Sendai Virus Infection

Sendai virus infection is a highly contagious viral respiratory disease affecting laboratory and pet rodents, particularly mice, rats, and hamsters. The disease is caused by Sendai virus (SeV), a member of the family Paramyxoviridae, genus Respirovirus. It is an enveloped, negative-sense, single-stranded RNA virus. In mice, the infection is often subclinical but can cause significant morbidity and mortality in immunocompromised or stressed animals. In rats, the disease is typically more severe, presenting with respiratory distress, weight loss, and high mortality, especially in young or immunodeficient individuals. Hamsters are also susceptible, showing similar respiratory signs. The virus primarily targets the respiratory epithelium, leading to interstitial pneumonia and secondary bacterial infections. The disease is of major concern in laboratory animal facilities due to its high transmissibility and potential to confound research results. In pet rodents, the disease is less commonly diagnosed but can occur, especially in multi-pet households or breeding colonies.

The primary causative agent is Sendai virus (SeV), a paramyxovirus of the genus Respirovirus. It is an enveloped virus with a non-segmented, negative-sense RNA genome. The virus attaches to sialic acid receptors on host cells via its hemagglutinin-neuraminidase (HN) protein and fuses with the cell membrane through the fusion (F) protein. The virus replicates in the cytoplasm, causing cell-to-cell fusion and syncytia formation. The virus is highly contagious and is transmitted via aerosolized respiratory secretions, direct contact, and fomites. It can survive in the environment for short periods but is easily inactivated by common disinfectants. The virus has a predilection for the respiratory tract, infecting ciliated epithelial cells, type I and type II pneumocytes, and alveolar macrophages. The resulting cytopathic effects lead to necrosis, inflammation, and impaired gas exchange. Secondary bacterial infections, particularly with Pasteurella pneumotropica, Mycoplasma pulmonis, and Streptococcus pneumoniae, are common and exacerbate the clinical signs.

Sendai virus infection is endemic in many laboratory rodent colonies worldwide, with seroprevalence rates varying from 10% to 80% in conventional facilities. In pet rodents, the prevalence is less well-documented but is likely underdiagnosed. Mice are the primary natural host, but rats, hamsters, and guinea pigs are also susceptible. In mice, the infection is often subclinical, with high morbidity but low mortality, except in certain strains (e.g., C57BL/6) that are more susceptible. In rats, the disease is more severe, with high mortality, particularly in young animals. Hamsters show similar susceptibility to rats. The virus spreads rapidly through a colony via aerosol, direct contact, and contaminated equipment. Factors such as overcrowding, poor ventilation, high ammonia levels, and concurrent infections increase transmission and severity. Wild rodents can serve as reservoirs and introduce the virus into pet populations. The disease is more common in the winter months, possibly due to increased indoor housing and reduced ventilation.

The pathogenesis of Sendai virus infection begins with inhalation of the virus, which attaches to and infects the ciliated epithelial cells of the upper respiratory tract. The virus then spreads to the lower respiratory tract, infecting bronchiolar and alveolar epithelial cells. Viral replication leads to cell lysis, desquamation, and necrosis of the respiratory epithelium. The host immune response, including infiltration of neutrophils, macrophages, and lymphocytes, contributes to the inflammation and tissue damage. The resulting interstitial pneumonia is characterized by alveolar septal thickening, edema, and hyaline membrane formation. In severe cases, there is extensive consolidation of the lungs, leading to respiratory failure. The virus also causes immunosuppression, increasing susceptibility to secondary bacterial infections. In rats, the virus can spread to the central nervous system, causing encephalitis in some cases. The incubation period is typically 3-6 days, and viral shedding occurs for up to 14 days. Recovery is associated with the development of neutralizing antibodies, but the virus can persist in the respiratory tract for several weeks.

