Canine influenza — dog flu — is a contagious respiratory disease caused by influenza A viruses that have adapted to dogs. Two strains circulate in North America: H3N8, recognized in racing greyhounds around 2004 and now rare, and H3N2, introduced during the 2015 Chicago outbreak and now the only strain actively circulating in the United States. Dog flu is NOT the same as kennel cough. Kennel cough (canine infectious respiratory disease complex, or CIRDC) is usually driven by Bordetella bronchiseptica and canine parainfluenza virus — a completely different virus from influenza A. The kennel cough overview covers the CIRDC complex in detail.

Canine influenza is highly contagious, and essentially every dog exposed is susceptible because the canine population lacks prior immunity to these viruses. The good news: most infected dogs recover over 2 to 3 weeks with supportive care, and the overall fatality rate is under 10 percent. Deaths are usually caused by secondary bacterial pneumonia, not the virus itself. The challenge is the shedding timeline: dogs infected with H3N2 can shed the virus for up to 24 days, which means an infected dog must be isolated for approximately 4 weeks to prevent transmission.

A vaccine is available and is classified as a lifestyle (non-core) recommendation for dogs that board, attend daycare, visit dog shows, or enter shelter environments. For a full breakdown of vaccine products and the outbreak landscape, see the dog flu vaccine options and outbreak data.

Is canine influenza the same as kennel cough or parainfluenza?

No. This is the single biggest point of confusion among dog owners, and it matters because the diseases are caused by different organisms, require different prevention strategies, and carry different risk profiles.

The three-way confusion

	Canine influenza (dog flu)	Canine parainfluenza	Kennel cough (CIRDC)
Cause	Influenza A virus (H3N8 or H3N2)	Canine parainfluenza virus (CPIV)	Complex: Bordetella, CPIV, adenovirus-2, and others
Virus family	Orthomyxoviridae (true influenza)	Paramyxoviridae (NOT influenza)	Multiple pathogens
Vaccine	Canine influenza vaccine (CIV) — lifestyle/non-core	CPIV is included in core DHPP vaccine	Bordetella vaccine — lifestyle/non-core
Severity	Moderate; can progress to pneumonia	Usually mild; contributes to CIRDC	Usually mild; rarely progresses
Shedding duration	H3N2 up to 24 days	Days to ~2 weeks	Variable by pathogen
Population immunity	None — novel virus in dogs	Widespread — part of core vaccination	Partial from vaccination

The critical distinction: canine parainfluenza virus (CPIV) is not influenza. It is a different virus entirely — same word, different biology. CPIV is a component of the CIRDC complex and is covered by the core DHPP (or DA2PP) vaccination that most dogs receive as puppies. The bordetella vaccine does not cover dog flu — it targets Bordetella bronchiseptica, another CIRDC pathogen. Neither the bordetella vaccine nor the core DHPP vaccine protects against canine influenza A.

The USDA APHIS veterinary biologics catalog confirms this separation at the product level: parainfluenza (CPIV) products are registered under different product codes as part of the CIRDC and bordetella combination vaccines, while true canine influenza A vaccines carry distinct product codes (15Q520 for the bivalent killed H3N2+H3N8 products and 15P5R0 for the recombinant H3N2 product). They are separate biological products because they target separate viruses.

What are the two strains of canine influenza, and which one is circulating?

H3N8 — the equine-origin strain

Canine influenza H3N8 was first identified in racing greyhounds in Florida in 2004. The virus originated in horses (equine influenza has been recognized for over 40 years) and jumped the species barrier into dogs. H3N8 spread through the US dog population over the following decade but never reached pandemic levels. By the mid-2020s, H3N8 has become rare and may no longer be actively circulating in the US dog population, though the possibility of sporadic reintroduction exists.

H3N8 characteristics:

• Shedding period: approximately 7 to 10 days
• Clinical signs similar to H3N2 but generally considered slightly less pathogenic in field conditions
• No sustained outbreaks reported in recent years

H3N2 — the avian-origin strain driving current outbreaks

Canine influenza H3N2 was first detected in the United States during a major outbreak in the Chicago area in February–March 2015 (Voorhees et al., 2017). The virus originated in birds in Southeast Asia and spread to dogs, likely through importation of infected dogs. The 2015 Chicago outbreak was the largest recorded canine influenza event in the US, with approximately 1,000 recognized infections and roughly 5 reported deaths. The virus subsequently spread to more than 30 US states.

