Arterită virală ecvină

Geographic distribution of EAV:

Equine arteritis virus is found in various horse populations throughout the world. It has been reported from countries in North and South America, Europe, Africa, Asia, Australia and New Zealand. EVA outbreaks don’t occur often, but when they do, they are frequently associated with the movement of horses or shipment of semen. The prevalence of EAV infection differs connsiderably both between countries and between particular horse breeds in the same country.

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.

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. 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.

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 months in foal to the virus may lead to abortion. Abortion will usually occur within 1-3 weeks following initial viral exposure. Abortion rates may range from 10 to 70 percent during an outbreak.

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 ≥ 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 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.

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. 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.