Swamp Fever, Mountain Fever (tick), Slow Fever, Equine Paludism (Malaria) Fever, Coggins Disease.

Transcription

1 Swamp Fever, Mountain Fever (tick), Slow Fever, Equine Paludism (Malaria) Fever, Coggins Disease. Importance Vallee's disease (equine infectious anemia, EIA) is a retroviral disease of equidae, that is characterized by acute clinical signs and/or chronic recurrences, that can include fever, anemia, edema and cachexia in some animals. Many horses present mild signs or inapparent signs in first time exposure and are subclinical carriers of the virus. It is likely that the owners are not aware of the infection in their horses, unless they have a serologic test performed. All infected horses, including those without symptoms of the disease, become carriers and sources of infection during the whole course of their lives. Animals that are infected should be sacrificed or should remain permanently isolated from all other equidae to avoid transmission of the disease. Etiology The EIA is caused by the equine infectious anemia virus (EIAV), a lentivirus from the Retroviral family (subfamily Orthoretroviridae). Affected Species It has been reported that the EIAV infects all members of the Equidae family. Clinical cases appear in horses and ponies (Equus caballus) and have been reported in mules. Some horse-adapted viral isolates are replicated in low levels without presenting clinical signs in donkeys. (Equus asinus); however, non-publicated evidence suggest that the isolated strains and serial passages in donkeys can be pathogens for this species. Geographical Distribution The EIA has been found nearly throughout the world. This virus exists in the United States and Canada but appears to be absent from a few countries, those such as Island and Japan. The EIAV transmission is

2 transmitted mechanically from the buccal parts of biting insects. In horses, this virus remains in the bloods leukocytes for life, and is also present in the plasma during febrile episodes. Symptomatic horses are the more likely to transmit the disease than those with inapparent infections. After resting on an asymptomatic carrier, only 1 out of 6 million of flies becomes a vector. There has also been reported high levels of viremia in mules, during the early stages of infection. Significantly lower titers have been reported in inoculated donkeys with certain horse adapted strains. Although other insects, including stable flies (Stomoxys calcitrans) can still transmit EIAV, vectors that are more effective are biting flies of the Tabanidae family, especially horse flies (Tabanus spp. and Hybomitra spp.) and deer flies (Chrysops spp.). The bites of these flies are painful, and the reaction of the animal is to interrupt its feeding. The fly attempts to continue feeding immediately, on the same animal or another host that is closest, resulting in the transfer of infectious blood. The EIAV survives during a limited period of time in the insects buccul apparatus and has less probability to spread to hosts that are further away. The virus can also be transmitted by blood transfusions or through needles, surgical instruments, and contaminated dental floats. It has been reported that it persists up to 96 hours in hypodermic needles. The EIAV can also be transmitted from a mare to her foal in the utero. There can be other possible minor routes of transmission. The EIAV doesn't appear to be excreted through saliva or urine. However, it can be found in a mare's milk or a stallion's semen, and horses can be infected by inoculating these secretions subcutaneously. It has been reported that transmission is possible through the milk in some nursing foals. Although venereal transmission does not seem to be a principle route of propagation, a stallion apparently transmitted the virus to a mare with a vaginal tear during breeding. From an epidemic outbreak in 2006 in Ireland, the possibility of an aerosol transmission by infectious material during close contact was raised. Period of Incubation The incubation period is from a week to 45 days or longer. Some horses remain asymptomatic until they are submitted to some type of stress. Clinical signs More severely affected horses can become weak, depressed and inappetent, with additional signs that may include jaundice, tachypnea, tachycardia, ventral pitting edema, thrombocytopenia, petechiae on the mucus membranes, epistaxis or blood stained feces. Anemia can occur, although it is more likely to be severe in chronically infected

