Canine Influenza

8 minutes

Recently, outbreaks of canine influenza across the United States and other parts of the world have become a growing concern for veterinarians. With its rapid transmission and potential for severe respiratory complications, canine influenza can affect dogs of all breeds and ages.  

Also known as ‘dog flu’, canine influenza is a highly contagious respiratory infection caused by two specific type A influenza viruses known to infect dogs. One is the H3N8 virus. The other is the H3N2 virus. 

In the US, canine flu is still considered an emerging disease. Most dogs lack immunity and are susceptible to infection once exposed to the virus. Virtually all dogs exposed to the virus become infected, and nearly 80% show clinical signs of disease, though most exhibit the mild form.  

The heterogeneity of zoonotic canine influenza viruses (CIVs) through genetic reassortment and the point mutations typical of RNA viruses and their capacity to cross the species barrier continuously have always been an important concern. 

As new strains may emerge and diagnostic and treatment protocols evolve, veterinarians must stay abreast of the latest developments for a proactive approach and continuous learning.   

Related: Canine Influenza 

Understanding canine influenza 

Canine influenza virus (CIV) is a highly contagious respiratory infection that affects dogs, causing symptoms similar to those of human flu. Two distinct strains cause this influenza: H3N8 and H3N2. H3N8, originally an equine influenza virus, infected canines in the United States around 2004, rapidly spreading among susceptible dog populations. H3N2, believed to have originated from avian influenza in Asia, was first identified in the U.S. in 2015. These two strains have since posed ongoing challenges for veterinarians in many regions. 

Like other influenza viruses, the canine influenza virus continues to evolve through genetic mutations, raising concerns about the potential emergence of new variants. These mutations could impact the sensitivity of diagnostic tests and the effectiveness of current vaccines, requiring continuous monitoring and updates in preventive strategies. 

A dog’s lifestyle determines the risk of that dog’s exposure to the canine influenza virus. Dogs that are frequently or regularly exposed to other dogs—for example, at boarding or day care facilities, dog parks, grooming salons, or social events with other dogs present—are at greater risk of encountering the virus. While the virus is rarely fatal, severe cases can lead to pneumonia, particularly in puppies, elderly dogs, and immunocompromised pets. 

Symptoms 

Canine influenza virus infections can manifest in two different clinical syndromes: a mild type and a more severe version that often includes pneumonia. 

Mild Form Severe Form 
– A soft, moist cough that persists for 10 to 30 days 
– Lethargy Fever 
– Inappetence 
– Sneezing 
– Ocular and/or nasal discharge (may be thick when secondary bacterial infection is present) 
– Some dogs have a dry cough like the traditional “kennel cough” 		– High fever (104ºF to 106ºF) 
– Lethargy 
– Anorexia 
– Pneumonia symptoms: increased respiratory rate, difficulty breathing, labored or rapid breathing, wheezing, productive cough, and nasal discharge 
– Poor exercise tolerance and cyanotic gums may be manifested 

The signs associated with most influenza virus infections, regardless of the H subtype, are not pathognomonic for an influenza virus infection. The onset of clinical signs is usually rapid, with incubation periods in natural settings of 2 to 3 days being common. 

Routes of transmission 

The primary transmission routes of canine influenza include respiratory secretions from coughing, barking, or sneezing and contaminated surfaces, hands, and clothing, making outbreaks difficult to control in high-density dog populations. Unlike human influenza, dog flu is not seasonal and can occur at any time of the year.  

Diagnosing canine influenza 

The success of laboratory testing for canine respiratory pathogens, particularly viruses like influenza, is highly dependent on the timing of specimen collection. Due to the short duration of viral shedding, often ceasing within seven days post-infection, samples for agent detection should be collected at the animal’s initial presentation. Delaying collection based on antibiotic response is not recommended. It will likely result in negative test results, even if a viral infection was present. 

Advancements in diagnostic techniques 

The accurate and timely identification of canine influenza virus (CIV) is essential for effective disease management and containment of an outbreak. Diagnostic tools have evolved over the years, strengthening the capacity of veterinarians to swiftly identify and verify CIV infections. 

Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) test 

RT-PCR is a traditional diagnostic method for canine influenza. It continues to be the most reliable method for diagnosing canine influenza. It detects viral RNA, providing rapid and accurate identification of active infections. Recent advancements have significantly improved PCR testing. Faster turnaround times allow for quicker diagnoses, enabling timely treatment and isolation. Increased sensitivity enhances the ability to detect even low viral loads, crucial for early-stage infections. 

The initial step is to determine whether an influenza virus is present in the clinical case. If the result is positive, additional tests can then identify the specific subtype. This approach allows for the detection of various influenza strains affecting dogs. 

The recommended sample for testing is a nasal swab, which can be collected using either cotton or Dacron swabs. Since RT-PCR does not require live virus for accurate detection, the choice of transport medium is not as vital. However, bacterial transport media that have not been validated for RT-PCR should be avoided. To preserve sample integrity, a few drops of saline are sufficient to keep the swab moist during transport. 

Serology 

Serology measures antibody levels, indicating past exposure or vaccine response. In the United States, the hemagglutination inhibition (HI) test is the standard test used for serologic determinations of CIV. The test has high sensitivity because it can detect antibody responses in dogs as early as 7 days post-infection. Antibodies to CIV develop rapidly, and by 10 days post-infection, there is a significant antibody titer. In the absence of a history of vaccination, the presence of CIV antibodies following a clinical illness is highly correlated with CIV being part of the clinical event. 

Serological testing also plays a vital role in epidemiological surveillance, tracking the spread of CIV and identifying at-risk populations. Also, next-generation sequencing is increasingly used to characterize viral strains, identify genetic mutations, and track the evolution of CIV. This provides significant insights for vaccine development and outbreak management. 

Prevention and treatment 

The treatment protocol for canine influenza focuses on supportive care. It includes rest, fluids, and potentially antibiotics for secondary bacterial infections, as there’s no specific antiviral treatment for the virus itself. Antiviral therapy (oseltamivir) is neither approved nor recommended for use in dogs with CIV. 

Vaccinations for canine influenza 

Inactivated vaccines are available for CIV. Vaccination is classified as risk-based by the American Animal Hospital Association. It should be considered based on exposure risk, such as dogs who are boarded, travel, participate in dog shows, or visit dog parks frequently. Given the potential for co-infection and increased respiratory disease severity, dogs with risk factors that necessitate non-core Bordetella bronchiseptica or parainfluenza vaccination should be considered for CIV vaccination as well. 

All offered products contain killed vaccines. They are specific for the H3N8 or the H3N2 strain of influenza A and can be administered separately. A vaccine combining the two virus strains into a single vaccine is now also available.  

Two doses of the vaccine are necessary to develop an effective immune response. Animals previously administered CIV vaccines should have detectable antibodies to the variants of the virus in the administered vaccine(s). Therefore, it is important to include vaccination history when requesting CIV HI antibody testing for dogs with respiratory illness. 

Prevention 

Dogs infected with CIV shed the virus through nasal and respiratory secretions. This makes it highly contagious among dogs in close contact. While viral shedding is most intense during the first four days after infection, intermittent and prolonged shedding can occur.  

Biosecurity and isolation protocols recommended by AVMA include the following: 

  • Isolate dogs with suspected or confirmed canine influenza, as well as those exposed to the virus. Continue this isolation for at least four weeks from the onset of illness to prevent further transmission. 
  • Ensure that dogs with known or suspected infections stay at least 20 feet away from other dogs and cats. This can help prevent the spread of the virus. 
  • Have infected or suspected dogs enter and exit the hospital through an alternate entrance, rather than the main door. Place them in a designated room away from other animals, ideally one with a separate air supply. 
  • Wear personal protective equipment, such as gowns, booties, and disposable gloves. 
  • Thoroughly clean and disinfect all areas where veterinarians examine and treat potentially infected dogs. Clean and disinfect any instruments used, to prevent contamination and the spread of the virus. 
  • Due to the potential for transmission of infectious agents, manage non-critical canine cases on an outpatient basis. This helps to protect all hospitalized animals, including felines, from unnecessary exposure. 
  • Adhere to proper hand hygiene protocols. 

References 

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Editor’s note: this post was originally published on and updated on .

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