insect-borne equine diseases

During the 2019 University of Kentucky (UK) Equine Showcase, held Jan. 26 in Lexington, Rebecca Ruby, BVSc, Dipl. ACVP, of the UK Veterinary Diagnostic Laboratory, educated horse owners about important equine diseases carried or caused by insects.

What is equine infectious anemia (EIA) and how does a horse get it?

EIA is caused by a virus that integrates into the horse’s DNA and results in life-long infection. There is no vaccine and no cure for the disease.

The disease presents in three phases:

  • Acute, characterized by high fever and low blood platelet count;
  • Chronic recurrent infection, which results in frequent fevers and illness; and
  • Inapparent carrier, in which the horse appears healthy outwardly but is infected and can still transmit disease.

The most common natural way EIA spreads is via blood-feeding insects, such as deerflies and horseflies, which take blood from infected horses and pass it to uninfected ones.

Additionally, an increasing number of EIA cases have been linked to sharing needles and blood doping. Once a horse is infected, it is infected for life.

What can I do to prevent EIA?

Many horse owners who travel and compete are be familiar with the Coggins test, a blood test that detects antibodies to the EIA.

“Only approximately 22% of the U.S. horse population is tested annually,” Ruby said.

To prevent the spread of disease, Ruby suggested that all horses be tested for EIA at least annually, especially new herd additions and those used as blood or plasma donors. She also stressed that needles and syringes should never be reused or used to inject more than one horse at a single time.

Property owners should use environmental and on-animal control, such as topical application of fly sprays to combat horse flies, deer flies, and stable flies, to help prevent EIA spread.

What happens if my horse tests positive for EIA?

If your horse tests positive, he or she will be quarantined and retested for confirmation. State regulatory officials are always involved in positive cases and will determine the exact steps taken in each case.

Typically, horses within 200 yards of the infected horse are considered exposed and are also quarantined. All exposed horses are tested at 30- to 60- day intervals, and the quarantine ends when horses are negative for at least 60 days.

Because there’s no cure for EIA, positive horses must be permanently quarantined at least 200 yards from all other equids and undergo periodic regulatory testing, moved under permit to a federally approved facility, or euthanized.

What is Potomac horse fever and how does a horse get it?

Potomac horse fever is caused by the bacteria Neorickettsia risticii and can be mild to life-threatening. Within 24 to 48 hours of infection, approximately 60% of infected horses produce moderate to severe diarrhea, often accompanied by colic, and some horses develop severe toxemia or dehydration. Laminitis can occur in as many as 40% of affected horses. A horse can present any or all clinic signs.

Pregnant mares that develop PHF can abort their foals or fetuses several months following infection, and foals and fetuses from infected mares often test positive for PHF.

Case-fatality rates vary from 5% to 30% and depend mostly on the strain involved.

The source of the infection includes aquatic insects, such as caddisflies and mayflies. Horses that graze near creeks or rivers can inadvertently ingest the adult insects. Horses can also consume insects attracted by stable lights and subsequently accumulate in feed or water.

insect-borne equine diseases

What can I do to prevent PHF?

There is a vaccine available to boost a horse’s immunity to N. risticii. While this vaccine might not prevent illness in some cases, many researchers believe it can reduce the severity of clinical signs if a horse is exposed to the organism. If PHF cases are infrequent in your area, consider working closely with your veterinarian to assess your horse’s risk level and develop the best course of action to prevent disease.

How can different parasites affect my horses?

Onchocerca is a parasite that carried by culicoides (midges or no-see-ums) that can cause dermatitis in horses. Midges prefer to bite certain areas on the horse, including the tail, mane, and belly. Once an Onchocerca enters the horse’s system via a midge bite, the adult organism lives in the nuchal ligament and can produce microfilaria, or larvae, for five years. Ruby said the best treatment is ivermectin, which kills the microfilaria, but not the adult.

Habronema is a nematode, or worm, that lives in a horse’s stomach and can cause summer sores. These sores typically occur on the legs, inner corners of the eyes, and moist areas where the skin’s surface is likely to be broken from irritation.

Ruby also described a common parasite horse owners should protect against—the tapeworm—which is hosted by the oribatid mite. The mite feeds off the organic material present in the feces and ingests the tapeworm egg. Eggs develop within the mite and, when the horse later consumes the mite, the parasites travel to the junction of the ileum and cecum. Tapeworms have been associated with colic and intussuceptions, where one section of the intestine telescopes into another.

What is insect bite hypersensitivity (IBH)?

Sometimes called “sweet itch,” IBH is the most common allergic skin disease in horses. Ruby said IBH has both a genetic and environmental component and can be identified by its clinical signs: hives, itching, hair loss, and inflammation.

What should I do when my horse shows signs of IBH?

If your horse shows IBH signs, first consult your veterinarian. He or she might administer an intradermal allergy test, which can help determine what is bothering your horse. For severely affected horses, veterinarians frequently use corticosteroids to treat acute signs. After diagnosis, your veterinarian might recommend histamine blockers and allergy shots to help control the disease, along with fly sheets, boots, and masks and insect repellents.

What common tick-borne diseases affect horses?

Technically, ticks are arachnids, not insects. But, they can still transmit bacteria and diseases to horses.

Anaplasma phagocytophilum is an intracellular bacteria that lives in a horse’s white blood cells, mainly neutrophils, and is transmitted by several species of ticks. They can infect horses within two hours of feeding, but it can take up to 36 hours for infection to occur. This condition can be diagnosed by visualizing the bacteria in the neutrophils or via blood testing. Affected horses generally have a high fever during the first day or two of infection; other clinical signs can include fever, depression, anorexia, limb edema (fluid swelling), and reluctance to move. Horses typically recover in about two weeks but relapses have been reported.

Additionally, Lyme disease, an infection caused by Borrelia burgdorferi transmitted via infected ticks, can cause a variety of clinical signs. Owners and veterinarians anecdotally report signs including stiffness and lameness in more than one limb, muscle tenderness, hyperesthesia (hypersensitivity to touch and sound), lethargy, behavioral changes, and muscle wasting and pain over the thoracolumbar area (the horse’s lower back). However, only three clinical signs have been experimentally produced with infection, Ruby said: uveitis, dermatitis and neurologic disease.

Horses that test positive for exposure range from 14.8% to 94% in different parts of the United States. All current tests detect antibodies against the causative agent (so, exposure rather than infection). Researchers are still working to determine the true role of Lyme disease in horses. The first step to diagnosing if your horse has Lyme disease is to rule out the presence of other diseases.

Samantha Geller, a senior double-majoring in equine science and management and environmental and sustainability studies, is a communications intern with UK Ag Equine Programs and the Gluck Equine Research Center.

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More information on Gluck Equine Research Center and UK Ag Equine Programs.