3 animals. Occasionally, horses become gravely ill and may die during the acute stage. After the initial bout, most horses become asymptomatic carriers; however, some animals develop recurring clinical signs that vary from mild illness and failure to thrive to fever, depression, petechial hemorrhages on the mucus membranes, weight loss, anemia and dependent edema. Inapparent infections may become symptomatic during concurrent illnesses, severe stress or hard work. Death is possible during these febrile episodes. Ophthalmic lesions, characterized by depigmentation with prominent choroidal vessels, have been reported in chronically infected horses. On occasion, horses become gravely ill and may die during the acute stage. After the first signs of the disease, the majority of horses become asymptomatic carriers; however, some animals develop recurring clinical signs that can vary from mild illness and fail to grow due to fever, depression, petechial hemorrhages in the mucus membranes, loss of weight, anemia and dependent edema. Inapparent infections may become symptomatic when other illnesses occur simultaneously, severe stress or intense workouts. It is possible that death occurs during these febrile episodes. In horses with chronic infection, cases of ophthalmic lesions have been reported, characterized by depigmentation with prominent choroidal vessels. It is less probable that donkeys and mules develop clinical signs. Mules can be infected asymptomatically, but cases of typical EIA signs have been reported in some affected animals in a natural or experimentally form. In a recent experiment, inoculated donkeys with 2 horse-adapted strain became infected but did not present symptoms. In China, it was reported that inoculated donkeys with a one adapted strain through serial passages, had developed clinical signs. Post Mortem Lesions The spleen, liver and lymph nodes may be enlarged, and the mucous membranes can be pale. In chronic cases, emaciation may also be observed. With frequency edema can be observed in the limbs and along the ventral abdominal wall. Petechia may be observed on the internal organs such as the spleen and kidney. Also mucosal and visceral hemorrhages and blood vessel thrombosis have been reported. Horses with chronic infection that die between clinical episodes, generally do not present important macroscopical lesions, but some animals may present proliferative glomerulonephritis or ocular lesions. Morbidity and Mortality The infection rate varies with the geographic region. The transmission of the virus is influenced by the number and the species of flies involved, their habits, the density of the horse population, the level of viremia in the

4 host and the quantity of blood transferred. Infections are more common in humid and swampy regions. Seroprevalence rates as high as 70% have been observed on farms where the disease has been endemic for many years. The morbidity rate and severity of the clinical signs are influenced by the strain, the dosis of the virus, and the health of the animal. It is more probable that horses develop clinical signs than that of donkeys or mules, although many horses are infected by subclinically forms. Frequently, the presence of EIAV in a herd often goes unnoticed until some horses develop the chronic form of the disease or if routine testing is done. Epizootics with high morbidity and mortality rates have been reported, but cases of deaths are not common in naturally infected horses. Experimental inoculation with a high viral dose can produce mortality rates as high as 80%. DIAGNOSIS Clinical The EIA should be among the differentials, in particular cases of horses that present weight loss, edema and intermittent fever. It should also be considered when various horses suffer from fever, anemia, edema, progressive weakness or weight loss, particularly when new animals have been incorporated into the herd or an integrant of the same herd has died. Differential diagnosis: The differential diagnosis includes other febrile illnesses such as the equine viral arteritis, hemorrhagic purpura, leptospirosis, babesiosis, severe strongyliasis or fascioliasis, intoxication with phenothiazine, autoimmune hemolytic anemia and other diseases that cause fever, edema and/or anemia. Laboratory analysis: Generally the EIA is confirmed through serological tests. Once the animal is infected, it becomes a carrier for life. The two most commonly used serological tests are the Agar gel immunodiffusion (AGID) also known as the Coggins test and the Enzyme-linked immunosorbent assays (ELISA). Horses are generally seronegative in the AGID test during the first 2-3 weeks after infection; in some cases, it is possible that they do not develop antibodies until after 60 days has passed. ELISAs can detect antibodies earlier than the AGID test and are more sensitive, but false positives are more than likely to occur. For this reason, ELISAs possible results are confirmed with the AGID test or with immunoblotting (Western blotting). Limited experimental evidence suggests that the antibody production may be delayed in cases of donkeys and mules. Tests of reverse-transcriptase polymerase chain reaction (RT- PCR) can also be used to detect infection in horses. These tests are valuable for

