Equine Viral Arteritis

Equine viral arteritis is one of those diseases that most backyard horse owners never deal with but can absolutely devastate breeding operations when it shows up. The virus causes respiratory symptoms that look like a dozen other things in most horses, but in pregnant mares it can trigger abortion storms where multiple mares lose foals in a short period. The really insidious part is that some stallions become permanent carriers after infection, shedding virus in their semen for years or even life without showing any signs of illness themselves.

For most adult horses that get infected, EVA causes fever, depression, limb swelling, and respiratory signs that resolve within a couple weeks and seem like just another viral respiratory infection. Some horses don't show any symptoms at all despite being infected. Young foals can develop severe pneumonia that can be fatal, but adult horses usually handle the infection without serious complications as long as they're not pregnant mares.

The breeding industry nightmare scenario involves a carrier stallion infecting mares through breeding, those mares aborting, and the virus spreading through the facility to other horses through respiratory transmission. This is why reputable breeding operations test stallions for EVA carrier status and vaccinate susceptible horses before breeding season. The regulations around EVA vary by country and even by state, with some requiring testing and vaccination for certain activities while others have minimal oversight.

Transmission happens both through respiratory secretions and through semen from carrier stallions. Horses at shows and sales can spread respiratory infection to each other just like influenza or herpesvirus. But the semen transmission route is unique to EVA and is what makes carrier stallions such a management challenge. A single positive stallion can infect dozens or hundreds of mares over multiple breeding seasons if nobody catches on that he's a carrier.

This guide will help you understand how EVA spreads, recognize the signs of infection, understand testing and carrier status, and implement biosecurity measures that protect breeding operations.

Equine viral arteritis spreads primarily through two routes - respiratory transmission and venereal transmission through breeding. The respiratory route works like other airborne viruses, with infected horses shedding virus in nasal secretions and respiratory droplets that other horses inhale. Close contact, shared airspace, and contaminated equipment all contribute to respiratory spread. During the acute phase of infection, horses shed significant virus through respiratory secretions for a week or more.

The venereal route through breeding is what makes EVA particularly problematic. Stallions that become persistent carriers shed virus continuously in their semen without showing any signs of illness. When they breed mares either through live cover or artificial insemination, the virus infects the mare through the reproductive tract. Frozen semen from carrier stallions retains infectivity, so mares can be infected from breedings years after the stallion was actually infected.

Horses at highest risk include those at breeding farms, stallion stations, show barns with breeding programs, and sales where breeding stock is concentrated. Imported horses may have been exposed to EVA strains not common in your region, and some countries have higher prevalence than others. Stallions used heavily in breeding programs face ongoing risk of exposure through the mares they breed, since infected mares can transmit the virus to stallions during natural breeding.

Natural infection versus vaccination creates different antibody patterns that testing can distinguish. Vaccinated horses develop antibodies but typically remain negative for viral shedding, while naturally infected horses may develop carrier status and shed virus despite having antibodies. This distinction matters because you can't just look at antibody titers and know whether a horse is safe to use in breeding.

Young horses that get infected sometimes develop more severe disease than adults, possibly because their immune systems are still developing. Pregnant mares face the highest stakes because EVA infection during pregnancy frequently results in abortion regardless of the stage of gestation. Some mares abort within days of infection while others carry compromised foals to term that die shortly after birth.

Carrier status in stallions appears to have some genetic component, with certain bloodlines seeming more likely to become persistent shedders after infection. Not every stallion that gets infected becomes a carrier - some clear the virus completely just like mares and geldings do. But there's currently no way to predict which stallions will become carriers until after infection has occurred and you test for shedding.

Fever is typically the first sign, often spiking to 102-106 degrees and lasting several days. Horses look depressed and may go off their feed partially or completely during the febrile period. The fever alone doesn't tell you much since lots of conditions cause fever, but when it occurs along with other characteristic EVA signs, it raises suspicion.

Limb edema is one of the more distinctive signs, with swelling developing in the legs, along the belly, around the sheath or udder, and sometimes in the head and neck region. The swelling can be quite dramatic, with legs stocking up significantly and fluid accumulating along the ventral midline. This edema develops as the virus affects blood vessel walls, increasing their permeability and allowing fluid to leak into tissues.

Respiratory signs include nasal discharge, coughing, and sometimes conjunctivitis with tearing and redness around the eyes. The nasal discharge starts clear but may become thicker and more mucoid as the infection progresses. Some horses develop skin rashes or hives, particularly around the head and neck, though this is less consistent than the other signs.

Pregnant mares infected with EVA may show the same fever, depression, and edema as other horses, but the critical sign is abortion which can occur anywhere from a few days to several weeks after infection. Some mares abort without showing obvious illness beforehand, while others have clear respiratory symptoms before losing the foal. The aborted fetus typically appears fresh and well-developed, not macerated or obviously abnormal.

Stallions that become carriers show no signs of illness related to their carrier status. They look perfectly healthy, perform normally, have normal semen quality on routine evaluation, and give no indication that they're shedding virus in every breeding. This is what makes carrier stallions so dangerous - there's absolutely nothing about them that would make you suspect a problem without specific testing.

Young foals infected with EVA can develop severe respiratory disease with rapid breathing, nostril flaring, and obvious respiratory distress. Some develop pneumonia that can be fatal despite aggressive treatment. The mortality rate in foals is significantly higher than in adult horses, though most foals that survive the acute infection recover without lasting problems.

