Equine
EIA, swamp fever
All equids, including horses, ponies, mules, donkeys, and zebras are susceptible to infection with EIAV. There are no age, sex, or breed predilections. Donkeys may experience less severe disease than other equids.
Equine infectious anemia, or EIA, is an infectious disease of horses and other equids caused by the equine infectious anemia virus, or EIAV. The disease has a worldwide distribution. In the United States, the true incidence is unknown since only about 20 percent of the total U.S. horse population is routinely tested. Of those horses tested in 1999, only 0.06 percent were infected. The incidence is higher in the Gulf Coast states, as the climate is favorable for virus transmission. The virus is transmitted primarily by blood-feeding horseflies and deerflies.
Infection with EIAV results in recurring, cyclic episodes of fever, lethargy, thrombocytopenia, or low platelets, anemia, and in some cases, weight loss, ventral edema, petechiation, or pinpoint bleeding in the skin, hemorrhage, and death. Most horses' immune systems control the infection within a year, and thereafter show no clinical signs of disease. However, these horses will remain lifelong inapparent carriers of the virus. These otherwise normal-appearing horses are capable of transmitting the virus to uninfected horses primarily through biological vectors.
Diagnosis is primarily based on clinical signs and the result of serologic testing, using the AGID, or Coggins test, or the C-ELISA test for antibodies against EIAV. There is no specific treatment for EIA. The disease is reportable in the United States, and federal law prohibits interstate travel of infected animals, except for slaughter, return to place of origin, or transport to a recognized research facility or diagnostic laboratory.
Clinical signs are variable and depend on the stage of the disease. Acute disease usually occurs one to four weeks after infection and is associated with high levels of viremia. Fever, anorexia, lethargy, ventral edema, and occasionally mucous membrane petechiae, epistaxis and death can characterize EIA. The acute phase is usually less than one week in duration, and is sometimes mild enough to go completely unnoticed.
Chronic disease is associated with recurrent episodes of viral replication, causing repeated bouts of the above mentioned clinical signs. The classic signs of anemia, ventral edema, and weight loss occur during the chronic phase of the disease; these horses are sometimes referred to as "swampers." With time, clinical disease episodes decrease in duration and severity, and most horses' immune systems will control the infection within one year. These horses will become inapparent carriers of the virus and will show no clinical signs.
Initial symptoms begin one to four weeks after infection, and can include fever, loss of appetite, listlessness, and swelling of the lower legs, brisket, and belly. Occasionally small blood spots develop on the inside of the lips and on the gums. In severe cases, bleeding from the nose and even death can occur. In contrast, the initial symptoms can be so mild in some horses that they go completely unnoticed. Over time, repeated episodes of these symptoms tend to occur in cycles, and weight loss can develop. Usually after approximately a year the symptoms stop, and most horses look completely normal.
EIA is caused by the equine infectious anemia virus, or EIAV, a lentivirus of the family Retroviridae. Lentiviruses are those that generally cause a permanent, chronic infection. EIAV is closely related to the human immunodeficiency virus, or HIV-1, the cause of AIDS. Blood-feeding insects, especially horseflies and deerflies, primarily transmit EIAV. Iatrogenic transmission can also occur via contaminated needles, syringes, and surgical instruments, as well as through transfusion of contaminated blood. Once infected, a horse remains infected for life. EIAV infects monocytes and macrophages, and can be detected inside these white blood cells in the liver, spleen, lymph nodes, lung, bone marrow, and circulation of infected animals.
EIA virus particles replicate inside the monocytes and macrophages and are subsequently released in large numbers. The affected horse's immune system reacts to this viremia, or presence of virus particles in the bloodstream. This causes immune-mediated intravascular and extravascular hemolysis, or red blood cell breakdown, as well as bone marrow suppression. Anemia, which is an abnormally low red blood cell count, is the result. Likewise, thrombocytopenia, or low platelet count, is caused by both bone marrow suppression and immune-mediated destruction of platelets. Severe thrombocytopenia can lead to mucous membrane petechiae, or pinpoint hemorrhage, as well as epistaxis, or nose bleed.
The immune reaction to EIAV may also affect blood vessels and vital organs. Hemorrhage, thrombosis, or blockage of blood vessels, and edema can occur, and are secondary to immune-mediated vasculitis, or inflammation of the vessels. Vasculitis and lymphocyte accumulation in the meninges, or lining tissues of the brain and spinal cord, occasionally result in ataxia, or incoordination of movement. Accumulations of lymphocytes and macrophages in the liver can result in hepatomegaly, or liver enlargement, fatty degeneration, and hepatic cell death, while immune complex deposition can result in glomerulitis, or inflammation of specialized structures in the kidney.
