Definition Equine infectious anemia (EIA) is an infectious viral disease of equidae characterized by a variety of symptoms related to anemia that accompany either an acute, subacute or chronic illness that may terminate in death. The disease may be subclinical in some individuals.
Prevalence Equine infectious anemia has been reported in all parts of the world. The disease is widespread on this continent and has been reported in all mainland states and most of the Canadian provinces. The United States Department of Agriculture has recorded the number of positive EIA tests since the early 1970's. In 1971, 3.9% of the EIA tests were positive. By 1977, only 0.6% of 793,536 official EIA tests were positive. Since 1977, there have been 600,000 to 900,000 horses tested annually in the United States with between 0.3% to 0.5% of the tests positive on immunodiffusion test. Clinical outbreaks of the disease have been greatly reduced from the number fund in the 1960's and early 1970's.
Horses, ponies, mules and donkeys are the only known natural hosts of EIA virus. Most naturally occurring outbreaks of acute and subacute cases of this disease develop during the late summer and early fall months in the temperate zone. This coincides with the peak of the biting insect population, especially blood sucking horseflies. Transmission of infective blood by these insect vectors is an important cause of natural outbreaks. Transmission via contaminated hypodermic needles or other instruments can produce new infections at any other time of the year.
Cause Equine infectious anemia is caused by a retrovirus that is related to the lentiviruses which cause AIDS in humans. However, as far as is known, the equine virus does not infect humans. The EIA virus is not infectious for the common laboratory animals and does not elicit a strong neutralizing antibody response in infected horses, although non-protective antibodies are abundantly produced. There are indications that several strains of the virus exist. The virus has been found in blood, milk, saliva, feces and semen of diseased horses but rapidly loses its infectivity outside the body. Clinically sick horses have higher titers of virus in their blood and tissues than in apparent carriers. The virus is readily destroyed by sunlight, boiling, and by most chemical disinfectants. Once infected, diseased horses carry the virus for life regardless of the severity of symptoms. Foals born to infected dams may carry a maternal antibody titer for 2-6 months after birth. If a foal is infected, the titer will persist indefinitely. There is evidence that foals have become infected in utero and from nursing infected dams.
Transmission The disease is readily transmitted by subcutaneous, intramuscular or intravenous injection of infective blood. Horses showing acute clinical signs of the disease are the most potent source of virus and are more likely to be involved in transmission of the infection. However, virus may be found in the blood of any infected animal regardless of clinical symptoms. Blood sucking insects, primarily horseflies and deerflies, contaminated hypodermic needles, surgical instruments, dental floats and bridle bits can be the mode of infection transfer in outbreaks of the disease.
Recent research data show that serums of some inapparent carriers of EIA are infectious. Thus, it is necessary to conclude that all horses positive for EIA are potential spreaders of EIA infection.
Symptoms The incubation period for EIA following the subcutaneous inoculation of infective blood into a susceptible horse is usually about 14 days but it may be up to several months. The initial illness is commonly acute or subacute with definite clinical signs, and lasts from 3 to 20 days followed by cycles of febrile periods and death or apparent recovery. The disease may occur in a subacute or chronic form characterized by recurring attacks at greatly variable intervals. The form it takes is influenced by the size of the infective dose, strain of virus, the presence of absence of antibody in the donor horse, and the physical stress the animal is under at the time.
The acute form of the disease may develop at the initial attack or at any subsequent attack and often terminates in the death of the horse within 3 to 14 days. It is characterized by a sudden onset, a high fever of 104 to 108 degrees F., severe depression, depressed appetite and a rapid loss in physical condition. A profound weakness may result in incoordination, recumbency or prostration. Jaundice and edema of the ventral abdomen, sheath and limbs may develop, with or without petechial hemorrhages of the mucous membranes at the base of the tongue and on the conjunctivae. The heart rate and intensity of heart sounds are greatly increased especially with moderate exercise. A yellowish or bloody nasal discharge may be present. An enlarged spleen may be palpated on rectal examination. During an acute attack, a severe anemia develops due to a destruction of red blood cells and reduced hemopoieses. This is characterized by a low hematocrit, low hemoglobin value, low red blood cell count, and a high sedimentation rate. However, immature red cells are usually not observed; the white blood cell picture may show a leuopenia or be normal. Death is usually related to the severity of the anemia.
