JPC SYSTEMIC PATHOLOGY
Signalment (JPC #1482029): 5-month-old guinea pig
HISTORY: This animal died following a brief period of depression and anorexia.
HISTOPATHOLOGIC DESCRIPTION (Slide 6a): Heart: The epicardium is diffusely replaced by an adherent, variably thick (up to 1mm) mat of pale eosinophilic, homogenous to fibrillar material (fibrin) that contains numerous enmeshed viable and degenerate heterophils, moderate numbers of erythrocytes (hemorrhage), eosinophilic cellular and karyorrhectic debris (necrosis), and fewer scattered lymphocytes, plasma cells, and macrophages. The mat multifocally extends into the subjacent myocardium where it separates, surrounds, and replaces cardiomyocytes. Subjacent to this infiltrate are reactive fibroblasts separated by minimal amounts of loose fibrous connective tissue and many perpendicular, evenly spaced, small caliber blood vessels lined by hypertrophic endothelium (granulation tissue). Multifocally, vessel lumina are partially to completely occluded by adherent fibrinocellular thrombi that contain enmeshed heterophils, macrophages, and fewer lymphocytes and plasma cells, and endothelial cells are often hypertrophic. There is focal mild chondrous metaplasia within tunica media of the proximal ascending aorta.
(Slide 6b): Heart (Brown & Brenn stain): The fibrinous mat contains abundant scattered extracellular and intraheterophilic Gram-positive diplococci.
MORPHOLOGIC DIAGNOSIS: Heart: Epicarditis, fibrinosuppurative, chronic-active, diffuse, severe, with mild subepicardial myocarditis, fibrin thrombi, and abundant intralesional Gram-positive diplococci, guinea pig, rodent.
ETIOLOGY: Streptococcus pneumoniae
ETIOLOGIC DIAGNOSIS: Diplococcal (pneumococcal) epicarditis
- Gram-positive, 1-2 µm, heavily encapsulated, alpha-hemolytic cocci in pairs (diplococci) and short chains; capsular polysaccharide types 4 and 19 most frequently isolated from guinea pigs
- pneumoniae is common inhabitant of upper respiratory tract of clinically normal guinea pigs, rabbits, horses, cattle, monkeys, rats, dogs, cats and humans
- Serotypes isolated from guinea pigs are identical to human isolates, however interspecies transmission has not been proven to date
- Infection in guinea pigs is most often associated with the lower respiratory tract; extra pulmonary lesions are rare in the absence of pneumonia
- Additional disease syndromes include fibrinopurulent bronchopneumonia; bacteremia; fibrinopurulent polyserositis; suppurative inflammation of the middle or inner ear, joints, peritoneum, lungs, heart, kidneys; and panophthalmitis
- Transmission: Aerosol and direct contact
- Predisposing factors: Stress (e.g. transportation), pregnancy, poor husbandry, inadequate nutrition (hypovitaminosis C), poor climate control
- Virulence factors:
- pneumoniae does not produce exotoxins,
- A thick protective polysaccharide capsule resists phagocytosis
- Many pneumococci can activate alternate complement pathway which may be important in early tissue changes
- Extrapulmonary lesions occur with bacteremia
TYPICAL CLINICAL FINDINGS:
- Up to 50% of infected guinea pigs may be subclinical carriers
- Epizootics occur most often during winter months
- Young animals and pregnant sows are most at risk
- Signs of disease include: ruffled coat, anorexia, dyspnea, inactivity, nasal and ocular discharge, vestibular signs (if middle ear infection), stillbirth, abortion, torticollis; death is often the only sign
TYPICAL GROSS FINDINGS:
- Fibrinopurulent polyserositis (e.g. pericarditis, pleuritis, peritonitis, periorchitis), meningitis, metritis
- Lesions may be confined to the meninges in some fatal cases
- Consolidation of affected lung lobe(s) (“lobar pneumonia”): dark red to dull tan, relatively firm, non-resilient
- Upper respiratory exudate, purulent otitis interna/media, abscessed lymph nodes
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Heart lesions: May see bacteria (diplococci), endocardium is typically not affected
- Mild: Fibrinous epicardial tags, neutrophilic and lymphocytic infiltration around epicardial vessels
- Severe: Thick epicardial mats of fibrin and debris infiltrated by abundant neutrophils, lymphocytes, and plasma cells; pulmonary thrombi
- Chronic: Epicardial granulation tissue, hemosiderosis, and hemorrhage
- Pulmonary lesions: acute bronchopneumonia with fibrinous exudate and neutrophilic infiltration
- Additional lesions: Fibrinopurulent polyserositis (pleuritis, peritonitis, pericarditis, perihepatitis), leptomeningitis, splenitis, lymphadenitis, endometritis, suppurative embolic lesions (e.g. liver, spleen, kidney, ovary), suppurative arthritis and osteomyelitis, centrilobular hepatic necrosis
ADDITIONAL DIAGNOSTIC TESTS:
- Gram stain: direct smears of exudate, histologic sections
- Culture of nasal passage washings
Pericarditis in the guinea pig:
- equi subsp. zooepidemicus (also causes bilateral suppurative lymphadenitis of the cervical lymph nodes, see H-B02)
- Salmonella spp.
- Klebsiella pneumoniae (see P-B13)
- Pasteurella multocida
Respiratory infection in the guinea pig:
- Acute Bordetella infection (see P-B09)
- Bacterial endocarditis, often valvular: Acute valvular endocarditis in many animals can be caused by many bacterial species, including Streptococcus , Staphyloccus sp, Escherichia coli, and more species-specific bacteria, e.g. Trueperella pyogenes (most common cause in cattle), Erysipelothrix rhusiopathiae (most commonly isolated in pigs), Actinobacillus equuli (horse), Bartonella spp. (dogs, cats),
- Fibrinous pericarditis: In cattle may be due to Pasteurella multocida, Clostridium chauvoei, Mycoplasma mycoides subsp. Mycoides, and as a component of neonatal coliform infections; in swine multocida, Mycoplasma hyopneumoniae or as part of Glassers disease; in sheep may be due to P. multocida; in horses Mycoplasma felis or Streptococcus spp.
- Meningitis/leptomeningitis (see N-B08): In many animal species, meningitis is most commonly caused by hematogenous dissemination of bacteria like coli and Streptococcus sp.; Streptococcus pneumoniae is the most common cause of bacterial meningitis in nonhuman primates, and is also reported in guinea pigs, rabbits, horses, cattle, rats, dogs, and cats
- Bronchopneumonia (see P-B07): Streptococcus pneumoniae is a major cause of bronchopneumonia in nonhuman primates especially secondary to viral infection (e.g. human metapneumovirus, parainfluenza-3, and human respiratory syncytial virus in gorillas and chimpanzees), and can be carried subclinically on the nasopharyngeal mucosa; it also causes pneumonia in rats, guinea pigs, hamsters, ferrets, mink, and foals; pleuritis with bronchopneumonia is common; the bacteria may cause acute primary disease but it is also an important secondary invader; bronchopneumonia may be followed by development of lesions in other organs (e.g. arthritis, meningitis)
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