Clinical Signs & Diagnosis of Canine Influenza

Although dogs were typically considered to be refractory to infection with influenza A viruses, two canine influenza virus (CIV) subtypes—H3N8 and H3N2—have emerged in the past 12 years. Studies have shown that H3N8 CIV originated from cross-species transmission of an equine influenza A virus subtype H3N8, while H3N2 originated in avian species.¹

CIV has, therefore, now been added to the list of several pathogens with a causative role in the canine infectious respiratory disease (CIRD) complex (CIRDC). However, although CIRD is typically associated with mild to moderate, self-limiting upper respiratory tract (URT) infection, outbreaks of canine influenza have resulted in significant morbidity and mortality in affected dogs.²

CANINE INFLUENZA VIRUS SUBTYPE H3N8

H3N8 CIV was first identified as a novel canine pathogen after the first recognized outbreak of canine influenza in the world, which occurred in 2004 in racing greyhounds in Florida.³ It was also later associated with outbreaks of acute respiratory disease in racing greyhounds and other dogs in other states.^4,5

CANINE INFLUENZA VIRUS SUBTYPE H3N2

H3N2 CIV was first identified in the United States in 2015, after an outbreak of canine influenza—which began in March in dogs in Chicago—affected more than 1,000 dogs in the Midwest.⁶ From March 8, 2015, to February 2, 2016, H3N2 CIV has been identified in 30 states.⁷

Studies have shown that H3N2 CIV can infect both dogs and cats, and although rare, feline infection with CIV has been documented in the United States.⁸ In particular, at one Indiana shelter in early 2016, multiple cats with signs of URT tested positive for H3N2 CIV. The virus was also shown to be able to spread from cat to cat.⁹ However, there is no evidence, to date, of transmission of CIV from dogs to people, and no cases of CIV infection in humans have been reported.⁶

TRANSMISSION OF CIV

CIV is highly contagious, and dogs of all ages and breeds are considered susceptible to infection.⁷ The virus spreads from dog to dog by both direct contact (via aerosolized droplets from coughing or sneezing) and indirect contact (via fomites, such as contaminated bowls). The incubation period is 2 to 5 days. Peak viral shedding occurs about 2 to 5 days after infection, but before the onset of clinical signs, making it difficult to prevent transmission to other dogs.¹⁰ In particular, such environments as boarding kennels, daycare facilities, and animal shelters—in which many dogs are in close contact, often in a restricted space—allow for rapid viral spread.⁷

CLINICAL MANIFESTATIONS OF CIV INFECTION

About 80% of dogs infected with CIV show clinical signs.¹⁰ Two clinical syndromes have been described: a mild URT form and a severe lower respiratory tract form.

Most infected dogs develop the mild form of the illness, which is usually self-limiting. It manifests with clinical signs associated with viral infection of the URT: cough for 10 to 30 days, nasal discharge, and low-grade fever—followed by recovery in 2 to 3 weeks. However, coinfection with other CIRDC pathogens—such as Pasteurella multocida, Mycoplasma spp, or Streptococcus spp—may also occur and lead to purulent nasal discharge.^4,5,10

One percent to 5% of infected dogs develop the severe pneumonia form, which is characterized by cough, high-grade fever (104°F-106°F), depression, anorexia, dyspnea, tachypnea, and purulent nasal discharge. During the initial outbreaks of CIV H3N8 in racing greyhounds, a few affected dogs (1%-8%) died acutely with hemorrhagic pneumonia.^4,5,9 However, this hemorrhagic pneumonia manifestation has not been reported in pet dogs.¹¹

It has generally been considered that dogs with H3N8 CIV infection are typically no longer contagious by 10 to 14 days after the onset of clinical signs.¹⁰ However, a recent study in dogs with H3N2 CIV infection showed that prolonged intermittent viral shedding can occur, even after resolution of clinical signs. In this study, despite 13 to 24 days between first and last positive, real-time reverse transcriptase polymerase chain reaction (PCR) assay results, the interval from first clinical signs to last positive assay results ranged from 15 to 26 days. Therefore, dogs infected with H3N2 CIV should be isolated for more than 21 days after the onset of clinical signs.¹²

DIAGNOSIS

Because most of the clinical signs associated with CIV infection are similar to those associated with other organisms in the CIRDC, canine influenza cannot be diagnosed on the basis of clinical signs alone.¹¹ Clinicians should therefore test for the most common causes of respiratory diseases in dogs.^7,11

However, to detect CIV, oropharyngeal and/or nasal swabs must be collected within 1 or 2 days after the onset of clinical signs. PCR testing is the most reliable method of detecting CIV, although virus isolation (VI) testing is also an option. Swabs should be placed in sterile tubes containing either a few drops of saline for PCR testing or viral transport medium for VI testing.⁷

After 4 days of illness, serologic testing is the most reliable and sensitive method to confirm CIV infection—paired acute (taken within the first 7 days of illness) and convalescent (taken 10 to 14 days later) serum samples are required. Hemagglutination inhibition (HI) testing is used to identify the presence of antibodies to CIV in the blood. And while dogs that are vaccinated against CIV will develop low HI antibody titers, a diagnosis of canine influenza is made on the basis of a four-fold increase in antibody titer between the acute and convalescent phase sera. However, a single serum sample, taken at least 7 days after the onset of clinical signs, may be sufficient if an acute serum sample is not available.⁷

In conclusion, it is important to consider serological test results, alongside history and clinical signs, in order to diagnose canine influenza. Because CIV seroprevalence and vaccination rates are low in many regions of the United States, detection of a moderate antibody titer in a dog after respiratory illness correlates highly with involvement of CIV.⁷

Dr. Parry graduated from the University of Liverpool, England, in 1997, and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC, where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals, and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.

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• UW Shelter Medicine, WVDL find canine influenza transmitted to cats in Midwestern shelter. University of Wisconsin-Madison School of Veterinary Medicine website. www. uwsheltermedicine.com/news/2016/3/uw-shelter-medicine-wvdl-find-canine-influenza-transmitted-to-cats-in-midwestern-shelter. Published 2016. Accessed May 16, 2016.
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• 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). J Am Vet Med Assoc. 2016;248(9):1022—1026.