The subacute form of EIA is reported to be the most common form of the disease. The signs are similar to the acute form but not as severe and death seldom occurs. Most horses with this form of the disease appear to recover after 7 to 20 days and remain free of symptoms for weeks or months, after which another subacute may occur. These relapses often develop during periods of stress and if repeated at frequent intervals result in weakness, loss of weight, marked unthriftiness and anemia.
The chronic form of the disease may follow initial or subsequent acute, subacute or mild episodes of illness. The chronic form may at any time revert to the subacute or acute and then become chronic once again. Some chronically affected horses appear unthrifty and lack stamina. Periodic signs of anemia may develop. Most infected animals are unable to perform hard work and may be unsatisfactory breeding animals.
Following the widespread use of the agar gel immunodiffusion test since 1971, a great many of the antibody positive horses have been reported to have apparently recovered completely and to have remained free of observable symptoms for many years. However, the blood from these asymptomatic animals is still infectious. Some horses with this asymptomatic "carrier" form of the disease can withstand severe stress without developing a recurrence of the clinical disease. After many asymptomatic years, some chronically infected horses will suffer relapses late in life following stress of concurrently with other severe disease conditions. Death may be associated with intravascular clotting of blood and thrombus formation.
It should be recognized that the classification of acute, subacute and chronic are arbitrary and only represent EIA infection with varying degrees of severity. Our present knowledge indicates that most horses with persistent clinical signs of EIA sooner or later die from the disease or are euthanized because of impending death.
Diagnosis The clinical diagnosis of EIA presents many difficulties no matter what form of the disease is present. In the face of an outbreak continuous observations are helpful and often necessary along with regular twice-daily recording of body temperatures and concurrent regular blood studies especially during and following recurrent acute or subacute episodes of the disease. A history of illness following shipping, advance training, foaling, surgery or other recent stress often provides a clue.
Equine infectious anemia should be differentiated from equine babesiosis seen in some areas of the southern United States. Babesiosis is a blood disease spread by ticks and can be diagnosed by blood studies and a complement fixation test. It is rare in North Carolina except perhaps in imported horses. Leptospirosis may resemble EIA but is a much milder disease with a short course and the agglutination-lysis blood test is highly diagnostic. Severe strongylosis (blood-worm infection) may also produce anemia, emaciation and weakness. Fecal examination for parasite eggs and the typical response after proper deworming procedures aid in the differential diagnosis. Purpura hemorrhagica, and acute often fatal disease that may follow streptococcic infections such as strangles, is characterized by a high fever, swelling of the limbs and hemorrhages in the mucous membranes. Purpura may be diagnosed by the history, symptoms and blood studies revealing a lack of platelets.
Laboratory Diagnosis Confirmation of a clinical diagnosis of EIA was handicapped until 1970 by the lack of a simple reliable diagnostic test. The successful adaption of the agar gel immunodiffusion (AGID) test to the diagnosis of EIA by Dr. Leroy Coggins and co-workers at the N.Y.S. Veterinary College, Cornell University, has provided a reliable and economical diagnostic tool. Prior to the development of this test, a clinical diagnosis could be confirmed only y inoculation of a known susceptible horse with suspect blood, a procedure costing about $500 and taking about 3 months.
During its development, samples from about 500 horses with known EIA status, based on animal inoculation, were checked by the immunodiffusion test with 100% correlation. Eighty-five random AGID test positive blood samples produced EIA when injected into susceptible ponies. Conversely, seventy-six test negative samples failed to produce EIA. Animals in the early incubation period and, in rare cases, those showing clinical signs soon after infection may not be detected by the test. However, with few exceptions animals infected three weeks or more prior to sampling will yield a positive result.
On the basis of the impressive accuracy of this relatively simple test the Committee on Infectious Diseases of Horses of the U.S. Animal Health Association, in its 1970 report, recommended use of the immunodiffusion test for the detection of EIA. During 1971, a committee of the American Association of Veterinary Laboratory Diagnosticians prepared a protocol of test procedures to insure that there would be uniformity in laboratory procedures and consistency in results. The immunodiffusion test was recognized in November 1972 by the United States Department of Agriculture as the official test for the detection of EIA.. A second test, the more rapid competitive ELISA test, was approved as an official test for EIA in 1986. The United States Department of Agriculture has monitored EIA reagent production and test performance by approved diagnostic laboratories in the United States to assure uniform test results throughout the country.
