By Susan Kauffmann

Whether we are pleasure riders who live to hit the trail or serious competitors gunning for the top prize, summer is prime horse time, and we want our horses to be at their very best. Fortunately, horses generally have fewer health problems in the summer months, but this doesn’t mean we can take their health for granted. In fact, there are a number of ailments associated with the insects, heat, sun and activities of summer – some of which can be quite serious. Therefore, if you want your horse to remain hale and hearty throughout the summer season, you need to know what problems to look out for, and what you can do to prevent them.

In part one of this series, we look at diseases your horse can catch from the summer pests merrily winging their way around your horses.

What’s Bugging You: Serious Insect-Borne Health Threats 

The presence of insects is an inescapable and irritating part of life in the summer — but insects can be more than just an irritation to your horse. Explains Bob Wright, DVM, Veterinary Scientist, Equine and Alternative Livestock, Ontario Ministry of Agriculture, Food & Rural Affairs (OMAFRA), “You’ve got to be aware of insects, especially mosquitoes, for their potential to spread serious diseases like West Nile Virus, Eastern Equine Encephalomyelitis and Western Equine Encephalomyelitis. That potential increases towards the end of summer, and into the Fall.” Because all of these diseases are potentially fatal, it is extremely important to be familiar with their clinical manifestations, as well as what measures to take to help keep your horse from contracting them.

West Nile virus (WNV) causes serious and potentially fatal neurological disease in horses. Mosquitoes bite infected birds and then spread the virus to other species. Although the number of equine cases has dropped off markedly, WNV continues to be a threat, with an equine mortality rate of about 30%. Those at greatest risk are the young, the old, and those with compromised immune systems. Neurological symptoms such as muscle twitching, incoordination and muscle weakness are the most common clinical manifestations of WNV in horses. Additional signs may include fever, somnolence (sleepiness), inability to eat and drink, recumbency, and seizures.

Mosquitoes can infect your horse with West Nile Virus, which is potentially fatal.

If a horse survives a WNV infection, it is protected from development of clinical disease for an extended period of time.  Nonetheless, prevention remains the best option. There are now several types of  vaccine available, and most veterinarians recommend vaccinating all horses, especially in areas where WNV is known to be present.

Encephalomyelitis refers to a disease that causes inflammation of the brain and spinal cord, usually as a result of an infectious agent. There are two variants of equine encephalomyelitis of concern in North America: Eastern (EEE) and Western (WEE), both of which are caused by viruses transmitted mostly by mosquitoes. Either disease can be serious, but EEE has a much higher fatality rate than WEE.

Typical symptoms of Eastern Equine Encephalomyelitis include loss of appetite, fever, and change in behavior. Within 12 to 24 hours of the initial symptoms, the horse will exhibit head pressing, circling, and will often become blind. As the disease progresses, the horse may become recumbent and might suffer seizures. In the worst cases, horses die from respiratory arrest two to three days after the onset of clinical signs. The fatality rate for horses with EEE may be as high as 90%. Horses that do survive are frequently left with permanent neurological impairment. Treatment is mainly supportive, as there are no anti-viral drugs available for EEE. Fortunately, EEE is relatively rare and is generallyrestricted to the eastern, southeastern and some southern states.

Western Equine Encephalomyelitis manifests quite differently. After an incubation period of one to three weeks, horses will develop a fever up to 105 degrees. In many cases, this will be the only symptom, as the horse’s natural immune response will clear the virus from the body within 48 hours after the onset of the fever. If the horse is unable to clear the virus, the situation becomes much more serious. The central nervous system is attacked, resulting in symptoms that include agitation, compulsive walking and circling,and sometimes crashing into walls. This stage is followed by extreme depression, reluctance to move, and drooping posture. The horse may also have tremors, difficulty swallowing, and an uncoordinated gait.  If the horse remains standing, it may fully recover, but horses that become recumbent have a poor prognosis.

