Canine Distemper

Canine distemper is a highly contagious, systemic, and often fatal viral disease caused by Canine morbillivirus (formerly canine distemper virus, CDV), a single-stranded RNA virus belonging to the genus Morbillivirus within the family Paramyxoviridae. The disease affects domestic dogs and a wide range of carnivores, including ferrets, raccoons, skunks, foxes, and large felids. Clinically, it is characterized by a biphasic fever, respiratory signs (serous to mucopurulent nasal and ocular discharge, cough, pneumonia), gastrointestinal signs (vomiting, diarrhea), and a variety of neurological manifestations (seizures, myoclonus, paresis, ataxia). The virus has a pantropic nature, targeting epithelial, lymphoid, and nervous tissues. The disease can be peracute, acute, subacute, or chronic, with neurological signs sometimes appearing weeks to months after recovery. Diagnosis is based on clinical signs, history, and laboratory confirmation via RT-PCR, ELISA, or immunofluorescence. Treatment is primarily supportive, as no specific antiviral therapy is available. Vaccination remains the cornerstone of prevention.

The causative agent is Canine morbillivirus (CDV), an enveloped, non-segmented, negative-sense single-stranded RNA virus. The viral genome encodes six structural proteins: nucleocapsid (N), phosphoprotein (P), matrix (M), fusion (F), hemagglutinin (H), and large polymerase (L), plus two nonstructural proteins (C and V). The H protein is responsible for attachment to host cellular receptors, primarily signaling lymphocyte activation molecule (SLAM/CD150) on immune cells and nectin-4 on epithelial cells. The F protein mediates membrane fusion. CDV is closely related to measles virus and rinderpest virus. The virus is relatively labile in the environment, being inactivated by heat (50-60°C for 30 minutes), ultraviolet light, and common disinfectants (e.g., 70% ethanol, 0.5% sodium hypochlorite, quaternary ammonium compounds). Transmission occurs primarily via aerosolized respiratory droplets, but also through direct contact with infected body fluids (urine, feces, ocular/nasal discharge) and fomites. The virus is shed in all secretions and excretions during the acute phase, and can persist in the central nervous system (CNS) for months. There is no carrier state in recovered dogs, but immunocompromised animals may shed virus intermittently.

Canine distemper has a worldwide distribution, with higher prevalence in unvaccinated or inadequately vaccinated dog populations, shelters, and wildlife reservoirs. Domestic dogs (Canis lupus familiaris) are the primary hosts, but the virus infects a broad range of carnivores, including ferrets (Mustela putorius furo), raccoons (Procyon lotor), red foxes (Vulpes vulpes), gray foxes (Urocyon cinereoargenteus), coyotes (Canis latrans), wolves (Canis lupus), and various large felids (e.g., lions, tigers). In endemic areas, the disease is enzootic with periodic epizootics. The incidence is highest in puppies aged 3-6 months, particularly those with waning maternal antibody. Breed predilections are not well-defined, but certain breeds (e.g., Rottweilers, German Shepherds) may have increased susceptibility or severity due to immunogenetic factors. Sex predilection is not reported. Seasonality is not pronounced, but outbreaks may occur more frequently in winter and spring in temperate regions, possibly due to increased indoor crowding and reduced ventilation. In wildlife, CDV can cause significant population declines, especially in endangered species. The case fatality rate in domestic dogs ranges from 30% to 80%, depending on viral strain, host immunity, and supportive care.

