Update on Infectious Anemia in Horses in California

Researcher looking through microscope to examine disease samples.
Researcher looking through microscope to examine disease samples. Unknown

Newsdate: Tuesday, February 6, 2024 – 11:30 am
Location: SACRAMENTO, California

Equine Infectious Anemia (EIA), also known as “Swamp Fever”, is a viral disease of all equidae (horses, donkeys, mules and zebras). The virus belongs to the family that includes the human immunodeficiency virus (HIV), and other animal immunodeficiency viruses. All equids are susceptible to EIA. The virus is not transmissible to humans.

Horse isolated in stall.

Horse isolated in stall.

Equine Infectious Anemia virus is an enveloped virus that does not persist in the environment and is easily killed by most common disinfectants.
© 2017 by Smerikal New window.

January 9, 2024: One of the twenty-two (22) remaining exposed horses tested positive for EIA on the sixty (60) day retest. This two-year-old Quarter Horse gelding was euthanized. The remaining twenty-one (21) exposed horses and three (3) foals will remain under quarantine for an additional sixty (60) days until they are retested.

October 12, 2023: Twenty-four (24) potentially exposed horses were sampled at the second Riverside County premises on October 5th and two (2) of have initially tested positive for EIA and were euthanized. The two positive horses were both 4-year-old Quarter Horse geldings. The remaining twenty-two exposed horses and three (3) foals will remain under quarantine until their sixty (60) day retest.

September 28, 2023: A 4–year–old Quarter Horse gelding residing in Riverside County was confirmed positive for Equine Infectious Anemia (EIA). The gelding was quarantined and later euthanized at its home private premises where it was the only equine present. The horse had a brief career in sanctioned racing. Epidemiological investigation led to the quarantine of twenty-four (24) exposed horses and three (3) dam’s side foals at a second Riverside County premises.

Equine Infectious Anemia (EIA) virus belongs to the family Retroviridae. This group of viruses includes human immunodeficiency virus (HIV), the immunodeficiency viruses of apes and monkeys (SIV), goats (CAEV), cats (FIV) and cattle (BIV). The virus persists in the horse’s white blood cells, and in the plasma (non-cellular portion of the blood) during febrile episodes.

The amount of virus in the infected animal’s blood varies. Stress due to racing, heavy exercise, transport or illness due to other causes may elevate the amount of virus in the blood.

EIA virus is an enveloped virus, does not persist in the environment and is easily killed by most common disinfectants. The virus is viable (infective) for up to 96 hours on contaminated needles. The virus stays viable in the mouthparts of a biting fly for less than four (4) hours. Thus, horses in close contact with an infected horse are at risk of infection via fly bite.

Natural transmission occurs when a deer or horse fly bites and consumes a blood meal from an infected horse and transfers the virus via its mouthparts to another horse. Additionally, pregnant mares may pass the virus to the foal in utero or through the milk. Stallions can infect mares at breeding.

People can introduce virus to a naïve horse through the use of infected blood or blood products, or through the use of blood contaminated equipment such as needles, syringes, surgical instruments, dental equipment, tattooing equipment, or any other equipment.

Clinical Signs

Clinical signs can vary widely. Most infected horses are asymptomatic, showing no signs of disease. Acutely infected animals may develop a fever, go off feed or die suddenly. A chronically infected equine may develop non-specific signs such as weight loss, weakness, anemia and swelling of the legs, chest and abdomen. Stress or immunosuppression can lead to recrudescence of disease or to increased levels of viremia.

Diagnosis & Treatment

A serologic (blood) test confirms the diagnosis of EIA. The two most commonly used serologic tests are the agar gel immunodiffusion (AGID), commonly known as the “Coggins” Test, and the enzyme-linked immunosorbent assay (ELISA). A positive test indicates the presence of EIA specific antibodies.

The ELISA test can detect antibodies earlier than the Coggins test. As the ELISA test may produce false positive results, the confirmatory test for EIA is the AGID test. Only USDA-approved laboratories can perform EIA testing in the United States.

There is no known treatment for EIA. Infected horses become lifelong carriers and pose a risk of infection to other horses.  Therefore, if a horse is confirmed to be positive for EIA, management options are limited to:
    a.    Euthanasia
    b.    Lifetime quarantine, with permanent isolation at a minimum of 200 yards from all other horses. Positive horses must be permanently identified (tattoo, brand or microchip).

An epidemiologic investigation will be performed to determine the movement history of the infected equine and to identify other exposed horses or equids. Exposed equines will be quarantined and tested, and be subject to a retest 45-60 days after the removal of the EIA positive horse. Quarantine will remain in effect until negative retest results are received.

Prevention and Control

There is no approved vaccine for EIA in the United States. In order to prevent infection, follow these guidelines:
    •    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.
    •    Only administer commercially licensed blood products.
    •    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.
    •    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 premises 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.
    •    Separate horses with fevers, reduced feed intake and/or lethargy from your other horses and contact your veterinarian.

Press release by California Department of Food and Agriculture

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