JPC SYSTEMIC PATHOLOGY

HEMOLYMPHATICS SYSTEM

February 2024

H-B08

 

SIGNALMENT: Male rat

 

HISTORY: Tissue from a 150 gm male rat inoculated intraperitoneally with an infectious agent and killed 72 hours later.

 

HISTOPATHOLOGIC DESCRIPTION: Spleen: Effacing over 85% of the splenic white pulp are multifocal to coalescing up to 2mm nodules of lytic necrosis composed of abundant eosinophilic cellular and karyorrhectic debris, eosinophilic beaded fibrillar material (fibrin), hemorrhage, and fewer lymphocytes and macrophages. Necrotic nodules are surrounded by moderate amounts of fibrin and edema that extends into the adjacent red pulp. Less affected areas of white pulp are hypocellular (lymphoid depletion) with increased numbers of tangible body macrophages. Multifocally, vessels contain fibrin thrombi or vessels are lined by hypertrophic endothelium (reactive endothelial cells). Multifocally, the capsule contains few lymphocytes and plasma cells, and mesothelial cells are hypertrophic (reactive mesothelium).

 

Lymph node: Up to 60% of the cortical follicles, paracortical lymphocytes, and subcapsular sinuses are effaced by multifocal to coalescing nodules of necrosis (lymphoid depletion).  Areas of necrosis are composed of abundant eosinophilic cellular and karyorrhectic debris, fibrin, edema, and rimmed by macrophages. Medullary and subcapsular sinuses are expanded by fibrin, edema, and numerous foamy macrophages often containing intracytoplasmic erythrocytes (erythrophagocytosis). Multifocally, small and medium caliber vessels contain one of the following changes: lined by hypertrophic endothelium (reactive endothelium), diffusely congested, or contain perivascular edema.  

 

MORPHOLOGIC DIAGNOSIS:

1. Spleen: Splenitis, fibrinonecrotizing, subacute, multifocal to coalescing, severe, with lymphoid depletion, breed unspecified, rat, rodent.
2. Lymph node: Lymphadenitis, fibrinonecrotizing, subacute, multifocal, severe, with lymphoid depletion, sinus histiocytosis, erythrophagocytosis, and edema.

 

ETIOLOGIC DIAGNOSIS: Splenic and lymphatic francisellosis

 

CAUSE: Francisella tularensis

 

CONDITION: Tularemia, Rabbit fever, deer fly fever, O’Hara's Disease 

 

GENERAL DISCUSSION:

• Francisella tularensis is a gram-negative, pleomorphic, aerobic, non-spore forming, intracellular bacillus that can infect humans (zoonotic), wild rabbits, rodents, and over 100 species of wild and domestic mammals birds, fish, and reptiles
  • The biovar tularensis (type A) found only in North America
    • The biovar A is associated with terrestrial tick-rabbit infection cycle and causes severe illness in human, cats, rabbits
  • The biovar holartica (type B) found throughout Northern Hemisphere
    • The biovar B associated with aquatic species – including beavers and muskrats
    • Strains of the clade B.FTNF002-00 in European brown hares are often associated with splenitis and hepatitis and not with the polyserositis affecting pleura, pericardium, and kidney reported in the literature for infections
    • Caused by the clade B.13
  • The biovar mediasiatica is restricted to Central Asia
• Organism is abundant in nature as infection in many species of rodents, and transmit directly or via insect to other animals
• Potential biowarfare/bioterrorism weapon: Airborne tularemia; can potentially cause outbreaks of acute, undifferentiated febrile illness with incipient pneumonia, pleuritis, and hilar lymphadenopathy in 3-5 days
• Cats – Severe systemic illness with various manifestations
• Dogs – Highly resistant
• Foals and sheep – Associated with heavy infestation by ticks

 

PATHOGENESIS:  

• Poorly understood
• Transmission:
  • Arthropod bite inoculation; penetration of intact skin or cuts/abrasions; exposure of mucous membranes (conjunctiva, oropharynx, respiratory); ingestion; inhalation
  • Dermacentor variabilis (dog tick), D. andersoni (wood tick), D. occidentalis and Amblyomma americanum (lone star tick); transstadial and transovarian passage (both a vector and a reservoir of infection)
  • Chrysops discalis (deer fly); important vector in North America
• Reports of transmission from domestic cats to humans
• Ability to multiply intracellularly
• Infection at site of infection > lymphadenopathy of regional lymph nodes > bacteremia and dissemination to spleen, liver, and bone marrow in 3-5 days > septicemia
• Host defense is cell-mediated immunity by CD4+ T cells
• Cell-mediated immunity remains for several years following acute illness or vaccination with F. tularensis live vaccine
• Strong antibody response is mounted during infection with F. tularensis; however it is unclear if humoral immunity is protective; studies with killed vaccines show that humoral immunity alone is of no value against a virulent strain

 

TYPICAL CLINICAL FINDINGS:

• Lymphadenomegaly (localized, regional, or generalized), splenomegaly, and hepatomegaly with abscesses or areas of lytic necrosis
• Non-specific clinical signs: Fever, anorexia, lethargy, ocular and nasal discharge, diarrhea, pneumonia, ataxia
• Occasionally, abscesses at site of inoculation/penetration; vesiculopapular rash
• Rabbits and rodents often found dead with no clinical signs
• Fatal cases have been reported in foals, sheep, and young pigs, involving heavy infestations of ticks

 

TYPICAL GROSS FINDINGS:  

