Equid Blog : Horse Health & Diseases : University of Guelph : Ontario Veterinary College : Equine Infectious Diseases

A horse from Missouri has been diagnosed with equine infectious anemia (EIA) as part of a routine Coggin's test. As is normal, the infected horse was euthanized and the farm was immediately quarantined so that all other horses could be tested.

EIA, which is sometimes also called swamp fever, is a caused by the un-originally named equine infectious anemia virus. It is a retrovirus (the same type of virus that causes HIV/AIDS in people and feline leukemia in cats) that can cause serious infection but also long-term carriage by apparently healthy horses. The ability of this virus to cause lifelong infection in some horses is a major problem and complicates its control. The infection is not treatable and there is no vaccine. For these reasons, aggressive control measure are in place in most regions. The main component of this is routine testing.

EIA testing is a standard requirement in many areas of the world for horses that are traveling, competing or being sold. Traditionally, the Coggins test has been used for EIA testing. More recently, another test (a cELISA test) has become available. This test is thought to be more sensitive and takes less time to perform, but positive results still need to be confirmed with a standard Coggins test.

Positive results are immediately reported to the appropriate government agency (in Canada, it's the Canadian Food Inspection Agency). Immediate euthanasia of positive horses is the typical response, although keeping the horse in permanent quarantine, with insect control and a minimum distance from any other horse (or donkey or mule) is also an option in some cases. It may seem draconian to euthanize an apparently healthy horse and quarantine the farm, but this is an important disease that is hard to control if it gets established in an area.

The most important EIA control measure is regular testing, which allows for early detection of cases and implementation of appropriate (albeit unfortunate for the horse) measures. Other measures that help reduce the risk of transmission if the virus is in the area include control of biting insects and ensuring that there is no re-use of needles or other equipment or biologics that can transfer blood. 

There's no indication as of yet where the infection originated from in the Missouri horse. That's a very important factor, because the horse had to get it from somewhere, and that means another infected horse, donkey or mule. Finding one positive animal means that there must have been another positive animal somewhere in the vicinity to infect the insect (the most likely vector) that infected this horse. Hopefully, the horse did not pick it up from another animal in the US, but careful tracking of this horse's travel history and detailed surveillance of equids in the area will need to be performed to figure this out.

Image: A black horse fly (Tabanus atratus); members of this genus are capable to transmitting EIA between horses when they bite.  (Photo credit: Mark Cassino, source: USDA Systematic Entomology Laboratory)

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Equine infectious anemia (EIA) virus, a chronic and potentially devastating bloodborne virus, was identified in two horses in Britain. The two affected horses were from a group of horses imported from Romania via Belgium. They were tested as part of standard import testing regulations designed to do exactly what happened here - diagnose this important infectious disease at the time of importation so that the infected horses cannot spread the disease in the country. The two positive horses will be euthanized, the unfortunate but standard response to this virus, because infected horses can pose a risk of infection to other horses for their entire lives. Other horses on the premises are under quarantine and are likely being tested further. The risk of transmission to other horses during the presumably short period of time they've been on the farm is probably low because, as an insect borne disease, biting fly activity has probably been pretty low during the cold British winter weather. (Transmission by sharing needles or other human-associated ways of cross-contamination of blood is also a concern, and has been a problem in other outbreaks.)

Chief veterinary officer Nigel Gibbens stated that these are the first imported cases of EIA identified since 1976. This is a good example of why we need to continue routine infection control measures such as import testing, even when nothing is found for years. Some people try to argue that since certain problems don't seem to be present, or at least are not identified, that infection control testing or activities should be decreased. This situation illustrates why that's bad thinking. Despite only picking up one incident in the past 34 years, this is a very important finding - failure to detect the positive horses could have lead to widespread infection in the country, which would ultimately make it very difficult and expensive to try to control. You never know when the next outbreak is lurking around the corner, and complacency is a big enemy of infection control.

Image source: www.collectgbstamps.co.uk

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A pony in New Jersey has been diagnosed with equine infectious anemia (EIA) and euthanized. Details are pretty scarce at this point and it's unclear whether the pony was sick or whether EIA was simply diagnosed through routine testing. There's also no indication of where the pony might have acquired the infection, which is a very important question.