CDV enters the host via the respiratory tract, initially replicating in macrophages and dendritic cells of the tonsils and bronchial lymph nodes. Within 24-48 hours, the virus spreads via the lymphatic system to systemic lymphoid tissues (spleen, thymus, lymph nodes, bone marrow), causing profound immunosuppression due to lymphopenia and depletion of T and B lymphocytes. The virus then disseminates via viremia (cell-associated) to epithelial tissues of the respiratory, gastrointestinal, urinary, and integumentary systems, as well as the CNS. The hallmark of CDV infection is the formation of intranuclear and intracytoplasmic inclusion bodies in epithelial cells, neurons, and glial cells. In the CNS, the virus enters via infected mononuclear cells crossing the blood-brain barrier or via direct infection of endothelial cells. It causes demyelination, neuronal necrosis, and gliosis, leading to white matter lesions in the cerebellum, periventricular areas, and spinal cord. The neurological phase may be acute (due to viral replication) or chronic (immune-mediated demyelination). The virus also induces apoptosis in lymphoid cells, contributing to immunosuppression and secondary bacterial infections. The biphasic fever (first peak at 3-6 days post-infection, second at 6-9 days) corresponds to viremia and systemic spread. The severity of clinical signs correlates with the degree of immunosuppression and the viral strain's neurotropism.

Intrinsic factors include young age (puppies <6 months), lack of maternal immunity (due to inadequate colostrum intake or early weaning), and genetic susceptibility (certain breeds may have impaired cell-mediated immunity). Immunocompromised states (e.g., concurrent infections, stress, malnutrition, corticosteroid therapy) increase susceptibility and severity. Extrinsic factors include unvaccinated or incompletely vaccinated status, overcrowded environments (shelters, kennels), poor hygiene, and exposure to wildlife reservoirs. Environmental factors such as cold, damp conditions and poor ventilation may facilitate viral survival and transmission. Concurrent infections (e.g., canine parvovirus, canine adenovirus, Bordetella bronchiseptica) can exacerbate clinical signs and worsen prognosis.

Clinical signs vary depending on the viral strain, host immunity, and organ systems affected. The incubation period is typically 1-2 weeks but can range from 3 days to 6 weeks. The disease progresses through several stages:

- **Peracute**: Sudden onset of high fever (39.5-41°C), lethargy, anorexia, and rapid progression to neurological signs (seizures, coma) and death within 1-2 days. - **Acute**: Biphasic fever (first peak at 3-6 days, second at 6-9 days), serous nasal and ocular discharge that becomes mucopurulent, conjunctivitis, cough, dyspnea, vomiting, diarrhea (often with blood), and dehydration. Tonsillitis and pharyngitis may be present. - **Subacute**: Respiratory and gastrointestinal signs may resolve, but neurological signs may emerge, including myoclonus (involuntary muscle twitching, often in the limbs or face), seizures (focal or generalized), ataxia, paresis, paralysis, head tilt, nystagmus, and behavioral changes. Hyperesthesia and cervical rigidity may occur. - **Chronic**: Neurological signs may persist or appear weeks to months after recovery, including chronic myoclonus, paresis, and cognitive deficits. Other chronic manifestations include enamel hypoplasia (in dogs infected before permanent tooth eruption), hyperkeratosis of the footpads (hard pad disease), and retinal lesions (chorioretinitis).

Systemic signs include fever, depression, anorexia, and weight loss. Secondary bacterial infections (e.g., bronchopneumonia, urinary tract infections) are common due to immunosuppression.

Differential diagnoses for canine distemper include:

1. **Canine Parvovirus Infection**: Causes severe hemorrhagic diarrhea, vomiting, and leukopenia, but lacks respiratory and neurological signs typical of distemper. Parvovirus is diagnosed via fecal antigen test or PCR. 2. **Canine Infectious Respiratory Disease Complex (CIRDC)**: Includes Bordetella bronchiseptica, canine adenovirus type 2, canine parainfluenza virus, and mycoplasmas. Causes cough and nasal discharge but no neurological signs. PCR panels can differentiate. 3. **Canine Adenovirus Type 1 (Infectious Canine Hepatitis)**: Causes fever, vomiting, abdominal pain, and hepatic dysfunction. May cause corneal edema (blue eye) and coagulopathy. Differentiated by liver enzyme elevation and PCR. 4. **Rabies**: Causes progressive neurological signs (behavioral changes, paralysis, hypersalivation) and is invariably fatal. History of exposure and fluorescent antibody testing on brain tissue are diagnostic. 5. **Toxoplasmosis**: Can cause neurological signs, fever, and respiratory signs. Serology (IgM/IgG) and PCR on CSF or tissue are helpful. 6. **Neosporosis**: Causes neuromuscular signs (paresis, muscle atrophy) in puppies. Serology and PCR differentiate. 7. **Granulomatous Meningoencephalomyelitis (GME)**: An immune-mediated CNS disease causing multifocal neurological signs. MRI and CSF analysis (lymphocytic pleocytosis) are key. 8. **Epilepsy**: Idiopathic epilepsy causes recurrent seizures without systemic signs. Diagnosis of exclusion. 9. **Lead Poisoning**: Causes gastrointestinal and neurological signs (seizures, ataxia). Blood lead levels confirm. 10. **Hepatic Encephalopathy**: Causes neurological signs (seizures, ataxia) due to liver dysfunction. Liver function tests and bile acids are diagnostic.

