further contributing to the spread of the virus in a newly exposed population. (Crawford, 2005) (American Veterinary Medical Association, 2015) Due to the possibility of asymptomatic infection and viral shedding, any dog exposed to canine influenza should be considered to be potentially contagious for 14 days. (Burrough et al., 2024) Isolation should be considered for exposed dogs, especially in multi-dog homes and other environments where the risk of transmission is high.
Canine influenza is not a widespread disease; therefore, many dogs have no immunity against infection with this virus. All dogs are considered to be susceptible to canine influenza virus, regardless of age, breed, or gender. Approximately 80% of infected dogs will show clinical signs when infected with canine influenza. Clinical signs associated with infection may vary from a mild cough to more severe cases of pneumonia requiring hospitalization. The remaining 20% of infected dogs will shed the virus without demonstrating any clinical signs of illness, thus
CLINICAL SIGNS
Some dogs with mild canine influenza infection may develop other clinical signs, which may include serous ocular or nasal discharge, sneezing, lethargy, low-grade fever, and/ or anorexia in addition to cough. (American Veterinary Medical Association, 2015) Purulent nasal discharge may also be observed; this is not due to the influenza infection itself, but instead is caused by secondary bacterial infection, usually involving Pasteurella and/or mycoplasma. (American Veterinary Medical Association, 2015) In more severe cases, canine influenza H3N2 has been associated with high fevers (104°F to 106°F) and the development of pneumonia. Thoracic radiographs in these severely affected cases may show evidence of consolidated lung lobes, indicating the presence of pneumonia. (American Veterinary Medical Association, 2015) This pneumonia is felt to be largely caused by secondary bacterial infection, although there are isolated reports of severe pneumonia in dogs with sterile bacterial cultures. (Crawford et al., 2010) Canine influenza virus-associated pneumonia can be life- threatening. In the 2015 Chicago outbreak, a mortality rate of approximately 5 in 1,000 dogs, or 0.05%, was observed. (Rishniw, 2015) The American Veterinary Medical Association currently reports an overall case fatality rate of 1-5% associated with canine influenza virus. (American Veterinary Medical Association, 2015) The most severe manifestations of canine influenza have been noted in racing greyhound facilities, where significantly higher mortality rates have been reported. (American Veterinary Medical Association, 2015) Often, greyhounds in these settings have died acutely. Necropsy of affected dogs often reveals evidence of fibrinous pleuritis, severe bronchopneumonia, and pulmonary/mediastinal/ pleural hemorrhage. This syndrome associated with canine influenza virus is not fully understood, but it is felt to be due to the unique environment and physical stresses placed upon racing greyhounds. Infection with Streptococcus equi zooepidemicus may also play a role in this condition. A similar syndrome has not been reported in pet dogs. (American Veterinary Medical Association, 2015)
Fortunately, many cases of canine influenza are mild or subclinical in nature. (Klivleyeva, 2022) These dogs may be completely asymptomatic, or they may only experience mild clinical signs. However, severe cases can and do occur. Canine influenza virus replicates in all of the cells of the respiratory tract, from the nasal lining down into the terminal airways of the lung. This viral replication can lead to significant neutrophilic and monocytic inflammation along the airways, resulting in rhinitis, tracheitis, bronchitis, and bronchiolitis. (Crawford, 2005) (American Veterinary Medical Association, 2015) In addition to the widespread inflammation that is found throughout the respiratory tract, airway epithelial cells begin to die due to viral infection. (Moyer, 2011) This exposes the underlying basement membrane and impairs mucociliary clearance, further perpetuating the effects of canine influenza infection. (Moyer, 2011) Accumulations of mucus and necrotic epithelial cells within the airways create an environment favorable for the growth of commensal and opportunistic bacteria, thus contributing to the secondary bacterial infections that can be associated with canine influenza infection. (Landolt, 2011) (American Veterinary Medical Association, 2015) Most dogs infected with canine influenza demonstrate mild signs of disease; their clinical signs are often indistinguishable from other common upper respiratory infections. The most common reported clinical signs of canine influenza infection is a cough. This cough is often reported to be identical to that seen with kennel cough or infectious tracheobronchitis. In some cases, however, the cough associated with canine influenza may be characterized as a moist or wet cough, in contrast to the dry cough that is often associated with infectious tracheobronchitis. (Klivleyeva, 2022) The cough associated with canine influenza typically persists for approximately 10 to 21 days, which is significantly longer than the typical clinical course of kennel cough. (Rishniw, 2015) (American Veterinary Medical Association, 2015) This persistent cough is regarded by some veterinarians as a helpful diagnostic feature when evaluating a dog for potential canine influenza infection.
DIAGNOSIS
lung consolidation. Radiographic disease may be noted unilaterally or bilaterally; however, the right middle lung lobe and caudal part of the left cranial lung lobe appear to be most commonly involved. (Crawford et al., 2010) (Crawford, 2005) If canine influenza is suspected early in the course of infection (within four days of the onset of clinical signs), nasal or pharyngeal swabs can be submitted for polymerase chain reaction (PCR) testing. Virus recovery rates have been noted to be higher with nasal swabs, so these samples are typically preferred over pharyngeal swabs. (Iowa State University, 2022) Once obtained, swabs can be placed into a red-top tube with a few drops of sterile saline (or in viral
When dogs with influenza initially present to the veterinary hospital for signs of upper respiratory disease, the first diagnostic testing to be performed will likely include a minimum database. In dogs with mild cases of influenza without secondary bacterial pneumonia, the complete blood cell count (CBC) and serum biochemistry analysis will typically show no abnormalities. (Crawford et al., 2010) (Crawford, 2005) If leukocytosis with neutrophilia and left shift is observed, the veterinarian should suspect pneumonia. In this case, thoracic radiographs should be performed. Radiographic findings may vary in these more severe cases, ranging from mild bronchointerstitial infiltrates to generalized
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