H3N2 characteristics:

• Shedding period: up to 24 days (Cornell AHDC) — this is dramatically longer than H3N8 and is the basis for the approximately 4-week isolation recommendation
• Continues to cause sporadic outbreaks, particularly in congregate settings (boarding kennels, daycare facilities, shelters, dog shows)
• Many recent outbreaks have been linked to dogs imported from other countries where H3N2 is endemic
• H3N2 can occasionally infect cats — an important consideration in mixed-species households

H3N2 is currently the only canine influenza strain with confirmed active circulation in North America. Almost all public health advisories, outbreak reports, and vaccine recommendations are driven by H3N2.

What are the symptoms of dog flu, and how long does it last?

Incubation period

After exposure, the incubation period is approximately 2 to 4 days before clinical signs appear. However, dogs can begin shedding virus during this pre-symptomatic incubation period, which is a key reason outbreaks spread so quickly in congregate settings — infected dogs are contagious before they look sick.

Mild form (majority of cases)

Most dogs develop a mild to moderate respiratory illness:

• Persistent cough — often described as soft and moist, sometimes harsh; can last 10 to 21 days, which is notably longer than the typical 7 to 10 day kennel cough
• Nasal discharge — initially clear (serous), may become thick and yellow-green (mucopurulent) if secondary bacterial infection develops
• Sneezing
• Lethargy and reduced appetite — variable
• Low-grade fever — often 103°F to 104°F (39.4°C to 40°C)

Most dogs with the mild form recover within 2 to 3 weeks. The cough can persist for several weeks even after the dog is otherwise feeling better.

Severe form (minority of cases)

A smaller subset of infected dogs develop a more severe illness characterized by:

• High fever — above 104°F (40°C)
• Rapid, labored breathing — suggesting lower respiratory tract involvement
• Hemorrhagic pneumonia — in the most severe cases, H3N2 can cause viral pneumonia with hemorrhagic lung lesions, or secondary bacterial pneumonia can develop
• Dehydration from fever, poor appetite, and increased respiratory rate

Dogs with the severe form require veterinary intervention, potentially including hospitalization, intravenous fluid therapy, oxygen supplementation, and antibiotics for secondary bacterial infections. When dog flu turns into pneumonia, the outcome depends heavily on how quickly treatment is initiated.

Overall fatality

The overall canine influenza fatality rate is under 10 percent, and fatal outcomes are nearly always caused by secondary bacterial pneumonia rather than the virus itself. Most healthy adult dogs recover uneventfully. Puppies, senior dogs, brachycephalic breeds (flat-faced dogs with compromised airways), and immunocompromised dogs are at higher risk for complications.

How contagious is dog flu, and how long should a sick dog be isolated?

Canine influenza is highly contagious. Because the virus is relatively new in dogs and the canine population lacks prior immunity, essentially every exposed dog is susceptible. In congregate settings like boarding kennels, outbreaks can spread through an entire facility within days.

Transmission routes

• Direct contact — nose-to-nose contact, shared water bowls, shared toys
• Respiratory droplets — coughing and sneezing spread viral particles up to approximately 20 feet
• Fomites — the virus can survive on clothing, hands, leashes, and surfaces for hours. It survives on hard surfaces for up to 48 hours, on clothing for about 24 hours, and on hands for approximately 12 hours
• Pre-symptomatic shedding — dogs can transmit the virus during the 2 to 4 day incubation period before signs appear

Isolation timeline

The key isolation parameter is the shedding duration:

• H3N8: shedding lasts approximately 7 to 10 days
• H3N2: shedding lasts up to 24 days (documented by Cornell AHDC)

Because most current cases are H3N2, the conservative isolation recommendation is:

Isolate infected dogs from all other dogs for at least 21 to 28 days (approximately 4 weeks) from the onset of clinical signs.

This is longer than the isolation period for most respiratory infections and can be a significant logistical challenge for multi-dog households and boarding facilities. During isolation:

• Keep the infected dog completely separated from other dogs
• Use dedicated bowls, bedding, and toys that are not shared
• Change clothing and wash hands thoroughly after handling the infected dog before interacting with other animals
• Disinfect surfaces with standard household disinfectants (influenza A is an enveloped virus and is easily killed by soap, bleach solutions, and most common disinfectants — unlike the much hardier parvovirus)
• Be aware that H3N2 can occasionally infect cats — isolate from feline household members as well

How dangerous is canine influenza, and when does it become pneumonia?

