Pneumonia in Rats and Mice – NAVLE Study Guide

Pneumonia in Rats and Mice

Overview

Pneumonia is a common and clinically significant respiratory disease in both rats and mice, caused by bacterial, viral, or mixed infections. The most important pathogens include Mycoplasma pulmonis (covered separately), Sendai virus (parainfluenza virus type 1), Streptococcus pneumoniae, and Pasteurella pneumotropica (now reclassified as Rodentibacter spp.). Pneumonia often results from complex interactions between multiple pathogens, with primary viral infections predisposing animals to secondary bacterial pneumonia. Clinical signs range from subclinical colonization to severe dyspnea and death, particularly in young, immunocompromised, or genetically manipulated animals. Understanding the specific pathogens, their interactions, and appropriate diagnostic and treatment strategies is essential for the NAVLE and clinical veterinary practice.

Major Pathogens Causing Pneumonia

Key Point: Severe respiratory disease in rats is typically M. pulmonis combined with Sendai virus and/or CAR bacillus. In mice, combined Sendai virus and M. pulmonis infections cause the most severe outbreaks.

Sendai Virus

Etiology and Significance

Sendai virus (SeV) is a murine parainfluenza virus type 1 that is endemic in many rodent colonies worldwide and is currently believed to be the leading cause of pneumonia in mice. It is largely non-cytolytic and selectively infects respiratory epithelium in the nose, trachea, and bronchioles, as well as type II pneumocytes. The virus is highly contagious and spreads rapidly through aerosol transmission.

Clinical Presentation

In clinically apparent infections, signs include chattering, mild to severe respiratory distress, labored breathing, decreased fecundity in adults, deaths (possibly whole litters) in neonates and sucklings, and poor growth in weanlings and young adult mice. High mortality rates occur in neonates. Infection can also be asymptomatic in adult immunocompetent mice.

Pathology

Gross Pathology: Lungs may appear plum-colored with sharply demarcated red and tan foci in the parenchyma, consolidation of the anteroventral lung or entire lobes. Exudate may be visible in major airways. In surviving mice, consolidated areas may turn gray.

Histopathology: Characteristic interstitial pneumonia with perivascular and peribronchiolar lymphoid infiltrates and hyperplasia of alveolar macrophages. Disease is characterized by necrotizing rhinitis, tracheitis, bronchiolitis, and interstitial pneumonia arising during the immune phase of infection, wherein cytotoxic T cells destroy virus-infected cells.

Immunosuppressive Effects

Sendai virus infection can alter antibody responses and other measures of immunity, making infected animals more susceptible to other respiratory pathogens such as Mycoplasma pulmonis. This immunosuppression is a critical factor in the pathogenesis of severe pneumonia outbreaks in mouse colonies.

Diagnosis

Commercially available multiplex fluorescent immunoassay (MFI) and indirect fluorescent antibody (IFA) tests can identify antibody titers in recovering mice. PCR of lung tissue can diagnose Sendai virus in acute infections. Serology is the most common diagnostic approach in surveillance programs.

[Image Placeholder: Histopathology of Sendai virus pneumonia showing interstitial infiltrates and peribronchiolar lymphoid hyperplasia]

Streptococcus pneumoniae

Etiology and Characteristics

Streptococcus pneumoniae (also known as Diplococcus pneumoniae, Pneumococcus pneumoniae) is an encapsulated, Gram-positive, lancet-shaped diplococcus. It grows on blood agar producing alpha-hemolysis. S. pneumoniae is one of three major respiratory pathogens causing overt clinical disease in rats (along with M. pulmonis and Corynebacterium kutscheri).

Role as Co-Pathogen

Bacterial pneumonia caused by S. pneumoniae nearly always requires M. pulmonis acting as a co-pathogen. Subclinical carriers of S. pneumoniae are common in rat colonies. Clinical disease emerges when mucosal immunity is compromised by primary M. pulmonis infection or other stressors.

Clinical Signs

In rats displaying signs, serous to mucopurulent nasal discharge, rhinitis, sinusitis, and conjunctivitis may be observed. Pneumonia signs include dyspnea, rapid breathing, audible respiratory noises (rattling), and in severe cases, open-mouth breathing or panting (indicating critical respiratory compromise). Chronic cases show weight loss, reduced appetite, poor body condition, and lethargy.

Pasteurella pneumotropica (Rodentibacter spp.)

Taxonomic Update and Epidemiology

Pasteurella pneumotropica has been reclassified into the new genus Rodentibacter, with seven new species identified (formerly known as the P. pneumotropica complex). This Gram-negative coccobacillus infects a variety of rodents including mice, rats, guinea pigs, hamsters, and gerbils. Rodentibacter spp. are found in many research and commercial rodent colonies.

