NAVLE Rats-Mice

Murine Mycoplasmosis (Dirty Rat Disease) – NAVLE Study Guide

March 28, 2026 22 min read 17 views
Murine Respiratory Mycoplasmosis (MRM), commonly called Dirty Rat Disease or Chronic Respiratory Disease (CRD), is caused by Mycoplasma pulmonis.

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.

Route Mechanism Significance
Aerosol (Primary) Inhalation of respiratory droplets from sneezing, snuffling Most common route; highly contagious in close quarters
Direct Contact Contact with infected nasal/ocular secretions Common in cage mates, grooming behavior
Vertical (Transplacental) Infected dam transmits to fetuses in utero Ensures perpetuation in breeding colonies
Fomites Contaminated bedding, cages, equipment Less common; organism doesn't survive long outside host

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
NAVLE TipMycoplasma = NO cell wall! This is why penicillins and cephalosporins DON'T work. Use fluoroquinolones or tetracyclines instead!
Age Pathology Stage Clinical Presentation
Birth to 2 months Colonization phase; microscopic lesions only ASYMPTOMATIC; no detectable signs
2-6 months Early chronic inflammation; bronchiolitis develops Mild signs emerge: occasional sneezing, slight nasal discharge
6-12 months Moderate to severe bronchiectasis, alveolitis CLASSIC SIGNS: Snuffling, red tears, nasal discharge, dyspnea
12-18+ months Chronic obstructive lung disease; pulmonary fibrosis, abscesses SEVERE: Labored breathing, gasping, chattering, weight loss, torticollis

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.

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