The Future of Equine Influenza

The Future of Equine Influenza

Photo: Anne M. Eberhardt/The Horse

Researchers are striving to better understand this fast-spreading virus before its next mutation.

Equine influenza is the most common respiratory disease in horses. We saw just how powerful the influenza virus can be when in 2007 an outbreak in Australia infected approximately 70,000 horses, costing the country nearly $100 million and halting equine activities. Australia historically had been influenza-free, so the horse population was not vaccinated against the disease--and it spread like wildfire.

It is always wise to be on the lookout for clinical signs of equine influenza and be proactive about preventing it. Luckily, the equine form of this virus has not changed or mutated very often, but researchers use these lulls in virus shift to research vaccine technologies and learn more about the virus and how it develops and responds to the immune system. Because influenza is such a fast-spreading virus, researchers want to be ready for when the next great mutation comes along.

About Influenza

Influenza is known as a "hit and run" virus. It hits hard and spreads very quickly via aerosol transmission, often leaving many damaged horses in its wake. Horses spread the disease every time they cough or sneeze, and the virus particles have been known to travel surprising distances (as much as 150 feet, according to the American Association of Equine Practitioners). Clinical signs often include high fever typically lasting three to four days, a dry, hacking cough, loss of appetite, muscle soreness, and nasal discharge.

Influenza damages protective epithelial cells in the respiratory tract lining, and infected horses take several weeks to recover. This damage to the immune defenses leaves horses vulnerable to acquiring other diseases they're generally equipped to fight off when healthy. Many of these secondary infections are bacterial, such as pneumonia. Much like we see flu spread through human child care centers, the equine virus occurs predominantly in younger horses. If your older horses are also getting sick, it might mean the virus has mutated (a process called antigenic drift; the virus survives by always changing itself/producing new strains, which undermines vaccine efficacy) and could end up having a more severe impact on the horse industry as a result.


A regular vaccination program is the best first line of defense. "The main thing you can do for flu is to make sure (your horses) are vaccinated," says Eric Miller, DVM, of Cornerstone Veterinary Services, in Rock Spring, Ga. "Especially if you have horses that travel--those that are traveling can bring back the virus to the farm."

Historically, equine influenza has not mutated as fast as the human version. In fact, three or four of the vaccines currently on the market protect against strains that have been circulating over the past few years and are still effective, according to Thomas Chambers, PhD, of the University of Kentucky's Gluck Equine Research Center and head of the Office International des Epizooties (OIE, the World Organization for Animal Health) influenza reference laboratory in Lexington.

Consult your veterinarian and refer to the American Association of Equine Practitioners' vaccination recommendations to determine the best protocol for your horses. Remember that vaccination does not supply absolute protection. And while horses coming into contact with the virus might still show signs of influenza, the vaccine will prevent them from becoming as severe as they would in unprotected animals. Miller adds that equine influenza is probably underdiagnosed. "Many times we don't have the resources for virus isolation and the clients don't either," he ¬says.

Response and Management

Act immediately if you see flulike signs in your horse, especially if you have multiple animals housed together. Being proactive can help halt disease spread and save money and time. Call your veterinarian and have him or her take nasal swab samples first to determine what pathogen you are dealing with (several diseases can cause similar signs in infected horses) and treat accordingly. This also helps monitoring agencies such as OIE and state agricultural departments track potential ¬outbreaks.

Also immediately place the horse in question in a quarantine area and thoroughly clean and disinfect any areas or equipment he has been in contact with. Because horses with influenza do not show signs right away, quarantine other horses that have been housed close to the sick one, as they are most likely already infected. Flu's incubation period, or time that a horse is infectious before showing signs of illness, is about three days.

Management techniques can also help reduce disease spread, including designating one person to handle/treat the sick horse, treating the horse at the end of the day/shift (when the handler won't be coming into contact with other horses), and setting up foot baths with disinfectant solution for handlers to dip their shoes in as they enter and exit the premises. Disinfecting equipment such as buckets and hoses as well as tack can also prevent disease spread. Regardless of your disinfection protocol, be sure to dedicate a separate water bucket and hose for the sick horse.

