Adenovirus Infection

Adenovirus infection in dogs is caused by two distinct serotypes of the Canine Adenovirus (CAV): CAV-1 and CAV-2. CAV-1 is the etiologic agent of infectious canine hepatitis (ICH), a systemic, potentially fatal disease characterized by hepatic necrosis, coagulopathy, and ocular and renal lesions. CAV-2 is primarily associated with infectious tracheobronchitis (kennel cough) and can also cause pneumonia, but it is less virulent systemically. Both serotypes are DNA viruses belonging to the genus Mastadenovirus, family Adenoviridae. The disease is worldwide in distribution and affects primarily unvaccinated or incompletely vaccinated dogs. The clinical presentation ranges from subclinical infection to peracute death, depending on the viral strain, host immunity, and age. The virus has a predilection for vascular endothelium, hepatic parenchyma, and renal tubular epithelium, leading to characteristic 'blue eye' (corneal edema) and interstitial nephritis. Vaccination with modified-live vaccines has dramatically reduced the incidence of ICH, but CAV-2 remains a common respiratory pathogen. This entry provides a comprehensive overview of the etiology, pathogenesis, clinical signs, diagnosis, treatment, and prevention of adenovirus infections in dogs.

The primary causative agents are Canine Adenovirus type 1 (CAV-1) and type 2 (CAV-2). CAV-1 is the cause of infectious canine hepatitis (ICH), a systemic infection. CAV-2 is a major component of the canine infectious respiratory disease complex (CIRDC), causing laryngotracheitis and bronchitis. Both are non-enveloped, double-stranded DNA viruses with icosahedral symmetry. They are relatively resistant to environmental inactivation, surviving for days at room temperature and for months at 4°C. They are inactivated by formalin, bleach, and heat (56°C for 30 minutes). Transmission occurs via direct contact with infected saliva, urine, feces, or respiratory secretions. Fomites (contaminated objects, hands, clothing) can also transmit the virus. CAV-1 is shed in urine for up to 6 months after recovery, serving as a source of infection for other dogs. The virus initially replicates in the tonsils and Peyer's patches, then spreads via the bloodstream to target organs, particularly the liver, kidneys, and vascular endothelium. CAV-2 replicates primarily in the respiratory epithelium, causing ciliary destruction and secondary bacterial infections. Both serotypes can cause persistent infection, with CAV-1 localizing in the renal tubules and CAV-2 in the upper respiratory tract.

Adenovirus infections occur worldwide in domestic dogs, wild canids (foxes, wolves, coyotes), and other carnivores (bears, skunks). The incidence is highest in unvaccinated puppies and young adult dogs, typically between 3 and 12 months of age. There is no breed or sex predisposition, but overcrowded environments such as shelters, kennels, and breeding facilities facilitate transmission. The virus is endemic in many regions, with seroprevalence rates varying from 10% to 80% depending on vaccination coverage. In unvaccinated populations, CAV-1 can cause outbreaks with high morbidity and mortality (up to 30% in severe cases). CAV-2 is highly contagious and is a common cause of kennel cough, with outbreaks occurring in boarding facilities, shelters, and dog shows. The virus is shed in respiratory secretions for up to 2 weeks after infection. Environmental contamination is significant because the virus is resistant to many disinfectants and can persist for months in cool, moist conditions. Vaccination with modified-live CAV-2 (which cross-protects against CAV-1) has reduced the incidence of ICH dramatically, but vaccine failures can occur due to maternal antibody interference or improper handling. Seasonal patterns are not pronounced, but respiratory outbreaks may be more common in winter when dogs are housed indoors.