The diagnostic approach for suspected canine distemper follows a stepwise algorithm:

1. **Clinical Triage**: Obtain a thorough history (vaccination status, age, exposure to unvaccinated dogs or wildlife) and perform a complete physical examination, noting fever, respiratory signs, ocular/nasal discharge, and neurological deficits. 2. **Initial Laboratory Testing**: Perform a complete blood count (CBC), serum biochemistry profile, and urinalysis. Lymphopenia is a common early finding. Biochemistry may reveal electrolyte imbalances, hypoalbuminemia, and elevated liver enzymes. 3. **Specific Viral Testing**: - **RT-PCR**: On whole blood, conjunctival swabs, nasal swabs, urine, or cerebrospinal fluid (CSF). This is the most sensitive and specific test. Positive results confirm infection. - **Immunofluorescence (IF)**: On conjunctival or nasal smears, or on buffy coat smears. Detects viral antigen. - **ELISA**: For detection of IgM (acute infection) or IgG (past exposure or vaccination). IgM positivity indicates recent infection. - **Virus Isolation**: Rarely performed due to time and technical requirements. 4. **CSF Analysis**: If neurological signs are present, CSF analysis may show lymphocytic pleocytosis and elevated protein. RT-PCR on CSF can be performed. 5. **Imaging**: Thoracic radiographs may reveal interstitial or alveolar patterns consistent with pneumonia. MRI of the brain may show demyelinating lesions in chronic cases. 6. **Histopathology**: On postmortem examination, intranuclear and intracytoplasmic inclusion bodies in epithelial cells, neurons, and glial cells are pathognomonic. Immunohistochemistry can confirm viral antigen. 7. **Rule Out Differentials**: Based on clinical signs and laboratory results, rule out other infectious, toxic, and metabolic causes.

A definitive diagnosis is made by positive RT-PCR or antigen detection in a clinically affected animal.

Laboratory findings in canine distemper are variable and non-specific but support the diagnosis:

- **Hematology**: Lymphopenia is the most consistent finding, often severe (<1,000/µL). Neutropenia may occur due to bone marrow suppression. In later stages, leukocytosis may be present due to secondary bacterial infections. Anemia may develop due to chronic disease or blood loss. - **Serum Biochemistry**: Hypoalbuminemia due to protein-losing enteropathy or malnutrition. Elevated liver enzymes (ALT, AST) may occur due to hepatic involvement. Electrolyte imbalances (hyponatremia, hypokalemia) due to vomiting and diarrhea. Blood urea nitrogen (BUN) and creatinine may be elevated if dehydration or renal involvement occurs. - **Urinalysis**: May show proteinuria, hematuria, or casts due to renal involvement. Specific gravity may be low if renal failure develops. - **Blood Gas Analysis**: May reveal metabolic acidosis due to diarrhea and lactic acidosis. - **Specific Biomarkers**: C-reactive protein (CRP) may be elevated as an acute-phase protein. Serum amyloid A (SAA) may also be elevated. - **Serology/PCR**: RT-PCR on blood, urine, or CSF is the most sensitive and specific test. IgM ELISA indicates recent infection; IgG indicates past exposure or vaccination. Paired serology (2-4 weeks apart) showing a four-fold rise in IgG confirms infection. - **CSF Analysis**: Lymphocytic pleocytosis (typically 10-100 cells/µL) and elevated protein (50-200 mg/dL) in neurological cases. RT-PCR on CSF can be positive even when blood is negative.

