The signs of canine influenza virus (CIV) infection are mild in most dogs and are similar to those of canine cough.

Pet care professionals must have a high index of suspicion to ensure appropriate identification, management, and prevention of disease transmission.

Clinical Signs

The clinical signs of canine influenza virus (CIV) infection usually begin less than 5 days after infection and are very similar to those of infectious tracheobronchitis or canine cough due to other causes such as Bordetella bronchiseptica or other respiratory viruses (eg, parainfluenza, adenovirus, canine respiratory coronavirus, canine herpesvirus).¹¹ CIV cannot be distinguished from these other causes of acute respiratory disease based on clinical signs alone.^8,9 The course of canine influenza is mild in most dogs, but some dogs develop a more severe form with pneumonia and/or systemic illness.^6,11 Veterinarians and other pet care professionals must have a high index of suspicion to ensure appropriate identification, management, and prevention of disease transmission.

Mild, Uncomplicated Disease

The most common clinical presentation of CIV infection is a mild upper respiratory tract infection.⁶ The typical presentation is:⁶

• Lethargy
• Anorexia
• Low-grade fever
• Nasal discharge
• Dry, nonproductive cough (more common in mild disease) or soft, moist cough

The nasal discharge is typically clear initially but becomes mucopurulent rather quickly. The nasal discharge usually resolves with appropriate antibiotic treatment, suggesting that secondary bacterial infection is an important contributor to CIV infection.⁶ Coughing may last for several weeks, even up to 3 or 4 weeks, regardless of treatment with antibiotics and antitussive therapy.^2,6,11

Severe, Complicated Disease

Dogs at higher risk for a more severe form of canine influenza include those that are:¹¹

• In poor condition and/or are debilitated
• Concurrently exposed to other pathogens
• Very young or very old

However, it is important to note that even healthy dogs can develop more severe disease. In a clinical study of CIV involving experimentally infected dogs with no secondary bacterial or viral infections, every dog developed lung consolidation and lesions by the study’s end.³

Dogs with more severe disease usually present with:^6,8

• A high fever (104-106°F)
• An increased respiratory rate
• Other signs of pneumonia

In most cases, the severe course is attributed to secondary infection with bacteria or Mycoplasma that manifest as compromised pulmonary defenses.¹¹ The recovery from this form may be prolonged.²

Hemorrhagic pneumonia has been reported in the Greyhound population but is less commonly seen in the pet population. Based on data from an outbreak in racing Greyhounds in Iowa, the severe hemorrhagic form of CIV was complicated by coinfection with Streptococcus equi subsp. zooepidemicus.¹¹ Deaths may occur in dogs with more severe CIV disease. Deaths may be peracute and have also been reported to be associated with longer treatment periods.¹¹

When dogs with fatal disease are examined, they often show hemorrhages in the lungs, mediastinum, and pleural cavity. The lungs often appear dark red to black and may have signs of severe interstitial or bronchointerstitial pneumonia.⁶ Data from puppies that were experimentally infected with CIV showed that the puppies that were most severely affected had diffuse, small localized areas of hemorrhage in their lungs, but no evidence of hemorrhagic pneumonia.⁶

Arrows indicate lung consolidation.