight the importance of taking appropriate safety precautions when working with the carcasses of deceased wildlife in endemic areas.(4)
The agent is highly infectious with as few as 10-50 organisms capable of causing an infection and it is classified as a category A bioterrorism agent. Experimentally induced lesions from inhalation in African green monkeys included necrotizing pyogranulomatous lesions which targeted the lung and lymphoid tissue.(9) The organisms are most commonly located intracellularly in macrophages, in both the inhalation form in primates(9) as well as in small mammals, but may also be present extracellularly in exudates and necrotic debris as well as in other cell types.(3) F. tularensis is able to evade the immune system by infecting and replicating in macrophages and disseminating to distal organs in the acute phase of the disease without being detected. It prevents activation of macrophages that would result in an appropriate host innate immune response, which is important in survival in infections with virulent F. tularensis.(7)
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