Equine infectious anemia is a viral disease of horses and other equids that is characterized by recurrent fever, lethargy, decreased appetite and decreased red blood cell numbers (anemia). Sometimes, the first episode of disease (acute EIA) can be fatal, but most horses recover. Most survivors experience recurrent episodes of illness. In some horses, these episodes become frequent and severe.

A major problem with EIA is that horses are infected (and are infectious) for life. Even if they recover from the acute disease and rarely or never get noticeable recurrent disease, they are still carrying the virus in their blood. Biting flies are the main means of transmission from horse-to-horse, although human-associated transmission through reuse of needles or other procedures that cross-contaminate blood between horses can also be involved.

The combination of life-long infection and an insect vector is BAD. That's why, in areas where EIA is not endemic, infected horses are usually immediately euthanized. In some regions, affected horses can be branded or tattooed to permanently indicate their EIA status and housed at least 200 yards from any other horse (and often with strict insect control measures).

Control of EIA focuses on identification of carriers. Most jurisdictions require regular EIA testing (formerly the "Coggins test") for horses that are traveling or competing, as well as imported horses. While this disease is extremely rare and the vast majority of routine tests are negative, it's a disease that can silently spread in a population if people are not looking for it - routine testing is critical for keeping this disease at bay.

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Equine infectious anemia (EIA) is a rare disease but one that a lot of time, effort and money are put into avoiding. Most people know about this disease by way of the most common test (formerly) used to diagnose it: the Coggin's test (see image).

Equine infectious anemia is caused by a virus of the same name ( equine infectiious anemia virus, EIAV). EIAV is a lentivirus which, like all other lentiviruses, causes persistent infection. Unlike most other lentiviruses that cause slow, gradual preogression of disease, EIAV infection usually causes a sudden onset of disease (acute phase) followed by recurrent disease. After the initial (acute) phase, horses can appear normal, which means they can be a silent reservoir of the disease. That's why routine testing for EIAV is required in many circumstances: to detect silent carriers so they cannot continue to transmit infection.

Signs of acute EIA vary, but usually include fever, lethargy and decreased appetite. Anemia (decreased red blood cell count) and thrombocytopenia (decreased platelet count) can be detected. Anemia is more common and pronounced with recurrent infections. Intermittent illness often develops after the first acute episode. Affected horses may experience short (3-5 day) periods of fever, lethargy and decreased appetite. The severity of the anemia often correlates with the severity and frequency of these disease episodes. In some horses, these episodes are very common, long and severe, and these horses often have severe weight loss and anemia. However, most infected horses stop developing obvious signs of disease after a year and seem perfectly healthy.  This may be good for the one horse, but it's bad for other horses in the area to which the virus may be transmitted.

EIAV is a bloodborne virus that can be transmitted by blood-feeding insects, especially tabanid flies (horseflies, deerflies). Stable flies can also transmit EIAV but do so less effectively. Contaminated medical supplies such as reused needles and syringes can also transmit EIAV.

Fortunately, EIA is now rare in most regions and positive tests are quite uncommon. Routine testing for EIA is usually required for shows, sales, transportation and other situations where horses are mixed, in order to detect and remove carriers. Unfortunately, identification of a horse as a carrier is not good for the horse or owner (or other horses in the area) - EIA is not treatable, and horses that have positive tests ("reactors") are quarantined, as are all other horses that are housed within 200 yards. Horses living close to reactors are tested (usually 30 and 60 days after removal of the reactor) and only released from quarantine after getting negative test results at least 60 days after the last reactor was removed. Reactors are usually euthanized. If prompt euthanasia is not chosen, reactors are usually prominently branded or tattooed. They must be kept under quarantine for the rest of their lives, and at least 200 yards from other horses. (200 yards is used because the flies that transmit the disease don't usually travel that far.)

The best way to reduce the risk of EIA is ensuring that all horses are regularly tested. New horses coming onto a farm must be tested BEFORE arrival or after arrival but while in quarantine at least 200 yards away from other horses. Needles and other items that might be contaminated with blood should never be re-used, both due to the EIA risk and to avoid other potential problems.