Imaging findings in canine distemper are not pathognomonic but can support the diagnosis:

- **Radiography**: Thoracic radiographs may show interstitial or alveolar patterns, especially in the cranioventral lung lobes, consistent with bronchopneumonia. In chronic cases, pulmonary fibrosis may be evident. Abdominal radiographs may show gas-filled bowel loops due to ileus. - **Ultrasonography**: Abdominal ultrasound may reveal thickened intestinal walls, mesenteric lymphadenopathy, and hepatomegaly. Thoracic ultrasound may show lung consolidation and pleural effusion. - **Computed Tomography (CT)**: CT of the thorax can better characterize pulmonary lesions. CT of the brain may show hypodense lesions in the white matter, particularly in the cerebellum and periventricular regions, in chronic neurological cases. - **Magnetic Resonance Imaging (MRI)**: MRI of the brain is the most sensitive imaging modality for detecting demyelinating lesions. T2-weighted images may show hyperintense lesions in the white matter, and contrast enhancement may be seen in active inflammation. - **Endoscopy**: Bronchoscopy may reveal mucopurulent exudate in the airways. Gastrointestinal endoscopy may show mucosal erythema and erosions. - **Fluoroscopy**: Not commonly used, but may be helpful in evaluating swallowing dysfunction in neurological cases. - **Echocardiography**: Not typically indicated unless cardiac involvement is suspected, which is rare.

Cytological and histopathological findings are important for diagnosis, especially postmortem:

- **Cytology**: Fine needle aspirates of lymph nodes may show reactive lymphoid hyperplasia or depletion. Conjunctival or nasal smears may reveal intracytoplasmic inclusion bodies in epithelial cells. Bronchoalveolar lavage (BAL) cytology may show neutrophilic inflammation with intracellular bacteria if secondary pneumonia is present. - **Histopathology**: On postmortem examination, characteristic findings include: - **Lymphoid tissues**: Depletion of lymphocytes in lymph nodes, spleen, and thymus, with necrosis. - **Epithelial tissues**: Intranuclear and intracytoplasmic eosinophilic inclusion bodies in respiratory, gastrointestinal, urinary, and integumentary epithelial cells. - **Central nervous system**: Demyelination, neuronal necrosis, gliosis, and perivascular cuffing with mononuclear cells. Inclusion bodies may be present in neurons and glial cells. - **Footpads**: Hyperkeratosis and inclusion bodies in epidermal cells. - **Lungs**: Interstitial pneumonia with mononuclear infiltration and inclusion bodies in alveolar epithelial cells. - **Special stains**: Immunohistochemistry (IHC) using anti-CDV antibodies can confirm viral antigen in tissues. In situ hybridization (ISH) can detect viral RNA.

There is no specific antiviral therapy for canine distemper; treatment is primarily supportive and symptomatic. The goals are to control secondary infections, maintain hydration and nutrition, and manage neurological signs.

