EQUINE VIRAL ARTERITIS (EVA)

Patrick McCue, DVM, PhD, Diplomate American College of Theriogenologists

Equine Reproduction Laboratory

Colorado State University

Ft. Collins, CO 80523

Introduction

Equine viral arteritis (EVA) is a contagious disease of horses caused by equine arteritis virus (EAV). The virus is present in horse populations throughout the world, aided by international movement of horses and infective cryopreserved semen. EAV has been associated with periodic outbreaks of clinical disease in the United States, most notably in Standardbred horses Ohio in 1953 and Thoroughbred horses in Kentucky in 1984. An outbreak of EVA occurred in Quarter Horses in New Mexico and Utah in 2006. Historically, the virus has been most prevalent in Standardbred horses and less common in other breeds. EVA is a reportable disease in the state of Colorado.

Clinical Signs

In most instances, horses exposed to EAV exhibit either mild clinical signs of an upper respiratory illness or no apparent clinical signs (i.e. an asymptomatic or subclinical infection). Affected horses may have an elevated body temperature, decreased appetite, depression, edema of the ventral trunk, scrotum, mammary gland or lower limbs, nasal discharge and possibly swelling and/or inflammation around the eye (‘pink-eye’). Additional clinical signs may include ocular discharge, photophobia, and skin rash primarily around the head and neck. The incubation period from exposure to onset of signs is typically 3 to 14 days. Clinical signs may be present for a week or more.

The primary pathologic feature of acute EAV infection is inflammation, edema and necrosis of blood vessels in multiple organ systems.

Clinical disease is most prevalent when large groups of horses are held in close confinement (i.e. racetracks and breeding farms). Horses with clinical EVA usually make a full recovery and mortality associated with the virus is rare in adult horses. Foals infected with EAV in utero may die within 2-4 days after birth due to severe progressive interstitial pneumonia.

A significant concern is the possibility that the virus may cause abortion in pregnant mares, illness and death in young foals and establishment of the carrier state in stallions. Carrier stallions may shed the virus in their semen for months, years or throughout their lifetime.

Mares bred with infective semen do not have any reduction in fertility. However, exposure of mares ³ 3 months in foal to the virus may lead to abortion. Abortion will usually occur within 1-3 weeks following initial viral exposure. There is no evidence that exposure to EAV at breeding can cause a mare to abort later in gestation. Abortion rates may range from 10 to 70 percent during an outbreak.

Stallions may experience a period of short-term infertility associated with an increased body temperature and scrotal edema following initial exposure to EAV. Subsequent fertility is not impaired by continued presence of the virus.

The virus may localize and persist in accessory sex glands (primarily the ampullae and bulbourethral glands) of stallions after an initial exposure. Approximately 30-60 % of stallions exposed to EAV will become chronically infected carriers. Carrier stallions shed virus in semen, but not in nasal secretions. Some carrier stallions have been reported to spontaneously eliminate the virus from their reproductive tract and consequently no longer shed virus in their semen. The carrier stallion is a natural reservoir of EAV in the horse population. Mares, geldings and sexually immature colts are not long-term carriers or reservoirs of the virus. Maintenance of the virus in accessory sex glands of males is testosterone dependent. The virus will not persist after castration of an infected stallion.

Transmission

The primary mode of transmission of the virus is by the aerosol or respiratory route when horses are in close contact with one another. The virus is present in nasal secretions in acutely infected horses for up to 16 days. It may also be transmitted by the use of infective semen from carrier stallions during live cover, or insemination with fresh, cooled or frozen semen. Virus transmission rate from a carrier stallion to a naïve mare is 85 to 95 percent.

EAV is most commonly introduced onto an equine facility by the arrival of an acutely infected horse, introduction of a carrier stallion, or a shipment of infective cooled-transported or frozen semen. In the latter circumstances, secondary spread occurs by the respiratory route from the mare inseminated with the infective semen to other susceptible horses.

Testing

Initial determination if a horse has been exposed to the virus or has been vaccinated is through a blood test. Specific EVA test submission forms are available from the Colorado Department of Agriculture. Similar to the equine infectious anemia (Coggins) test, samples must be collected and submitted by an accredited veterinarian.

A serum neutralization (SN) test is performed on a serum sample to detect presence of antibodies against equine arteritis virus. An antibody titer of ³ 1:4 is considered a positive reaction and the horse will be considered seropositive for exposure to EAV. Antibody titers of < 1:4 are considered to be negative and the horse will be considered seronegative. It is currently not possible to distinguish antibodies that develop following vaccination from antibodies that develop following natural exposure to the field virus.

