JPC SYSTEMIC PATHOLOGY

HEMOLYMPHATIC SYSTEM

February 2024

H-B02

 

Signalment (JPC #1618601): Guinea pig.

 

HISTORY: Cervical (neck) swellings.

                                                                                        

HISTOPATHOLOGIC DESCRIPTION: Lymph node: Effacing approximately 95% of lymphoid architecture is an extensive abscess composed of numerous necrotic heterophils, fewer macrophages, and rare lymphocytes and plasma cells mixed 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 mildly 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, with suppurative steatitis and myositis, guinea pig (Cavia porcellus), rodent.

2) Salivary gland: Acinar atrophy, diffuse, moderate.

 

ETIOLOGIC DIAGNOSIS: Streptococcal lymphadenitis

 

CAUSE: Streptococcus equi subsp. zooepidemicus

 

CONDITION: Cervical lymphadenitis, “Lumps”

 

GENERAL DISCUSSION:

• Gram-positive encapsulated coccus that produces beta hemolysis on blood agar plates
• Streptococcus equi subsp. 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

 

PATHOGENESIS:

• Virulence factors include an antiphagocytic capsule (M-like antigen) and several others (C5a peptidase, invasins, fibronectin-binding protein)
• 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; Bilateral cervical lymph node enlargement; freely moveable, firm to soft, nonfluctuant lymph nodes with a thick purulent exudate
• 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
• Lesions in adults typically in regional lymph nodes
• Localized form or acute septicemic form

 

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 exophthalmos 
• Otitis media, arthritis and abortions can occur
• Acute septicemic form can occur in younger animals and causes depression, anorexia, pyrexia, or sudden death
• Dyspnea, cyanosis, and nasal/ocular discharge occur with pneumonia

 

TYPICAL GROSS FINDINGS:

• Cervical lymph and submandibular nodes are markedly enlarged, irregular, and filled with a thick, purulent, nonodorous yellow-white to red-gray exudate; node is often destroyed; rupture
• Acute systemic form – Thoracic lesions: Fibrinous pleural adhesions, fibrinopurulent bronchopneumonia, pleuritis, and pericarditis may be present at necropsy

 

TYPICAL LIGHT MICROSCOPIC FINDINGS: 

• Chronic suppurative lymphadenitis
  1. Effaced > 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 

 

ADDITIONAL DIAGNOSTIC TESTS:

 

DIFFERENTIAL DIAGNOSIS:

For cervical lymphadenitis

 

For acute septicemic disease

• Streptococcus pneumoniae (diplococcal or pneumococcal pneumonia): Gram (+) cocci in pairs or short chains
• S. pyogenes: Gram-positive, β-hemolytic streptococcus that is catalase negative; hemorrhagic septicemia; adults primarily affected; necrohemorrhagic and fibrinopurulent pneumonia w/abscessation, hemopericardium, hemothorax, petechiation of heart and kideneys and suppurative metritis  
• Acute Bordetella infection: Gram (-) rods, septicemic lesions

 

Neoplasm

 

COMPARATIVE PATHOLOGY:

• Horses: Streptococcus equi subsp. 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; sporadic outbreaks of S. equi subsp. zooepidemicus
• Camelids: Common cause of endometritis and alpaca fever 

 

REFERENCES:

1. Agnew D. Camelidae. In: Terio KA, McAloose D, St. Leger J ed. Pathology of Wildlife and Zoo Animals.  Cambridge, MA: Elsevier Inc. 2018:198.
2. Barthold SW, Griffey SM, Percy DH. Guinea pig. In: Pathology of Laboratory Rodents and Rabbits. 4th ed. Ames, IA: Wiley Blackwell; 2016:229-230. 
3. Bemis DA, Johnson BH, Bryant MJ, et al.  Isolation and identification of Caviibacter abscessus from cervical abscesses in a series of pet guinea pigs (Cavia porcellus).  J Vet Diagn Invest.  2016; 28(6):763-769.
4. Caswell JL, Williams KJ. Respiratory System. In: Maxie MG, ed. Jubb, Kennedy & Palmer's Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2016:465-591. 
5. Delaney MA, Treuting PM, Rothenburger JL. Rodentia. In: Terio KA, McAloose D, St. Leger J ed. Pathology of Wildlife and Zoo Animals.  Cambridge, MA: Elsevier Inc. 2018:510.
6. Matz-Rensing K and Lowensine LJ. New World and Old World Monkeys. In: Terio KA, McAloose D, St. Leger J ed. Pathology of Wildlife and Zoo Animals.  Cambridge, MA: Elsevier Inc. 2018:363.
7. Stanton JB, Zachary JF. Mechanisms of Microbial Infections. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:210-211. 

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