5 determining the status of infection in foals born to infected mares, seeing that young animals may present maternal antibodies up to the age of 6-8 months. PCR tests can also be used to supplement or confirm serological tests, particularly when there are conflicting results or an infection is suspected, but the serology is negative or the results have been inaccurate (for ex: in the early stages, when antibodies have not developed). In addition, this technique ensures blood donors that they will not be infected and the horses used for vaccine or antiserum production, are not infected. The RT-PCR appears to be an effective method to diagnose in mules as well as in horses. In general, it is not required the isolation of the virus to carry out the diagnosis, although it can be done. The EIAV can be found as much as in the plasma as in sanguineous leukocytes during febrile episodes; between these periods, this virus is cell-associated. The isolation of the virus is done by the culturing of equine leukocytes. As it is difficult cultivating these cells, it is possible that this test is not available in all laboratories. The identity of the virus can be confirmed through specific ELISA antigen tests, immunofluorescence or PCR test. If the condition of immunity in an equine cannot be determined through other methods, blood may be inoculated in a susceptible horse. Antibodies and clinical signs in a test animal are controlled for at least 45 days. Samples to collect: Serum should be collected for serology. Blood samples are taken for RT-PCR, viral isolation or inoculation in an animal (for testing). Recommended actions in case of Equine Infectious Disease suspicion: Official Notice to the authorities: The EIA should be reported to the World Organization for Animal Health (OIE, for its acronyms in french). Each state (country) should be checked for specific regulations. The requirements for notification of the disease to the member nations of the OIE and the guidelines for import/export can be consulted in the Sanitary Code Book for Terrestrial Animals of the OIE [ Veterinarians who detect a case of equine infectious anaemia should follow the national and/or local guidelines for the notification and the corresponding diagnostic tests. Control Many countries have control programs that require equines to be tested for the EIA. State regulations vary in the U.S.A. but the majority of these states require

6 one or more tests done, particularly before the horse is to participate in organized activities and/or sale of the horse. To keep a herd free of the IEA, it is useful to carry out voluntary testing of horses of an establishment (farm) and testing of new animals before their integration. There is no vaccine available. Infected equines become carriers for life, and should remain isolated from other susceptible animals or be sacrificed. In the U.S.A. a reactor must be marked with a brand, freezemarking or lip tattoo before to be transported from one state to another. Reactors can only be transported between states, if they are going to their original establishments (home farms), a slaughterhouse or a diagnostic or investigation (research) facility, and should be transported under quarantine conditions. Furthermore, the majority of these states require that reactors be marked, if they remain within their state. The risk of transmission from carriers vary, seeing that it is impossible to measure the risk, all infected horses are treated in the same manner. Mare asymptomatic carriers can birth uninfected foals. Congenital infection risk is high if the mare shows clinical signs before to give birth. Foals that are born from infected mares should be isolated from other equines until it has been determined if they are free of infection. During an outbreak, fumigating to control insect vectors, the use of insect repellents and insect-proofing stables and barns, can facilitate the interruption of the transmission. By separating small animal groups by at least 200 yards away might be beneficial when the virus is being transmitted within a farm. Caution should be taken to prevent iatrogenic transmission. In countries where the EIA is not present, they contain outbreaks with quarantines and movement controls, case follow-ups and surveillance. Enveloped viruses such as the EIAV are rapidly destroyed by common disinfectants. This virus does not persist in insects, which are mechanical vectors. Public Health No evidence exists that suggests that the EIA is a potential threat to humans. Internet Resources American Association for Horsemanship Safety (AAHS). Your Horse, Equine Infectious Anemia and the Law AAHS. Equine Infectious Anemia Statutes and Administrative Regulations for Individual States Food and Agriculture Organization of the United Nations (FAO). Manual for the Recognition of Exotic Diseases of Livestock