Diagnosing EVA requires laboratory testing because the clinical signs overlap with so many other diseases. Blood tests can detect antibodies, but remember that antibodies can come from either natural infection or vaccination. Virus isolation or PCR testing on nasal swabs, blood, or semen can confirm active infection or shedding. For stallions suspected of being carriers, semen testing is the definitive way to determine if they're actively shedding virus.

Testing protocols for breeding stallions typically involve checking semen for viral shedding before the breeding season starts. If a stallion tests positive for shedding, decisions need to be made about whether to use him for breeding and under what conditions. Some breeding programs will breed carrier stallions only to vaccinated mares, while others retire carrier stallions from breeding entirely.

There's no specific antiviral treatment for EVA, so management focuses on supportive care during acute infection. Most horses recover on their own within two to three weeks with basic nursing care - rest, good nutrition, clean water, and monitoring for complications. Anti-inflammatory drugs can reduce fever and make horses more comfortable. The limb edema typically resolves as the infection clears, though it can take several weeks for all the swelling to go down.

For pregnant mares diagnosed with EVA, there's unfortunately no treatment that prevents abortion once infection has occurred. The focus shifts to managing the mare through the abortion if it happens and preventing spread to other horses in the facility. Aborted fetuses and placental material should be handled carefully to avoid contaminating the environment, and the mare should be isolated until she's no longer shedding virus.

Quarantine and isolation of infected horses is critical to preventing spread within a facility. Horses with confirmed or suspected EVA need complete separation from pregnant mares especially, with separate handlers, equipment, and ideally separate barns. The quarantine period typically lasts until the horse is no longer febrile and nasal shedding has stopped, which is usually two to three weeks from the onset of clinical signs.

Vaccination is available and is commonly used in breeding operations. The vaccine can prevent infection entirely or reduce disease severity if infection occurs. Importantly, vaccination before exposure is much more effective than trying to vaccinate after exposure has already happened. Breeding operations typically vaccinate all horses on the property annually, timing it before breeding season starts.

For carrier stallions, decisions about continued breeding use depend on many factors including the stallion's value, owner preferences, and breeding program setup. Some owners vaccinate all mares before breeding to carrier stallions, accepting the risk because the stallion's genetics are valuable enough to justify it. Others use these stallions only for frozen semen collected under careful biosecurity. Still others retire carrier stallions from breeding entirely to eliminate the risk.

Managing breeding operations to minimize EVA risk involves multiple layers of biosecurity and testing. Incoming breeding stock should ideally be tested for EVA before arriving at the facility, with documentation of vaccination status or antibody titers. Stallions should have semen tested for viral shedding before being used for breeding, especially if they're new acquisitions or have unknown exposure history. These protocols can feel excessive until you've dealt with an abortion storm.

Isolation of new arrivals for a quarantine period allows you to monitor for clinical disease before mixing them with resident horses. Two to three weeks gives adequate time for most infections to become apparent if the horse was incubating disease when they arrived. During quarantine, take temperatures daily and watch for any signs of illness. For pregnant mares, quarantine becomes even more critical because the consequences of EVA infection are so severe.

During an outbreak, aggressive isolation of affected horses prevents further spread. Breeding activities should cease immediately until the outbreak is contained and all horses have been tested to determine their infection status. This can be financially devastating for breeding operations during peak breeding season, but continuing to breed during an outbreak spreads infection further and makes the overall damage worse.

Cleaning and disinfection after an outbreak should be thorough because the virus can persist in the environment on contaminated surfaces. Fortunately, EVA virus is relatively fragile outside the host and is susceptible to common disinfectants. Standard barn disinfectants used properly will kill the virus on equipment, stall surfaces, and other contaminated areas. Pay special attention to breeding equipment, examination areas, and anywhere bodily fluids from infected horses may have contaminated surfaces.

Record keeping becomes important when dealing with EVA because tracking breeding dates, exposures, and test results helps identify how infection entered the facility and which horses may have been exposed. If a carrier stallion is identified, you need records of every mare bred to that stallion to notify owners of potential exposure. Good records also help demonstrate due diligence if legal issues arise from infected breedings.

Prevention in breeding operations centers on vaccination, testing, and biosecurity protocols that minimize exposure risk. Vaccinating breeding stock before breeding season provides good protection against infection and reduces disease severity if breakthrough infections occur. The vaccine needs to be given at least three weeks before breeding to allow time for immunity to develop. Annual boosters maintain protection, though some high-risk operations vaccinate more frequently.

Testing stallions for shedding status before using them for breeding identifies carriers before they infect mares. This testing should happen annually or more frequently for stallions used heavily or standing at public stud. Some stallions that test positive can eventually clear the infection and become negative shedders, though others remain positive indefinitely. Retesting at intervals helps track whether a positive stallion has cleared or remains a carrier.

Quarantine protocols for new arrivals and horses returning from breeding or shows reduce the chances of infected horses mixing with resident stock before illness becomes apparent. While this requires extra facilities and management, the cost is minimal compared to dealing with an EVA outbreak in a breeding operation.

The prognosis for individual horses that get infected with EVA is generally good - most recover completely without lasting effects. The prognosis for breeding operations dealing with an outbreak is more complicated, involving lost breeding seasons, aborted foals, and potential reputation damage. Stallions identified as carriers face uncertain breeding futures depending on how owners and breeding managers choose to handle the situation.

International trade in horses and semen has increased EVA exposure risk globally as strains circulate between countries. Some regions that previously had minimal EVA now see more cases as horses and breeding stock move around the world. Staying current with EVA status in regions where you're sourcing horses or semen helps you assess risk appropriately.