Understanding and controlling the risk of swamp fever

Horses that test positive must be euthanized or quarantined for life

Understanding and controlling the risk of swamp fever
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The recent increase in confirmed cases of equine infectious anemia (EIA), also known as swamp fever, in Alberta is a reminder to horse owners that this disease maintains a constant presence in equine populations.

Often evidence for infection is noted only after routine surveillance testing for EIA.

EIA is a blood-borne and potentially fatal viral disease affecting the immune system of horses, donkeys, and mules worldwide.

EIA can present in an acute or chronic disease state or as an inapparent carrier. With initial exposure, the virus reproduces and traffics throughout the animal’s body within white blood cells.

Viral particles are then released into the blood and become attached to red blood cells. The horse’s immune system mounts a vigorous attack against the virus by producing antibodies.

This attack inadvertently also causes destruction of the horse’s own red blood cell components, resulting in anemia and organ-damaging inflammation. During this time the horse will be acutely ill, with heavy concentrations of virus in its bloodstream.

Clinical signs vary

The clinical signs, morbidity, and mortality of EIA vary with the strain of the virus and the immune system of each individual horse. Symptoms include fever, loss of appetite, depression, anemia, dependent swelling, jaundice, inco-ordination and/or muscle weakness and loss of condition.

Horses may die from the direct effects of the virus or from secondary infections as the body is weakened by the virus. Horses that survive the acute phase go on to experience chronic cyclic flare-ups of clinical signs as the horse alternates between remission and the disease state.

The onset of symptoms is often associated with stressors. Within a one-year period many horses begin to control the infection and show no clinical signs. These inapparent carriers may serve as a source of infection for other horses. On occasion an apparently healthy horse will show serological evidence of viral infection yet never exhibit any symptoms of the disease.

Evidence for infection is noted when a horse develops a clinical bout of illness or after routine surveillance testing for EIA. Much about the pathogenesis of EIA is not completely understood, including the mechanism which allows the virus to survive the immunological response of the horse.

In a natural setting, the EIA virus is most commonly transmitted from an infected horse to a non-infected horse by the mechanical transfer of blood from blood-feeding insects such as horseflies and deer flies.


The virus is passed from one horse to another on the insect’s mouthparts as it feeds. The painful bite of the flies causes the horse to react and interrupt the insect’s feeding. The fly seeks to immediately resume feeding on either the same animal or on another nearby horse.

If an uninfected horse is in close proximity to the infected horse, the completion of this blood meal is likely to effectively transfer virus between horses. Because of their strong instinct to immediately complete a blood meal, horseflies do not travel long distances before biting again.

Through the insect vector pathway, the disease can enter a herd and slowly and silently infect an increasing number of horses. Since these insects are mainly active in the summer months and prefer wet, marshy and wooded areas, outbreaks of EIA are often associated with horses pasturing in swampy areas, hence the name swamp fever.

Another means of blood transmission of EIA amongst horses is through the practice of sharing needles, and other blood-contaminated equipment such as dental floats, surgical instruments and tattooing tools.

There is currently no cure for EIA, nor is there an effective vaccine available to protect horses from getting EIA. As a result many countries including Canada and the U.S. have established control and containment programs based on voluntary serological testing.


The Coggins test is used to consistently and reliably detect the presence of EIA-specific antibodies. Confusion often arises because reference to the Coggins name does not appear anywhere on the EIA Serum test report and certificate.

The original test was developed by Dr. Leroy Coggins in 1970 and thus was aptly named the Coggins test. Currently a c-ELISA test is also employed, as it offers the advantage of rapid results. Blood sample can only be taken from horses by veterinary practitioners accredited by the CFIA.

A negative Coggins test is a snapshot of a horse’s health status at a particular point in time. A negative Coggins test means there were no detectable antibodies at the time of testing. A positive test indicates the horse does have detectable EIA-specific antibodies.

Venues involving the movement, co-habitation, and commingling of horses recognize different time frames regarding the eligible status of a negative Coggins test. Its eligibility can range from 30 days to a year.

Proof of a negative EIA test is an entry requirement for many competitions, horse-related events, boarding facilities, and is necessary for border crossings. For export purposes a negative Coggins is sufficient for 180 days. For the most part, the requirement for EIA testing is based on and determined by members within the horse industry itself.

EIA is a reportable disease under the Health of Animals Act. Horses confirmed to have EIA have limited options.

The CFIA requires that positive horses be either euthanized or quarantined for life. Strict quarantine measures considerably limit the quality of life for the affected animal.

Owners of euthanized horses are compensated by the CFIA at a specified rate for the loss. In the case of a positive horse being identified, CFIA conducts focal testing where other positives may be identified.

The many stakeholders in the equine industry all share equal responsibility and vigilance for educating themselves regarding the management of EIA. In truth, there are many unknowns when assessing the risk EIA poses to any one individual horse.

Certain management and geographic factors do put particular horses at a greater risk for contracting EIA. Environments with a steady influx of new horses, horses pastured in damp, swampy areas, and animals in frequent contact with outside horses that live and travel in regions known for EIA outbreaks or which may have contact with feral horses chance a higher level of exposure to the EIA virus.

About the author


Carol Shwetz is a veterinarian focusing on equine practice in Millarville, Alberta.



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