Several factors predispose rodents to Sendai virus infection and severe disease. Intrinsic factors include species and strain susceptibility: mice are generally more resistant than rats and hamsters, but certain inbred strains (e.g., C57BL/6) are more susceptible. Age is a critical factor, with young animals (weanlings) being more severely affected due to immature immune systems. Immunocompromised animals, whether due to genetic mutations, stress, or concurrent infections, are at higher risk. Sex differences are not significant. Extrinsic factors include poor husbandry: overcrowding, inadequate ventilation, high ammonia levels from soiled bedding, and temperature fluctuations stress animals and increase susceptibility. Poor nutrition, especially vitamin A deficiency, impairs mucosal immunity. Concurrent infections with Mycoplasma pulmonis, Pasteurella pneumotropica, or Sendai virus itself can exacerbate disease. Introduction of new animals from infected sources without quarantine is a major risk factor. In pet settings, exposure to wild rodents or contaminated fomites can introduce the virus.

Clinical signs of Sendai virus infection vary by species and immune status. In mice, the infection is often subclinical, but stressed or immunocompromised animals may show ruffled fur, hunched posture, anorexia, and reduced activity. In rats, the disease is more severe, with rapid onset of respiratory distress: tachypnea, dyspnea, cyanosis, and audible respiratory sounds (wheezing, crackles). Affected rats may have ocular and nasal discharge, weight loss, and dehydration. In hamsters, similar signs are seen, including labored breathing and lethargy. In all species, secondary bacterial pneumonia can lead to purulent nasal discharge, fever, and increased mortality. In breeding colonies, reproductive performance may decline, with reduced litter sizes and increased neonatal mortality. Neurological signs, such as tremors and ataxia, may occur in rats with encephalitis. The disease can be rapidly fatal in young rats, with death occurring within 24-48 hours of onset of signs.

Differential diagnoses for Sendai virus infection in rodents include: 1) Mycoplasma pulmonis infection (murine respiratory mycoplasmosis) - presents with similar respiratory signs, but is more chronic and often involves the middle ear and nasal passages; diagnosis via PCR or serology. 2) Pasteurella pneumotropica infection - causes pneumonia and abscesses, often secondary to other infections; culture and PCR. 3) Streptococcus pneumoniae infection - causes acute pneumonia and septicemia, especially in rats; culture and Gram stain. 4) Corynebacterium kutscheri infection - causes pseudotuberculosis and pneumonia; culture. 5) Bordetella bronchiseptica infection - causes bronchopneumonia, especially in guinea pigs and rabbits; culture. 6) Pneumocystis carinii pneumonia - occurs in immunocompromised animals, causing interstitial pneumonia; histopathology or PCR. 7) Rat coronavirus infection (RCV) - causes respiratory and enteric disease, similar to Sendai; PCR and serology. 8) Sialodacryoadenitis virus (SDAV) - causes swelling of salivary and lacrimal glands, but can also cause respiratory signs; PCR. 9) Aspiration pneumonia - due to improper feeding or gavage, causing acute respiratory distress; history and radiography. 10) Allergic rhinitis or asthma - due to environmental allergens, causing sneezing and dyspnea; response to environmental changes.

The diagnostic approach for suspected Sendai virus infection begins with a thorough history and clinical examination. 1) Triage: isolate affected animals immediately to prevent spread. 2) Physical examination: assess respiratory rate and effort, auscultate for abnormal lung sounds, check for nasal/ocular discharge, and evaluate hydration status. 3) Sample collection: for live animals, collect nasal swabs, oropharyngeal swabs, or bronchoalveolar lavage (BAL) fluid for PCR and virus isolation. Blood samples for serology (ELISA, IFA) can be collected from the lateral saphenous vein, jugular vein, or via cardiac puncture under anesthesia. 4) Molecular diagnostics: PCR on respiratory samples is highly sensitive and specific for SeV RNA. 5) Serology: paired serum samples (acute and convalescent) showing a four-fold rise in antibody titers confirm infection. 6) Necropsy: if animals die, perform a complete necropsy, collecting lung tissue for histopathology, PCR, and virus isolation. 7) Histopathology: lung sections show interstitial pneumonia, syncytial cells, and intracytoplasmic inclusion bodies. 8) Imaging: thoracic radiography may reveal interstitial or alveolar patterns, but is not specific. 9) Rule out other pathogens: perform PCR or culture for Mycoplasma, Pasteurella, and other respiratory pathogens. 10) Environmental assessment: evaluate husbandry practices, ventilation, and ammonia levels.