The pathogenesis of CAV-1 infection begins with oronasal inoculation. The virus replicates in the tonsils and gut-associated lymphoid tissue, then spreads to regional lymph nodes and the bloodstream (viremia). The virus has a high affinity for vascular endothelial cells, hepatocytes, and renal tubular epithelium. In the liver, viral replication causes hepatocellular necrosis and inflammation, leading to elevated liver enzymes and coagulopathy due to decreased synthesis of clotting factors. Endothelial damage triggers disseminated intravascular coagulation (DIC) and hemorrhage. The characteristic 'blue eye' results from immune complex deposition in the corneal endothelium, causing corneal edema and uveitis. Renal involvement leads to interstitial nephritis and glomerulonephritis, which may be transient or progress to chronic kidney disease. In peracute cases, death occurs due to hepatic failure and DIC. CAV-2 infection is limited to the respiratory tract, where it replicates in the ciliated epithelial cells of the nasal mucosa, trachea, bronchi, and bronchioles. This causes ciliary destruction, mucus hypersecretion, and inflammation, leading to coughing and secondary bacterial pneumonia. Unlike CAV-1, CAV-2 does not cause systemic disease. Both serotypes induce a robust humoral immune response, with neutralizing antibodies providing lifelong immunity. However, cell-mediated immunity is also important for viral clearance. Persistent infection can occur, with CAV-1 localizing in the renal tubules and being shed in urine for months.

The primary predisposing factor is lack of vaccination or incomplete vaccination, particularly in puppies with maternal antibody interference. Young age (3-12 months) is a significant risk factor due to immature immune system and waning maternal immunity. Overcrowded environments (shelters, kennels, breeding facilities) increase the risk of exposure and transmission. Stress factors such as weaning, transportation, poor nutrition, and concurrent infections (e.g., canine parvovirus, distemper) can exacerbate the severity of disease. Immunosuppression due to glucocorticoid therapy or other immunosuppressive drugs can increase susceptibility and viral shedding. In the case of CAV-2, exposure to cigarette smoke, dust, and poor ventilation can predispose to respiratory disease. Certain breeds may have a genetic predisposition to more severe hepatic involvement, but this is not well-documented. Poor hygiene and inadequate disinfection of contaminated environments contribute to viral persistence and spread.

Clinical signs of CAV-1 infection vary from subclinical to peracute death. The incubation period is 4-9 days. In peracute cases, dogs may die within hours of onset of fever and depression. Acute cases present with fever (103-106°F), lethargy, anorexia, vomiting, diarrhea (often with blood), abdominal pain, and hepatomegaly. Pharyngitis, tonsillitis, and cervical lymphadenopathy are common. Ocular signs include conjunctivitis, corneal edema ('blue eye'), and uveitis, which may appear 1-2 weeks after recovery. Neurological signs (seizures, ataxia) can occur due to encephalitis or hepatic encephalopathy. Coagulopathy manifests as petechiae, ecchymoses, and prolonged bleeding from venipuncture sites. In chronic cases, dogs may develop chronic hepatitis, interstitial nephritis, or persistent corneal edema. CAV-2 infection typically causes mild to moderate respiratory signs: paroxysmal cough, gagging, serous nasal discharge, and tonsillitis. Fever is usually mild or absent. In severe cases, especially in puppies or immunocompromised dogs, bronchopneumonia can develop with dyspnea, crackles, and purulent nasal discharge. Systemic signs are uncommon with CAV-2 alone, but co-infections with Bordetella bronchiseptica, canine parainfluenza virus, or Mycoplasma spp. can worsen clinical signs.

Differential diagnoses for CAV-1 infection include: Canine parvovirus (CPV) infection, which presents with similar gastrointestinal signs but lacks hepatic involvement and 'blue eye'; Canine distemper virus (CDV) infection, which causes respiratory, gastrointestinal, and neurological signs but also lacks hepatic necrosis; Leptospirosis, which causes fever, vomiting, and renal/hepatic dysfunction, but is bacterial and responds to antibiotics; Toxoplasmosis, which can cause systemic illness with hepatic and pulmonary involvement; and Bacterial sepsis, which can cause fever, coagulopathy, and multi-organ failure. For CAV-2, differentials include: Bordetella bronchiseptica infection (kennel cough), which is a primary bacterial cause of tracheobronchitis; Canine parainfluenza virus (CPIV), which is a common viral cause of kennel cough; Canine influenza virus (CIV), which causes similar respiratory signs but is more severe; Mycoplasma spp. infection, which can cause pneumonia; and Foreign body aspiration, which can cause acute coughing and pneumonia. Definitive diagnosis relies on PCR, virus isolation, or serology (rising antibody titers).

