JPC SYSTEMIC PATHOLOGY

RESPIRATORY SYSTEM

August 2023

P-B06

 

SIGNALMENT (JPC #4069782): 3-month-old miniature horse filly

 

HISTORY: This filly presented with a one-week history of non-resolving respiratory congestion. High fever and increased respiratory rate were noted and (unspecified) treatment was initiated. The patient was initially responsive to care, but subsequently had two seizures and died.

 

HISTOPATHOLOGIC DESCRIPTION: Effacing approximately 60% of the normal pulmonary parenchyma and compressing adjacent pulmonary parenchyma are multiple variably sized pyogranulomas up to 14 mm in diameter composed of a central core of necrosis composed of eosinophilic cellular and karyorrhectic debris, eosinophilic beaded fibrillar material (fibrin), multifocal mineralization, and abundant necrotic neutrophils and macrophages, surrounded by epithelioid macrophages and fewer multinucleated giant cells with a loose, indistinct fibrous capsule. The cytoplasm of macrophages and multinucleated giant cells is often packed with numerous 1-2 µm basophilic coccobacilli. Bronchiolar lumens and adjacent alveoli spaces are filled with previously described inflammatory cells (including macrophages packed with coccobacilli), necrotic cellular debris, and flocculant eosinophilic material (fibrin and edema). In the most affected areas adjacent to bronchioles, alveolar septa are necrotic and replaced by cellular debris or in less affected areas are expanded up to 5 times normal by fibrin, edema, macrophages, and neutrophils. Bronchiolar epithelial cells are multifocally necrotic and sloughed into the lumen. The pleura and interlobular septa are expanded up to 4 times normal by fibrin and edema and few macrophages and neutrophils. 

 

MORPHOLOGIC DIAGNOSIS: Lung: Bronchopneumonia, pyogranulomatous and necrotizing, multifocal to coalescing, chronic-active, severe, with intrahistiocytic coccobacilli, miniature horse, equine.

 

ETIOLOGIC DIAGNOSIS: Rhodococcal pneumonia

 

CAUSE: Rhodococcus equi

 

GENERAL DISCUSSION: 

• Rhodococcus equi is a common, facultative intracellular, Gram-positive coccobacillus
• Cell wall contains complex lipids similar to Mycobacterium sp. and Nocardia sp.; increased resistance to chemicals, dehydration, oxidative stress, and lowered pH
• Causes two primary forms of disease, typically in foals 1-6 months old:
  1. Pyogranulomatous bronchopneumonia 
  2. Ulcerative and pyogranulomatous enterotyphlocolitis 
• Worldwide distribution with highest incidence in temperate climates, neutral soil pH, and high stocking density in dry, dusty, manure-contaminated paddocks
• Sporadic disease in cattle, goats, pigs, dogs, cats, and immunocompromised humans

 

PATHOGENESIS:  

• Normally inhabits soil and gastrointestinal tract of herbivores and is shed in horse manure
• Transmission via inhalation of soil-borne bacteria
• Bacterium is engulfed by macrophages, but survives by preventing maturation of the phagosome and fusion with the lysosome; bacteria replicate within and rupture macrophage to release bacteria
  1. Macrophages can kill bacterium when opsonized or when macrophages are activated by IFN-gamma
• Humoral immunity is protective, but strong cell-mediate T_H-1 response is necessary to clear an established infection, as CD4+ T-cells produce IFN-gamma to activate macrophages and CD8+ T-cells lyse infected macrophages
  1. Hyperimmune plasma protects against development of disease
• Neonatal foals are most susceptible when compared to those >6 weeks old due to waning passive humoral immunity and an underdeveloped cell-mediated immune response; most are infected by 2-weeks of age
  1. Failure of passive transfer of colostral immunoglobulins is a predisposing factor
• Virulence differs between isolates and correlates with a 80-90 kb virulence-associated plasmid (pVAP); lipoprotein vapA and its positive regulators (VirR and ORF8) is associated with virulence and survival/replication within host macrophages
  1. vapA (circular): Horses
  2. vapB (circular): Swine
  3. vapN (linear): Cattle and goats
• Other virulence factors: Glycolipids containing long-chain mycolic acids, capsular polysaccharide, “equi factors” cholesterol oxidase and choline phosphohydrolase, and iron-binding protein requichelin 
• Neutrophils can kill R. equi, but also generate significant tissue damage
• Intestine: Dome epithelium that overlies lymphoid follicles is the primary route of infection; bacteria infiltrate the M cells overlying the GALT; neutrophils infiltrate epithelium and exude into lumen; histiocytic infiltration of lymphoid follicles à caseous necrosis à ulceration of mucosal surface (this histologic finding corresponds to the gross lesion of a “volcano ulcer”); frequent coinfection with helminths (migrating larvae may aid in distribution of bacteria throughout body)
• Joints: Hematogenous spread may lead to polyarthritis

