Canine Influenza Virus: Diagnosis, Treatment, and Control
Canine Influenza virus (CIV) is a highly contagious respiratory disease that is of particular concern for shelters (as well as other facilities where large numbers of dogs are housed in close proximity, such as boarding facilities). CIV can result in serious disease if not quickly addressed. Prompt identification and diagnosis of cases, treatment of infected dogs, and implementation of control measures are critical.
Two strains of CIV have been identified in the U.S.: H3N8 and H3N2. The H3N8 strain was originally detected in racing greyhounds in Florida in 2004, although there is evidence it has been circulating in the dog population since at least 1999; infection has been reported in most states. Previously reported only in Asia, H3N2 was first noted in Chicago in 2015 and has since been reported in most states. H3N2 was documented to affect cats in one Indiana shelter in 2016.
Currently, H3N2 is the most commonly circulating strain and the cause of several outbreaks in 2022, including in Texas and North Carolina. These outbreaks, while significantly impacting affected shelters, can burn out relatively quickly. The virus moves rapidly through the susceptible population and, with timely implementation of control measures, community transmission decreases and levels return to a low baseline or even zero. However, in some shelters, CIV becomes endemic. For this reason, shelters must remain vigilant and monitor all dogs for signs of illness from the time of intake through their stay in the shelter.
The clinical signs seen in affected dogs are very similar to other pathogens that are part of the Canine Infectious Respiratory Disease Complex, so laboratory testing is necessary to reach a definitive diagnosis to allow for proper treatment and management.
Prevention
Three keys to preventing widespread CIV in your dog population are vaccination, crowding reduction, and sanitation.
Vaccination
Vaccination against CIV will not completely prevent infection, but it can reduce the severity and duration of disease. There are currently two bivalent CIV vaccines effective against the H3N8 and H3N2 strains. All currently available vaccines are inactivated (killed) virus products and require two doses given 2-4 weeks apart. Because an optimal immune response should not be expected until approximately one week following the second dose (i.e., 3 weeks following the initial vaccination), CIV vaccination provides limited benefit in the sheltering setting unless exposure can be prevented during this time.
Dogs should receive core vaccinations in accordance with current shelter guidelines. These vaccinations provide protection against other pathogens in the Canine Infectious Respiratory Disease Complex and can reduce the morbidity associated with co-infections.
Crowding and Stress Reduction
Perhaps the most critical of all control measures is the reduction of crowding and stress. Crowding is an extremely potent stressor for dogs and exacerbates many other negative factors already present in shelters. High-density housing, particularly when young and immunologically naïve dogs are mixed with other dogs in the facility, is a significant risk factor for disease.
Keeping fewer dogs in the facility at any given time, particularly by reducing the length of time they spend in the shelter, generally improves the level of animal care and sanitation and reduces the risk of direct or indirect contact with infected animals. It is critical that organizations operate within their capacity for care to ensure they are able to provide a humane level of care to the animals in their facility.
Direct physical benefits of stress reduction include lowering circulating cortisol levels; increased levels have a negative effect on animals’ immune systems and their ability to fight off infection. Reducing stress also decreases barking, which can help to reduce respiratory irritation and aerosol spread of pathogens.
Sanitation
Although CIV is not particularly resilient in the environment, adequate cleaning and disinfection practices are necessary to limit transmission. Most disinfectants routinely used in animal shelters are effective against CIV when used at proper dilutions and for the appropriate contact time.
Staff assignments should be made to limit the spread of disease and reduce fomite transmission. This includes assigning staff to specific subsets of the animal population (e.g., affected dogs or non-affected dogs), using appropriate personal protective equipment, and providing separate supplies for each section of the facility.
The virus moves rapidly through the susceptible population and, with timely implementation of control measures, community transmission decreases and levels return to a low baseline or even zero.
Clinical Signs
The most common signs of CIV include:
- Coughing
- Sneezing
- Nasal discharge
- Mild fever
It is important to note some infected dogs will remain asymptomatic; it has been estimated that up to 20% of those infected with CIV will not show clinical signs of disease. More severe infections, which may occur in a small number of cases, can lead to pneumonia and even death. Risk factors for serious disease include age, vaccination history, genetics, co-morbidities, and environmental factors such as crowding.
The incubation period is less than 1 week for both H3N8 and H3N2. Viral shedding for H3N8 is short, typically lasting no more than 1 week following infection; however, dogs infected with H3N2 have been shown to shed the virus for up to 3-4 weeks. Peak shedding of both strains occurs very early in the course of infection and can actually precede the development of clinical signs by a few days.
Transmission
CIV is most commonly transmitted through:
- Aerosolization
- Direct contact
- Fomite transfer
Because canine influenza outbreaks tend to be sporadic and vaccination is not widespread, most dogs will be susceptible to infection. As a result, a very high incidence of upper respiratory signs can occur when the virus is introduced.
Zoonotic Potential
While there have been no reports of human infections with CIV, influenza viruses have the ability to reassort and mutate into new variants. The general recommendation is that immunocompromised individuals should limit contact with sick animals.
Diagnosis
Canine influenza virus can be diagnosed by PCR performed on deep nasal and/or pharyngeal swabs; samples should be collected within 1-2 days of the onset of clinical signs. Because viral shedding peaks early in the course of disease, the PCR test is of little value in dogs who have been ill for several days. Instead, blood samples should be used to test for antibodies against CIV.
When disease spreads rapidly through the facility, large numbers of dogs are affected, or disease is particularly severe, then a respiratory PCR panel should be performed at a diagnostic laboratory to determine the specific causative agent. Respiratory PCR panels using combined samples from both nasal and oropharyngeal swabs are recommended.
Treatment/Management
Individual Animal Care
Treatment is usually symptomatic and supportive in nature. It is aimed at preventing secondary bacterial infections as well as providing definitive treatment, if appropriate, for any primary pathogens and other medical conditions. If a secondary bacterial agent is identified or highly suspected, then a course of antibiotics may be indicated; however, antibiotics will not be effective against CIV.
More severely affected dogs require more aggressive treatment, which may include hospitalization, intravenous fluid therapy, oxygen support, nebulization, and coupage. If intensive treatments are not possible, or if inadequate response to treatment is noted in individual cases, consider transferring these dogs to a private veterinary facility or humane euthanasia.
Population Considerations
Humane conditions and medical care, as well as adequate isolation space to prevent transmission to the rest of the population, must be available. Depending on the size of the facility, number of staff, and number of infected dogs, it may become necessary to temporarily halt intake to avoid jeopardizing the health of incoming animals and avoid the need for depopulation. Utilization of depopulation in outbreak management is rare and should be utilized only as a last resort and after consultation with a veterinarian with shelter medicine expertise.
Disease Outbreak Management
Isolation
Timely removal of clinically affected dogs to strict isolation must be undertaken to limit the spread of disease. Dogs should be moved as soon as possible once clinical signs are noted, as the intensity of shedding does not necessarily correlate with the severity or duration of clinical signs observed. For this reason, complete separation of exposed dogs from unexposed and newly arrived dogs, often referred to as a "clean break," is essential.
Contact with infected dogs should be restricted to key staff and volunteers. Whenever possible, these individuals should not work with other dogs. If this is not feasible, appropriate personal protective equipment (PPE) must be worn and dogs should be handled in isolation only after handling healthy animals in the general population.
Depending on the nature of the disease and available resources for treatment, placement into foster care, release to rescue organizations, or hospitalization at a veterinary clinic may be appropriate options for infected dogs.
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