EVA

Equine Viral Arteritis
Serum (clotted blood)
 Infection with equine arteritis virus is notifiable under the Equine Viral Arteritis Order 1995
(Animal and Plant Health Agency Tel. 03000 200 301) when:
o you suspect the disease in a stallion, either on the basis of clinical signs or following
blood or semen testing;
o you suspect disease, either on the basis of clinical signs or following blood testing, in a
stallion or mare that has been mated or artificially inseminated within the past 14
days.
 spread by respiratory and venereal routes
 Mainly subclinical and unrecognised.
 Diagnosis may be made via a combination of serology (VN, ELISA) and viral identification
(PCR).
 ELISA is a far cheaper method preferred for screening. VN can then be used as a further
check in certain cases
 EVA is a common contagious respiratory disease worldwide and it is not unusual to
encounter seropositivity in horses in the UK that have been imported from abroad.
 EVA is rarely encountered in the UK as a clinical disease and therefore finding a seropositive
result in a horse that has never left the UK is very unusual and indicates exposure to the
virus within the UK.
 Vaccination is available following documentation of seronegativity.
 imperative that regular 6 monthly boosters are maintained or subsequent seropositivity may
require expensive PCR and test mating to confirm that it is indeed vaccine-related.

Equine arteritis virus is spread by respiratory and venereal routes and outbreaks typically arise
following transmission during breeding a carrier stallion with a susceptible mare (natural breeding or
artificial insemination). Further transfer to any in-contact horses due to aerosolised virus follows due
to respiratory infection in the mare. Horses may shed virus for approximately two weeks from the
respiratory tract and then recover with solid immunity (detected later as seropositivity). Only stallions
carry the risk of longer term infection. Importantly, semen remains infectious after chilling or freezing.

SIGNS

The majority of EVA infections are subclinical and unrecognised. Clinical EVA infection is associated
with a variable incubation period of between 2-14 days. It results in widespread vasculitis and may
manifest as any or all of the following:
 pyrexia, anorexia and depression
 respiratory signs
 nasal discharge
 conjunctivitis, epiphora and photophobia (“pink-eye”)
 oedema of the legs, scrotal, preputial, mammary and periorbital areas
 general urticarial rash
 abortion in mares (single cases or outbreaks)
 fatal neonatal pneumonia/enteritis
 oral ulcers

Respiratory disease within a week or two of breeding (natural breeding or chilled or frozen AI) is a
big alarm for EVA. Abortion accompanied by respiratory signs is also indicative. Conjunctivitis is also
a sign not otherwise seen in many respiratory cases.

DIAGNOSIS
Diagnosis may be made via a combination of serology and viral identification, with serology being by
far the most commonly employed method.
Gender and country of origin are very important in interpreting EVA serology results:

Gender
 Infections in mares, geldings, fillies and sexually immature colts is only of relevance if very
recent as a carrier status does not occur and the infection will be eliminated within a few
weeks. Thus, in this population of horses, if it can be established that the serologic titre is
stable and not still rising then it can be assumed that the infection was a long time ago and
the horse presents no further risk. Indeed, the majority of seropositive results will simply
reflect previous exposure from which they have recovered and poses no ongoing risk to
other horses.
 In contrast, around 30–50% of infected stallions become persistent carriers for variable
periods of time and are a key natural reservoir of the virus. Thus, when seropositivity is
found in a stallion it is important to determine whether or not the horse is a carrier by
further testing described below. All carrier stallions will be seropositive but not all
seropositive stallions will be carriers. When stallions are vaccinated it is crucial that they are
established as seronegative first, and then an uninterrupted course of vaccination then
carries on, otherwise there may be doubt about whether the seropositivity is rom
vaccination or natural infection, with further testing then being required.

Country of origin
 EVA is a common contagious respiratory disease worldwide especially in Standardbred and
Warmblood populations. Thus, it is not unusual to encounter seropositivity in horses in the
UK that have been imported from abroad. EVA is rarely encountered in the UK as a clinical
disease and therefore finding a seropositive result in a horse that has never left the UK is
very unusual and indicates exposure to the virus within the UK.

Serology (virus neutralization (VN), ELISA – red top tube)
 a four-fold increase in antibody titres in serum samples collected two to four weeks apart
offers evidence of active infection. The virus neutralisation (VN) test may be problematic in
horses recently vaccinated against EHV1 and so the ELISA test may be preferable as a
screening test.
 Stable or declining seropositive titres in samples collected 2-3 weeks apart in geldings and
mares is innocuous and simply reflects recovered infection in the past. However,
seropositive unvaccinated stallions present further concerns as a proportion of these might
be persistently infected carriers.

Virus identification (Nasopharyngeal and conjunctival swabs (in viral transport medium), EDTA-
blood samples, semen or placentae)

This should be performed on any horse showing respiratory signs following breeding, and also in
stallions found to be seropositive in the absence of a solid vaccination record:
 Virus isolation (VI) test can be used to test stallion’s semen for international trade purposes.
 PCR can be used on respiratory swabs, blood, semen and placentae. Semen testing must be
carried out in an APHA laboratory. Two ejaculates are examined 7 days apart; if either is
positive then the stallion is a shedder; if both are negative then test mating is recommended
to ensure the stallion is clear.
 Test mating is used for final confirmation of shedding status in a stallion (if negative by PCR).
This must be done in strict isolation and under veterinary supervision. The following
procedure should be followed:
o Identify 2 seronegative mares;
o Take and store blood samples from each and then isolate the mares;
o Consult the testing laboratory about storage conditions;
o Mate each mare twice a day with the stallion on 2 consecutive days;
o Keep the mares in isolation;
o After 28 days, take blood samples and send them, with the pre-isolation samples, to
the laboratory.

If the mares remain seronegative, the stallion is unlikely to be a shedder and can be released after a
clinical examination. If one or more mares become seropositive, the stallion is a shedder. He must be

kept in isolation and not be used for breeding activities while he is shedding, unless permitted under
an official licence issued by Defra.
Seropositive mares must remain in isolation until they have a stable or declining antibody level in
two sequential blood tests taken at an interval of at least 14 days.

VACCINATION

Vaccination is available for protection against EVA infection following documentation of
seronegativity in blood samples. Following the initial primary course (2 doses 3-6 weeks apart) it is
imperative that regular 6 monthly boosters must be maintained or subsequent seropositivity may
require expensive PCR and test mating to confirm that it is indeed vaccine-related.