The pneumonia threshold

The transition from a mild, self-limiting respiratory infection to life-threatening pneumonia is the critical clinical boundary. Warning signs that a dog with suspected canine influenza is developing pneumonia include:

• Rapid or labored breathing (respiratory rate above 40 breaths per minute at rest)
• Blue or pale gums (cyanosis) indicating poor oxygenation
• Persistent high fever (above 104°F / 40°C) despite supportive care
• Nasal discharge that becomes thick, green, or bloody
• Complete refusal to eat or drink
• Marked lethargy — the dog does not respond normally to stimuli

Any of these signs warrant immediate veterinary attention. Pneumonia in the context of canine influenza can be:

• Primary viral pneumonia — direct lung damage from the influenza virus, often with hemorrhagic lesions
• Secondary bacterial pneumonia — bacterial infection of lungs already compromised by the viral illness; this is the more common cause of fatal outcomes

Secondary bacterial pneumonia develops when the virus damages the respiratory epithelium, destroying the normal mucociliary clearance mechanism and allowing bacteria that are normally present in the upper airway to invade the lower respiratory tract. This is why most canine influenza deaths are attributed to bacterial pneumonia rather than the virus alone.

Risk factors for severe disease

• Puppies with immature immune systems
• Senior dogs with declining immune function
• Brachycephalic breeds with compromised airway anatomy
• Dogs with pre-existing respiratory disease (chronic bronchitis, tracheal collapse)
• Immunocompromised dogs (on corticosteroids, chemotherapy, or with immunosuppressive conditions)
• High viral exposure in overcrowded, poorly ventilated facilities

How is canine influenza diagnosed and treated?

Diagnosis

Canine influenza cannot be definitively diagnosed by clinical signs alone — the symptoms overlap significantly with kennel cough/CIRDC. Definitive diagnosis requires laboratory testing:

• PCR testing on nasal or pharyngeal swabs — most reliable during the first 3 to 5 days of illness, when viral shedding is highest. PCR identifies the specific influenza strain (H3N8 or H3N2).
• Serology (paired titers) — a fourfold rise in antibody titer between acute and convalescent samples (2 to 3 weeks apart) confirms recent infection. This is retrospective and not useful for guiding acute treatment decisions.
• Point-of-care rapid tests — available in some clinics but less sensitive than PCR.

In practice, many veterinarians treat suspected canine influenza cases empirically based on clinical signs and exposure history, reserving PCR confirmation for outbreak investigations, facility management decisions, or cases with public health implications.

Treatment

There is no specific antiviral treatment for canine influenza. Management is supportive:

• Rest and isolation — minimize activity and separate from other dogs for the full 4-week isolation period
• Hydration — ensure adequate fluid intake; intravenous fluids for dogs that are dehydrated or not drinking
• Nutrition — encourage eating; warming food may increase palatability for inappetent dogs
• Cough management — cough suppressants (such as hydrocodone) may be prescribed for comfort if the cough is non-productive and distressing, though they are used cautiously because coughing helps clear secretions
• Antibiotics — prescribed when secondary bacterial infection is suspected or confirmed (mucopurulent nasal discharge, fever not resolving, radiographic evidence of bacterial pneumonia). Antibiotics do not treat the virus but address the bacterial complication that causes most fatalities
• Anti-inflammatory support — NSAIDs may be used for fever and discomfort under veterinary supervision
• Oxygen therapy — for dogs with pneumonia and hypoxemia, often requiring hospitalization

Most dogs recover with supportive care alone. Hospitalization is reserved for dogs with pneumonia, severe dehydration, or high fever that does not respond to outpatient management.

Should my dog get the canine influenza vaccine, and which product?

The canine influenza vaccine is classified as non-core (lifestyle) by the AAHA vaccination guidelines. It is not recommended for every dog — it is recommended for dogs whose lifestyle puts them at higher exposure risk.

Which dogs should be vaccinated?

• Dogs that attend boarding kennels or daycare facilities
• Dogs that visit dog parks regularly
• Dogs that compete in dog shows, agility trials, or field events
• Dogs that enter shelters or rescue organizations
• Dogs that travel to areas with known outbreaks
• Dogs in households with frequent dog-to-dog contact (dog walkers, foster homes)

Many boarding facilities and daycare centers now require proof of CIV vaccination, just as they require the bordetella vaccine. If your facility requires the "dog flu shot," this is where canine influenza fits in the vaccine schedule.

USDA-licensed CIV vaccine products

As of the USDA APHIS veterinary biologics product export (July 25, 2026), the licensed canine influenza vaccine products include:

Bivalent killed H3N2 + H3N8 (product code 15Q520):

• Elanco TruCan Ultra CIV H3N2/H3N8 — a whole inactivated virus vaccine using PureFil purification technology; 0.5 mL dose
• Zoetis Vanguard CIV H3N2/H3N8 — a whole inactivated virus bivalent vaccine

Both bivalent products provide coverage against both strains with a two-dose initial series (2 to 4 weeks apart) followed by annual boosters.

