Canine influenza or Canine flu is a contagious respiratory infection in dogs. Here’s more about this dreaded disease.
Canine influenza is an emerging contagious respiratory infection of dogs. It has a high morbidity (close to 100 percent), but the mortality, as with most other influenza infections, is relatively low (less than 8 percent). A novel influenza-A virus that appears to be a mutation from a previously recognised strain of equine influenza virus, the H3N8 strain, causes this disease. Dog-to-dog transmission does occur and therefore this infection must be distinguished from kennel cough.
Influenza viruses are enveloped viruses with segmented single stranded RNA genomes that belong to the Orthomyxoviridae. family Influenza viruses that cause disease in domestic animals belong to Ggenus Influenza virus A, whereas influenza B and influenza C viruses primarily circulate among humans. Influenza A viruses are classified based on the genetic composition of their haemagglutinin (H) and neuraminidase (N) genes. To date, 16H types and 9 N types have been identified, each of which are antigenically distinct. Canine influenza virus usually causes fever, joint pain and respiratory signs. CIV infection can cause respiratory disease by itself or in conjunction with other respiratory pathogens such as Distemper virus, respiratory coronavirus, Parainfluenza virus and Bordetella bronchiseptica, etc. Death is unusual but causes respiratory complications, which are most common in very old and very young ones. It is important to note that influenza virus is not related to parainfluenza virus and infection or vaccination for one does not induce cross-protective immunity against the other
Like other respiratory pathogens, CIV is most efficiently transmitted by direct contact with infected dogs and by aerosols as a result of coughing and sneezing. Dogs have the highest level of virus in their secretions 2-4 days after they are exposed to virus. Often they are not yet showing clinical signs when they are most potent source of transmitting the virus. Dogs may be able to spread the virus for up to 10 days. The virus can contaminate kennel surfaces, food and water bowls, collars and leashes and the hands & clothing of people who handle the infected dogs. Influenza virus can remain viable on surfaces for up to 48 hours, on clothing for 24 hours and on hands for 12 hours. Fortunately, washing hands with simple disinfectant easily inactivates the virus.
Canine influenza virus replicates in epithelial cells lining the airways from nose to the terminal airways in the lungs. Peak viral shedding starts from the upper respiratory tract during the incubation period of 2-4 days, therefore, dogs are most contagious prior to showing obvious clinical signs. Dogs with subclinical infection also shed virus. Virus shedding decreases substantially during the first four days of illness, but continues up to seven days in most dogs, and to 10 days in some dogs. Once virus shedding ceases, the dog is no longer contagious. Therefore, it is dodgy that dogs pose a significant infectious risk after 10-14 days of onset of clinical signs. Following viral replication in necrosed epithelial cells expose the basement membrane to secondary infections by a variety of gram-positive and gram-negative commensal bacteria including Streptococcus sp, Staphyloccocus sp, E. coli, Klebsiella, Pasteurella multocida, and Mycoplasma spp. These bacteria contribute to development of purulent nasal discharge and productive cough. The viral and secondary bacterial infections initiate an intense inflammatory response resulting in rhinitis, tracheitis, bronchitis, and bronchiolitis. Fortunately, most of the infected dogs recover within 2-3 weeks without any further health complications. However, some dogs progress to pneumonia, which is usually due to secondary bacterial infections. The overall mortality rate for canine influenza is very low. The secondary pneumonia develops due to contamination by invaders can be life threatening.
Signs and symptoms
Fortunately, most of the sufferers have the mild form of this illness. Those animals who are more susceptible to serious illness are young puppies, old dogs and dogs with weakened immune systems. Infected animals may show symptoms within two to five days post exposure. Two clinical syndromes have been seen in dogs infected with the canine influenza virus–a mild form and a more severe form of the disease.
- Mild form–Dogs suffering with the mild form of canine influenza develop a soft, moist cough that persists for 10 to 30 days. Some dogs have a dry cough similar to the ‘kennel cough’ caused by Bordetella bronchiseptica/parainfluenza virus complex. For this reason, canine influenza virus infections are frequently mistaken for ‘kennel cough’. Dogs with the mild form of influenza may also have a thick nasal discharge, which is usually caused by a secondary bacterial infection.