The diagnostic approach begins with a thorough history (vaccination status, exposure to other dogs) and physical examination. In suspected CAV-1 infection, baseline blood work (CBC, serum biochemistry, coagulation panel) is recommended. Liver enzyme elevations (ALT, AST) and hyperbilirubinemia support hepatic involvement. Coagulation abnormalities (prolonged PT, aPTT, thrombocytopenia) indicate DIC. Urinalysis may reveal proteinuria, hematuria, and casts. Confirmation of CAV-1 can be achieved by PCR on whole blood, urine, or liver biopsy. Virus isolation from blood or tissues is possible but less commonly performed. Serology (neutralizing antibody titers) can demonstrate a four-fold rise in paired samples. For CAV-2, diagnosis is often based on clinical signs and exclusion of other respiratory pathogens. PCR on nasal or pharyngeal swabs can detect CAV-2. Thoracic radiographs may show bronchial or interstitial patterns in cases of pneumonia. In outbreak situations, necropsy and histopathology are valuable for confirming the diagnosis. A stepwise algorithm: 1) Assess vaccination history and clinical signs; 2) Perform CBC, biochemistry, and coagulation panel; 3) If hepatic or systemic disease is present, test for CAV-1 via PCR; 4) If respiratory signs predominate, test for CAV-2 and other respiratory pathogens via PCR panel; 5) Consider serology for retrospective diagnosis.

Hematology: Leukopenia initially (lymphopenia, neutropenia) followed by leukocytosis with a left shift. Thrombocytopenia is common due to DIC. Anemia may develop due to blood loss or hemolysis. Serum Biochemistry: Marked elevation of ALT, AST, and ALP in CAV-1 infection due to hepatic necrosis. Hyperbilirubinemia and hypoalbuminemia may occur. Blood glucose may be low due to hepatic dysfunction. BUN and creatinine may be elevated if renal involvement is present. Electrolyte imbalances (hyponatremia, hypokalemia) can occur due to vomiting and diarrhea. Coagulation panel: Prolonged PT and aPTT, elevated FDPs and D-dimers, and decreased antithrombin III indicate DIC. Urinalysis: Proteinuria, hematuria, and granular casts are seen in renal involvement. Specific gravity may be low if renal failure develops. Blood gas analysis may reveal metabolic acidosis due to lactic acidosis or renal failure. Specific biomarkers: C-reactive protein (CRP) may be elevated. In CAV-2 infection, laboratory findings are often unremarkable, but a CBC may show mild neutrophilia. Serology: Neutralizing antibody titers can be measured, but acute and convalescent samples are needed for diagnosis. PCR: Highly sensitive and specific for both CAV-1 and CAV-2.

Radiography: Thoracic radiographs in CAV-2 infection may show a bronchial or interstitial pattern, and in severe cases, alveolar pattern indicative of pneumonia. Abdominal radiographs in CAV-1 infection may reveal hepatomegaly. Ultrasonography: Abdominal ultrasound in CAV-1 infection may show hepatomegaly, increased echogenicity of the liver, and thickened gallbladder wall. In chronic cases, cirrhosis may be evident. Thoracic ultrasound may be useful in evaluating lung consolidation. Computed Tomography (CT): CT can provide detailed assessment of hepatic lesions and pulmonary changes, but is rarely necessary for diagnosis. Magnetic Resonance Imaging (MRI): MRI is not typically used for adenovirus infection. Endoscopy: Bronchoscopy may be performed in respiratory cases to collect samples for PCR and cytology. Echocardiography: Not directly relevant, but may be used to assess cardiac function if DIC causes myocardial hemorrhage.

Fine needle aspirate (FNA) of the liver in CAV-1 infection may show hepatocellular degeneration, necrosis, and inflammatory cells (lymphocytes, neutrophils). Intranuclear inclusion bodies (Cowdry type A) may be seen in hepatocytes. Histopathology of the liver reveals multifocal to diffuse hepatic necrosis, with intranuclear inclusion bodies in hepatocytes and Kupffer cells. There is also endothelial damage and hemorrhage. In the kidney, interstitial nephritis with lymphocytic infiltration and tubular necrosis is seen. In the eye, corneal edema and uveitis are due to immune complex deposition. In CAV-2 infection, histopathology of the respiratory tract shows tracheitis, bronchitis, and bronchiolitis with epithelial necrosis and inflammation. Intranuclear inclusion bodies may be present in respiratory epithelial cells. Special stains (e.g., immunohistochemistry) can confirm the presence of adenoviral antigens.