- **Emergency Stabilization**: If the dog presents in shock or with severe dehydration, immediate intravenous fluid therapy with isotonic crystalloids (e.g., lactated Ringer's solution) at a rate of 60-90 mL/kg/hour for the first hour, then adjusted based on hydration status and ongoing losses. Colloids (e.g., hetastarch) may be considered for hypoproteinemia. - **Antimicrobial Therapy**: Broad-spectrum antibiotics are indicated to prevent or treat secondary bacterial infections, especially pneumonia. Recommended protocols include: - Amoxicillin-clavulanate: 12.5-25 mg/kg PO q8-12h, or 20 mg/kg IV q8h. - Doxycycline: 5-10 mg/kg PO q12h (for respiratory infections). - For severe pneumonia, combination therapy with a fluoroquinolone (e.g., enrofloxacin 5-10 mg/kg IV/PO q24h) and a beta-lactam (e.g., ampicillin 20-40 mg/kg IV q8h) may be used. - **Antiemetics**: For vomiting, maropitant (Cerenia) at 1 mg/kg SC q24h or 2 mg/kg PO q24h is effective. - **Gastrointestinal Support**: Gastroprotectants such as omeprazole (0.7-1.5 mg/kg PO q12h) or famotidine (0.5-1 mg/kg PO/IV q12h) may be used. Probiotics may help restore gut flora. - **Nutritional Support**: If the dog is anorexic, a high-quality, easily digestible diet is recommended. In severe cases, a feeding tube (nasoesophageal, esophagostomy) may be necessary. - **Neurological Management**: For seizures, diazepam (0.5-1 mg/kg IV) can be used acutely, followed by maintenance with phenobarbital (2.5-5 mg/kg PO q12h) or levetiracetam (20 mg/kg PO q8h). For myoclonus, no specific treatment is effective, but some dogs may respond to phenobarbital or potassium bromide. - **Immunomodulatory Therapy**: The use of immunosuppressive doses of corticosteroids (e.g., prednisone 0.5-1 mg/kg PO q12h) is controversial. They may be beneficial in chronic demyelinating disease but can worsen immunosuppression. Some studies suggest that high-dose vitamin A (100,000 IU/day for 3 days) may reduce mortality, but evidence is limited. - **Nursing Care**: Maintain airway clearance, provide humidified oxygen if hypoxemic, and ensure a clean, warm, and quiet environment. Eye care with lubricants and antibiotics for conjunctivitis. - **Physical Rehabilitation**: For dogs with paresis or ataxia, physical therapy may help maintain muscle mass and joint mobility.

The prognosis for canine distemper is guarded to poor, especially in puppies and dogs with neurological signs. Mortality rates range from 30% to 80%, with higher rates in young, unvaccinated dogs. Factors associated with a worse prognosis include: - Young age (<6 months) - Severe lymphopenia (<1,000/µL) - Presence of neurological signs, especially seizures or myoclonus - Secondary bacterial pneumonia - Lack of adequate supportive care

Dogs that survive the acute phase may develop chronic neurological signs, which can be progressive and debilitating. Myoclonus may persist lifelong but is not necessarily fatal. Some dogs recover completely, but they may have residual deficits such as enamel hypoplasia or hard pad disease. The prognosis for chronic neurological cases is poor, with many dogs requiring euthanasia due to poor quality of life.

Follow-up care for dogs with canine distemper is essential for monitoring recovery and managing complications:

- **Re-check Intervals**: Initially, re-check every 1-2 weeks during the acute phase. Once stable, re-check every 4-6 weeks for 3-6 months. - **Serial Lab Monitoring**: Repeat CBC and biochemistry to monitor for resolution of lymphopenia and organ function. If on anticonvulsants, monitor serum drug levels (e.g., phenobarbital) and liver enzymes. - **Imaging**: Repeat thoracic radiographs if pneumonia was present, to ensure resolution. MRI may be repeated if neurological signs persist or worsen. - **Vaccination**: After recovery, dogs should be vaccinated according to standard protocols (e.g., modified live vaccine for CDV) to prevent reinfection, but only after full recovery and with veterinary guidance. - **Long-term Management**: For dogs with chronic neurological deficits, provide a safe environment, assist with mobility, and manage seizures with appropriate medications. Regular veterinary assessments are needed to adjust therapy. - **Infection Control**: Isolate infected dogs from other susceptible animals for at least 2-4 weeks after clinical recovery, as viral shedding may persist.

**Pearls**: - Always consider distemper in any unvaccinated dog with fever, respiratory signs, and neurological signs, especially if there is a history of exposure to wildlife or shelters. - Lymphopenia is a key early laboratory finding; a CBC is essential. - RT-PCR on conjunctival swabs or whole blood is the most sensitive diagnostic test; a negative result does not rule out distemper if clinical signs are strong. - Myoclonus is pathognomonic for distemper when present; it may persist after recovery. - Enamel hypoplasia in a young dog is a historical indicator of distemper infection. - Vaccination is highly effective; ensure puppies receive a series of vaccines and boosters.

