Vaccine Helps Reduce Duration & Severeity of Canine Influenza

The discovery of the highly contagious H3N8 canine influenza virus (CIV) by researchers at the University of Florida and Cornell University veterinary schools in a community of racing Greyhounds in 2004 represented a rare event in adaptive evolution.1 Experts studying the genetic composition of the virus were amazed to learn that the entire genome of the H3N8 horse influenza virus had transferred to dogs, where it adapted to become a new canine-specific virus.

Cynda Crawford, D.V.M., Ph.D., clinical assistant professor of shelter medicine at the University of Florida College of Veterinary Medicine, led the team that first discovered CIV as the cause of a respiratory disease outbreak in January 2004 in Greyhounds at a racetrack in Jacksonville, Fla. This outbreak was followed by more CIV outbreaks involving thousands of Greyhounds at tracks across the country from 2004 to 2006.1       

The potential severity of the respiratory disease was realized when eight of the 23 Greyhounds infected in the first outbreak in 2004 died of severe pneumonia. Crawford consulted Edward J. Dubovi, Ph.D., professor of virology at Cornell University College of Veterinary Medicine, whose team analyzed samples collected from the Greyhounds and isolated the virus.

Working with experts from the Centers for Disease Control and Prevention, they determined that the racing Greyhounds were infected with a novel influenza A subtype H3N8 virus. Until this discovery, dogs were not known to be susceptible to influenza virus infections causing clinical disease. 

“The discovery was novel because the virus was transferred from its natural host, the horse, to another host, the dog,” Crawford says. “The virus adapted to the new host over time and no longer needed the original host to spread. Since it is a relatively new virus, dogs have no natural or pre-existing immunity to fight it.”

Eventually, Crawford and her colleagues determined that all dogs, not just Greyhounds, can be infected by CIV. Since CIV is not a zoonotic disease, dogs do not transmit the virus to humans.

“This virus adapted to dogs through a series of mutations,” Dubovi says. “If the virus continues to circulate unchecked, it would be like trying to predict the spread and overall impact of pandemic flu in people. Because this virus could become even more adaptable, canine influenza potentially could become more widespread.”

Since the initial outbreaks in Greyhounds, the researchers have identified CIV in thousands of dogs in at least 38 states. Canine influenza virus has become established in states in the Northeast as well as Colorado. Recently, several Northeastern states and Texas have reported CIV outbreaks in dogs in veterinary clinics, boarding kennels and animal shelters, Crawford says.

Dogs at risk for exposure to the virus are those that live in close proximity to one another and those that participate in canine activities, such as dog shows, field trials, agility and obedience. Training, boarding and breeding kennels, as well as shelters, are possible sites for transmission of the virus. Grooming salons, dog day care facilities and even veterinary clinics are others.   

“Our studies along with those of several other groups have confirmed the prevalence of naturally occurring CIV outbreaks in confined dog populations,” Crawford says. “This virus is spread through respiratory secretions, contaminated objects and aerosols generated by coughing and sneezing. Infected dogs that come into contact with other dogs through licking, nuzzling, coughing and sneezing spread the virus. Contaminated surfaces of bowls, toys and leashes and people who have touched infected dogs spread the virus as well.”

Influenza viruses remain active on surfaces for up to 48 hours and on hands and clothing for eight to 12 hours. Viral shedding with transmission to other dogs starts before the infected dog becomes ill and can continue for about seven days. The most contagious period occurs simultaneously with the onset of clinical signs at two to four days after exposure.

The virus causes an influenza-like illness characterized by coughing, sneezing and discharge from the eyes and nose. Fortunately, most dogs recover after about two weeks, but some can progress to pneumonia requiring hospital care. Mortality is associated with severe pneumonia, but death is uncommon with appropriate care.

“Virtually every dog without any immunity will become infected upon exposure to the virus, and about 80 percent of infected dogs will develop the typical influenza-like illness,” says Crawford. “The 20 percent that do not become sick can still spread the virus to other dogs. Infected dogs should be quarantined from other dogs for 14 days from the first day of clinical signs in order to be sure that they are no longer contagious.”

Infectious Respiratory Disease Pathogens

Canine influenza virus is one of at least nine pathogens that can cause respiratory infections in dogs. Infections caused by these pathogens commonly are referred to as canine infectious respiratory disease (CIRD). Formerly referred to as kennel cough, CIRD primarily was linked to infections by Bordetella bronchiseptica, canine parainfluenza virus type 5, and canine adenovirus type 2. Newer data show the involvement of canine respiratory coronavirus, canine pneumovirus and H3N8 canine influenza virus. Because the clinical signs associated with infections by these pathogens may be similar, the canine influenza virus in CIRD can be overlooked.

The most devastating effect of canine influenza is promoting secondary infections, especially bacterial infections. “Just like in humans who develop the flu, canine influenza sets up opportunities for secondary disease,” says Ronald D. Schultz, Ph.D., DACVIM, professor and chair of the Department of Pathobiological Sciences at the University of Wisconsin-Madison School of Veterinary Medicine. “It is common for dogs to have bacteria already in their systems when they become infected with CIV. Alone, those bacteria would rarely cause much of a problem, but combined with CIV, they can become deadly.”

Canine influenza virus infects the epithelial cells lining the respiratory tract from the nose to the airways deep in the lungs.1 As the epithelial cells die, the respiratory tract becomes exposed to secondary bacterial infections. The inflammatory response to the virus contributes to disease by causing rhinitis, tracheitis, bronchitis and bronchiolitis.  

Confirming CIV Diagnosis

Canine influenza virus is similar to other canine infectious respiratory diseases, so diagnosis must be confirmed through blood samples and nasal or throat swabs. The virus can be detected by polymerase chain reaction (PCR) assays in nasal or throat swabs collected from dogs during the first five days of illness.1 If the PCR test 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 virus shedding period. 

