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Equine Infectious Anemia (EIA) Description | Transmission | Clinical Signs | Detection | EIA Control Program | Recommendations | EIA Control Program Policy | Statistical Report
DescriptionEquine Infectious Anemia (EIA) or “swamp fever” is an infectious, worldwide disease of horses, donkeys and mules caused by a virus. Once infected, animals carry the virus for the remainder of their lives. There is currently no effective treatment or approved vaccine available in North America. The EIA virus does not infect people or other animals outside the family Equidae. In Canada, the disease was first recognized in Manitoba in 1881. Transmission of diseaseThe natural spread of EIA is predominantly by biting insects, especially by horseflies and deerflies when their feeding is interrupted (by tail switching, etc), and they move from an infected horse to an uninfected horse. The virus is thought to last approximately 30 minutes to four hours within these insects. Horses can also become infected by the transfer of virus in blood-contaminated needles, blood-contaminated instruments and blood transfusions. The EIA virus can survive for several months at room temperature in dried blood and serum. In an infected pregnant mare, the virus can sometimes cross over to infect the foal before it is born. Foals can also occasionally get the disease by drinking milk containing EIA virus from an infected mare. Although uncommon, EIA can be spread by breeding infected animals to uninfected animals. Clinical signsEIA virus, once it establishes infection inside the horse, is thought to live and grow primarily in macrophages, a type of white blood cell used to help defend the body against disease. An infected horse, in its attempts to control the virus, usually shows up as having one of four forms of the disease as follows: 1. Peracute – overwhelming infection that causes few if any signs before the horse suddenly dies. This form of the infection is not common. 2. Acute – the virus, and the horse’s immune system called in to fight it, causes destruction of red blood cells, destruction of platelets (used to help clot the blood) and inflammation in various tissues. The result is usually high fever, anemia (low numbers of red blood cells), edema (fluid build up under the skin), hemorrhages, loss of condition, jaundice (yellowing of the tissues), sometimes incoordination and often death. Levels of virus are high in the blood and they can transmit the virus readily to other horses. 3. Subacute to Chronic – continuous but milder signs as described in the acute form, or horses may cycle in and out of the acute stage and look relatively normal between episodes. The subacute to chronic stage is the form that is usually seen in horses showing clinical signs of the disease. Weight loss is often more evident and the horse may die during a more severe episode. These horses have generally moderate levels of virus in the blood and are a major source of transmission to other horses. 4. Subclinical or Inapparent – the horse appears well and may have never been noticed to be sick. This is the most common form of the disease detected by routine testing. Some of these horses may remain subclinical or inapparent carriers for much of their life. Others have recurring lapses into the subacute to chronic form. These lapses may be caused by stress, another illness, certain drugs such as steroids or by the virus mutating to a more damaging strain within the horse’s body. Subclinical or inapparent carriers generally have low numbers of virus in their blood and do not pose as great a threat for transmission to other horses. There is however documented evidence of EIA being spread from these inapparent carriers to other horses by insects, infected mare’s milk and breeding. Also, these inapparent carriers may sometimes relapse to the chronic or subacute form where they are more likely to spread the disease. Although there are methods to monitor and evaluate these carriers (permanent identification as carrier, daily temperature taking, counts of iron-laden macrophages in blood stream, etc.), with our current technology it is cumbersome to do so. DetectionAs the virus itself is not easy to detect in infected horses, a test to find antibodies to the virus was developed in 1970 by Dr. LeRoy Coggins. This type of test is based on the horse’s antibodies to the EIA virus in its serum (the clear fluid part of the blood), precipitating in a specific way in an agar gel with a known purified form of the virus. This Agar Gel Immunodiffusion test or AGID is commonly called the Coggins test after Dr. Coggins. It is a very specific test for the antibodies to the virus and is only done by accredited labs that have been trained by the Canadian Food Inspection Agency (CFIA) to perform the test. These labs must undergo proficiency panels every year to keep their accreditation. Vaccinations or other illnesses do not cause the specific lines of precipitation as found with EIA antibodies. When an animal is found to have antibodies to EIA by this test, it is repeated by the lab to ensure the accuracy of the result. Due to the serious consequences of a positive result, this positive sample is then sent to CFIA who repeat the test once again as another check. Rarely does a horse give an atypical reaction on the test that is not clearly negative or positive. These samples are also repeated by the accredited and CFIA lab; if they remain atypical the horse is retested in 30 days by which time they usually become clearly negative or positive. Once a clear positive sample has been through all these checks, an owner can be very confident that his or her horse truly has antibodies to the EIA virus. It then must be determined if there are any underlying circumstances which may cause the horse to have only antibodies to the virus but not the virus itself. This occurs in two circumstances as follows:
In a) or b) the horse is retested after a certain length of time in which the antibodies should have been eliminated from its system. The initial presence of antibodies in these animals indicates still that there is an infected animal(s) out there that should also be tested (dam, donor, herd mates, etc.). Once these factors (a and b) have been eliminated as possible causes of antibodies being present in the serum, a positive Coggins test is a very reliable marker that the horse truly has the virus somewhere in its body. Unfortunately, the reverse is not always true; one negative Coggins test cannot always guarantee the horse does not have infection. It takes approximately 14 to 42 days after infection by the virus for a horse to produce enough antibodies to detect by the Coggins test. During this time a horse may show up negative on the test. If the test, however, is repeated after 60 days and kept in the company of other EIA negative horses or if the test is done 60 days after the end of fly season, you can be reasonably sure that the horse is free of EIA. There are also a few reported cases of EIA infected horses that persistently have come up negative on the Coggins test. Although these horses appear to be uncommon, they are a cause of concern. More sensitive tests have been developed or are in the process of being developed to increase our ability to detect these horses. EIA is a reportable disease of horses regulated by the federal Health of Animals Act since 1971. Numbers of positive horses decreased under the policy from 2.9 per cent of 49,114 horses tested in 1972 to 0.09 per cent of 70,984 horses tested in 1992. EIA statistics after 1992, broken down into provinces, are in Table Two. According to the CFIA, EIA has almost been eradicated from that segment of the horse industry that voluntarily requested testing. The total number of horses tested in Western Canada is low compared to horses tested in Eastern Canada (Table Two). Although testing for EIA is low in Western Canada, the percentage of positive horses is the highest. An increase in surveillance of Western Canadian horses has the potential to significantly reduce the risk and spread of EIA in the region. The increase in positive EIA horses has started a trend in 1999 to require EIA testing at exhibition grounds and fairs. Some horse associations, fairs and exhibitions have funded clinics this year to help the horse industry increase the amount of EIA testing. CFIA has implemented a revised EIA Control Program as of April 1, 1998. Table One provides the policy in full. Information on EIA and the Domestic Disease Control Policy is available from CFIA district offices located throughout the province. A presentation on the disease is also available from CFIA district offices. Recommendations to ownersFor an individual owner, some of the control measures to prevent EIA from coming into their herd are as follows:
Table OneTable Two[1] From Agriculture Canada’s 1991 and 1996 Agriculture Census (The following are CFIA statistics on test results in Saskatchewan from Sept. 1998 to April 1999, as a result of testing traceout positive horses: 153 owners; 1652 horses tested; 40 horses tested positive; on 15 premises.) Material presented in this publication was written by: Dr. Shelagh Copeland and Dr. LeeAnn Forsythe, Inspection and Regulatory Management – Saskatchewan Agriculture, Food and Rural Revitalization. |
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