JPC SYSTEMIC PATHOLOGY

RESPIRATORY SYSTEM

September 2023

P-F07

 

Slide A: Signalment (JPC #2018004): Unknown age, female Sprague‑Dawley rat

 

HISTORY: This Sprague‑Dawley rat was sacrificed at the termination of a steroid dose‑range‑finding study. This animal was in the high‑dose group fed a diet providing approximately 20 mg steroid/kg BW/day for 13 weeks.

 

HISTOPATHOLOGIC DESCRIPTION: Lung: Affecting 80% of the section, alveolar lumina are expanded by eosinophilic foamy to granular material mixed with low numbers of necrotic neutrophils, macrophages, sloughed epithelial cells, and fewer lymphocytes and plasma cells. These inflammatory cells also mildly expand the alveolar septa. Within the eosinophilic exudate and rarely within macrophages are numerous indistinct, poorly-defined, 4-8 µm, pale staining, round to oval fungal cysts. Multifocally alveoli are lined by plump cuboidal epithelium (type II pneumocyte hyperplasia). Multifocally there are perivascular and peribronchiolar aggregates of moderate numbers of plasma cells, fewer lymphocytes and macrophages, and mildly ectatic lymphatics (edema). Multifocally, the pleura is lined by hypertrophic mesothelium.     

 

Slide B: Lung (GMS): There are numerous 3-4 µm round to oval fungal cysts within alveoli and macrophages that stain black with central pallor. 

 

MORPHOLOGIC DIAGNOSIS:  Lung:  Pneumonia, interstitial, neutrophilic and histiocytic, diffuse, mild, with type II pneumocyte hyperplasia and abundant foamy alveolar exudate with numerous intraalveolar and intrahistiocytic fungal cysts, Sprague‑Dawley rat, rodent.

 

ETIOLOGIC DIAGNOSIS:  Pulmonary pneumocystosis

 

CAUSE:  Pneumocystis carinii 

 

GENERAL DISCUSSION: 

• Atypical fungus of worldwide distribution causing pneumonia in immunocompromised animals of a variety of species
• Related to ascomycetous yeasts (e.g. Candida albicans) based on genetic sequencing; biologically, behaves like a protozoan
• Individual strains are morphologically indistinguishable but retain high species specificity (not zoonotic and not transmitted between different species)   
• Organism occurs in two forms (both occur in the lung):     
  • Trophic form: 1-4 µm, irregularly shaped, uninucleate
  • Cysts: 4-8 µm, thick walled, multinucleate with 8 intracystic bodies  
• Infective stage unknown and life cycle not completely understood; most commonly appears as an opportunistic infection in an immunocompromised patient (SCID mice, HIV/SIV)

 

PATHOGENESIS: 

• In immunosuppressed animals, intracystic bodies are released from cyst and adhere to type I pneumocytes, mediated by fibronectin-binding integrins and major surface glycoprotein (glycoprotein A), then develop into trophic forms
• Alveolar-capillary membrane is disrupted causing epithelial cell necrosis and sloughing
• Host response leads to type II pneumocyte hypertrophy and hyperplasia
• In severe cases, significant alveolar-capillary disruption can lead to dissemination and extrapulmonary pneumocystosis
• Macrophages and cell-mediated immune responses are essential to control Pneumocystis, hence clinical disease implies impairment of these responses

 

TYPICAL CLINICAL FINDINGS:

 

TYPICAL GROSS FINDINGS:

• Diffuse or patchy, red to pale-pink to yellow-brown, firm rubbery consolidation
• Failure to collapse
• May be altered by coexisting viral or bacterial infection

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Diffuse interstitial pneumonia with alveolar flooding by an eosinophilic foamy or “honeycomb” exudate (edema + organisms) with fungal cysts containing fungal bodies
• P. carinii stains poorly with H&E; however, large amounts of eosinophilic foam without significant inflammation should put this at the top of the rule out list 
  • Use GMS stain to better visualize yeast
• Variable type II pneumocyte hyperplasia and fibrosis

 

ULTRASTRUCTURE FINDINGS:

 

ADDITIONAL DIAGNOSTIC TESTS:

• Trophozoites stain with Giemsa (red nucleus, blue cytoplasm)
• Cysts stain with Gomori Methenamine Silver (GMS) (black with central pallor), as well as PAS
• Bronchoalveolar lavage to recover cysts or trophozoites
• Cytologic findings:
  1. Cysts are extracellular, approximately 5 μm in diameter and contain four to eight round, 1- to 2-μm basophilic bodies; trophozoites are pleomorphic, ranging from 2 to 7 μm in length