 

TYPICAL CLINICAL FINDINGS:  

• Can have acute or insidious onset, though infection and lesions are invariably chronic leading to high fatality rates in untreated foals; many foals with pulmonary abscesses recover without clinical disease
• Fever, cough, dyspnea, mucopurulent nasal discharge and increased respiratory rate
• Intestinal form: Diarrhea, weight loss, ascites (from hypoproteinemia), colic
• Sudden death (subacute form)
• Metritis and abortion in mares; fetus expelled with membranes 

 

TYPICAL GROSS FINDINGS:   

• Lung: Multiple 1-10 cm diameter, coalescing, firm, pyogranulomatous foci with friable or caseous centers that are predominantly in the cranioventral lung lobes that can progress to large caseonecrotic masses with extensive fibrosis; intervening areas are reddened and firm; lack the thick capsule of an abscess; pleuritis; enlarged bronchial lymph nodes +/- caseous nodal necrosis 
• Gastrointestinal tract: Ulcerative enterocolitis; begin as focal mucosal ulcers centered on lymphoid follicles; ulcers often have a raised periphery with a depressed ulcerated center (“volcano ulcers”); present in 50% of cases; 5% are restricted to the intestine; colon, cecum, and associated lymph nodes are most common sites; intestine is markedly thickened and corrugated
• Tracheobronchial and colonic lymph nodes: Swollen, pale, and edematous, often with pyogranulomatous lymphadenitis; mesenteric lymph node infections can progress to large abscesses
• Joints: Polyarthritis (1/3 of affected foals), suppurative synovitis and positive joint culture; especially of the tibiotarsal and stifle joints
• Bone: Osteomyelitis; vertebrae or metaphysis of long bones
• Eye: Hypopyon
• Skin: Cutaneous ulcerative lymphangitis (via contamination of a wound)
• Peritoneum: Purulent peritonitis and mesenteric lymphadenitis 
• Abscesses in mesenteric or mediastinal lymph nodes, liver, spleen or skin

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Lung: Early in disease suppurative bronchopneumonia that progresses to pyogranulomatous bronchopneumonia; gram-positive bacteria are present in cytoplasm of plump uninucleated or multinucleated macrophages in the exudate
• Intestine: Pyogranulomatous inflammation of lymphoid tissue; fibrinonecrotic ulceration of dome epithelium; neutrophilic luminal exudate; macrophages under dome epithelium contain intracytoplasmic bacilli
• Lymph nodes: Pyogranulomatous lymphadenitis +/- necrosis; large numbers of macrophages and multinucleated giant cells
• Joints: Septic arthritis or immune-mediated polyarthritis that is sterile and characterized by plasmocytic synovitis with mononuclear cells in the joint fluid and immunoglobulin deposits in the synovium 
• Placenta: Perivascular macrophages that contain numerous intracytoplasmic gram-positive coccobacilli; inflammation is extensive on the allantoic surface with hyperplasia and hypertrophy of the allantoic epithelial cells

 

ADDITIONAL DIAGNOSTIC TESTS:

• Bacterial culture 
• +/- Weakly acid fast (Fite)
• Radiographic and/or ultrasonographic evidence of lung abscesses

 

DIFFERENTIAL DIAGNOSIS:

Granulomatous pneumonia:

• Fungal pneumonia (uncommon in horses): Aspergillus fumigatus, Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides spp., Cryptococcus neoformans  
• Mycobacterium sp.: Rare in horses
• Foreign body

 

Foal pneumonia:

Bacterial:

• Streptococcus equi subsp. zooepidemicus: Most common cause of bacterial pleuropneumonia in foals, common cause of pleuropneumonia in older horses
• Embolic pneumonia: Streptococcus equi subsp. equi, Actinobacillus equuli, Pasteurella multocida, Bordetella bronchiseptica, Escherichia coli, Bacteroides sp., Salmonella sp., and Chlamydophila psittaci; can be differentiated on bacterial culture

 

Viral:

• Equine viral rhinopneumonitis (EHV-1, subfamily Alphaherpesviridae, P-V10, R-V01): Intranuclear inclusions; mild transient bronchointerstitial pneumonia
• Equine influenza virus (Family Orthomyxoviridae): Mild, self-limiting bronchopneumonia
• Rhinovirus (Family Picornaviridae): Typically affects the upper respiratory system, but not the lungs
• Adenoviral pneumonia (Family   Adenoviridae): Necrotizing pneumonia; intranuclear inclusions; usually in SCID foals (H-M02)
• Equine viral arteritis (Genus Arterivirus): Interstitial pneumonia with arteritis and edema

 

Fungal:

 

COMPARATIVE PATHOLOGY:

• Mice: Classification of R. equi as virulent (vapA positive), intermediately-virulent (vapB positive), or avirulent (no virulence plasmid) is based on behavior of isolate in mice
• Cattle: Metritis; tubercle-like lesions in lymph nodes; pneumonia in calves; ulcerative, granulomatous glossitis and enteritis (Saied, JVDI. 2019)
• Sheep: Caseous bronchial lymphadenitis
• Goats: Pyogranulomatous lesions in the liver and lungs; osteomyelitis of the vertebra and skull; fibrinous enterocolitis, majority caused by VapN plasmids, one was avirulent (Stranahan, Vet Pathol. 2017)
• Pigs: Tubercle-like lesions in lymph nodes; swine cervical lymphadenopathy 
• Llama: Pyogranulomatous enteritis and mesenteric lymphadenitis (Löhr, JVDI. 2019)

 

References:

1. Bianchi MV, Mello LS, Ribeiro PR, Wentz MF, Stolf AS, Lopes BC, de Andrade CP, Snel GGM, Ssonne L, Driemeier D, Pavarini SP. Causes and Pathology of Equine Pneumonia and Pleuritis in Southern Brazil. J Comp Path. 2020;179:65-73.
2. 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; 2016:569-571.
3. Craig LE, Dittmer KE, Thompson KG. Bones and Joints. In: Maxie MG, ed. Jubb, Kennedy & Palmer's Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016:99-100.
4. Löhr, C., O’Neill, T., Daw, D., Pitel, M. and Schlipf, J. Pyogranulomatous enteritis and mesenteric lymphadenitis in an adult llama caused by Rhodococcus equi carrying virulence-associated protein A gene. J Vet Diagn Invest. 2019;31(5): 747-751.
5. Lopez A, Martinson SA. Respiratory System, Thoracic Cavities, Mediastinum, and Pleurae. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:604-605.
6. Saied, AA, Bryan, LK, Bolin DC. Ulcerative, granulomatous glossitis and enteritis caused by Rhodococcus equi in a heifer. J Vet Diagn Invest. 2019;31(5):783-787. 
7. Schlafer DH, Foster RA. Female Genital System. In: Maxie MG, ed. Jubb, Kennedy & Palmer's Pathology of Domestic Animals. Vol 3. 6th ed. St. Louis, MO: Elsevier; 2016:418.
8. Spagnoli ST, Gelberg HB. Alimentary System and the Peritoneum, Omentum, Mesentery, and Peritoneal Cavity. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:460-461.  
9. Stanton JB, Zachary JF. Mechanisms of Microbial Infections. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:205, 211. 
10. Stranahan LW, Plumlee QD, Lawhon SD, Cohen ND, Bryan LK. Rhodococcus equi infections in goats: characterization of virulence plasmids. Vet Pathol. 2017;55(2):273-276.
11. Uzal FA, Arroyo LG, Navarro MA, Gomez DE, Asin J, Henderson E. Bacterial and viral enterocolitis in horses: a review. J Vet Diagn Invest. 2022;34(3):354-375.
12. Uzal FA, Plattner BL, Hostetter JM. Alimentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. Philadelphia, PA: Elsevier; 2016:197-198, 251.

 

 

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