Overall, the fatality rate for WEE is close to 50%. WEE is the most common strain of equine encephalomyelitis, with outbreaks recorded in the western and mid-western states. Variants of WEE have also caused sporadic cases in the northeast and southeast. The risk of exposure and the geographic distribution of WEE and EEE vary from year-to-year depending on the distribution of mosquitoes and birds that act as reservoirs for the virus.

Prevention: To reduce the likelihood of horses contracting WNV, WEE, and EEE, the American Association of Equine Practitioners (AAEP) advises vaccination of all horses against these diseases. In addition to vaccinations, a good health program will include minimizing your horse’s exposure to mosquitoes, and mosquito control. To reduce mosquito exposure, stable horses at dusk and dawn; use fly blankets, masks, and leg wraps; turn barn lights off; use fans to move air; and use mosquito repellents.  Mosquito control programs include reduction of standing water, encouraging natural predators (fish, birds, etc.), and use of chemicals for larval and adult mosquito control.

However, there are other serious diseases that may be spread not only by mosquitoes, but also by biting flies. Equine Infectious anemia (EIA), also called swamp fever due to the higher incidence in warm, wet areas, is just such a disease. Says Dr. Wright, “The EIA virus is mechanically transmitted from one horse to another by the bloodsucking horse flies, deer flies (Tabanids), stable flies (Stomoxys spp.), mosquitoes and possibly midges. For transmission of the EIA virus to occur, the horse fly (vector) must first find and begin feeding on an infected horse, be interrupted in its feeding, and then transfer the virus during a subsequent feeding on a susceptible horse. Feeding must occur within a short period of time.”


Deer flies do more than just inflict a painful bite: they can also be carriers of Equine Infectious Anemia.

Deer flies do more than just inflict a painful bite: they can also be carriers of Equine Infectious Anemia.

The first symptom of EIA is usually fever, which comes on 7-30 days after infection. In most cases, the initial fever goes undetected, and the disease is not recognized until it becomes chronic and the symptoms are more obvious. As Dr. Wright explains, “Chronically infected horses  experience episodes of fever, weight loss, depression, progressive weakness, anemia and edema. These signs occur every two weeks in recurring cycles. Other symptoms which may occur during the course of the disease include: loss of appetite, frequent urination, diarrhea, weakness, paralysis of the hindquarters, paleness of the mucous membranes, yellowish discoloration of the conjunctiva, small pinpoint hemorrhages beneath the tongue, rapid breathing and accelerated pulse. Pregnant mares may abort.”

Most frightening is that any symptomatic episode can be fatal, even if the horse has survived many of them in the past.An unusual characteristic of the EIA virus is that it remains permanently in the body. Therefore, even though a horse may be in symptom-free remission,  it is still a source of possible infection for other horses. Because of this, recommendations are that affected horses be euthanized or permanently segregated in a fly-proof building.

Unfortunately, there is currently no vaccine available to prevent EIA, and no specific treatment for the disease. What we do have is the Coggins test, a blood test that detects antibodies to the EIA virus. Utilization of the Coggins test has helped reduce the incidence of EIA, and many facilities now require a negative Coggins test certificate before a horse is allowed onto the property.

In reduce the incidence of EIA infection, the American Association of Equine Practitioners (AAEP) encourages horse owners to:

  •  Require proof of a recent negative Coggins test at time of purchase or for new horses entering the premises. Require an EIA test for horses which have spent time at a premise where EIA-positive horses have been identified.
  •  Only participate in events that require evidence of a negative Coggins test for every horse entering the event to prevent disease introduction and spread.
  •  Practice good fly control by regular mucking of stalls, proper disposal of manure away from horse stabling areas and use of fly sprays or natural predators to minimize fly presence.
  •  Use a sterile needle and syringe for all injections or treatments.
  •  Disinfect any surgical or dental equipment thoroughly between horses. Remove all debris and blood with soap and water before disinfection.
  •  Use a sterile needle each time when puncturing a multi-dose medication bottle. Consult a veterinarian to demonstrate how to use sterile technique when drawing up medications.
  •  Separate horses with fevers, reduced feed intake and/or lethargy from your other horses and contact your veterinarian.

Stay tuned for Parts 2 and 3 of this important series!

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