Opportunistic Pathogen

P. pneumotropica is best viewed as an opportunistic pathogen. Colonization typically does not result in clinical disease in immunocompetent animals. However, it can cause fatal disease in immunocompromised mice. P. pneumotropica complicates pneumonias due to M. pulmonis or Sendai virus, acting as a secondary bacterial pathogen.

Clinical Manifestations

Clinical manifestations include conjunctivitis, panophthalmitis, dacryoadenitis, orbital abscesses, pneumonia, otitis, mastitis, and genital tract infections. Pneumonia and soft-tissue abscesses are common presentations. P. pneumotropica biotype Jawetz is consistently isolated from the upper respiratory tract, lungs, and abscesses.

Diagnosis

Rodentibacter is best detected by culture or specific PCR. Cultures should be taken from the usual colonization sites: nasopharynx, vagina, and intestines. Screening via serology is NOT recommended, as animals with subclinical infections are often seronegative.

Treatment

Enrofloxacin delivered in drinking water (85 mg per kg body weight daily) for 14 days was sufficient to clear bacterial infection in normal, breeding, and immune-deficient mice. Combination therapy with enrofloxacin and doxycycline hyclate for 7 days is also effective.

Zoonotic Potential

Occupational exposure is one of the main causes of human infection by P. pneumotropica. Clinical presentation in humans includes subcutaneous abscesses, respiratory tract colonization, and systemic infections. Laboratory personnel working with rodents should use appropriate personal protective equipment.

Clinical Signs of Pneumonia: General Overview

Diagnosis of Pneumonia

Diagnosis is often made based on clinical history and physical examination in acutely ill patients to avoid delaying treatment. Diagnostic tools can provide additional information:

Physical Examination: Assess respiratory rate and effort, auscultate for abnormal lung sounds (crackles, wheezes), check for nasal or ocular discharge, evaluate body condition.

Radiography: X-rays can assess lung consolidation, infiltrates, and overall respiratory health. Useful in chronic or ambiguous cases.

Culture and Sensitivity: Nasal swabs, tracheal washes, or lung tissue samples can be cultured to identify bacterial pathogens and determine antibiotic sensitivity.

PCR Testing: Molecular diagnostics can detect M. pulmonis, Sendai virus, and Rodentibacter spp. with high sensitivity.

Serology: Antibody testing for M. pulmonis and Sendai virus is useful for surveillance and diagnosis in convalescent animals.

Treatment of Pneumonia

Antibiotic Therapy

Antibiotic selection should target the most likely bacterial pathogens. Common regimens include:

Moderate to Advanced Lung Infections:

Doxycycline 10 mg per kg every 12 hours PO for 10 to 30 days

PLUS Enrofloxacin (Baytril) 15 mg per kg every 12 hours PO for 10 to 30 days

Mild Upper Respiratory Illness:

Tylosin, tetracyclines, or enrofloxacin monotherapy may be sufficient

Supportive Care

• Oxygen therapy for severe dyspnea or cyanosis
• Fluid therapy (subcutaneous or intravenous) to maintain hydration
• Nutritional support (syringe feeding or high-calorie supplements) for anorexic animals
• Anti-inflammatory drugs (NSAIDs or corticosteroids) to reduce airway inflammation
• Nebulization therapy with saline or bronchodilators to humidify airways and facilitate mucus clearance
• Warm, clean, low-stress environment

Clinical Pearls for the NAVLE

• Mycoplasma pulmonis is the most important respiratory pathogen in RATS; Sendai virus is the most important in MICE.
• Severe pneumonia outbreaks typically involve co-infections: M. pulmonis plus Sendai virus and/or CAR bacillus in rats; Sendai plus M. pulmonis in mice.
• Streptococcus pneumoniae causes bacterial pneumonia in rats but nearly always requires M. pulmonis as a co-pathogen.
• Pasteurella pneumotropica (now Rodentibacter spp.) is an opportunistic pathogen that complicates primary viral or mycoplasmal infections.
• Sendai virus is immunosuppressive, increasing susceptibility to bacterial superinfections.
• Open-mouth breathing/panting in rats or mice indicates SEVERE respiratory compromise and is an EMERGENCY.
• Treatment of choice for moderate-to-severe pneumonia: Doxycycline 10 mg/kg q12h PLUS Enrofloxacin 15 mg/kg q12h PO for 10-30 days.
• Rodentibacter screening by serology is NOT recommended (often seronegative despite infection); use culture or PCR instead.