 

DIFFERENTIAL DIAGNOSIS:

• Fairly diagnostic when H&E combined with GMS
• Lesions of P. carinii have been previously attributed to “rat respiratory virus”
• In mice, prior to staining with GMS or Giemsa, consider: Sendai virus, pneumonia virus of mice, and pulmonary edema secondary to congestive heart failure

 

COMPARATIVE PATHOLOGY:

• Immunosuppressed laboratory animals: Athymic rats and mice, SCID mice, SIV monkeys
  1. Can be carried without clinical signs by a variety of wild rodents
• Species specific
  • P. carinii & wakefieldiae (rat)
  • P. murina (mouse)
  • P. carinii & jirovecii (nonhuman primates)
  • P. carinii sp. nov. orytolagi (rabbit)
  • P. carinii special form (sp. f.) equi (horse, especially SCID Arabian foals; often associated with Rhodococcus equi)
  • P. carinii sp. f. suis (pig- secondary to PRRS)
• Dogs: Often associated with canine distemper virus
  • Hereditary immunodeficiency in miniature dachshunds and Cavalier King Charles spaniel
  • Case report of disseminated pneumocystosis in a toy poodle (Sakashita, J Comp Pathol. 2020)
• Nonhuman primates: Associated with debilitation due to importation, concurrent bacterial infection, neoplasia, or retroviral infection; can be endemic in tamarin colonies; clinical signs include weight loss, anorexia and failure to thrive
• Humans: Found in 60% of AIDS patients (P. jiroveci). 

 

REFERENCES:

1. Barthold SW, Griffey SM, Percy DH, eds. Pathology of Laboratory Rodents & Rabbits, 4th ed. Ames, IA: Blackwell Publishing Ltd; 2016:146-147. 
2. Boes KM. Respiratory System. In: Raskin RE, Meyer DJ, eds. Canine and Feline Cytology: A Color Atlas and Interpretation Guide. 4th ed. St. Louis, MO: Elsevier; 2023:189, 220-221. 
3. Caswell JL, Williams KJ. Respiratory system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. Philadelphia, PA: Elsevier; 2015:535-536. 
4. Delaney MA, Treuting PM, Rothenburger JL. Rodentia. In: Terio KA, McAloose D, St. Leger J, eds. Pathology of Wildlife and Zoo Animals. London, UK: Academic Press; 2018:510-511. 
5. De Terlizzi R, English K, Cowell RL, Tyler RD, Meinkoth JH. Transtracheal and Bronchoalveolar Washes. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and Hematology of the Dog and Cat. 5th ed. St. Louis, MO: Elsevier Mosby; 2020:263. 
6. Grimes CN, Fry MM, LeBlanc CJ, Hecht S. The Lung and Intrathoracic Structures. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and Hematology of the Dog and Cat. 5th ed. St. Louis, MO: Elsevier Mosby; 2020:278. 
7. Lane LV, Yang PJ, Cowell RL. Selected Infectious Agents. In: Valenciano AC, Cowell RL, eds. Diagnostic Cytology and Hematology of the Dog and Cat. 5th ed. St. Louis, MO: Elsevier Mosby; 2020:52, 59. 
8. Lopez A, Martinson SA.  Respiratory system, mediastinum, and pleurae, In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:563. 627, 637.
9. Lowenstine LJ, Osborn KG. Respiratory system diseases of nonhuman primates. In: Abee CR, Mansfield K, Tardif S, Morris T, eds. Nonhuman Primates in Biomedical Research: Diseases. Vol 2. San Diego, CA: Academic Press; 2012:450-452.
10. Matz-Rensing K and Lowenstein L. New world and old world monkeys. Pathology of Wildlife and Zoo Animals, San Diego, CA: Elsevier 2018: 364.  
11. Sakashita T, Kaneko Y, Izzati UZ, et al. Disseminated pneumocystosis in a toy poodle. J Comp Pathol, 2020; 175:85-89.
12. Simmons J, Gibson S. Bacterial and mycotic diseases of nonhuman primates. In: Abee CR, Mansfield K, Tardif S, Morris T, eds. Nonhuman Primates in Biomedical Research: Diseases. Vol 2. San Diego, CA: Academic Press; 2012:147-148.

 

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