**Pitfalls**: - Do not rely solely on clinical signs; many diseases mimic distemper. - Avoid using corticosteroids in the acute phase, as they may worsen immunosuppression. - Do not assume a dog is non-infectious after clinical recovery; viral shedding can occur for weeks. - Do not neglect supportive care; aggressive fluid therapy and nutritional support improve survival. - Be cautious with the use of live vaccines in immunosuppressed dogs; they may cause disease.

Based on Plumb's Veterinary Drug Handbook, the following drug protocols are commonly used for canine distemper:

- **Antibiotics**: - Amoxicillin-clavulanate: 12.5-25 mg/kg PO q8-12h, or 20 mg/kg IV q8h. - Doxycycline: 5-10 mg/kg PO q12h. - Enrofloxacin: 5-10 mg/kg IV/PO q24h (avoid in puppies <8 months due to cartilage damage). - Ampicillin: 20-40 mg/kg IV q8h. - **Antiemetics**: - Maropitant: 1 mg/kg SC q24h or 2 mg/kg PO q24h. - Ondansetron: 0.1-0.2 mg/kg IV q8-12h. - **Gastroprotectants**: - Omeprazole: 0.7-1.5 mg/kg PO q12h. - Famotidine: 0.5-1 mg/kg PO/IV q12h. - **Anticonvulsants**: - Diazepam: 0.5-1 mg/kg IV for acute seizures. - Phenobarbital: 2.5-5 mg/kg PO q12h; monitor serum levels (target 15-45 µg/mL). - Levetiracetam: 20 mg/kg PO q8h. - Potassium bromide: 20-40 mg/kg PO q24h (for refractory seizures). - **Immunomodulators**: - Prednisone: 0.5-1 mg/kg PO q12h (use cautiously, only for chronic neurological disease). - **Nutritional Support**: - Vitamin A: 100,000 IU/day PO for 3 days (may reduce mortality, but evidence is weak). - **Fluid Therapy**: - Isotonic crystalloids (LRS) at maintenance (60-100 mL/kg/day) plus deficits and ongoing losses. - Colloids (hetastarch) 10-20 mL/kg IV over 1-2 hours for hypoproteinemia.

All dosages should be adjusted based on renal/hepatic function and clinical response. Contraindications: Enrofloxacin should be avoided in growing puppies. Corticosteroids are contraindicated in acute infection. Drug interactions: Phenobarbital may induce hepatic enzymes, affecting metabolism of other drugs.

Key evidence and guidelines for canine distemper include:

- **Vaccination**: The World Small Animal Veterinary Association (WSAVA) guidelines recommend core vaccination for canine distemper using modified live vaccines (MLV) for all dogs. Puppies should receive a series of vaccines starting at 6-8 weeks, with boosters every 2-4 weeks until 16 weeks of age, and a booster at 1 year, then every 3 years. - **Diagnosis**: RT-PCR is the most sensitive diagnostic test, as shown in studies by Elia et al. (2006) and others. Immunofluorescence on conjunctival smears has lower sensitivity. - **Treatment**: No specific antiviral therapy is approved. A study by Greene (2012) in Ettinger's Textbook of Veterinary Internal Medicine emphasizes supportive care. A randomized controlled trial by Koutinas et al. (2004) evaluated the use of vitamin A and found no significant benefit, but a later study by Rikula et al. (2007) suggested a possible reduction in mortality. - **Prognosis**: A retrospective study by Amude et al. (2007) found that the presence of neurological signs and lymphopenia were negative prognostic indicators. - **Pathogenesis**: Research by von Messling et al. (2003) and others has elucidated the role of SLAM and nectin-4 receptors in viral entry and spread. - **Consensus Guidelines**: The ACVIM consensus statement on infectious diseases (2015) provides recommendations for diagnosis and management of canine distemper.

Overall, the evidence supports early diagnosis, aggressive supportive care, and prevention through vaccination.