It is strongly recommended that a test be performed on all stallions to determine if they have been exposed to the virus. Stallions that are seronegative (i.e. no antibodies in their blood) should be vaccinated within 10 days of the test. This will result in development of protective antibodies and prevent the vaccinated stallion from becoming a long-term carrier if subsequently exposed.

Stallions that are seropositive from natural exposure should have semen collected and tested for evidence of viral shedding by either culture of the virus or by detection of the virus using a polymerase chain reaction (PCR) test. A stallion would be considered to be a carrier if virus is detected in his semen. A stallion would also be considered to be a carrier if seronegative mares convert to seropositive status within 28 days after breeding or insemination.

It is recommended that all breeding stallions be tested for EAV prior to breeding by natural cover or collection for artificial insemination. In addition, the EAV status of semen to be frozen should also be determined, since the virus is not killed during the freezing process.

In the future, it may become common for mare owners and breeding managers to request confirmation of the EAV status of a stallion before booking, ordering semen or breeding.

Documentation of EAV exposure status (i.e. blood test) prior to vaccination will be important in both mares and stallions in regards to future sales, shows, and potential international shipment of horses and/or semen.

Table 1. Fees for serologic and semen testing for EVA through the Equine Reproduction Laboratory, Colorado State University.

Diagnostic Test	Sample Type	Cost
Serology (Serum neutralization test)	Blood (serum)	$ 25.00
Polymerase chain reaction (PCR) test	Semen	$ 40.00
Virus isolation/culture	Semen	$ 50.00

Vaccination

The recent outbreaks of EVA in the western United States has led to considerable concern regarding vaccination of horses against this disease.

There is only one vaccine currently approved for vaccination of horses against equine arteritis virus (Arvac^â, Fort Dodge Laboratories, Inc., Fort Dodge, IA). Distribution of the vaccine may be regulated in some states. Proper documentation of the vaccination procedure, including accurate horse identification, serologic status prior to vaccination, and vaccination date(s) is critical.

Guidelines for vaccination of horses against EAV have been prepared by the American Association of Equine Practitioners (AAEP) and are presented in the table below. Modifications of the original guidelines are described later.

Horse	Vaccination Guideline
Foals and Weanlings	Intact colts intended to be breeding stallions should be vaccinated at 6-12 months of age
Yearlings	Annual vaccine for colts intended to be breeding stallions
Pleasure and Performance Horses	Annual vaccine for colts intended to be breeding stallions
Broodmares	Annual for sero-negative, open mares before breeding to a carrier stallion
Stallions	Annual for breeding stallions and teasers 28 days before start of breeding season

One of the original strategies for prevention and control of EVA was vaccination of all intact young colts and adult stallions. Prevention of the carrier state in stallions would eliminate the viral reservoir in the horse population. A second strategy was to vaccinate mares that are to be bred to carrier stallions to reduce transmission of the virus. A current strategy in the face of large outbreaks of EVA is to vaccinate all horses on a farm to reduce or prevent initial infection and transmission.

Mares should be vaccinated a minimum of 21 days prior to breeding to a carrier stallion to allow sufficient time for development of protective antibodies. It is often recommended that mares be isolated for a period of time after being vaccinated for the first time. Potential reasons for isolation after vaccination are: 1) allow sufficient time for mares vaccinated for the first time to develop a protective immune response, 2) prevent exposure of mares to field virus before they are protected, and 3) limit the possibility of transmitting vaccine virus (by nasal shedding) to other horses. A significant percentage of horses do not shed vaccine virus after vaccination. When present, the amount of vaccine virus detectable in nasal secretions is up to 10,000 fold less than field virus present after natural infection. In any event, the vaccine virus should not cause clinical disease even if transmitted to another horse. However, it could potentially cause seroconversion in a previously seronegative horse that would be indistinguishable from natural exposure (i.e. a false positive). Mares do not need to be isolated after subsequent vaccinations.

Mares should be physically separated from other horses in an isolation area for a minimum of 21 days after breeding by either live cover or artificial insemination to a carrier stallion. The goal of isolation is to prevent the field virus present in the semen from being transmitted from the recently bred mare to other horses (i.e. lateral transmission from mare to mare). An additional risk is posed by the infective semen used in the breeding process. Invariably some semen is discharged out of the reproductive tract in the first 24 to 48 hours after breeding. It is not known whether the discharged infective semen would cause sufficient environmental contamination to cause infection in other horses. In addition, if a mare with a foal at side is bred, the infective semen could potentially cause viremia and subsequent seroconversion of the foal. This may not be a serious health concern for the foal, but seroconversion of a colt foal may be a concern when it is tested prior to first vaccination at 6 to 9 months of age.

