Murine Mycoplasmosis (Dirty Rat Disease) – NAVLE Study Guide

Overview and Clinical Importance

Murine Respiratory Mycoplasmosis (MRM), commonly called Dirty Rat Disease or Chronic Respiratory Disease (CRD), is caused by Mycoplasma pulmonis. This is the most common and important infectious disease of pet rats and mice, characterized by chronic progressive respiratory tract infection.

M. pulmonis is essentially ubiquitous in non-SPF rat populations and causes slowly developing, chronic disease that may not manifest until 2-18 months of age. The hallmark clinical sign is red tears (porphyrin staining) around the eyes and nose, along with snuffling, sneezing, and respiratory distress.

Etiology and Organism Characteristics

Mycoplasma pulmonis

• Classification: Mycoplasma (class Mollicutes)
• Unique Feature: LACKS CELL WALL (smallest free-living bacteria)
• Size: 0.2-0.3 micrometers
• Culture: Requires special mycoplasma media; slow-growing
• Antibiotic Resistance: Intrinsically resistant to beta-lactams and cell wall synthesis inhibitors

Epidemiology and Transmission

Prevalence

M. pulmonis is essentially ubiquitous in rats other than SPF laboratory stocks. Up to 90-100% of conventional pet rat colonies are infected. Mice are less commonly affected than rats.

Transmission Routes

Pathogenesis

Step 1: Initial Colonization

M. pulmonis is inhaled and colonizes ciliated respiratory epithelium of the nasal cavity, trachea, and bronchi. It attaches to cilia as an extracellular parasite.

Step 2: Ciliostasis and Epithelial Damage

Mycoplasma produces toxins and enzymes that cause ciliostasis (ciliary paralysis) and epithelial cell damage. Mucociliary clearance is impaired.

Step 3: Chronic Inflammation

Persistent infection triggers chronic lymphoplasmacytic inflammation with infiltration of lymphocytes, plasma cells, and neutrophils into respiratory mucosa and submucosa.

Step 4: Progressive Lung Disease

Over months to years, chronic inflammation leads to bronchiectasis, bronchiolitis, alveolitis, and pulmonary abscesses. Airway remodeling and fibrosis develop.

Step 5: Secondary Infections

Damaged respiratory defenses allow secondary bacterial infections (Streptococcus, Pasteurella, Corynebacterium, Bordetella) which exacerbate disease.

Step 6: Spread to Middle Ear

Infection can ascend via eustachian tube causing otitis media and otitis interna, leading to torticollis (head tilt) and vestibular signs.

Clinical Signs and Presentation

Age-Dependent Presentation

Critical Concept: Murine mycoplasmosis is typically SILENT in young animals. Clinical signs emerge as the rat ages:

Classic Clinical Signs

• Snuffling/Sneezing: Audible rattling or congested breathing sounds ("snuffles")
• Red Tears (Chromodacryorrhea): PATHOGNOMONIC - Rust-colored porphyrin staining around eyes and nose
• Nasal Discharge: Serous to mucopurulent discharge
• Dyspnea: Labored breathing, open-mouth breathing, abdominal breathing
• Respiratory Sounds: Rattling, wheezing, chattering (in mice), moist lung sounds
• Weight Loss: Progressive weight loss, unthrifty appearance
• Head Tilt (Torticollis): If otitis media/interna develops; vestibular signs
• Ruffled Fur, Hunched Posture: Signs of chronic illness

Pathologic Findings

Gross Pathology

• Lungs: Dark red to gray consolidation; failure to collapse; abscessation in severe cases
• Airways: Mucopurulent exudate in trachea and bronchi
• Middle Ear: Purulent exudate in tympanic bulla if otitis media present

Histopathology

• Upper Respiratory Tract: Rhinitis, epithelial hyperplasia, lymphoid infiltrates
• Lower Respiratory Tract: Bronchiolitis, bronchiectasis, peribronchial lymphoid cuffing (hallmark)
• Alveoli: Alveolitis, type II pneumocyte hyperplasia, neutrophilic infiltration
• Chronic Changes: Pulmonary fibrosis, abscess formation

Diagnosis

Treatment

CRITICAL: Mycoplasma pulmonis CANNOT be eliminated. Treatment goals are to control clinical signs, improve quality of life, and prevent acute exacerbations.

Antibiotic Therapy

GOLD STANDARD PROTOCOL: Enrofloxacin 10-20 mg/kg PO q12h + Doxycycline 5-10 mg/kg PO q12h for 4-6 weeks minimum

Supportive Care

• Corticosteroids: Dexamethasone or prednisolone to reduce inflammation; rapid relief
• Bronchodilators: Albuterol, aminophylline for severe dyspnea
• Nebulization: Saline or with gentamicin/enrofloxacin to humidify airways
• Oxygen Therapy: For severe respiratory distress
• Nutritional Support: High-calorie diet, syringe feeding if anorexic
• Environmental Management: Dust-free bedding, good ventilation, reduce stress

Prognosis and Prevention

Prognosis

• Early/Mild Disease: Good with chronic antibiotic therapy; may live normal lifespan
• Moderate Disease: Fair; requires lifelong management; quality of life variable
• Severe/Advanced Disease: Guarded to poor; progressive decline despite treatment

Prevention

• Source SPF (specific pathogen-free) rats and mice from reputable breeders
• Quarantine new animals for 30 days; test before introducing to colony
• Barrier housing with HEPA filtration
• Good husbandry: clean cages, dust-free bedding, low ammonia levels
• Avoid overcrowding and stress