7 The Merck Veterinary Manual USDA APHIS. Equine Infectious Anemia Disease Information World Organization for Animal Health (OIE) OIE Manual of Diagnostic Tests and Vaccines for Terrestrial Animals OIE Terrestrial Animal Health Code References Brangan P, Bailey DC, Larkin JF, Myers T, More SJ. Management of the national program to eradicate equine infectious anaemia from Ireland during 2006: a review. Equine Vet J. 2008;40(7): Cook SJ, Cook RF, Montelaro RC, Issel CJ. Differential responses of Equus caballus and Equus asinus to infection with two pathogenic strains of equine infectious anemia virus. Vet Microbiol. 2001;79(2): Dawson RO. Your horse, equine infectious anemia and the law [online]. American Association for Horsemanship Safety. Available at: Accessed 3 Jul Garner G, Saville P, Fediaevsky A. Manual for the recognition of exotic diseases of livestock: A reference guide for animal health staff [online]. Food and Agriculture Organization of the United Nations [FAO]; Equine viral arteritis. Available at: Accessed 1 Jul Hall FR, Pursell AR, Cole JR, Youmans BC. A propagating episode of equine infectious anemia on a horse farm. 1988;193(9): Issel CJ, Rushlow K, Foil LD, Montelaro RC. A perspective on equine infectious anemia with an emphasis on vector transmission and genetic analysis. Vet Microbiol. 1988;17(3): More SJ, Aznar I, Myers T, Leadon DP, Clegg A. An outbreak of equine infectious anaemia in Ireland during 2006: the modes of transmission and spread in the Kildare cluster. Equine Vet J. 2008;40(7): United States Government. Code of Federal Regulations Title 9, Chapter I, Subchapter C, Part 75. Communicable diseases in horses, asses, ponies, mules, and zebras. Equine infectious anemia (swamp fever). Interstate movement of equine infectious anemia reactors and approval of laboratories, diagnostic facilities, and research facilities. Available at:

8 idx?c=ecfr&sid=ef347dc0612a15832a2d28710b258ea7&rgn=div5&view=t ext&node=9: &idno=9#9: Accessed 13 Jul Kahn CM, Line S, editors. The Merck veterinary manual [online]. Whitehouse Station, NJ: Merck and Co; Equine infectious anemia. Available at: 0.htm. Accessed 21 Jun Kemen MJ Jr, Coggins L. Equine infectious anemia: transmission from infected mares to foals. J Am Vet Med Assoc. 1972;161(5): Motie A. An outbreak of suspected equine infectious anaemia in Guyana. Br Vet J. 1986;142:36-40 Murphy FA, Gibbs EPJ, Horzinek MC, Studdert MJ, editors. Veterinary virology. San Diego, CA: Academic Press; Equine infectious anemia;. p Sellon DC. Equine infectious anemia. Vet Clinic North American Equine Practice 1993;9(2): Spyrou V, Papanastassopoulou M, Psychas V, Billinis Ch, Koumbati M, Vlemmas J, Koptopoulos G. Equine infectious anemia in mules: virus isolation and pathogenicity studies. Vet Microbiol. 2003;95(1-2): Stein CD, Osteen OL, Mott LO, Shahan MS. Experimental transmission of equine infectious anemia by contact and body secretions and excretions. Vet Med. 1944;39: Stein CD, Mott LO. Studies on congenital transmission of equine infectious anemia. Vet Med. 1942;37: Tashjian RJ. Transmission and clinical evaluation of an equine infectious anemia herd and their offspring over a 13-year period. J Am Vet Med Assoc. 1984;184(3): United States Department of Agriculture. Animal and Plant Health Inspection Service, Veterinary Services, Centers for Epidemiology and Animal Health [USDA APHIS, VS, CEAH]. National Animal Health Monitoring System. Equine infectious anemia [online]. USDA APHIS, VS, CEAH; Oct Available at: Accessed 26 Sept World Organization for Animal Health (OIE): World animal health information database (WAHID) [database online]. Equine infectious

9 anemia. Paris:OIE;2009. Available at: Accessed 20 Jun World Organization for Animal Health (OIE): Manual of diagnostic tests and vaccines for terrestrial animals [online]. Paris: OIE; Equine infectious anemia. Available at: Accessed 21 Jun From: The Center for Food Security and Public Health Iowa State University