Recombinant H3N2-only (product code 15P5R0):

• Merck/Intervet Nobivac NXT Canine Flu H3N2 — a self-amplifying RNA (saRNA) vaccine targeting the H3 hemagglutinin protein; stimulates both antibody and T-cell responses. Covers H3N2 only.

For a detailed comparison of these products, including the 2025 approval timeline and efficacy data, see the dog flu vaccine options and outbreak data.

What the vaccine does and does not do

The CIV vaccine does not provide sterilizing immunity — a vaccinated dog can still become infected. What it does:

• Reduces severity and duration of clinical signs
• Reduces viral shedding, lowering transmission risk
• Reduces the probability of progression to pneumonia
• Provides population-level protection in congregate settings by reducing overall viral load

The vaccine requires two doses for initial immunization, followed by annual boosters. Protection is not immediate — dogs are considered protected approximately 2 to 4 weeks after the second dose, which is important to factor in before travel or boarding.

Canine Influenza FAQs

Can my dog catch canine influenza from me, or give me the flu?

No. Canine influenza A (H3N8 and H3N2) viruses are adapted to dogs and are not known to infect humans. There have been no confirmed cases of dog-to-human canine influenza transmission. Similarly, human seasonal influenza viruses (H1N1, H3N2 — note the same subtype name, different virus lineage) do not infect dogs. The naming overlap between human H3N2 seasonal flu and canine H3N2 causes confusion, but these are different viruses that adapted to different host species.

For how canine influenza differs from avian H5N1, which is a different influenza A subtype with different zoonotic characteristics, the bird flu article covers the relevant distinctions.

Does the bordetella (kennel cough) vaccine protect against canine influenza?

No. The bordetella vaccine targets Bordetella bronchiseptica, a bacterium that is part of the CIRDC complex. The core DHPP vaccine covers canine parainfluenza virus (CPIV), which is a different virus from influenza A. Neither vaccine protects against canine influenza. A dog needs the specific CIV vaccine for canine influenza protection.

What is the difference between H3N8 and H3N2 dog flu?

H3N8 originated in horses and jumped to dogs around 2004; it is now rare or possibly no longer circulating in the US. H3N2 originated in birds and was introduced to the US in 2015; it is the only strain currently causing outbreaks. H3N2 has a longer shedding period (up to 24 days vs. about 7 to 10 days for H3N8) and can occasionally infect cats, which H3N8 generally does not.

My boarding facility requires the dog flu vaccine — what does that mean?

It means you need to have your dog vaccinated with the canine influenza vaccine (CIV) — not the bordetella vaccine or the core DHPP, which cover different pathogens. The CIV vaccine requires two initial doses 2 to 4 weeks apart, with annual boosters. Plan ahead: full protection takes about 2 to 4 weeks after the second dose, so start the vaccination series at least 4 to 6 weeks before boarding. Ask your facility whether they require coverage for both strains (bivalent) or H3N2 only.

Can cats get canine influenza?

Yes — H3N2 can occasionally infect cats, causing mild respiratory signs. H3N8 is not known to infect cats. In a household with both dogs and cats, an H3N2-infected dog should be isolated from cats as well as from other dogs during the shedding period.

How do I clean my home after a canine influenza outbreak?

Influenza A is an enveloped virus and is relatively fragile in the environment compared to parvovirus. It is easily killed by:

• Soap and water
• Dilute bleach solution (1:30)
• Standard household disinfectants (quaternary ammonium compounds, accelerated hydrogen peroxide)
• Heat and drying

The virus survives up to 48 hours on hard surfaces, about 24 hours on clothing, and approximately 12 hours on hands. After the isolation period, a thorough cleaning of the dog's living area, bedding, bowls, and toys is sufficient. For canine influenza vaccine safety reporting, the biologics adverse event article covers the reporting framework.

Sources

• Centers for Disease Control and Prevention (CDC) — About Dog Flu (Canine Influenza). cdc.gov
• American Veterinary Medical Association (AVMA) — Canine Influenza: Veterinary Resources. avma.org
• Cornell University Animal Health Diagnostic Center — Canine Influenza H3N2 Updates. cornell.edu
• Voorhees IEH, et al. Spread of Canine Influenza A(H3N2) Virus, United States. Emerg Infect Dis. 2017;23(12):1950-1957. ncbi.nlm.nih.gov
• USDA APHIS — Center for Veterinary Biologics, Licensed Products. aphis.usda.gov
• Merck Veterinary Manual — Canine Influenza (Flu). merckvetmanual.com