Understanding Influenza

What is flu? "It's a hit-and-run disease," said Tom Chambers, PhD, who heads The OIE international influenza reference laboratory at the University of Kentucky's Gluck Equine Research Center in Lexington. "When it hits, it causes big problems, but then it disappears again." Many horse owners are concerned about equine influenza following announcement of an outbreak in Japan.

The Japan Racing Association released information stating that a flu outbreak is occurring at two Thoroughbred training centers, and it has shut down racing throughout the country. Chambers has no further information about the outbreak.

Here are some flu facts:

  • Equine influenza is one of the most common causes of upper respiratory disease in horses;
  • Flu is a viral disease;
  • Flu is highly contagious;
  • Flu is spread by the aerosol route (i.e., sneezing, coughing);
  • Flu often causes a fever;
  • Infected horses can shed flu virus prior to having a fever or other clinical signs;
  • Flu can cause subclinical infections (no clinical signs);
  • Horses with no clinical signs can shed virus and infect other horses;
  • Flu does not produce chronic (long-lasting) infections;
  • Flu does not produce latent infections (the virus does not persist in the body and become reactivated at a later time due to stress);
  • Flu can spread down a barn aisle in a matter of days;
  • Flu changes over time;
  • A horse can get flu many times during his life;
  • Horses that get flu usually don't get it again for 1-1½ years;
  • Flu is not a hardy virus, it can be easily killed through disinfection;
  • Vaccines might protect horses from getting sick (showing clinical signs), but might not prevent exposed horses from shedding the virus and infecting other horses;
  • Horses can shed flu virus for a week or more, so quarantine recommendations are for 10 days to two weeks for exposed horses;
  • Nasal swabs can be tested for the presence of flu virus in about an hour at a laboratory.
  • Because flu changes, vaccines need to be updated to reflect those changes.

Researchers know horses can spread flu virus from direct contact or coughing. "I strongly suspect flu can be spread by fomite transmission, meaning casual contact by inanimate objects such as hands, bits, or anything that comes into contact with a horse's nose," said Chambers.

He suspects there are many cases of equine influenza that are never diagnosed. "It wouldn't surprise me if sooner or later that nine out of 10 horses are exposed," said Chambers. "It very rarely kills a horse. It gives them a fever, cough, runny nose, and with no complications, a week later they are starting to feel better." There can be secondary bacterial infections in horses that have the flu that can be dangerous if not treated. For that reason, antibiotic therapy is widely employed, he said.

One other characteristics of flu is that it can produce subclinical infections, especially if horses are vaccinated, he said. The vaccine might protect the horse from getting sick, but not necessarily protect him from shedding virus. Chambers said, "The horses might not show (clinical) signs, but can shed virus. That makes them a risk. You think everything's safe, but it's not safe."

Since the flu virus is not hardy, it is easily killed. Washing hands with soap and warm water is important when handling sick horses. (Dr. Roberta Dwyer of the Gluck Center recommends singing "Happy Birthday" twice—a total of 30 seconds--while scrubbing your hands with liquid soap as a good way to time the length of washing.)

Other management tips:

  • Handle healthy horses before sick horses;
  • Use equipment dedicated to each horse, or don't mix equipment from sick and healthy horses;
  • If you have to share equipment, decontaminate it after use on sick horses;
  • Use soap, bleach, or Lysol to disinfect equipment, rinse well;
  • Vaccinate with a product from a reputable manufacturer (there are injectable and intra-nasal flu vaccines).

Each horse farm should have routine quarantine for horses coming onto the farm, noted Chambers. This is the farm's first defense to prevent flu from spreading to horses at the farm. Horses should be quarantined for a minimum of 10 days.

The National Animal Health Monitoring System (NAHMS) collected data on equine health and management practices from a representative sample of equine operations in 28 states from four regions. Blood and nasal swabs were tested for flu.

On operations with at least 20 resident horses, 81.5% percent of horses had a detectable equine influenza virus antibody titer and 52.6% had a high equine influenza virus antibody titer. The percentage of horses on small operations (1-6 horses) with a detectable equine influenza virus antibody concentration was 63.1%, while 30.6% had high titers. An estimated 65.4% of horses in the study were reported to have been vaccinated previously. Horses in the study vaccinated for equine influenza virus were more likely to have a detectable equine influenza antibody titer than horses that had never been vaccinated.

(To read the fact sheet from NAHMS visit www.aphis.usda.gov/vs/ceah/ncahs/nahms/equine/equine98/eq98iurd1.pdf.)

Testing for Flu

There are a variety of tests for equine influenza. Chambers emphasizes the need for veterinarians to take nasal swabs and submit them to their state diagnostic laboratories or the Gluck Center in order to keep track of what strains of flu are circulating. There is no charge for testing at the Gluck Center. (a virus that was isolated from a sick horse, thus the need to do testing).

Swab samples should be taken as soon as a horse appears ill, he said. You don't want to swab a horse that had a fever a week ago. "It's too late then to get a positive test," he said. If you have a contagious upper respiratory disease going through your barn, you need to test early.

Horse owners and veterinarians need to help researchers keep track of which equine flu strains are circulating. There are two main types of strains, American and Eurasian. The American strain can be found world-wide, but the United States does not have the Eurasian strain in circulation.

"In the United States the Eurasian strain is not in all vaccines because we haven't had outbreaks of the Eurasian strain in the United States," said Chambers. "Could that change tomorrow? Yes."

He recommends if owners have horses going oversees to look for vaccines with the Newmarket/2/93 strain (Eurasian).

Since 2004, world-wide flu experts have recommended strains resembling South Africa 2003 be put in all vaccines. The American strain Ohio03 fits that criteria, and the Gluck Center has provided vaccine manufacturers that strain. Because of the expense and time to have a vaccine approved, "So far I haven't seen any vaccine with Ohio03 come on the market," said Chambers, "but we hope they will not be long in coming."

Other Flu Concerns

Springing out of the canine flu experience in 2005 are studies to see if the equine flu (that mutated and adapted in the dog) could be passed back to horses, and if so, if the current equine flu vaccines protect against it.

Chambers explained, "The (equine influenza) virus mutated and adapted in the dog and now it can be transmitted dog-to-dog by the aerosol route. Now a whopping dose of virus isn't needed anymore for dogs to become infected. That's what we're afraid of happening with the bird flu in Southeast Asia. If the virus can mutate to make itself less lethal perhaps, but more contagious, control of the disease could be difficult.

"Speaking as a virologist, influenza is actually a simple virus, and the way it survives is by a very simple strategy of always changing itself, always mutating," said Chambers. "That's a simple, but very effective strategy. Ninety-nine percent of flu is going to die before it can get in to a different species. It's the one mutant virus that causes all the problems. For that reason, we have to continue and improve surveillance, and continue with research in order to fight it."

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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