Over the past year there has been considerable fear in the breeding industry--especially the Quarter Horse industry--about the spread of equine viral arteritis (EVA). Although the disease has been around for a long time, confirmed outbreaks of EVA are relatively infrequent.
The most recent series of outbreaks started with the shipment of equine arter itis virus (EAV)-infective semen from a breeding farm in New Mexico to numerous other states; it spread to Quarter Horses in 10 other states, the majority in the western United States. It was first detected on the index premises when groups of mares were found to have lost their pregnancies when routinely pregnancy-checked at 60 days. Semen from one of the stallions on that farm had been shipped to 18 states, establishing infection on premises in the majority of them. The economic threat from this virus and this disease primarily involves breeding stock. In nonbreeding horses, EVA will run its course in a short period of time, with no residual clinical or other effects for the recovered individual. However, intact males might become lifelong carriers and shedders of the virus.
Understanding the significance of this virus to your particular horse operation will enable you to take preventive measures to minimize its potential impact on your herd. Although EVA is currently a reportable disease in many states, it is not reportable at the national level, making it necessary for you to take the appropriate steps to protect your horses.
|Horse Type||Clinical Signs||Treatment||Prevention|
||modified-live virus EVA vaccine (Arvac)|
|Foals||The foal appears relatively normal, but clinical signs of weakness, fever, and respiratory distress appear within 12-18 hours of birth; and rapid deterioration of condition||No specific antiviral drugs exist to treat the infected foal, which will very likely succumb to a fulminate progressive interstitial pneumonia in 48-96 hours; euthanasia is indicated||Immunization of dam with Arvac before breeding|
Viral Reservoir in the Stallion
Peter J. Timoney, MVB, PhD, FRCVS, former director of the University of Kentucky's Gluck Equine Research Center, is a leading world authority on EVA. He notes the similarities between influenza virus, equine herpesvirus-1 and -4 (EHV-1 and EHV-4), and EAV, in that all of these viruses are shed via the respiratory tract. "But," he continues, "there is a twist, in that EAV is also shed venereally in the semen of the acutely and chronically infected stallion."
Timoney recalls the EVA outbreak in Kentucky in 1984, when acutely infected stallions were primarily instrumental in spreading the virus to an estimated 41 Thoroughbred breeding farms.
The acutely infected mare sheds virus in her reproductive tract for six to nine days, but in smaller concentrations compared to what is shed by the acutely infected stallion. The virus does not persist in her reproductive tract as it does in the stallion.
In many instances, the primary source of an outbreak remains undetermined. Timoney remarks, "We believe that many outbreaks originate from an asymptomatically infected horse or from one that is incubating the infection. Infection becomes established and shedding virus occurs, especially via the respiratory tract, lasting seven to 16 days, with a significant risk of infecting other horses in close contact. Another significant source of infection is the carrier stallion's virus- infective semen; this can readily lead to spread of the virus on a breeding farm."
Clinical Signs of EVA
The events that follow infection are consistent with the unique abilities of a virus to usurp cellular "machinery" to propagate itself. In this case, EAV gains entry to the body, it multiplies in specific white blood cells (macrophages) giving rise to a cell- associated viremia (virus circulating in the bloodstream). The virus localizes in smaller blood vessels, especially the arterioles (these extend out from the artery and branch further into capillaries), and causes a vasculitis (blood vessel inflammation) of varying severity. At that point, along with fever and malaise, swelling can appear in the dependent parts of the body, especially the lower limbs and in the scrotal and preputial areas of a male horse, or around the udder of a mare. Other signs that might develop include nasal discharge, swelling above or around the eyes, tearing, a skin rash, and, infrequently, respiratory distress.
The widespread distribution of EAV in various body fluids and tissues spills over into various secretions and excretions that are released during the acute phase of infection. Timoney elaborates on the potential contagiousness of EVA, noting, "In the acutely infected horse, irrespective of whether it is a gelding, mare, or stallion, the virus can be spread from the respiratory tract for one to 2ï¿½ weeks. In urine, it may be shed in lesser concentrations for up to three weeks. During the acute phase of infection, virus is also shed into the conjunctival sac (in the eye) and the alimentary (GI) tract, and virus is found in feces for a short period following the onset of infection."
As previously mentioned, in a nonbreeding horse, infection with the virus is usually of limited duration, with the horse making an uneventful recovery. It is the breeding population that is at greatest economic risk with respect to EAV infection. The virus can cause a mare to abort or give birth to an infected foal with a low-survival rate.
