NAVLE Guinea Pigs

Guinea Pig Streptococcosis Study Guide

📅 March 28, 2026 ⏱ 25 min read

Streptococcosis in guinea pigs (Cavia porcellus) represents one of the most clinically significant bacterial infections encountered in this species.

Overview and Clinical Importance

Streptococcosis in guinea pigs (Cavia porcellus) represents one of the most clinically significant bacterial infections encountered in this species. The disease is primarily caused by Streptococcus equi subspecies zooepidemicus (the most common pathogen) and Streptococcus pneumoniae. These infections manifest as multisystemic disease with various clinical presentations including cervical lymphadenitis (also known as "lumps"), pneumonia, septicemia, otitis media, and reproductive disorders.

Streptococcosis is a high-yield topic for the NAVLE examination due to its zoonotic potential, characteristic clinical presentation, and unique treatment considerations in guinea pigs. Understanding antibiotic safety in this species is critical, as many commonly used antibiotics are contraindicated due to disruption of the normal gastrointestinal flora.

Route	Mechanism	Clinical Outcome
Oral/Oropharyngeal	Mucosal abrasions from coarse feed, dental disease, or bite wounds allow bacterial entry	Cervical lymphadenitis (most common)
Nasal/Conjunctival	Direct inoculation through intact nasal or conjunctival mucosa	Cervical lymphadenitis, respiratory infection
Respiratory Aerosol	Inhalation of droplets from infected animals (sneezing, coughing)	Pneumonia, systemic infection
Genital	Sexual transmission or during parturition	Metritis, mastitis, stillbirths, abortions
Cutaneous	Wounds, bite injuries, or skin abrasions	Localized abscess, septicemia

Etiology and Causative Agents

Guinea pigs are susceptible to infection by several Streptococcus species, with two being most clinically significant:

Streptococcus equi subspecies zooepidemicus

Gram-positive, beta-hemolytic coccus arranged in pairs or chains
Lancefield Group C streptococcus
Encapsulated with hyaluronic acid capsule (antiphagocytic)
Most common cause of cervical lymphadenitis (lumps) in guinea pigs
Normal commensal of oropharyngeal and nasopharyngeal mucosa
Zoonotic pathogen - can cause severe disease in humans

Streptococcus pneumoniae

Gram-positive, alpha-hemolytic, lancet-shaped diplococcus
Not classified by Lancefield grouping (capsular typing used instead)
Serotypes 4 and 19F most commonly isolated from guinea pigs
Primary cause of pneumonia and fibrinopurulent polyserositis
Carrier state common; clinical disease often triggered by stress

High-YieldOn the NAVLE, remember S. zooepidemicus = BETA-hemolytic (Lancefield Group C) causing CERVICAL LYMPHADENITIS, while S. pneumoniae = ALPHA-hemolytic causing PNEUMONIA. Both can cause septicemia.

Syndrome	Clinical Signs	Primary Agent
Cervical Lymphadenitis	Unilateral or bilateral swelling of ventral cervical lymph nodes Firm, encapsulated abscesses (up to several cm) Yellow-white to red-gray caseous pus Often otherwise healthy appearing	S. zooepidemicus
Pneumonia	Dyspnea, nasal discharge, sneezing Conjunctivitis (pink eye) Fever, anorexia, weight loss Depression, sudden death possible	S. pneumoniae
Otitis Media/Interna	Head tilt (torticollis) Circling, nystagmus, ataxia Rolling, loss of balance Possible deafness	Both species
Septicemia	Acute onset, high fever Weakness, lethargy, anorexia Multi-organ involvement Peracute death (especially young)	Both species
Reproductive Disease	Abortions, stillbirths Metritis, mastitis Uterine abscessation	S. zooepidemicus

Pathogenesis

Routes of Infection

Pathophysiology of Cervical Lymphadenitis

Following mucosal penetration, S. zooepidemicus is transported via lymphatic vessels to the regional cervical lymph nodes. The bacteria proliferate within the nodes, leading to suppurative inflammation and abscess formation. The virulence factors responsible for pathogenicity include:

