Ectromelia Virus (Mousepox) – NAVLE Study Guide

Overview and Clinical Importance

Ectromelia virus (also called mousepox) is a highly contagious viral disease affecting mice, caused by an orthopoxvirus in the family Poxviridae. While rare in modern research facilities due to strict biosecurity, ectromelia remains an important disease for the NAVLE because of its devastating impact on mouse colonies and its potential to confound research results.

Ectromelia is species-specific to mice and does not naturally infect other rodent species or humans. The disease can present in multiple forms ranging from subclinical to acute fatal depending on mouse strain, virus virulence, and immune status.

Etiology and Viral Characteristics

Viral Classification

• Family: Poxviridae
• Genus: Orthopoxvirus (same genus as smallpox, vaccinia, cowpox)
• Genome: Double-stranded DNA virus
• Morphology: Brick-shaped or ovoid virion, 200-400 nm
• Replication: Cytoplasmic replication (unique among DNA viruses)

Environmental Stability

• Highly stable in environment; can survive in dried scabs and bedding for months
• Resistant to desiccation and many disinfectants
• Susceptible to: Bleach (10% solution), phenolic disinfectants, formaldehyde, autoclaving

Epidemiology and Transmission

Host Specificity

Ectromelia virus naturally infects ONLY mice. It does not cause disease in rats, humans, or other species under normal circumstances. Susceptibility varies by mouse strain:

• Highly susceptible: BALB/c, A, DBA/2 strains (high mortality)
• Moderately susceptible: C3H, CBA strains
• Relatively resistant: C57BL/6, C57BL/10 strains (often subclinical)

Routes of Transmission

Pathogenesis

Step 1: Viral Entry

Virus enters through skin abrasions, respiratory tract, or gastrointestinal tract. The footpad is a common entry site.

Step 2: Primary Replication

Virus replicates locally at the site of entry, causing primary lesion (papule, ulcer, or footpad swelling).

Step 3: Lymphatic Spread

Virus spreads to regional lymph nodes, causing lymphadenopathy and necrosis.

Step 4: Primary Viremia

Virus enters bloodstream and disseminates to liver, spleen, and other organs.

Step 5: Secondary Replication

Massive viral replication occurs in liver and spleen, causing hepatic and splenic necrosis. This is the critical phase determining survival.

Step 6: Secondary Viremia and Skin Lesions

Second wave of viremia seeds skin, causing characteristic maculopapular rash that progresses to pustules, scabs, and eventual scarring (in survivors).

Step 7: Outcome

In susceptible strains: Death from hepatic necrosis and multiorgan failure within 7-14 days. In resistant strains: Recovery with immunity or chronic subclinical infection.

Clinical Presentation

Clinical Forms

Pathologic Findings

Gross Pathology

• Liver: Multifocal to coalescing pale foci of necrosis (most characteristic finding)
• Spleen: Splenomegaly with white necrotic foci
• Lymph nodes: Enlarged, necrotic, hemorrhagic
• Skin: Papules, pustules, ulcers, scabs on tail, limbs, face
• Limbs: Severe edema, necrosis, possible gangrene

Histopathology

Pathognomonic Finding: Large, eosinophilic intracytoplasmic inclusion bodies (Marchal bodies or Guarnieri bodies) in epidermal cells, hepatocytes, and other tissues.

• Skin: Epidermal hyperplasia, ballooning degeneration, intracytoplasmic inclusion bodies
• Liver: Coagulative necrosis, hepatocellular degeneration, inclusions in hepatocytes
• Spleen: Lymphoid depletion, necrosis
• Lymph nodes: Necrosis, inflammation

Diagnosis

Diagnostic Approach

Differential Diagnoses

• Ulcerative dermatitis: No systemic signs, no hepatic necrosis
• Fight wounds/bite injuries: Localized, traumatic history
• Bacterial dermatitis: Responds to antibiotics, no inclusions
• Sendai virus: Respiratory signs, no skin lesions
• Mouse hepatitis virus: Hepatic necrosis but no skin lesions

Treatment and Management

Treatment

There is NO specific antiviral treatment for ectromelia. Management is supportive only in research or pet settings (though euthanasia of entire colonies is typical in research).

Supportive Care (if treating individual pets):

• Isolation of affected animals
• Nutritional support, fluid therapy
• Wound care for skin lesions
• Antibiotics for secondary bacterial infections
• Analgesia if needed

Control Measures in Research Facilities

Depopulation and Decontamination: In research settings, the standard protocol is complete depopulation of affected and in-contact colonies, followed by thorough facility decontamination.

Decontamination Protocol:

• Euthanize all mice in affected rooms/facilities
• Remove all bedding, food, cages, equipment
• Disinfect with 10% bleach, formaldehyde vapor, or phenolic disinfectants
• Autoclave or incinerate all contaminated materials
• Allow 30-day quarantine period before restocking

Prevention

• Strict Biosecurity: Quarantine new arrivals for 30 days; serologic testing
• Barrier Housing: HEPA-filtered cages, dedicated equipment
• Source Mice from SPF Facilities: Specific pathogen-free colonies
• Regular Sentinel Monitoring: Serology every 3-6 months
• Personnel Training: Proper PPE, handwashing, cage-changing procedures

Prognosis and Public Health

Prognosis

• Highly susceptible strains: Greater than 90% mortality within 7-14 days
• Moderately susceptible strains: Variable mortality (20-60%); survivors develop immunity
• Resistant strains: Low mortality; often subclinical with seroconversion

Public Health Considerations

Ectromelia virus does NOT naturally infect humans. There are no documented cases of human infection. However, laboratory personnel should use standard precautions when handling infected mice due to potential zoonotic risk from other orthopoxviruses.