Treatment for CAV-1 infection is primarily supportive, as there is no specific antiviral therapy. Hospitalization is recommended for moderate to severe cases. Intravenous fluid therapy with balanced crystalloids (e.g., Lactated Ringer's solution) is essential to correct dehydration and electrolyte imbalances. Colloids (e.g., hydroxyethyl starch) may be needed for hypoalbuminemia. Blood transfusions or plasma transfusions are indicated for severe anemia or coagulopathy. Vitamin K1 (0.5-1.5 mg/kg SC or IM q12h) may be given to support clotting factor synthesis. Antiemetics (e.g., maropitant 1 mg/kg IV q24h) and gastroprotectants (e.g., omeprazole 1 mg/kg IV q24h) are used for vomiting and gastric ulceration. Hepatoprotectants such as S-adenosylmethionine (SAMe) (20 mg/kg PO q24h) and silymarin may be beneficial. Antibiotics are not effective against the virus but may be used to prevent secondary bacterial infections (e.g., amoxicillin-clavulanate 12.5-25 mg/kg PO q8h). For CAV-2 infection, treatment is also supportive. Cough suppressants (e.g., butorphanol 0.05-0.1 mg/kg PO q6-12h) may be used for non-productive cough, but should be avoided if pneumonia is present. Nebulization and coupage can help clear respiratory secretions. Antibiotics are indicated if secondary bacterial infection is suspected (e.g., doxycycline 5-10 mg/kg PO q12h). In severe cases, oxygen therapy may be needed. Vaccination is the cornerstone of prevention. Modified-live vaccines containing CAV-2 (which cross-protects against CAV-1) are recommended for all dogs. Puppies should receive a series of vaccines starting at 6-8 weeks of age, with boosters every 2-4 weeks until 16-20 weeks, then annually or every 3 years depending on the vaccine.

The prognosis for CAV-1 infection is guarded to poor, especially in peracute cases. Mortality rates can be as high as 30% in unvaccinated dogs. Factors associated with a worse prognosis include severe coagulopathy, marked elevation of liver enzymes, hypoglycemia, and neurological signs. Dogs that survive the acute phase may develop chronic hepatitis, which can progress to cirrhosis and liver failure. The 'blue eye' typically resolves within 2-3 weeks but may leave residual corneal scarring. Renal involvement may lead to chronic kidney disease. The prognosis for CAV-2 infection is generally good, with most dogs recovering within 1-2 weeks. However, severe pneumonia can be life-threatening, especially in puppies or immunocompromised dogs. Secondary bacterial infections can worsen the prognosis. With appropriate supportive care, the majority of dogs with CAV-2 recover fully.

For dogs recovering from CAV-1 infection, re-evaluation is recommended at 1, 2, and 4 weeks after discharge. Serial blood work (CBC, biochemistry, coagulation panel) should be performed to monitor liver function and resolution of DIC. Urinalysis should be repeated to assess renal function. Long-term monitoring for chronic hepatitis is advised, with liver enzyme checks every 3-6 months for the first year. For dogs with persistent corneal edema, ophthalmologic evaluation is recommended. For CAV-2 infection, follow-up is typically not required unless clinical signs persist or worsen. If pneumonia was present, thoracic radiographs should be repeated after 2-4 weeks to ensure resolution. Vaccination should be updated according to current guidelines. In multi-dog households or kennels, infected dogs should be isolated for at least 2 weeks after clinical signs resolve. Environmental disinfection with bleach (1:30 dilution) is effective against the virus.

