Canine influenza has caused much concern in veterinarians, dog owners and public health personnel. From the emergence of H3N8 canine influenza virus (CIV) in the US in the early 2000s, to the ongoing and broad international dissemination of H3N2 CIV, this disease has had a significant impact. The scope of disease is not known, but it is estimated that thousands (or, more likely, tens of thousands) of dogs have been infected in the US. Serious infections, including fatal infections, have occurred and while the overall medical and economic impacts are unknown, they are clearly substantial. CIV has also highlighted various areas such as risk of imported pathogens, spread of disease in a naïve population, the need for a coordinated human/veterinary response to this potentially zoonotic pathogen and the need to consider optimal vaccination strategies for an emerging disease.
Canine influenza refers to an influenza virus that is host-adapted to live in dogs and circulate in the dog population. Dogs are susceptible to a range of influenza viruses, including human influenza viruses,1 but infections with those are sporadic and not sustained in the dog population. Canine influenza viruses can spread readily between dogs, without the need for any other hosts. As is common for influenza viruses, CIV can be shed for a short time (e.g., 24 h) prior to the onset of clinical signs, with peak shedding during early disease. Duration of shedding differs between H3N8 and H3N2, with H3N8 being shed typically for only a few days and H3N2 being shed for up to a few weeks. This may account for the wider and more sustained transmission of H3N2, as longer shedding creates more opportunities for infection.
Transmission of CIV can be via direct contact, droplet transmission (transmission over a short distance from infectious aerosols generated during breathing, barking or coughing) and indirect transmission via fomites or the environment. The relative roles of these results are not known, but direct contact is probably the most important route. Influenza viruses are not particularly stable in the environment and do not persist for long periods of time, so indirect transmission is probably short term and mainly involving items and surfaces that have direct contact with respiratory secretions (e.g., bowls, human hands).
Cats can be infected with a range of influenza viruses but appear to have low susceptibility to CIV and while infections have been reported, they are rare.
Most clinically affected dogs develop upper respiratory tract disease with varying degrees of cough, nasal discharge, ocular discharge, lethargy and anorexia. Fever may be present but is inconsistent. Secondary bacterial pneumonia may develop in a minority of cases. Severe peracute lower respiratory tract disease has also been reported and can result in rapid deterioration (including death) within a short period of time. How much of that is due to an abnormal presentation of primary viral disease versus severe peracute secondary bacterial infection is usually difficult to discern.
Clinical signs are not adequate for diagnosis since CIV infection is not distinguishable from disease caused by other CIRDC pathogens. Suspicion should be raised in some situations, such as high morbidity outbreaks in groups of dogs vaccinated against Bordetella bronchiseptica and canine parainfluenza virus and in dogs imported from Asia, but diagnostic testing is required.
PCR is the most widely used test. Testing can involve one or more of nasal, pharyngeal and conjunctival swabs. Conjunctival swabs are likely the lowest yield and nasal swabs are most often used. Testing of 2 or 3 sites in parallel likely increases sensitivity of diagnosis.
Serological testing is less useful clinically because of the need to test convalescent titres. A four-fold or greater increase in serum antibody titre in samples collected ∼4 weeks apart is diagnostic in the absence of recent (probably within the past few weeks) vaccination.
Most cases of CIV infection respond well to supportive care, such as cough suppression. The type and level of care that is needed (e.g., IV fluids, oxygen support) is dictated by the severity of CIRDC, not by the pathogen. Most affected dogs are easily managed at home. Secondary bacterial infections are not common but may occur, and should be managed as per standard approaches.7 Antiviral drugs are uncommonly used in humans and the approach should be similar in animals. While drugs such as oseltamivir might be useful in some situations, they are not likely indicated in the vast majority of cases. They could be considered in severe, acute cases, particularly in high risk (e.g., brachycephalic) patients, but evidence is currently lacking.
Monovalent H3N2 or H3N8, and bivalent vaccines, are available. It is questionable whether H3N8 vaccination is required since this virus seems to be very rare (if it is even still circulating). As is typical for killed vaccines, an initial two dose series is required.
The timeframe required for two doses and subsequent development of a protective immune response means that vaccination is not a highly effective initial outbreak response tool, since much transmission can happen during this window of developing resistance. While vaccination can be considered for any patient, vaccination is perhaps most important in dogs at higher risk of exposure (e.g., those exposed to imported dogs or dogs from other endemic regions, dogs that have frequent contact with other dogs) and those at increased risk of severe infection (e.g., seniors, dog with underlying respiratory or cardiovascular disease, brachycephalics).
Influenza vaccines are labelled to aid in reduction of disease. They are most reasonably expected to reduce the risk of severe disease and help establish herd immunity, to hopefully reduce the risk of transmission. It is not uncommon for vaccinated dogs to develop influenza after exposure, so client counselling must emphasize the reasons for vaccination (e.g., to reduce the risk of severe disease, rather than to provide a high likelihood that the dog will not develop influenza).
The public health risks with H3N2 CIV seem to be very low. Human infections with H3N2 CIV have not been reported. However, it cannot be completely ruled out so practical infection control measures to improve hygiene (e.g., hand hygiene) and limit direct contact with respiratory secretions of infections dogs are prudent. The greater concern is the potential for dogs to act as a ‘mixing vessel’ for influenza viruses.
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