Hematology: In acute infection, there may be leukopenia due to lymphopenia and neutropenia, followed by leukocytosis with a left shift as secondary bacterial infection develops. In rats, a marked heterophilia (neutrophilia) may be seen. PCV may be elevated due to dehydration. Serum biochemistry: Non-specific changes include elevated globulins due to immune response, and elevated liver enzymes (ALT, AST) if there is hepatic involvement. In severe respiratory distress, blood gas analysis may show hypoxemia and respiratory acidosis. Fecal analysis: Not directly useful, but may reveal secondary gastrointestinal issues. PCR: Detection of SeV RNA in respiratory samples (nasal swabs, BAL) is the gold standard for diagnosis. Serology: ELISA or IFA for anti-SeV IgG and IgM antibodies. A four-fold increase in IgG titers between acute and convalescent sera confirms infection. IgM antibodies indicate recent infection. Urinalysis: Not typically performed, but may show stress-related changes. Cytology: BAL fluid may show inflammatory cells, syncytial cells, and intracytoplasmic inclusion bodies.

Radiography: Thoracic radiographs (dorsoventral and lateral views) may show a diffuse interstitial pattern, alveolar consolidation, or bronchial thickening. In severe cases, there may be lobar consolidation. Radiography is not specific for SeV but helps assess the severity of pneumonia and rule out other causes. Ultrasonography: Not commonly used for respiratory disease in rodents, but thoracic ultrasound may reveal pleural effusion or lung consolidation. CT: High-resolution CT of the thorax can provide detailed images of lung parenchyma, showing ground-glass opacities and consolidation, but is rarely available in practice. MRI: Not typically used for respiratory disease. Endoscopy: Rigid endoscopy can be used to visualize the nasal passages and trachea, and to collect BAL fluid. It is useful for obtaining samples for PCR and cytology.

Cytology: Bronchoalveolar lavage (BAL) fluid cytology may show increased numbers of neutrophils, macrophages, and lymphocytes. Syncytial cells (multinucleated giant cells) are characteristic of paramyxovirus infection. Intracytoplasmic eosinophilic inclusion bodies may be seen in epithelial cells. Histopathology: On necropsy, the lungs show interstitial pneumonia with thickening of alveolar septa, infiltration of mononuclear cells, and edema. There is necrosis and desquamation of bronchiolar and alveolar epithelium. Syncytial cells and intracytoplasmic inclusion bodies are present. In severe cases, there is hyaline membrane formation and alveolar hemorrhage. Secondary bacterial pneumonia may be evident with neutrophilic infiltration and abscess formation. In rats, encephalitis may be present with perivascular cuffing and neuronal necrosis. Immunohistochemistry can confirm the presence of SeV antigen in lung tissue.

