Canine influenza is a newly emerging viral respiratory disease endemic to the US and parts of Asia, with outbreaks documented in Canada and the UK.
The majority of cases present with mild clinical signs: pyrexia and a persistent cough lasting to 2-3 weeks. Inappetence, lethargy, and a purulent nasal discharge due to secondary bacterial infection may also be seen. Up to 8% of dogs develop severe disease with pneumonia and respiratory distress, which may result in mortality. Infection can also be asymptomatic.
As new outbreaks and strains of this highly contagious disease appear, it is crucial that veterinarians maintain up-to-date knowledge of control methods. Here, we provide a practical guide to preventive strategies and the effective use of canine influenza vaccines.
History and epidemiology of canine influenza
There are currently two strains of canine influenza virus (CIV), both of the influenza A group:
- H3N8 CIV was first identified in a population of racing greyhounds in Florida, 2004, and has since established endemic status in almost all US states. It is believed that an H3N8 equine influenza virus first infected dogs in the late 1990s and adapted into a dog-to-dog pathogen.
- H3N2 CIV appeared in the US in 2015. Previously only seen in Asia and thought to be due to importation of infected dogs, this avian origin strain rapidly spread across the US. In 2018, H3N2 CIV was confirmed in Ontario, Canada, causing focal outbreaks throughout the year which appear to have been contained.
Canine influenza outbreaks occur year-round with no seasonality. As a novel pathogen in the US and Canadian canine populations, almost all dogs of all ages are immunologically naïve to the virus.
Prevention protocols for canine influenza
Despite widespread canine susceptibility to infection, CIV outbreaks are most commonly (though not exclusively) seen where large numbers of dogs reside, such as in shelters or boarding kennels.
CIV is spread through aerosolised respiratory secretions, tactile nose/mouth contact, and contaminated objects or surfaces such as veterinary tables and supplies. Influenza A viruses can survive for up to 48 hours in the environment, depending on the fomite. As an enveloped virus, CIV is vulnerable to common disinfectants including quaternary ammonium compounds, phenols, bleaches and aldehydes.
At-risk establishments should practice consistent infection control including regular disinfection of surfaces, bedding and bows between each new boarding dog, and routine staff handwashing. Care should be taken in mixed species premises, as H3N2 CIV can also infect cats.
In veterinary practices, dogs suffering with suspected or confirmed CIV infection may be hospitalised. Here, strict isolation protocols and PPE should be used during possible virus shedding. Dogs shed virus in the incubation period as well as after the onset of clinical signs. H3N8 CIV can be shed for up to 10 days post-onset, and H3N2 CIV for just over 3 weeks. Thus infected dogs should be quarantined for a minimum of 21 days where logistically possible.
What canine influenza vaccines are available?
The most effective way to protect against canine influenza is through vaccination.
Merck Animal Health offers a bivalent Nobivac® vaccine for both H3N8 and H3N2 CIV, as well as monovalent versions covering the strains separately. Zoetis Inc. offers the same three vaccines in its Vanguard® range.
How do the vaccines work?
CIV vaccines are killed/inactivated and contain an adjuvant. The exact immunological correlates of protection are not known. In humans vaccinated against influenza A viruses, protection is generally conferred from anti-hemagglutinin humoral immune responses. In the elderly, protection is at least in part conferred from cellular immunity.
What is the efficacy of canine influenza vaccination?
CIV vaccines do not always prevent infection. However, vaccinated dogs that become infected typically have reduced duration and severity of clinical signs. In addition, less virus is shed, and for a shorter period of time.
Which dogs should be vaccinated?
CIV vaccines are non-core vaccines given according to lifestyle. Dogs expected to mix with large numbers of other dogs are particularly at risk, overlapping with populations at risk from other respiratory pathogens such as Bordetella bronchiseptica.
In the US, and in central and southern Ontario*, dogs in the following settings should be considered for CIV vaccination:
- Boarding kennels
- Grooming salons
- Competitive sporting dogs
- Dog shows
- Known local geographical epicenters with a high incidence of cases
- Travelling dogs (especially those going to endemic areas)
Vaccination may also be considered for dogs at a higher risk of disease complications, i.e. brachycephalics, geriatrics, and those with underlying structural cardiopulmonary abnormalities.
*In Canada, the current situation is highly dynamic and may change at any time if outbreaks arise in new areas or if endemicity establishes.
Which CIV strains should be vaccinated against?