SzP protein (M-like protein): Anti-phagocytic and anti-opsonization properties
FNZ protein: Fibronectin-binding protein facilitating cell adhesion
Hyaluronic acid capsule: Inhibits phagocytosis by mimicking host tissue
Hyaluronidase: Facilitates tissue invasion (spreading factor)
Streptokinase: Activates plasminogen, promoting bacterial spread

Test	Method	Expected Findings
Fine Needle Aspirate	Aspirate lymph node contents	Caseous, white-yellow pus; cytology shows degenerate neutrophils, bacterial cocci
Gram Stain	Stain of abscess material or tissue impression	Gram-positive cocci in chains or pairs
Bacterial Culture	Blood agar (5% sheep blood), aerobic and CO2 enriched	S. zooepidemicus: Beta-hemolysis S. pneumoniae: Alpha-hemolysis, mucoid colonies
Sensitivity Testing	Disk diffusion or MIC determination	Essential for guiding antibiotic selection
Radiography	Thoracic radiographs for respiratory disease	Consolidated lung lobes; tympanic bulla opacity (otitis)
CBC	Complete blood count	Leukocytosis with left shift; heterophilia

Clinical Signs and Presentations

NAVLE TipWhen you see a guinea pig with large, firm ventral neck swellings but otherwise appearing healthy, think CERVICAL LYMPHADENITIS caused by S. zooepidemicus first. The classic presentation is unilateral neck swelling in an otherwise well guinea pig.

SAFE Antibiotics	CONTRAINDICATED	Use with CAUTION
Chloramphenicol Trimethoprim-sulfa Enrofloxacin Ciprofloxacin Marbofloxacin Azithromycin Doxycycline	Penicillins (all) Ampicillin Amoxicillin Cephalosporins Clindamycin Lincomycin Erythromycin Bacitracin (oral)	Metronidazole Gentamicin (nephro/ototoxic) Tetracyclines (oral)

Diagnosis

Clinical and Physical Examination

Diagnosis begins with thorough physical examination. Key findings include:

Cervical lymphadenitis: Palpable firm, often fluctuant swellings in the ventral neck region
Respiratory disease: Nasal discharge, increased respiratory rate, abnormal lung sounds
Systemic signs: Fever, weight loss, depression, rough hair coat

Laboratory Diagnostics

Differential Diagnoses

When presented with cervical masses or respiratory disease in guinea pigs, consider:

Cavian leukemia/lymphosarcoma: Most common tumor; causes generalized lymphadenopathy
Bordetella bronchiseptica: More common cause of respiratory disease; beta-lactamase producing
Subcutaneous abscess: Other bacterial causes (Staphylococcus, Pasteurella)
Thyroid mass: Rare in guinea pigs but should be considered
Trichofolliculoma: Common benign skin mass; usually on back/flanks

High-YieldAlways include CAVIAN LEUKEMIA in the differential for any guinea pig with lymphadenopathy. It is the most common tumor in guinea pigs and can mimic cervical lymphadenitis. Cytology or biopsy differentiates these conditions.

Drug	Dosage	Route	Notes
Chloramphenicol	50 mg/kg q12h	PO	First-line for streptococci; 7-21 days
Trimethoprim-sulfa	30-50 mg/kg q12h	PO, SC, IM	Good for UTI; 7-14 days
Enrofloxacin	5-10 mg/kg q12h	PO, SC	Avoid in young growing animals; 7-21 days
Azithromycin	15-30 mg/kg once daily	PO	Stop if soft feces develop
Doxycycline	2.5-5 mg/kg q12h	PO	Combination therapy with enrofloxacin
Meloxicam	0.2-0.5 mg/kg q24h	PO, SC	Pain relief; post-surgical

Necropsy and Histopathology Findings

Cervical Lymphadenitis

Encapsulated abscesses up to several centimeters in diameter
Yellow-white to red-gray, nonodorous, caseous pus
Destruction of the lymph node architecture

Systemic Streptococcosis

Fibrinopurulent pleuritis, pericarditis, peritonitis
Suppurative bronchopneumonia with thrombosis of pulmonary vessels
Splenomegaly, hepatomegaly with focal necrosis
Suppurative arthritis, nephritis, myocarditis

Treatment

Antibiotic Safety in Guinea Pigs - CRITICAL CONCEPT

Guinea pigs have a unique hindgut fermentation system with a predominantly Gram-positive bacterial flora. Antibiotics with a primarily Gram-positive spectrum can disrupt this flora, leading to overgrowth of Gram-negative bacteria and Clostridium difficile, causing fatal enterotoxemia.