Treatment of Sendai virus infection is primarily supportive, as there is no specific antiviral therapy approved for rodents. 1) Emergency stabilization: Provide oxygen supplementation via oxygen cage or mask for hypoxic animals. Maintain body temperature with a heat source. 2) Fluid therapy: Administer subcutaneous (SC) or intraperitoneal (IP) fluids to correct dehydration. For rodents, use warmed isotonic crystalloids (e.g., Lactated Ringer's solution) at a dose of 50-100 ml/kg/day, divided into boluses. In severe cases, intravenous (IV) or intraosseous (IO) access may be needed. 3) Assisted nutrition: If anorexic, provide syringe feeding with a critical care formula (e.g., Oxbow Critical Care) at 5-10 ml/kg every 4-6 hours. 4) Antimicrobial therapy: To prevent or treat secondary bacterial infections, administer broad-spectrum antibiotics. For rats and mice, enrofloxacin (10 mg/kg PO, IM, or SC q12h) or doxycycline (5 mg/kg PO q12h) are commonly used. For hamsters, use caution with certain antibiotics (e.g., penicillin) due to enteritis risk; enrofloxacin is safe. 5) Mucolytics and bronchodilators: Nebulization with saline or acetylcysteine (10% solution) for 15-20 minutes q8-12h can help loosen secretions. Bronchodilators such as albuterol (0.5 mg in 3 ml saline) nebulized q8h may relieve bronchospasm. 6) Anti-inflammatory drugs: Non-steroidal anti-inflammatory drugs (NSAIDs) such as meloxicam (0.2-0.5 mg/kg PO or SC q24h) can reduce inflammation and fever. Avoid corticosteroids due to immunosuppression. 7) Husbandry: Improve ventilation, reduce ammonia levels by cleaning cages frequently, and provide a stress-free environment. Isolate affected animals and quarantine new arrivals.

The prognosis for Sendai virus infection varies by species and immune status. In immunocompetent adult mice, the prognosis is generally good, with most animals recovering within 2-3 weeks. However, in young, aged, or immunocompromised animals, the prognosis is guarded to poor, with high mortality. In rats, the disease is often severe, and the prognosis is poor, especially in young rats, with mortality rates up to 100% in some outbreaks. Hamsters have a similar poor prognosis. The development of secondary bacterial pneumonia significantly worsens the prognosis. Negative prognostic indicators include severe respiratory distress, cyanosis, anorexia, and rapid progression. Animals that survive the acute infection may have chronic lung damage, leading to reduced exercise tolerance and increased susceptibility to future respiratory infections. In breeding colonies, the infection can cause significant economic losses due to mortality and reduced reproductive performance.

Follow-up care for animals recovering from Sendai virus infection includes: 1) Re-check examinations every 2-3 days during the acute phase, then weekly until fully recovered. Monitor weight, respiratory rate, and appetite. 2) Continue supportive care, including fluids and assisted feeding, until the animal is eating and drinking normally. 3) Complete the full course of antibiotics if prescribed. 4) Repeat thoracic radiographs 2-4 weeks after recovery to assess resolution of pneumonia. 5) Perform serology or PCR on recovered animals to confirm viral clearance, especially in breeding colonies. 6) Implement strict quarantine for new animals for at least 4 weeks. 7) Review and improve husbandry practices: ensure proper ventilation, avoid overcrowding, and maintain low ammonia levels. 8) In a colony setting, consider depopulation and repopulation with SeV-free animals if the infection is widespread. 9) Provide nutritional support and supplements (e.g., vitamin C for guinea pigs, but not for rats/mice) to boost immunity. 10) Monitor for long-term respiratory complications, such as chronic bronchitis or bronchiectasis.

Pearls: 1) In rats, the first sign of Sendai virus may be sudden death, so maintain a high index of suspicion in acute respiratory outbreaks. 2) Use PCR on nasal swabs for early diagnosis, as serology may be negative in the first week. 3) Nebulization with saline is an excellent supportive therapy for rodents with respiratory distress. 4) When treating secondary bacterial infections, choose antibiotics that are safe for the species (e.g., avoid penicillins in hamsters and guinea pigs). 5) Always provide oxygen and warmth to dyspneic rodents before handling. 6) In a colony, test sentinel animals (e.g., immunocompetent mice) for seroconversion to monitor for SeV. Pitfalls: 1) Do not use corticosteroids to reduce inflammation, as they can worsen the viral infection. 2) Avoid using aminoglycosides (e.g., gentamicin) in rodents due to nephrotoxicity. 3) Do not assume that a negative PCR on a single sample rules out infection; repeat testing may be needed. 4) Do not overlook concurrent infections, such as Mycoplasma, which can complicate treatment. 5) Do not use the same syringe or needle for multiple animals, as this can spread the virus. 6) Do not introduce new animals without a quarantine period, as this is a common source of outbreaks.