As both H3N8 and H3N2 are present in the US, it is recommended to vaccinate for both strains. The easiest way to do this is with a bivalent vaccine. It should be assumed that monovalent CIV vaccines offer no cross-strain protection.
What is the canine influenza vaccine schedule?
Nobivac®: initial cover with two subcutaneous doses 2-4 weeks apart in healthy dogs ≥6 weeks of age (monovalent vaccines) or ≥7 weeks of age (bivalent vaccine), with single annual booster doses if still considered at-risk.
Vanguard®: initial cover with two subcutaneous doses 3 weeks apart in healthy dogs ≥8 weeks of age (for both monovalent and bivalent vaccines), with single annual booster doses if still considered at-risk.
Dogs are only considered protected 2 weeks after the second initial dose, regardless of age. Dogs prone to re-location (e.g. in rescue shelters) should be diligently followed up for their second dose.
What if a dog misses or is delayed in receiving a dose
Initial course: a maximum of 6 weeks can pass between the first and second CIV vaccine doses, at which point the initial course must be restarted.
Booster: the annual canine influenza vaccine booster can lapse up to a maximum of 18 months after the last vaccination, at which point an initial course of two doses must be restarted.
How do I phase patients with a history of monovalent vaccination into bivalent cover?
The simplest way is to give an initial two-dose course of the bivalent vaccine, regardless of if the dog is still covered for one strain from monovalent vaccination.
How should at-risk dogs with an unknown or incomplete history be vaccinated?
These dogs should be treated as if immunologically naïve and receive an initial two-dose course of CIV vaccine.
What side effects or adverse events have been reported?
All available CIV vaccines have a good track record for safety. A field study for Nobivac® bivalent CIV vaccine showed safety in 300 dogs. In a study using the Vanguard® H3N8 vaccine in 691 dogs, no serious adverse events were reported. 4% of dogs showed minor discomfort.
Up to ten days post-vaccination, 1.3% of dogs showed minor adverse events considered typical for vaccination.
Do canine influenza vaccines carry any contraindications?
It is not recommended to vaccinate pregnant bitches due to risk of fetal injury. This risk may be outweighed for diseases with significant risk of mortality, though the AAHA do not include CIV in this list.
Dogs receiving immunosuppressive treatments or chemotherapy should not receive the canine influenza vaccine.
Does vaccination cause live virus shedding?
All available CIV vaccines are killed/inactivated, with no risk of shedding.
In the event of infection, does previous vaccination interfere with diagnosis by serology?
Canine influenza can be diagnosed with a ≥4-fold temporal increase in antibody titre between samples in acute infection vs 3-4 weeks later, with suggestive clinical signs.
However, serology is unable to distinguish between antibodies present from vaccination vs natural infection, so other methods may need to be considered. This should be discussed with your local diagnostic lab.
Can cats be vaccinated against H3N2 CIV?
There are no CIV vaccines licensed for use in cats.
Canine influenza is a recently emerged, profoundly contagious disease. In places where dogs are found in high density numbers, infection control protocols should be vigilantly applied.
This, combined with prophylactic vaccine cover against both H3N8 and H3N2 strains for at-risk canine populations, is the most effective way to prevent infection and decrease severity of disease.
Sources and additional reading
- Canine influenza fact sheet. Center for Food Security and Public Health, Iowa State University, 2016 [Online]. Available at : http://www.cfsph.iastate.edu/Factsheets/pdfs/canine_influenza.pdf
- Newbury S, Godhardt-Cooper J, Poulsen KP, et al. Prolonged intermittent virus shedding during an outbreak of canine influenza A H3N2 virus infection in dogs in three Chicago area shelters: 16 cases (March to May 2015). Journal of the American Veterinary Medical Association 2016. 248: 1022-1026.
- Plotkin SA. Correlates of protection induced by vaccination. Clinical and Vaccine Immunology 2010. 17:1055-1065.
- Canine vaccination guidelines. The American Animal Hospital Association, 2017 [Online]. Available at: https://www.aaha.org/public_documents/guidelines/vaccination_recommendation_for_general_practice_table.pdf
- Nobivac® Vaccine Premier Promise. Merck Animal Health, 2015 [Online]. Available at: https://www.henryscheinvet.com/docs/default-source/landing-pages/companion-vaccinations/nobivac-guarantee-march-2018.pdf
- Canine vaccination guidelines. The American Animal Hospital Association, 2011 [Online]. Available at: https://www.aaha.org/public_documents/professional/guidelines/caninevaccineguidelines.pdf