Influenza treatment primarily involves making the horse comfortable (i.e., providing him with plenty of water and rest and administering non-steroidal anti-inflammatory medications as needed) and keeping him isolated until the disease has run its course. A good rule of thumb is for every day a horse has a fever he will need another week to recover. While horses generally survive the flu, they often need three to six weeks to recover fully. It's the secondary infections that can be life-threatening.

Continued Research

As there have been no major outbreaks lately to manage and study, equine influenza researchers around the world are working on critical vaccine-related research, according to Chambers. One significant research path is DNA vaccine technology, which includes using the virus' DNA (rather than relying on proteins derived from disease-causing agents) and an adjuvant to boost immune response. Researchers believe this vaccine offers better immunity against disease, more rapid protection, and fewer adverse reactions because there's no antigen for the horse to react to. Further, "it offers practical benefits like ease of handling and ease of use," Chambers says.

Chambers relates that researchers are also focusing on needle-free alternatives to traditional intramuscular and intranasal influenza vaccine administration methods. Some are actively experimenting with spring-loaded shooters that propel liquid into the horse's skin, and they are adjusting the projection's depth and strength to determine optimal results. At this point in the research process, the sprays are causing skin irritation but no actual puncture. This administration method might be beneficial as far as immune response efficiency is concerned, because it penetrates the skin rather than accessing the muscle. Skin is accustomed to cuts and scrapes, so the immune system is primed with immediate response. Chambers adds that this line of research could have far-reaching benefits for multiple species.

Aside from not-yet-public proprietary research, studies of the horse's innate immune response to flu are ongoing. Scientists are looking at things like cytokine (a mediator of inflammation) response, which triggers fevers, and the secondary bacterial infections to which flu cases are predisposed.

A major area of interest is interspecies transmission of equine flu. Similar instances of this phenomenon have been seen with avian and swine flu infecting humans. And in 2004 equine influenza jumped species and infected dogs in Florida. This virus strain has been circulating for about seven years now and no longer appears identical to equine flu, having mutated since its introduction to dogs.

Chambers explains that researchers look for potential roadblocks the virus will face in making successful mutations or jumps--the first being opportunities for exposure. Is there regular contact between species? Once the virus is introduced in another host, can it successfully infect that host's respiratory tissue cells? Can it then be transmitted within a new species and, if so, why is it able to do this? The bottom line is scientists still don't know why some viruses can infect and then be transmitted, but others can't. Avian flu, for instance, which infected humans in Asia a few years ago, made the jump across species, but it was not transmitted from person to person.

Virus mutations occur at random. Many of the mutations are bad for the virus strain, while others don't affect its virulence. And occasionally a mutation makes a virus less recognizable to the immune system, such as with antigenic drift, triggering the need to update vaccines. "We are worried that it will take only a handful of such mutations before the virus learns how to be successful (in transmitting rapidly from host to host)," says Chambers.

Scientists are continuing research in earnest to better understand flu mutations. In the face of an outbreak of a new virus strain it is almost impossible to update, test, and produce enough vaccine to inoculate a large population quickly enough to be effective. Researchers are hoping to uncover tangible vaccination solutions before the virus mutates to the point that influenza again becomes a deadly threat.

Take-Home Message

Be on the lookout for signs of influenza, especially in young animals and congested environments such as horse shows and racetracks. It pays to be particularly vigilant because the loss of performance days can cost owners and the industry a lot of money. Vaccinating wisely, responding rapidly to clinical disease, and managing cases are your best tools to combat influenza. In the meantime, researchers are working to make sure when the next challenge from equine influenza hits, the animal health industry will be ready to respond quickly.

About the Author

Liza Holland

Liza Holland is a freelance writer and voice talent based in Lexington, Ky.

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