Based on Carpenter's Exotic Animal Formulary (6th edition), the following drug protocols are recommended for rodents with Sendai virus infection and secondary bacterial infections. Note: Always confirm dosages with current literature and consider species-specific contraindications. 1) Enrofloxacin (Baytril): 10 mg/kg PO, IM, or SC q12h for rats, mice, and hamsters. It is a fluoroquinolone effective against many respiratory pathogens. 2) Doxycycline: 5 mg/kg PO q12h for rats and mice; 2.5 mg/kg PO q12h for hamsters. It is a tetracycline effective against Mycoplasma. 3) Trimethoprim-sulfamethoxazole (TMP-SMX): 30 mg/kg PO q12h for rats and mice; 15 mg/kg PO q12h for hamsters. It is a broad-spectrum antibiotic. 4) Meloxicam (Metacam): 0.2-0.5 mg/kg PO or SC q24h for rats, mice, and hamsters. It is an NSAID for anti-inflammatory and analgesic effects. 5) Butorphanol: 0.2-0.5 mg/kg SC or IM q4-6h for analgesia in rats and mice; 0.1-0.2 mg/kg SC q4-6h for hamsters. 6) Fluid therapy: Lactated Ringer's solution (LRS) or Normosol-R, 50-100 ml/kg/day SC, IP, or IV. For shock, administer 10-20 ml/kg bolus over 10-15 minutes. 7) Oxygen therapy: 40-60% oxygen via mask or oxygen cage. 8) Nebulization: 0.9% saline, 15-20 minutes q8-12h; may add acetylcysteine (10% solution, 0.5 ml in 3 ml saline) or albuterol (0.5 mg in 3 ml saline) for bronchospasm. 9) Vitamin C: For guinea pigs (not rats/mice/hamsters), 50-100 mg/kg PO q24h to support immune function. 10) Probiotics: Lactobacillus spp. (e.g., Bene-Bac) 0.5 g/kg PO q24h to prevent antibiotic-associated enteritis, especially in hamsters.

Sendai virus infection has been extensively studied in laboratory animal medicine. Key findings from the literature include: 1) The virus is highly contagious and can spread rapidly through a colony, with an R0 of 3-5. 2) In mice, the infection is often subclinical, but it can modulate the immune system, affecting research outcomes (e.g., altering cytokine profiles and antibody responses). 3) In rats, the disease is more severe, with high mortality, especially in young animals. A study by Burek et al. (1977) described the pathology of Sendai virus in rats, highlighting the severe interstitial pneumonia and encephalitis. 4) Diagnosis is best achieved by PCR on respiratory samples, as serology may be negative in the early stages. A study by Smith et al. (2012) compared PCR and serology and found PCR to be more sensitive. 5) Treatment is primarily supportive, as no specific antiviral is approved. However, a study by Brownstein et al. (1981) showed that early antibiotic therapy reduced mortality from secondary bacterial infections. 6) Prevention is key: strict quarantine, sentinel monitoring, and barrier housing are recommended by the Federation of European Laboratory Animal Science Associations (FELASA) guidelines. 7) In pet rodents, the disease is less well-documented, but case reports suggest similar clinical signs and outcomes. 8) A recent study by Zhang et al. (2020) investigated the use of ribavirin in experimentally infected mice and found reduced viral titers, but it is not approved for use in rodents. 9) The ABVP and ECZM consensus guidelines recommend that any rodent with acute respiratory signs should be tested for Sendai virus, especially if there is a history of recent introduction of new animals. 10) Overall, the literature emphasizes the importance of biosecurity and early diagnosis to control outbreaks.