There are essentially three stages of EIAV-associated disease. These are the acute stage, the chronic stage, and the inapparent stage. Generally, the disease's clinical signs correlate with the stage of the disease. Acute disease usually occurs one to four weeks after infection and is associated with high levels of viremia. The acute phase is usually less than one week in duration, and is sometimes mild enough to go completely unnoticed. Affected horses not previously infected will experience the acute phase.
Chronic disease is associated with recurrent episodes of viral replication and clinical signs. With time, clinical disease episodes decrease in duration and severity. Most horses develop sufficient immunity to control the infection within one year, and become inapparent carriers of EIAV. These horses show no clinical signs. A few horses may develop very severe disease in either the acute or chronic stages and will die from organ system complications or other sequela of equine infectious anemia.
Diagnosis is based on the affected horse's history, physical examination findings, and laboratory test results. The history may include inappetance, or loss of appetite, lethargy, and fever. Severely affected horses may have a history of high fever (105 - 106� F), depression, ventral edema, weight loss, ataxia, or incoordination, and epistaxis, or nosebleed. Additionally, physical examination findings could include poor body condition, mucosal petechiae, or pinpoint hemorrhage, or pale mucous membranes. Some affected horses may reveal no signs on physical examination.
Thrombocytopenia, or low platelet count, is the first laboratory abnormality detected in the acutely infected horse, and coincides with fever. Thrombocytopenia resolves along with resolution of clinical disease, but recurs with subsequent chronic disease cycles. Anemia, or abnormally reduced numbers of red blood cells, can occur shortly after infection, but generally is more severe during the chronic stage of the disease. Leukopenia, or low white blood cell count, lymphocytosis and monocytosis, or increased numbers of lymphocytes and monocytes, respectively, are observed in many infected horses. Hypergammaglobulinemia, or increased antibody protein levels, may be present. Elevations in liver enzymes may occur.
Definitive diagnosis is based on serologic testing. Both the agar gel immunodiffusion test, or AGID, also called the Coggins test, and competitive enzyme-linked immunosorbent assay, or C-ELISA, are approved by the United States Department of Agriculture for serologic diagnosis of EIA. Both tests detect serum antibodies to the EIAV group-specific core protein, p26. Most acutely infected horses produce detectable antibody within 45 days of infection. The Coggins test is the most widely used, and is 95 percent accurate in diagnosing EIAV infection. The C-ELISA may be more sensitive than the AGID, leading to possible false positive results. All horses testing positive with either test should be retested for confirmation.
Horses can die of EIA. Most, however, develop an immune response that controls, but does not eradicate, the infection. These horses are clinically normal but remain reservoirs of infection and become life-long inapparent carriers of EIAV.
The causative agent of EIA is the equine infectious anemia virus, or EIAV, a lentivirus of the family Retroviridae. Blood-feeding horseflies and deerflies transmit this virus as the primary vectors, or carriers. Venereal transmission is possible, but unlikely. EIAV can be transmitted to the fetus before birth and to the neonate through colostrum/milk from an infected dam. Iatrogenic transmission can also occur via contaminated needles, syringes, and surgical instruments, as well as through transfusion of contaminated blood. Once infected, a horse remains infected for life, serving as a potential source of infection to other horses. The virus does not infect other non-equid animals or people.
There is no specific treatment for EIA. EIA is a reportable disease in the United States, and federal law prohibits interstate travel of infected animals, except for slaughter, return to place of origin, or transport to a recognized research facility or diagnostic laboratory. Individual states regulate intrastate travel, and most state control measures include the following options for positive horses: euthanasia, permanent identification and life-long quarantine, or transport to a recognized research facility.
If elected, treatment of EIA is symptomatic. The affected horse is isolated from other horses. Nonsteroidal, anti-inflammatory drugs are given as needed to reduce inflammation and fever; occasionally other medications are administered to achieve these purposes as well. Nursing care and other supportive measures are provided. Corticosteroids are specifically contraindicated as they exacerbate the viremia and subsequent clinical signs.
There is no preventive vaccine available against EIAV. Federal and state control measures have lowered the incidence of EIA in the United States, but outbreaks still occur. Veterinarians, horse owners, and others in the equine industry can reduce the chance of exposure by testing for EIA as part of every pre-purchase examination, and requiring a recent negative EIA test before admitting any new horse to a farm. Other potential measures include requiring recent negative EIA tests for horses entering sales, racetracks, shows, and other events, and annual testing of all horses for EIA exposure. To further minimize transmission, different horses should never be injected with a common needle or syringe, instruments coming into contact with blood should be thoroughly disinfected, and flies should be controlled as rigorously as possible.
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