Following the introduction of the immunodiffusion test in the early 1970's, approximately 800,000 horses have been tested annually in the United States. Few problems have been encountered with the test and positive results have invariably correlated with infectiousness of the blood when animal inoculations have been attempted.
Submission of Samples For the diagnosis of EIA, North Carolina Veterinarians should send 10 cc. of aseptically collected whole blood to the Rollins Animal Disease Diagnostic Laboratory, P.O. Box 12223, Raleigh, NC 27605 or to some other officially approved laboratory. Samples may be submitted as serum (preferable) or whole blood. Whole blood should be collected in vacutainers without anti-coagulant using disposable needles and sterile syringes. Samples must be properly identified, carefully packed, and sent as promptly as possible by first class mail or the express services. The Diagnostic Laboratory fee is presently $4 per sample which must be submitted along with the sample. The EIA test is an official test. Therefore, samples must be submitted with an official form by an accredited veterinarian. Reports will be sent only to the veterinarian. Copies of all positive test reports are sent to the Office of State Veterinarian, North Carolina Department of Agriculture and Consumer Services. A full-time state veterinarian will then collect a second sample for confirmation before further action is taken.
The North Carolina Department of Agriculture and Consumer Services EIA Control Program With the designation of the immunodiffusion test as the official test for the detection of EIA, an important legal barrier to an effective control program was removed. The horse industry is vitally important to North Carolina's economy and the Department of Agriculture has a statutory responsibility to take all reasonable steps to protect it. There, in consultation with and the strong support of industry leaders, the Department devised a regulatory program to control EIA in the state.
The control program became effective July 1, 1974. Present regulations require that horses entering the States test negative to an official AGID test within one year of importation.
Blood samples must be submitted by a licensed and accredited veterinarian to a laboratory approved by the United States Department of Agriculture and by the North Carolina Department of Agriculture and Consumer Services. The sample must be accompanied by a completed official EIA test record form. In the case of a positive test, the horse will be quarantined to the premises by a state veterinarian who will collect a second blood sample to verify the identity of the test-positive animal.
If the second sample is positive, the horse will be permanently identified within 30 days by a freeze brand at public expense or by lip tattoo at owner's expense. The freeze brand is a painless procedure which leaves the hair white at the brand site. The brand used is a "55A" followed by two digits. The prefix "55" is North Carolina's designation in the national tagging system. Owners of infected horses have several options: 1) immediate euthanasia, 2) sale for slaughter purposes under written permit, 3) brand and permanently quarantine in isolation on its home farm or other facility approved by the State Veterinarian. Because of the probability of recurrent acute attacks and the fact that horses remain infected for life, euthanasia is the option generally selected.
Any animal that is exposed to EIA or is suspected of being an EIA carrier must be quarantined for 60 days then tested for EIA. If the test is negative, the horse is released from quarantine. If the test is positive, the horse is retested, permanently identified and isolated.
Management of Infected Animals Positive test (i.e., antibody titer) can be considered synonymous with infection regardless of clinical signs. The severity of symptoms is related to the amount of virus in the blood at any given time. Under conditions of stress, the asymptomatic carrier is likely to show clinical signs of the disease. No treatment is effective and spontaneous recovery has not been reported. A vaccine is not available. Current research at North Carolina State University is concentrated on two areas: elimination of the virus from carriers and development of a vaccine to protect against this infection.
With the present state of knowledge about the spread of EIA, it is possible to make specific recommendations for its control:
- Securely isolate any horse showing clinical signs of the disease pending laboratory confirmation. If the immunodiffusion test is positive, immediate euthanasia should be considered.
- Purchase only horses known to be negative to the immunodiffusion test; insist that you be furnished a copy of the test report.
- Do not use any instruments or equipment capable of transmitting blood cells from one horse to another without thorough cleaning and disinfecting between each use. This includes surgical instruments, needles and syringes, hoof knives, dental floats, stomach tubes, twitches and curry combs.
