At the barn, horse fly spray is often your first line of defense against annoying insects. Fly spray helps repel and kill flies, but the best fly spray for horses helps protect your horse against potentially disease-causing ticks and mosquitoes, too.
Tricky Tick-Borne Diseases
Tick-borne diseases are among the most difficult to diagnose and challenging to treat. Lyme disease, equine piroplasmosis and anaplasmosis are three of the better known equine diseases spread largely by ticks, but there are others as well.
Equine piroplasmosis (EP), also called babesiosis, is found around the world. It is currently not endemic to the United States, Canada, Australia, Japan, England, Ireland or Iceland. Horses imported into the United States from endemic areas must undergo quarantine and testing to ensure that they are not carrying antibodies to either of the two protozoa that cause EP. If a horse in the United States were diagnosed with EP, there would be two priorities—clearing the organism from his system through treatment and preventing the disease from becoming established in this country.
The protozoa that cause EP damage red blood cells, reducing their ability to carry oxygen and causing anemia. Signs develop within one to four weeks after exposure. Horses with mild EP may become weak, lethargic and lose weight. In acute cases, signs include fever, jaundice, roughened coats, colic, a swollen abdomen, edema in the legs and red urine. Some infected horses may show no signs of illness. Unless the parasites are eliminated through treatment, a horse may carry the parasites for the rest of his life, remaining a potential source of infection for others.
EP is a reportable disease, meaning that veterinarians who diagnose a case must notify state and/or federal authorities. Infected horses can be enrolled in a treatment program to receive an antiprotozoal medication that can eliminate the parasites from the horse’s system. If treatment fails to clear the organisms, the horse may be euthanized, put into permanent quarantine or be exported to an endemic area.
Probably the most commonly known tick-borne disease in the U.S. is Lyme disease. Named for Lyme, Connecticut, where it was first identified in 1975, Lyme disease cases are most common in New England and Mid-Atlantic regions as well as the upper Midwest, but they can appear in other states as well. According to the CDC, the number of cases diagnosed in the United States has been increasing over the past decade.
A horse bitten by an infected deer tick will experience a localized inflammatory reaction—the characteristic “bull’s-eye rash” at the site of the bite—which in horses is often missed under the hair coat. In some cases, the bacteria then enter the bloodstream and are carried throughout the horse’s body to cause localized infection in various tissues including joints, muscles or organs.
Lyme disease is considered acute if it is diagnosed within the first weeks after exposure and chronic if the infection persists for months. Signs can be vague and varied, depending on the tissues affected. They include generalized stiffness, lameness that shifts from limb to limb, swollen joints, weight loss, sensitive skin, lethargy, behavioral changes or rarely uveitis (inflammation of the eye). The horse may just generally not feel good or develop a surly attitude.
Lyme disease is difficult to diagnose because exposure to the bacterium alone is not proof of illness and the outward signs mimic so many other problems. For that reason, other illnesses and injuries are generally ruled out before a Lyme diagnosis is made. Lyme disease can be treated with antibiotics.
The same ticks that transmit the Lyme disease organism can also carry the bacterium responsible for anaplasmosis (previously known as ehrlichiosis). It’s possible for a horse to have both at once. Fortunately, anaplasmosis is rarely fatal and is easy to treat.
The bacterium infects white blood cells and causes signs like high fever as well as depression, lethargy, poor appetite, jaundice, ataxia and edema in the legs. Antibiotics—primarily oxytetracycline and tetracycline—can eliminate the infection. The corticosteroid dexamethasone can help to alleviate more severe signs. The good news is horses remain immune once the infection clears.
Diseases Spread by Mosquitoes
Mosquitoes can also be carriers of several deadly equine diseases. Eastern equine encephalitis (EEE), Western equine encephalitis (WEE) and West Nile virus (WNV) are the most common.
Eastern Equine Encephalitis
Eastern equine encephalitis generally occurs east of the Mississippi River in the U.S. Culiseta melanura mosquitoes transmit it to horses, causing serious neurologic disease with 90% fatality. Horses with encephalitis appear sleepy, develop a fever, involuntary muscle twitching and an uncoordinated gait. Eventually the horse goes down and is unable to get up. Most cases begin to appear in late summer or early fall months in temperate climate regions, but cases can appear year-round in southern states.
Western Equine Encephalitis
Western equine encephalitis is a similar neurologic disease to EEE. It is transmitted by the Culex tarsalis mosquito, which lives in the western part of the U.S. The fatality rate for WEE is about 40–50%. Recent years have seen a dramatic drop in equine cases, with none reported in the western states since 2004. However, this virus is still found in birds, mosquitoes and jackrabbits in this geographical area.
West Nile Virus
A West Nile virus infection does not always cause clinical signs but has a 35% fatality rate when it does. Along with fever, many horses experience problems chewing, swallowing and blinking among other functions. The infection also causes hind-limb weakness, muscle tremors, behavior changes, ataxia, an inability to stand or rise and paralysis. It is possible that other signs of encephalitis—head pressing, aimless wandering, seizures, hyper-excitability and coma—will develop in a horse infected with WNV. About 40% of the horses that recover from clinical signs of WNV will continue to experience neurologic deficit.
Although eliminating ticks and mosquitoes from your horse’s environment is practically impossible, you can take steps to safeguard his health. Using a powerful, proven tick repellent for horses or a proven fly spray that both repels and kills ticks and mosquitoes is key to preventing life-threatening diseases.