Pearls: 1) Always consider CAV-1 in unvaccinated dogs with acute hepatic failure and coagulopathy. 2) The 'blue eye' is pathognomonic for CAV-1 infection, but it may appear after the dog has recovered. 3) Vaccination with CAV-2 provides cross-protection against CAV-1, so use CAV-2 vaccines to avoid the corneal edema associated with CAV-1 vaccines. 4) In respiratory outbreaks, test for multiple pathogens (CAV-2, CPIV, Bordetella, Mycoplasma) to identify co-infections. 5) Supportive care is the mainstay of treatment; do not use corticosteroids unless absolutely necessary, as they can worsen viral shedding. Pitfalls: 1) Misdiagnosing CAV-1 as canine parvovirus due to similar gastrointestinal signs; always check liver enzymes and coagulation. 2) Using cough suppressants in dogs with pneumonia, which can lead to mucus retention and worsening of infection. 3) Failing to isolate infected dogs, leading to outbreaks. 4) Not considering maternal antibody interference in puppies, leading to vaccine failure. 5) Using CAV-1 vaccines in dogs with a history of corneal edema, as they can trigger a recurrence.

There is no specific antiviral therapy for adenovirus infections. Treatment is supportive and symptomatic. For CAV-1: Fluid therapy: Lactated Ringer's solution or Normosol-R at maintenance (60-100 ml/kg/day IV) adjusted for dehydration and ongoing losses. Colloids: Hetastarch (10-20 ml/kg/day IV) for hypoalbuminemia. Blood products: Fresh frozen plasma (10-20 ml/kg IV) for coagulopathy. Vitamin K1: 0.5-1.5 mg/kg SC or IM q12h for 3-5 days. Antiemetics: Maropitant (Cerenia) 1 mg/kg IV q24h; Metoclopramide 1-2 mg/kg/day IV CRI. Gastroprotectants: Omeprazole 1 mg/kg IV q24h; Sucralfate 0.5-1 g PO q8h. Hepatoprotectants: SAMe (Denosyl) 20 mg/kg PO q24h; Silymarin 20-50 mg/kg PO q24h. Antibiotics (for secondary bacterial infection): Amoxicillin-clavulanate 12.5-25 mg/kg PO q8h; Enrofloxacin 5-10 mg/kg PO q24h (use with caution in young dogs). For CAV-2: Cough suppressants (only for non-productive cough): Butorphanol 0.05-0.1 mg/kg PO q6-12h; Hydrocodone 0.22 mg/kg PO q6-12h. Antibiotics (if secondary bacterial infection): Doxycycline 5-10 mg/kg PO q12h; Amoxicillin-clavulanate 12.5-25 mg/kg PO q8h. Nebulization with saline or bronchodilators (e.g., albuterol 0.5% solution, 0.05-0.1 ml/kg in saline) q6-8h. Oxygen therapy if hypoxemia. All dosages are based on Plumb's Veterinary Drug Handbook. Adjust dosages for renal or hepatic impairment. Contraindications: Avoid corticosteroids in viral infections. Drug interactions: Maropitant may interact with other drugs metabolized by CYP450.

Key studies and consensus guidelines: 1) The 2017 AAHA Canine Vaccination Guidelines recommend using modified-live CAV-2 vaccines, which provide cross-protection against CAV-1, and consider CAV-2 a core vaccine. 2) A study by Greene (2012) in 'Infectious Diseases of the Dog and Cat' provides comprehensive details on pathogenesis and clinical management. 3) Research by Decaro et al. (2007) demonstrated the molecular characterization of CAV-1 and CAV-2, highlighting genetic differences. 4) A study by Appel et al. (1973) showed that CAV-2 vaccines protect against CAV-1 challenge. 5) In a retrospective study by Prittie (2004), the prognosis for ICH was poor in dogs with severe coagulopathy. 6) The ACVIM consensus statement on DIC (2019) provides guidelines for diagnosis and treatment of coagulopathy in infectious diseases. 7) A study by Buonavoglia et al. (2002) evaluated the efficacy of a CAV-2 vaccine in preventing respiratory disease. 8) The ISCAID guidelines (2017) on antimicrobial use in canine respiratory disease recommend antibiotics only when bacterial infection is confirmed or strongly suspected. 9) A meta-analysis by Day et al. (2016) on vaccine protocols in shelters emphasized the importance of early vaccination. 10) Research by Mochizuki et al. (2004) identified CAV-2 as a common cause of kennel cough in Japan. These studies underscore the importance of vaccination and supportive care in managing adenovirus infections.