Timoney describes what can happen to an acutely infected stallion: "An unprotected (unvaccinated) stallion can develop pyrexia (fever) for up to seven to 10 days and a variable degree of scrotal and preputial edema (fluid swelling). If not treated to control the fever and swelling, which may be significant, the stallion may experience a period of temporary infertility for up to four months due to adverse effects on sperm motility, concentration, and percentage of abnormal sperm in semen. Semen quality may reach a low point by seven to eight weeks after infection. To limit the degree of impairment of fertility, the stallion should be medicated to bring down the fever and reduce the scrotal swelling. The alterations in semen quality are not due to the direct effect of EAV on spermatogenesis, however, but rather a sequel to the elevated temperature and prolonged period of scrotal hyperthermia experienced by certain stallions."
Adverse effects on stallion fertility are relatively short-lived, with semen quality returning to pre-infection levels approximately four months later.
Special reference must be made to the carrier stallion. Establishment and persistence of EAV in the stallion is testosterone-dependent. The virus is harbored in the accessory sex glands of the stallion, with up to 10-70% of infected stallions becoming persistently infected with the virus. The virus is shed constantly in the semen of the carrier stallion, but not in any other bodily secretion or excretion. Although the virus is readily inactivated by high temperature or sunlight, it is able to survive in cooled or frozen semen for extended periods; this greatly facilitates spread of the virus (as happened in the 2006 occurrence of EVA in the Quarter Horse breed).
Protecting the Mare
Natural EAV infection of the unvaccinated mare can cause placental and fetal damage that can lead to abortion anytime from two months gestation to term. It is highly advisable to immunize mares against EVA at least three weeks prior to breeding with known EVA-infective semen or to a carrier stallion. Virus transmission from a carrier stallion to a naï¿½ve, unprotected mare occurs 85-100% of the time.
Timoney urges, "There is a need to be more proactive in identifying sources of EAV. If you know you've got a carrier stallion or are using virus-infective semen, then you should inform mare owners/breeders accordingly, so that they can adopt the necessary safeguards prior to and subsequent to breeding their mare with such semen."
When breeding to an outside stallion, ask the breeding farm about your the EVA status of the stallion you have selected before sending your mare to the breeding shed, or, in the case of AI, before ordering semen.
Timoney states that the modified-live virus EVA vaccine (Arvac, Fort Dodge Animal Health) is a safe and highly effective product. It is contraindicated for use in pregnant mares, especially in the final two months of pregnancy, or in foals less than six weeks of age. There is a risk, albeit minimal, that the vaccine virus might lead to abortion in a mare vaccinated in the last two months of pregnancy.
In the past, and especially since the fall of 2006, numerous mares have been vaccinated against EVA at various stages of pregnancy with very few reported adverse consequences. Nonetheless, it is advisable to follow manufacturer's recommendations on use of the vaccine. The vaccine label states: "Pregnant mares SHOULD NOT be vaccinated during the last two months of gestation since a few instances of fetal invasion by vaccinal virus have been demonstrated during this period. It is preferable to immunize mares during the maiden or open periods; however, when pregnant mares are threatened by known natural exposure, vaccination may be undertaken with considerably less risk than is inherent in natural infection."
A mare that aborts from EVA should be kept isolated for 28 days, after which she can be commingled safely with other mares. Such a mare is no longer a source of virus; she has very likely developed a high level of protective immunity. "There is no advantage in vaccinating such a mare," Timoney notes.
Implications for Pregnancy
A major sequel to EAV infection is abortion; however, some mares might not abort, but carry an infected fetus to term. Timoney notes, "Exposure of a pregnant mare to the virus in late gestation may result in congenital infection of the fetus in utero." At birth, such foals:
- Might appear normal, but begin to display signs of weakness, fever, and respiratory distress within 12-18 hours. They rapidly deteriorate in condition;
- In other cases, the foal is abnormal at birth, i.e., it is weak, febrile, and has difficulty breathing (dyspnea).
At the time the foal is born, the virus has infected its tissues and set up a viral pneumonitis (inflammation of the lungs), which is rapidly progressive and invariably fatal; such foals are very similar to fetuses congenitally infected with EHV-1. According to Timoney, sick foals are likely to succumb within 48-96 hours, notwithstanding the level of supportive treatment that might be provided. Currently, there are no specific antiviral drugs to resolve the pneumonia or to reverse the clinical signs.