- Control biting flies and other insects through good sanitation and judicious use of pesticides.
- It is is absolutely necessary to keep an infected horse, the animal should be isolated from others on the farm.
Federal regulations require that known EIA reactors moving interstate must be permanently identified, be accompanied by a permit and be moved only to slaughter, back to the farm of origin or to approved research facilities.
Summary Equine infectious anemia is a viral disease of equidae transmitted chiefly by transfer of infective blood from one animal to another. Its manifestations may be acute, chronic or inapparent and may terminate sooner or later in the death of the animal. No specific therapy or immunization is available. Diagnosis is based on clinical signs and immunodiffusion, ELISA or animal inoculation tests. The immunodiffusion test has been used on thousands of blood samples throughout the world with a ver high degree of accuracy. On the basis of this test, it appears that the prevalence of the disease in the United States approaches 0.5%. Horses positive to this test are considered infected and should be euthanized or placed in strict isolation. Further measures to control spread of this disease are insect-vector control and disinfection of surgical and other equipment between use on successive animals.
Complete cooperation of the equine industry is necessary. Furthermore, because of the rapid and constant movement of horses from state to state the regulatory program should be applied uniformly and concurrently throughout the country. Current regulations are designed to stop the disease from coming into North Carolina and from spreading from known infected animals within the state. Further control of the disease will depend on leadership from the horse industry.
EQUINE INFECTIOUS ANEMIA This publication was originally prepared by the Advisory Committee to the Director of the Division of Animal Industry, New York State Department of Agriculture and Markets in March, 1966. This article is an adaptation of the original publication prepared by Dr. Leroy Coggins, North Carolina State University College of Veterinary Medicine and Dr. Tom McGinn, North Carolina Department of Agriculture and Consumer Services Veterinary Division.
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Dreguss, M.N. and Lombard, L.S.: Experimental Studies in Equine Infectious Anemia, (1954), Univ. of Penn. Press, Philadelphia, PA.
Hall, R.F., Pursell, A.R., Cole, J.F., Jr. and Youmans, B.C.: A propagating epizootic of equine infection anemia on a horse farm. J.A.V.M.A. 193:1082-1084, 1988.
Issel, C.J. and Coggins, L.: Equine Infectious Anemia: Current knowledge. J.A.V.M.A. 174:727-733,1979.
Kemen, M.J. and Coggins, L.: Equine Infectious Anemia: Transmission From Infected Mares To Foals. J.A.V.M.A. 161:496-499, 1972.
Pearson, J.E. and Knowles, R.C.: Standardization Of The equine Infectious Anemia Immunodiffusion Test And Its Application To The Control Of The Disease In The United States. J.A.V.M.A. 184:298-301, 1984.
Timoney, J.F., Gillespie, J.H., Scott, F.W. and Barlough, J.E.: Hagan and Bruner's Microbiology And Infectious Diseases Of Domestic Animals, 8th Edition (1988), Comstock Pub. Co., Ithaca, NY.
North Carolina Department of Agriculture and Consumer Services,Veterinary Division P.O. Box 26026, Raleigh, NC 27611
North Carolina Department of Agriculture and Consumer Services, Rollins Disease Diagnostic Laboratory
2101 Blue Ridge Road Raleigh, NC 27607
North Carolina Department of Agriculture and Consumer Services, Division of Marketing Western NC Agricultural Center
1301 Fanning Bridge Road Fletcher, NC 28732
North Carolina Department of Agriculture and Consumer Services, Division of Marketing Horse Specialist
P.O. Box 27647 Raleigh, NC 27611
North Carolina Department of Agriculture and Consumer Services, N.C. State Fair Hunt Horse Complex
1025 Blue Ridge Boulevard Raleigh, NC 27607
United States Department of Agriculture Veterinary Services, NCSU Centennial Campus
1017 Main Campus Drive Partners Bldg. 1, Suite 2500 Raleigh, NC 27606-5202
North Carolina State University College of Veterinary Medicine
4700 Hillsborough Street Raleigh, NC 27606
North Carolina Horse Council
Box 12999 Raleigh, NC 27611
In state: (800) 529-9206