Exam Focus: NEVER prescribe penicillins, ampicillin, amoxicillin, cephalosporins, clindamycin, or lincomycin to guinea pigs orally. These cause fatal antibiotic-associated enterotoxemia. Safe choices are chloramphenicol, trimethoprim-sulfa, and fluoroquinolones.

Recommended Drug Dosages

Surgical Management of Cervical Lymphadenitis

Surgical intervention is often the definitive treatment for cervical lymphadenitis:

Complete surgical excision of the affected lymph node is preferred over incision and drainage
If excision is not possible, incise and debride the abscess, flush copiously with sterile saline
Leave wound open for drainage and daily flushing
Administer systemic antibiotics for 2-4 weeks (based on culture and sensitivity)
Provide vitamin C supplementation (50-100 mg/day) to support healing
Monitor for recurrence and systemic spread

NAVLE TipGuinea pig pus is CASEOUS (thick, cheese-like) and does not drain well. Simple lance-and-drain techniques used for cat abscesses are NOT effective. Complete surgical excision of the lymph node is the treatment of choice.

Prognosis

Localized cervical lymphadenitis: Good with complete surgical excision and appropriate antibiotics
Pneumonia: Guarded; requires aggressive, prolonged therapy (4-6 weeks)
Septicemia: Poor; often fatal despite treatment
Otitis media/interna: Guarded; neurological signs may persist
Colony/herd situation: Guarded; culling affected animals often recommended

Prevention and Control

Quarantine new animals before introducing to colony
Maintain excellent husbandry: clean cages, fresh bedding
Provide non-abrasive feed; avoid excessively coarse hay
Regular dental checks to prevent malocclusion
Minimize stress (overcrowding, temperature extremes, handling)
Do NOT house guinea pigs with rabbits (Bordetella transmission risk)
Palpate ventral neck regularly for early detection
Remove affected animals from colony; cull in herd situations

Zoonotic Considerations

Streptococcus equi subsp. zooepidemicus is a recognized zoonotic pathogen. Human infections from guinea pigs are rare but can be severe, particularly in immunocompromised individuals.

Human Disease Manifestations

Necrotizing fasciitis
Meningitis
Septicemia
Pneumonia
Endocarditis

Recommendations for Pet Owners

Wear disposable gloves when handling infected guinea pigs or cleaning cages
Wash hands thoroughly after handling guinea pigs
Avoid contact with draining abscesses
Immunocompromised individuals should take extra precautions

High-YieldS. zooepidemicus infection should be considered in any patient with severe illness (necrotizing fasciitis, meningitis, sepsis) who reports guinea pig exposure. This is particularly important for elderly or immunocompromised individuals.

Memory Aids

"SAFE" Antibiotics for Guinea Pigs

S - Sulfa drugs (Trimethoprim-sulfa) A - Azithromycin F - Fluoroquinolones (Enrofloxacin, Ciprofloxacin) E - (Chloramph)Enicol

"LUMPS" Mnemonic for Cervical Lymphadenitis

L - Lancefield Group C (S. zooepidemicus) U - Unilateral ventral neck swelling M - Mucosal abrasions allow entry P - Pus is caseous (thick, cheesy) S - Surgery (excision) is treatment of choice

"NO PACS" - Antibiotics to AVOID in Guinea Pigs

NO P - Penicillins A - Ampicillin/Amoxicillin C - Cephalosporins, Clindamycin S - Streptomycin, oral

Practice NAVLE Questions

Test your knowledge with 10,000+ exam-style questions, detailed explanations, and timed exams.

Start Your Free Trial →