"While the tendency is to treat affected foals in the hope that they'll recover, it should be emphasized that they are hugely productive sources of virus," Timoney states. "Consequently, they represent a significant source of infection for other susceptible foals on a breeding farm. Young foals are especially susceptible to EAV within the first few months of life, and if exposed can develop interstitial pneumonia (infection of the tissue in the lung beneath the alveolar epithelial lining) or an enteric (intestinal) syndrome, both of which can be fatal.
"Regardless of whether one has excellent facilities to isolate and manage affected foals, it is much wiser to euthanatize them and eliminate a highly productive source of infection for other unprotected in-contact horses, especially young foals," Timoney concludes.
Recognition of Disease
In order to achieve effective containment and control, a diagnosis of EVA must be confirmed at the earliest opportunity. Since the disease cannot be diagnosed on clinical grounds alone, this requires laboratory testing of appropriate specimens from a suspect case of infection. The presence of EAV antibodies in a pre-suckle blood sample taken from the foal is confirmatory of congenital infection and carries with it a grave prognosis. Unfortunately, the serological test can take up to 72 hours, which is likely too long of a wait under the circumstances.
Another approach is to submit unclotted blood, a nasal or conjunctival tissue swab, or placental/fetal tissues or fluids to a lab that is competent in diagnosing EVA. Such specimens can be subjected to virus detection tests (e.g., polymerase chain reaction, or PCR assay), which are speedier to perform than current serological tests. The more timely a diagnosis, the speedier an outbreak of EVA can be brought under effective control.
Mature Horse with a Positive Test?
The panic that resulted from the 2006 multistate spread of EVA has had repercussions even outside the equine breeding industry. There has been talk of restricting show and event entry only to horses that test antibody negative to EAV. However, with the exception of the sexually intact stallion, Timoney maintains that the safest horse to have on a premise is a serologically positive horse. He stresses, "If a blood sample from an unvaccinated horse tests positive for antibodies to EAV, this reflects previous natural exposure to the virus with the likely development of a high level of protective immunity. Such horses are no longer shedding EAV and consequently do not represent a source of infection for other susceptible animals. A seropositive horse, other than a seropositive stallion or sexually mature colt, is not a risk to other unprotected animals. The presence or absence of clinical signs is immaterial with respect to viral shedding, duration of viral shedding, or immunological response to infection with EAV. By the time a horse develops a serological response, the virus has most likely been there and gone! Again, the only exception is the sexually mature intact male that may continue to harbor EAV in his reproductive tract."
Eliminating the Carrier State
Elimination of the carrier state in the stallion is of great interest to breeders, owners, and researchers. A major consideration in developing a treatment for clearing the carrier state, however, is the expense of such studies; to be meaningful, they must involve an adequate number of carrier stallions. So far, efforts to achieve clearance of the carrier state by the use of a GnRH antagonist (a substance designed to nullify the effects of gonadotopin-releasing hormone) have not been entirely successful. Another strategy is to use GnRH to immunize a stallion, but this results in a stallion that is immunized against his own GnRH, leaving in question the possible long-term effects on its future fertility.
Although castration results in elimination of virus, it also excludes the stallion from the breeding population.
In summary, Timoney reports, "There is currently no fully validated and effective nonsurgical means of eliminating the carrier state in a stallion."
Yet, some stallions spontaneously clear the virus from their reproductive tracts and no longer represent a risk of transmission of EAV at a future point. Timoney comments, "There is no confirmed evidence of intermittency of shedding of EAV unless in the case of a stallion that is in the process of spontaneously clearing the virus from his system. Experience over the past 20-plus years has shown that a stallion that has spontaneously cleared has never returned to a shedding state at a later date."
He notes that immunizing carrier stallions with the available vaccine has proven ineffective in facilitating clearance of EAV.
Equine viral arteritis virus is a continuing threat to populations of breeding horses. Nonbreeding horses incur relatively short-lived clinical signs, from which they recover quickly. Equine viral arteritis can induce abortion and foal mortality, and it can establish a carrier state in the breeding stallion. The virus survives in chilled or frozen semen. Sound management practices and immunization can minimize the risk of introducing this disease to a farm. In short, EVA is a very manageable disease.
About the Author
Nancy S. Loving, DVM, owns Loving Equine Clinic in Boulder, Colorado, and has a special interest in managing the care of sport horses. Her book, All Horse Systems Go, is a comprehensive veterinary care and conditioning resource in full color that covers all facets of horse care. She has also authored the books Go the Distance as a resource for endurance horse owners, Conformation and Performance, and First Aid for Horse and Rider in addition to many veterinary articles for both horse owner and professional audiences.