Overall, EIA is a very rare disease, but the severe implications of a positive test mean that we need to be vigilant.

Image from http://www.aht.org.uk/science_eia.html

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The latest version of the DEFRA/AHT/BEVA Equine Quarterly Disease Report has been released. This is a joint venture of three British organizations, and often contains some interesting infectious disease information. This edition contains brief information about diagnoses of selected diseases such as equine herpesvirus (EHV) -1 abortion, EHV-1 neurological disease and equine influenza. There is also a brief review of the eradication of equine infectious anemia (EIA) following the 2006 outbreak in Ireland, which was also discussed in a previous equIDblog post.

Another useful piece of information in this report is the data regarding strangles (Streptococcus equi subsp. equi) diagnoses. They report 280 positive S. equi culture results and 167 positive S. equi PCR test results during the last quarter. This shows once again that strangles is still a rather common disease. This is important to remember when considering routine infection control practices, because the risk of strangles exposure is ever-present, it is not just concern during outbreaks.   Also, people shouldn’t overreact to single cases of strangles (as they often do), since this disease is far from rare.

The full Equine Quarterly Disease Report can be downloaded by clicking here, or through the Animal Health Trust’s website.
 

Image from http://snapshot.parade.com/mainemb.php?g2_itemId=783542.

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Another presentation from the recent 2008 AAEP Convention, this one by Dr. Simon More, described the outbreak and successful eradication of equine infectious anemia (EIA) from Ireland in 2006.  A review of the outbreak itself, its investigation and management was also published in the most recent issue of the Equine Veterinary Journal (2008; 40(7):702-711).

Equine infectious anemia (EIA) (aka "swamp fever") is a very important disease in horses, and can also affect donkeys and mules.  This is the disease for which horses are given a Coggins test, which is required every six months for horses attending most shows and competitions, and for horses traveling internationally.  EIA is caused by a retrovirus, more specifically a lentivirus.  Other important viruses in this group include several immunodeficiency viruses such as HIV.  When the EIA virus infects a cell, it actually permanently incorporates its genes into cell's DNA, so affected animals are infected for life.  Horses go through repeated bouts of illness - the earliest episodes are usually the most severe.  Horses that survive the acute phase become persistent, often inapparent carriers, and serve as a reservoir for the virus.  The primary means of transmission is via transfer of blood from infected horses.  This can occur through blood-feeding insects such as horse flies and deer flies, or through use of blood-contaminated equipment such as dirty needles, surgical instruments and dental equipment.

Signs of EIA during acute episodes include fever, depression, lack of appetite, decreased red blood cell count (anemia) and decreased platelet count  (thrombocytopenia).  Lack of platelets can lead to petechial hemorrhages (tiny dots of blood, often first noticed on the gums or inside of the lips).  Episodes typically last 3-5 days, but weeks or even months may go by between episodes.  Illness may also be precipitated by stressful events or treatment with immunosuppressive drugs.  In some horses the episodes become more frequent, and signs of chronic EIA develop.  In addition to anemia and thrombocytopenia, these horses (often called "swampers") become very thin and may develop edema over their lower abdomens.  If the condition becomes very severe, a horse may have very pale or yellow mucous membranes (e.g. gums, around the eyes), and some may have nose bleeds or even neurological signs.

Prior to the outbreak of EIA in June 2006, Ireland was free of EIA, an advantageous status for the horses who lived there, as well as their owners.  Between June and December, 38 horses became infected with EIA.  It is believed that the outbreak started with four foals that were given hyperimmune plasma for prevention of Rhodococcus equi infection that had been illegally imported from Italy.  It is likely that the plasma was contaminated with the EIA virus, but unfortunately by the time the outbreak was identified there was no remaining plasma that could be tested.  Transmission of the virus was thought to have occured through mechanical transfer of blood during veterinary procedures and vector transmission via horse flies. Additionally, it was suspected that several mares may have been infected through close contact with foals.  Ongoing surveillance in Ireland has not identified any additional cases in the last two years, so it is believed that the disease has been eradicated.  Investigation and control of the outbreak took a lot of cooperation between many different organizations and individuals.  The study of the epidemiology was also very important in controling the disease, as not all of the cases followed the book!

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