- Severe form–Dogs with the severe form of canine influenza develop high fevers (104º-106ºF) and have clinical signs of pneumonia, such as increased respiratory rates and effort. Pneumonia in these dogs is not caused by the influenza virus, but by secondary bacterial infections. The fatality rate of dogs who develop pneumonia secondary to canine influenza can reach 50 percent if not given proper treatment.
Solely clinical signs cannot diagnose canine influenza because the clinical signs (coughing, sneezing and nasal discharge) are similar to those associated with all of the other respiratory pathogens and cannot be differentiated from them. Antibodies to canine influenza virus may be detected in the blood as early as seven days after onset of clinical signs and the virus may be identified in nasal or pharyngeal swabs during the first four days of illness. The most reliable and sensitive method for confirmation of infection is serological testing. Paired acute serum samples (taken within the first seven days of illness) and convalescent serum samples (taken 10-14 days later) are necessary for diagnosis of recent infection. If an acute sample is not available, a convalescent sample will indicate whether a dog has been exposed to the virus at some point in the past. A diagnosis of CIV is made based on a four-fold increase in antibody titer from the acute to the convalescent sample. If a dog has been ill for less than four days, nasal and pharyngeal swab submission for Polymerase Chain Reaction (PCR) testing can be performed. If the PCR indicates a positive result, the dog is most likely infected. Negative PCR results may be falsely negative if the swabs are not collected during the time of peak virus shedding. After four days of illness, PCR results are less likely to be reliable. Serology should be performed to confirm infection, especially if the PCR results are negative and the case is highly suspicious for CIV infection. Other diagnostic options applicable to dogs who have died from pneumonia are viral culture and PCR analysis using fresh (not formalin-preserved or frozen) lungs and tracheal tissue. Virus detection in respiratory secretion specimens from acutely ill animals by use of viral culture, PCR analysis and rapid chromatographic immunoassay is possible, but usually unrewarding. Prevent your dog from influenza and get him treated as early as possible.
(Dr Rahul Kumar and Dr Vinod Kumar Singh work at the Department of Veterinary Pathology, College of Veterinary Science & Animal Husbandry, DUVASU, Mathura, UP.)
Since canine influenza is a viral infection, treatment consists mainly of supportive care based on clinical signs and laboratory tests. There is no specific antiviral treatment for canine influenza. A variety of secondary bacterial infections may play a significant role, therefore, the antibiotics are indicated for dogs having fever, productive cough and purulent nasal discharge. Nasal secretion usually responds within days to treatment with a broad-spectrum bactericidal antibiotic, but cough may persist for 10 to 30 days. Antitussives are not very effective in reducing frequency and duration of coughing, hence contraindicated in dogs with productive cough. Dogs who develop pneumonia usually require hospitalisation for intravenous fluids and parenteral antibiotics. Ideally, a trans-tracheal or endotracheal wash for bacterial culture and antibiotic sensitivity testing should be performed to target the choice of antibiotic. For dogs in which cultures are not performed, empirical treatment with a broad-spectrum combination of bactericidal antibiotics may be recommended to cover gram positive, gram negative, aerobic and anaerobic bacteria. For more severe cases of pneumonia, oxygen supplementation and nebulisation has been very beneficial.
Vaccine is available for reduction of disease due to CIV. The available vaccine is incapable to completely prevent infection and shedding, but they can lessen the severity of disease, provided other factors such as overcrowding and appropriate disinfection and reduction of other stressors are also addressed. Inactivated, parenteral vaccines are available for reduction of disease and shedding caused by H3N8 CIV. One vaccine also reduced the severity of illness caused by co-challenge with CIV and Streptococcus equi subspecies zooepidemicus. The use of these vaccines could be considered for dogs who are likely to contact other dogs in regions where CIV is endemic, especially those who enter shelters, boarding kennels, shows, sporting competitions, popular dog parks, or pet daycare facilities. The initial vaccine can be given as early as six weeks of age. Because CIV vaccines are inactivated, two initial doses are required three to four weeks apart, and maximum immunity does not occur until one week after the second dose.