Authored by: Becky Lundgren, DVM : from VeterinaryPartner on 6/18/15
The spring 2015 Chicago-based outbreak of canine influenza is a strain called H3N2, whereas this article is about a milder strain called H3N8. The vaccine for H3N8 is not thought to be effective against H3N2, which can cause a more severe case of influenza.
Influenza A virus in dogs (canine influenza virus, CIV, canine flu) is a respiratory tract disease that mimics bordetellosis (Bordetella bronchiseptica infection, kennel cough, infectious tracheobronchitis). However, unlike many cases of bordetellosis, the dog needs veterinary care.
Canine influenza is caused by a highly contagious virus that was identified in Florida in 2005 when it caused several severe respiratory outbreaks in racing greyhounds. The disease appears to occur most frequently in high-density dog populations: dogs who are housed with numerous other dogs in places such as shelters, boarding facilities, breeding kennels, pet stores, rescue groups, dog shows, and greyhound racing tracks. The disease is thought to have originated as a mutation of an influenza strain that affects horses and is not related to typical human influenza strains or the avian flu.
The virus is efficiently spread between dogs by aerosol, direct transmission, and fomite transmission of respiratory secretions (snot). A fomite is an object capable of carrying the organism; common fomites include toys, chew toys, bedding, etc. The actual virus persists for less than one week in the environment and is easily killed with bleach and other common disinfectants. No known transmission to humans has occurred.
There does not appear to be a carrier state of the disease, and no one knows how long immunity lasts after natural exposure. Influenza viruses can change over time, allowing them to evade host defenses, but whether this will happen with canine influenza is difficult to determine. All previously unexposed dogs are susceptible, regardless of age, sex, breed, or vaccination status, but the disease is more likely to become clinically apparent in dogs that are housed in populations such as animal shelters, boarding facilities or day care settings.
After infection, there is a 2-5 day incubation period. Nasal virus shedding peaks during this time. Clinical signs generally do not become apparent until day 5-7 and in most cases shedding wanes by 7-10 days after infection. Clinical signs are generally very mild to inapparent during peak viral shedding. A soft, moist, sometimes-productive cough is seen. The cough often persists for several weeks, even with appropriate therapy. Dogs may lose their appetite, develop a fever, and produce a pus-like nasal discharge. Up to 10% of dogs may develop a more severe form of illness, with high fever, lethargy (tiredness), rapid breathing, and secondary bronchopneumonia. The fatality rate related to pneumonia/bronchopneumonia is reported to be around 5-8% in selected high-risk populations. Acute, fatal hemorrhagic pneumonia tends to occur only in greyhounds. After day five, approximately 10-20% of affected dogs are have no symptoms but are still shedding infectious virus.
Distinguishing CIV from other causes of acute respiratory disease based on clinical signs can be difficult.
Diagnosis includes physical examination, blood tests looking for antibodies to the disease, virus isolation, thoracic radiographs, etc.
- Milder cases are often self-limiting, and may require only isolating the dog and providing supportive care, such as nutrition, rest, prevention of dehydration, and so on.
- With more severe signs, treatment is aimed at preventing secondary infection and is also largely supportive. Fluid support is important. Antimicrobials that target secondary bacterial infections are needed, and are ideally chosen based upon culture and sensitivity tests.
- It is generally recommended to NOT use human antiviral drugs that are neuraminidase inhibitors like Tamiflu. They require use early in the course of influenza infection when it is unlikely that disease will even have been noticed; there are no efficacy or safety studies done on animals; and there are potential resistance mechanisms that could develop with implications for human medicine.
Control of CIV relies primarily on reducing the spread of virus between dogs. Obviously, the best way to prevent widespread CIV is to prevent individual animal exposure. This is difficult, for several reasons. CIV is spread in part through aerosolized droplets and thus is difficult to contain. Also, some dogs may show obvious clinical signs while others can appear healthy but are still shedding infectious organisms.
Dogs that are in high risk categories, such as those scheduled for boarding or other group events, should be vaccinated against Bordetella bronchiseptica, parainfluenza, and adenovirus-2 at least 2 weeks prior to boarding, and vaccinating for CIV should also be considered. Although vaccination may not prevent infection, it tends to reduce the severity and duration of the disease. Your veterinarian is the best judge of which preventive measures your dog may need in your particular situation.