Blood testing should be performed to confirm infection, especially in cases where the PCR results are negative, but the dog is suspected of being infected. Antibodies to canine influenza virus are detected in the blood as early as seven days after the onset of clinical signs.1 Virus-specific antibody titers in samples taken within the first seven days of illness and 10 to 14 or more days later are compared; diagnosis is confirmed by finding at least a fourfold increase in the antibody titer in the second sample compared to the first. 

Though there is no specific therapy for dogs infected with H3N8, supportive treatment includes rest and antibiotics for secondary coughing and pneumonia. Hydration via intravenous fluids may be necessary for dogs with pneumonia. Optimal nutrition and clean, well-managed kennel environments help to promote healthy immune function. 

Veterinarians can vaccinate at-risk dogs for H3N8 vaccine using an inactivated whole virus, or killed, vaccine. Intervet/Schering-Plough Animal Health (now Merck Animal Health) introduced a H3N8 canine influenza virus vaccine in 2008. In published vaccine efficacy trials, Intervet/Schering-Plough established the vaccine’s ability to protect against disease and demonstrated safety in more than 700 dogs representing over 300 breeds from 6 weeks to 10 years of age. More recently, in 2011, Pfizer Animal Health introduced a CIV vaccine. 

Studies by Merck Animal Health have shown that the vaccine does not prevent infection but does reduce the number of dogs with clinical signs, the duration of illness and the severity of viral damage to the lungs.1 The vaccine also reduces the amount of virus shed and shortens the shedding interval. Vaccinated dogs that become infected develop less illness and are less likely to spread the virus to other dogs. 

“These studies showed that vaccination can reduce the number of infected dogs and dogs with pneumonia,” Crawford says. “This means that the vaccine can potentially help protect facilities from full-blown outbreaks and individual dogs from hospitalization.” 

“It is likely that greater than 90 percent of the canine population is immunologically naïve to H3N8,” Schultz says. “This makes the vaccine a valuable tool. We found that the vaccine significantly reduces the severity of CIV even when a dog also has a Streptococcus bacterial infection or other concurrent bacterial infection.” 

The canine influenza H3N8 vaccine is recommended for dogs at risk for exposure to the virus or those that live in communities where an outbreak has been reported. Experts recommend giving the vaccine to dogs that receive the kennel cough vaccine because the risk groups are similar.1 

Puppies should be at least 6 weeks of age when they receive their first dose of CIV vaccine. For dogs not previously vaccinated, two initial doses two to three weeks apart are required. Optimal immunity is usually established in most dogs within two weeks after the second vaccine. Ideally, dogs being vaccinated for the first time should start the two-dose vaccine series at least five weeks before going to a high-risk environment, such as a boarding or training kennel. 

“This is a major hurdle for the vaccine,” Schultz notes. “For dogs already in high-risk environments or those that need to go to a high-risk facility, the vaccine can’t work in less than five weeks after the first dose or two weeks after the second dose. Revaccination is recommended annually because maximum duration of immunity studies have not been done.” 

The benefits of vaccination are considered noteworthy. “Even if only 20 to 30 percent of dogs were vaccinated, it would make a difference in population immunity,” Schultz says. “The vaccine is about prevention within groups of dogs. It only takes one CIV outbreak to make people start to evaluate the necessity of prevention. Fortunately, so far, the virus has not been a major problem in most areas of the country.” 

“I am optimistic that this vaccine will help control canine influenza virus and limit its transmission from facility to facility and community to community,” Crawford says. “I am hopeful that the vaccine will be used and thus will help dogs that are housed in kennels and other environments with large numbers of dogs.” 

Signs of CIV

Canine influenza virus (CIV) H3N8 has signs similar to kennel cough, which can make it challenging for veterinarians to diagnose. Blood testing and nasal and throat swabs are used to confirm diagnosis. Among the signs of CIV are:  

  • Persistent coughing that lasts several days or weeks. The cough may be congested, soft and moist or dry, similar to kennel cough. 
  • Sneezing and nasal discharge usually accompany the cough. 
  • Low-grade fever can occur, but a high fever of 104 to 106 degrees Fahrenheit is likely in cases with pneumonia.
  • Loss of appetite and lethargy are common.

How to Handle a CIV Outbreak

When canine influenza virus strikes a kennel, here are key steps to help reduce spread of infection and curtail a serious outbreak. 

  • Prompt recognition that canine influenza virus may be the cause of coughing and sneezing, especially if the number of sick dogs increases rapidly within a few days. 
  • Prompt removal of sick dogs to an isolation room to reduce the amount of virus in the kennel environment. Once in isolation, nasal and throat swabs from the dogs can be collected for the CIV PCR test. Once CIV is confirmed, the dogs should be isolated from other dogs for two weeks.
  • Limited staff should be assigned to care for dogs in isolation. Full protective clothing, including gloves, gowns and boots or shoe covers, should be used in the isolation room and not worn outside the room. Feeding, cleaning and treatment supplies should be kept in the isolation room.
  • Dogs exposed to sick dogs should be quarantined. Even though they are not yet sick, they may be infected and contagious if they are in the incubation period or have subclinical infection. The dogs can be quarantined in the kennel for at least 10 days, but no other dogs should enter the kennel during this time. Dogs can be discharged from quarantine earlier than 10 days if they are negative for the virus by PCR testing of nasal and throat swabs.
  • Separate staff should be assigned to care for the dogs in quarantine using the same type of protective clothing as for isolation. This is important since the infection status of exposed dogs is unknown and thus the potential for contamination is unknown.

1 Canine Influenza: Influenza Updates from the AVMA. September 7, 2009. www.avma.org/public_health/influenza/canine_bgnd.asp

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