JPC SYSTEMIC PATHOLOGY
Signalment (AFIP #1618601): Guinea pig
HISTORY: Cervical (neck) swellings
HISTOPATHOLOGIC DESCRIPTION: Lymph node: Effacing and replacing approximately 95% of lymphoid architecture is an extensive abscess composed of numerous degenerate heterophils, fewer macrophages, and rare lymphocytes and plasma cells admixed with abundant eosinophilic cellular and karyorrhectic debris (lytic necrosis) surrounded by an up to 2 mm thick fibrous capsule. Multifocally the abscess extends through the capsule and infiltrates adjacent adipose tissue and skeletal muscle. The subcapsular and medullary sinuses in the adjacent normal remaining lymph node are mildy expanded by edema.
Salivary gland: Diffusely within the adjacent salivary gland, there is acinar atrophy, characterized by clusters of closely-spaced ducts and few intervening glands, as well as moderate fatty infiltration admixed with few lymphocytes and plasma cells.
MORPHOLOGIC DIAGNOSIS: 1) Cervical lymph node: Lymphadenitis, necrosuppurative, chronic, diffuse, severe (abscess), with suppurative steatitis and myositis, guinea pig (Cavia porcellus), rodent.
2) Salivary gland: Acinar atrophy, diffuse, mild to moderate.
ETIOLOGIC DIAGNOSIS: Streptococcal lymphadenitis
CAUSE: Streptococcus equi subsp. zooepidemicus
CONDITION: Cervical lymphadenitis, “Lumps”
- Gram-positive encapsulated coccus that produces beta hemolysis on blood agar plates
- Streptococcus equi zooepidemicus, Lancefield’s group C is the most common cause of cervical lymphadenitis in guinea pigs
- Commensal organism that is normally present in the nasopharynx and conjunctiva
- Females are more susceptible to the disease than males
- Stress increases susceptibility to infection
- Virulence factors include an antiphagocytic capsule (M-like antigen) and several exotoxins (hyaluronidase, a protease, a streptokinase)
- Invasion through abraded oral mucosa (often caused by malocclusion, hay stems, or bite wounds) is the most common route of infection, but inhalation (aerosol), skin abrasions, and invasion of the genital tract at farrowing can occur
- Penetration of oral mucosa > invasion of underlying tissue > transport to draining cervical lymph nodes via lymphatics > proliferation in cervical lymph nodes > chronic suppurative inflammation > surgical drainage or spontaneous rupture followed by healing with granulation tissue
- Cervical lymph nodes are most often affected due to entry of organism being most likely via the oral mucosa, though any lymph node and nearly any organ can be affected
- Associated suppurative processes may occur, including: pneumonia, retrobulbar abscesses, otitis media and interna, localized abscesses in various organs and other lymph nodes, fibrinous pleuritis, peritonitis, pericarditis, focal myocardial degeneration, focal hepatitis, nephritis, arthritis
- Rarely causes abortion
- An acute septicemia, usually affecting young animals, causes fibrinopurulent bronchopneumonia, pleuritis, and pericarditis
TYPICAL CLINICAL FINDINGS:
- The presence of bilaterally enlarged cervical lymph nodes in otherwise healthy guinea pigs is usually the only sign
- Another manifestation presents with retro-orbital abscessation accompanied by exophthalmus
- The acute septicemic form causes depression, anorexia, pyrexia, or sudden death
- Torticollis is seen with middle and inner ear infections
- Dyspnea, cyanosis, and nasal/ocular discharge occur with pneumonia
TYPICAL GROSS FINDINGS:
- Cervical lymph nodes are enlarged, irregular, and filled with a thick, purulent, nonodorous yellow-white to red-gray exudate; node is often destroyed
- In the acute systemic form, fibrinopurulent bronchopneumonia, pleuritis, and pericarditis may be present at necropsy
TYPICAL LIGHT MICROSCOPIC FINDINGS:
- Chronic suppurative lymphadenitis with central necrosis, peripheral fibrosis, and marked inflammation with heterophils
- May contain short chains of gram-positive cocci
- In the acute systemic form, fibrinopurulent bronchopneumonia, fibrinopurulent pericarditis, focal myocardial degeneration, focal hepatitis, and acute lymphadenitis may be evident
ADDITIONAL DIAGNOSTIC TESTS:
- Typical beta-hemolytic streptococci can usually be cultured on blood agar from affected tissues except in cases of chronic lymphadenitis of some duration
For cervical lymphadenitis
- Occasionally, cervical lymphadenitis is caused by Caviibacter abscessus (formerly Streptobacillus moniliformis)
For acute septicemic disease
- Streptococcus pneumoniae (diplococcal or pneumococcal pneumonia): Gram (+) cocci in pairs or short chains
- Acute Bordetella infection: Gram (-) rods, septicemic lesions
- Horses: Streptococcus equi equi causes strangles and guttural pouch abscesses; Streptococcus equi subsp. zooepidemicus is an opportunistic pathogen of respiratory and reproductive tract and is the most common bacteria causing placentitis in mares
- Swine : Streptococcus porcinus causes jowl abscesses
- Cats: Streptococcus canis can cause cervical lymphadenitis, pneumonia, as well as necrotizing fasciitis, sinusitis and myositis in laboratory colonies
- Nonhuman primates: Streptococcus pneumoniae is the primary cause of bacterial meningitis in nonhuman primates
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