Canine Influenza

What is canine influenza?

Canine influenza is an enveloped RNA virus. Dogs are the only known susceptible species; there is no evidence at this time that infected dogs pose a risk to humans or other species. Greyhounds are the only breed thus far reported to develop hemorrhagic pneumonia and acute death following infection. There are no other known age or breed risk factors for infection, severe disease or death. Healthy, well vaccinated dogs of all ages may be affected.

Disease course (incubation and recovery)

The incubation period is 2-5 days from exposure to onset of clinical signs. Peak viral shedding occurs 2-4 days post-infection, meaning that dogs may be at their most infectious prior to showing signs of disease. This represents a slightly shorter incubation period than is usually seen with other common causes of canine respiratory disease. In experimentally and naturally infected dogs, viral shedding ceases by 7 days post infection. This relatively short shedding period is typical of influenza infection in other species. Although a percentage of dogs may be subclinically infected as described below, there is no true carrier state for canine influenza. The short shedding period and absence of a carrier state is helpful for shelters trying to minimize disease spread within the shelter and community – it is unlikely that dogs pose a significant infectious risk a week or more after infection.

Clinical signs

When canine influenza first strikes a given population, virtually 100% of dogs will become infected. However, up to ~ 20% will show no signs of disease. Therefore, all exposed dogs must be considered an infectious risk, whether or not they are showing signs of respiratory infection. In most dogs, signs of infection are similar to “kennel cough” from other causes, and may include:

* Mild low-grade fever
* Soft, moist (productive) or dry cough lasting 10-30 days
* Unresponsive to antibiotics or antitussives
* +/- thick, purulent/mildly bloody nasal discharge

After the first week or so of coughing, 10-20% of dogs may progress to a more severe form of infection, including:

* High fever (104-106 F)
* Pneumonia

Death (overall fatality rate varies and is between 1-5%)

Is it flu or something else?

There is no way to distinguish canine influenza from respiratory disease caused by other infections based on clinical signs alone. Ultimately, diagnostics should be performed to rule canine influenza in or out. Some factors which raise the suspicion of influenza include:

* History
o Recent introduction of a high risk dog into the shelter population, such as a dog transferred from a canine influenza endemic area, or recent admission of a dog from a boarding kennel or racing track
* Percentage of dogs affected
o Since no dogs will have immunity when influenza first strikes, it is likely that over half the dogs will be infected. If only a few dogs are sick – even if the clinical signs are consistent with flu – it is probably not canine influenza.
* All ages affected
o Puppies are often more severely affected in outbreaks of respiratory disease from other causes, but canine influenza can strike all ages
* Vaccinated dogs affected
o Vaccination provides good protection against canine distemper and moderate protection against other agents of canine respiratory disease. If a high percentage of well vaccinated dogs are severely affected, influenza is more likely
* Possibly more severe signs, more likely fever
* Possibly less responsive to antibiotics
o But secondary infections may respond to antibiotics
* Don’t forget co-infections
o Background disease will continue to occur in concert with canine influenza – so a diagnosis of canine distemper, Bordetella, etc. does not rule out canine influenza infection.

Diagnosis

At this time, confirmation of canine influenza relies on paired serum samples. The first sample should be taken within 7 days after onset of clinical signs, and the 2nd 2-3 weeks later. A 4 fold rise in titers equals infection. One positive convalescent sample confirms a history of exposure, but does not indicate whether infection was recent. PCR (polymerase chain reaction) testing is also available and can provide more rapid results, but false negatives may occur due to the short shedding period. PCR on oropharyngeal swabs is most likely to be accurate in the first 2-4 days after exposure (as soon as or even a little before clinical signs appear). Lungs and distal trachea from dogs that died of pneumonia can also be tested by PCR. For more information on diagnostic testing. Although a positive test is indicative of infection, canine influenza should not be ruled out based on negative PCR or viral isolation.

Prevention and control

Transmission

As noted, virus may be excreted for up to three days before clinical signs develop. So once disease is diagnosed, it will be necessary to count back several days to determine which dogs may be at risk from exposure. The virus is shed primarily in respiratory discharge and transmission can be via direct contact, fomites (contaminated objects such as hands, surfaces, clothing, etc.), droplet, and aerosol spread. In a few cases, dog handlers have carried virus home on clothing to infect their own dogs, so a change of clothes between work and home should be routine in any shelter in which canine flu is a concern (as it should be at all times, anyway). Although separate ventilation areas are ideal to control spread, some shelters and veterinary clinics have managed isolation in areas without separate air supply provided strict attention was paid to fomite control. See notes on aerosol transmission at the end of this document for further information.

Disinfection

Canine influenza persists < 1 week in the environment. It will be inactivated by most any commonly used disinfectant such as alcohol, bleach, quaternary ammonium compounds, and potassium peroxymonosulfate (e.g. Trifectant ®).

Vaccination

Although many researchers are actively working towards a canine influenza vaccine, there is none available as yet. The vaccine for equine influenza is not effective and should not be used in dogs. Of course, vaccination against the respiratory pathogens for which we have the option should be continued in order to minimize background disease (e.g. canine distemper, adenovirus-2, Bordetella bronchiseptica, parainfluenza).

Treatment

Although there is no specific treatment for canine influenza viral infection at this time, secondary bacterial infection may play a significant role and antibiotics are often indicated. Antibiotics commonly used for treatment of Bordetella bronchiseptica kennel cough, such as doxycycline, Clavamox, or Baytril, are generally not as effective for treatment of secondary infections associated with canine flu. Cephalosporins may be a good choice for treatment of the secondary infections associated with the milder form of disease. Remember that Bordetella is very frequently resistant to Cephalexin, so if a co-infection is suspected, treatment with multiple antibiotics may be indicated. The pneumonia associated with the severe form of disease can be caused by a variety of bacteria including Pasteurella multocida, Staphylococcus intermedius, Streptococcus canis, and Mycoplasma spp. Ideally, a transtracheal wash and culture and sensitivity testing should be performed to choose an antibiotic for treatment of severely ill dogs. If an empirical treatment choice must be made, good choices should include a combination of broad spectrum antibiotics such as a fluoroquinolone + penicillin, either orally if eating or parenterally if inappetant. Supportive care such as IV fluid therapy is of course helpful. Cough suppressants do not tend to be helpful and should be avoided in dogs with a productive cough. Anecdotally, a one-time injection of steroids was helpful in treating severely affected dogs in a shelter outbreak. The real efficacy of this treatment is unknown.