• Multiple, white-gray, up to 2 mm diameter foci of necrosis on the surface of the spleen, liver and lymph nodes, and indistinguishable from lesions by Yersinia spp. 
• Lymph nodes: Wedge-shaped areas of cortical necrosis and a narrow zone of intense hyperemia
• Cats: Miliary white foci 2 mm or more in liver, spleen, and lymph nodes   Prominent ulceration in Peyer’s patches and colonic lymphoid follicles 

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:  

• Focal to multifocal areas of necrosis in spleen, liver and lymph nodes surrounded by zone of lymphocytes, neutrophils, and macrophages
• Older lesions also have fibroblasts that form a sharp border around areas of necrosis
• Thrombosis of small blood vessels occasionally in areas of necrosis
• Occasionally, there are bacilli in the center of necrotic foci or within macrophages
• Peyer’s patch lymphoid depletion and abscess

 

ADDITIONAL DIAGNOSTIC TESTS:

• Bacterial culture performed in laboratories with adequate biocontainment facilities (risk of laboratory-acquired infection)  
• Serology, fluorescent antibody, immunohistochemical identification of F. tularensis antigen
• Organism identification in aspirates or lesions (methylene blue)
• IHC to demonstrate large clumps of bacteria within necrotic foci and in macrophages
• PCR

 

DIFFERENTIAL DIAGNOSIS:

Multifocal necrotizing lymphadenitis, splenitis, and hepatitis in laboratory animals:

• Tyzzer’s disease (Clostridium piliformis):  
  • Mice, gerbils, and other laboratory and wild rodents
  • Multifocal foci of hepatic necrosis (older mice), catarrhal enterocolitis with mesenteric lymph node abscesses (younger mice); myocardial lesions occasionally (especially gerbils) 
• Yersiniosis (Yersinia pseudotuberculosis):  
  • Cats, Guinea pigs, rabbits, rats, other rodents 
  • Acute: Enteritis (ileum) with mucosal ulceration
  • Subacute to chronic: Discrete miliary to caseous lesions in lymph nodes, spleen, liver and lungs
  • Bacteria are easy to visualize, in contrast to F. tularensis
• Salmonellosis (Salmonella sp.):  
  • Guinea pigs, mice, rats, other rodents
  • Acutely- Focal areas of necrosis in liver, spleen, and lymphoid tissue and intestine
• Bubonic plague (Yersinia pestis):  
  • Rats, squirrels, and other rodents 
  • Very similar grossly

 

COMPARATIVE PATHOLOGY:  

• Humans: Seven distinct clinical forms are recognized
  • Ulceroglandular form(85% of cases): Ulcerated skin lesion, lymphadenopathy
  • Glandular form (5-10% of cases): Fever, lymphadenopathy without ulceration
  • Typhoidal form (5-15%): Fever, prostration, weight loss, no lymphadenopathy
  • Oculoglandular form (1-2%): Unilateral conjunctivitis, preauricular or cervical lymphadenopathy
  • Oropharyngeal form: Pharyngotonsillitis, cervical lymphadenopathy
  • Pneumonic and Septicemic forms
• Guinea pig: Easily infected, frequently used in diagnostic work
• Sheep: Endemic disease in areas with reservoir rodents and ticks
• Foals and sheep: Fatalities have occurred associated with heavy tick infestation

• Foals may have systemic disease with enlargement of liver, spleen and kidneys and small necrotic foci
• Sheep have lesions in superficial lymph nodes or more classic localization; pneumonia from other causes and anemia from ticks may be present

• Dogs: After ingestion of infected rabbit; may show disease but in general dogs are highly resistant to disease
• Cynomolgus monkeys: Local lymphadenopathy, gingivostomatitis, and systemic disease (necrotizing hepatitis, granulomatous splenitis, pneumonia, and tubulonephritis)
• Gray squirrels: Subclinical infection is possible
• European Brown Hares: Urinary tract and mammary glands are important routes for shedding

 

REFERENCES: 

1. Delaney MA, Treuting PM, Rothenburger JL. Lagomorpha. In: Terio KA, ed. Pathology of Wildlife and Zoo Animals. Elsevier Inc.; 2018; 492.
2. Frasca S, Wolf JC, Kinsel MJ, Camus AC, Lombardini ED. Osteichthyes. In: Terio KA, ed. Pathology of Wildlife and Zoo Animals. Elsevier Inc.; 2018; 978.
3. Lowenstine LJ, McManamon R, Terio KA. Apes. In: Terio KA, ed. Pathology of Wildlife and Zoo Animals. Elsevier Inc.; 2018; 396.
4. Mätz-Rensing KA, Lowenstine LJ . New World and Old World Monkeys. In: Terio KA, ed. Pathology of Wildlife and Zoo Animals. Elsevier Inc.; 2018; 358-359.
5. McAloose D, Stalis IH. Prosimians. In: Terio KA, ed. Pathology of Wildlife and Zoo Animals. Elsevier Inc.; 2018; 335.
6. Percy DH, Barthold SW. Pathology of Laboratory Rodents and Rabbits. 4th ed. Oxford UK, Wiley Blackwell Publishing; 2016; 185; 278.
7. Simmons J, Gibson S. Bacterial and Mycotic Diseases of Nonhuman Primates. In: Abee CR, ed. Nonhuman Primates in Biomedical Research. 2^nd ed. Elsevier, 2012: 127-128.
8. Van Wettere AJ, Brown DL. Hepatobiliary System and Exocrine Pancreas. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Mosby/Elsevier; 2022:514;516.
9. Valli VEO, Kiupel M, Bienzle D. Hematopoietic system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. 6th ed. Vol 3. Philadelphia, PA: Elsevier; 2016:184-186.

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