In high-risk situations, it may be beneficial to vaccinate pregnant mares. The vaccine is not labeled for use in pregnant mares. In fact, the information insert that accompanies the vaccine states that ‘Mares in foal should not be vaccinated until after foaling…’. However, many hundreds of pregnant mares have been vaccinated with this modified live virus vaccine without adverse effects (i.e. no abortion or congenital infections in foals). Consequently, if the risk of exposure to the field virus is high and an owner wants to limit the possibility of abortion and/or potential infection when breeding the mare back after foaling, vaccination of the pregnant mare should be considered. Vaccination during pregnancy will also provide the neonate with immune protection via passive transfer of maternal antibodies in colostrum.

Given the scale of the recent outbreaks, vaccination of all horses on a farm may be beneficial to limit or prevent transmission of virus. This would include stallions (after testing), broodmares, geldings, yearlings, weanlings (³ 6 months of age) and show/performance horses.

In other countries, restrictions regarding EVA are considerably greater than in the United States. For example, within the European Union (EU) a carrier stallion cannot be present on a stud farm that engages in trade of semen within the EU. Semen and stallions imported into the EU must be certified as seronegative for EAV or if seropositive, must not have virus present in semen (i.e. not a carrier). In addition, importation of semen from a carrier stallion is prohibited. Consequently, if export of stallions or semen from the United States is under consideration, it is important to document results of all serologic tests and vaccinations.

Environment

Equine arteritis virus is not considered to be very stable or resistant outside a horse ‘host’. The virus is sensitive to sunlight, high temperatures and low humidity. It may persist in the environment for several days in cold or damp conditions and in the presence of infective discharges or semen. EAV may be transmitted on hands, halters, lead ropes, and other equipment that may be in contact with infective discharges or semen. A broad-spectrum disinfectant such as Vircon-S^â may be used in contaminated examination areas, while indoor areas such as laboratories or clinics may be disinfected with a 1:32 dilution of sodium hypochlorite (Chlorox^â) and water. It may be important to disinfect breeding phantoms periodically or even between uses.

Control and Prevention of EVA

All Horses

Isolate new arrivals for 3 to 4 weeks. This would also be advantageous in the control of other infectious diseases such as rhinopneumonitis (Equine Herpesvirus-1) and Strangles.
In the event of an outbreak, movement of horses on and off the facility should be restricted.
On a high-risk farm, consider vaccination of all horses ³ 6 months of age.

Stallions, Colts and Semen

Perform serologic tests on all intact male horses (maintain permanent record).
Vaccinate seronegative male horses ³ 6 months of age (maintain permanent record).
Test semen from seropositive stallions for presence of EAV by culture or PCR test.
Isolate EAV carrier stallions.
Carrier stallions should only be bred to seropositive mares (vaccinated or naturally exposed).
Maintain strict hygiene when collecting semen from carrier stallions or working with known infective cooled-transported or frozen semen.

Mares

Separate and isolate pregnant mares from all other horses.
Vaccinate mares to be bred to a carrier stallion or infective semen at least 3 weeks prior to breeding.
Isolate or quarantine mares bred with infective semen for 3 weeks after breeding. This may be most important in mares that have been vaccinated for the first time.
Consider vaccination of pregnant mares on high-risk farms; vaccination may be administered 4 to 6 weeks or more prior to the potential due date.

Acknowledgements

Dr. Peter Timoney, Gluck Equine Research Center, University of Kentucky, provided valuable scientific information and expert advice on EVA.

References

Del Piero F. Equine viral arteritis. Vet Pathol 2000;37:287-296.

Glaser AL, Chirnside ED, Horzinek MC, de Vries AAF. Equine viral arteritis. Theriogenology 1997;47:1275-1295.

Swerczek TW, Dennis SM. Equine Viral Arteritis. In: Current Therapy in Large Animal Theriogenology, Youngquist RS, Threlfall WR (eds). Saunders Elsevier, St. Louis, 2007, pp. 181-183.

Timoney PJ. Equine viral arteritis. International Veterinary Information Service (www.ivis.org), 2002.

Timoney PJ. Equine viral arteritis: is the disease a cause for industry concern? Impulsion 2005:4-10.

Updated: December 12, 2006