EQUIVET AUSTRAILA - EQUINE INFO - EHV-1 (HERPES VIRUS) ABORTION

(MODIFIED GUIDELINES FOR CONTROL FROM AEVA RECOMMENDATIONS)

EHV-1 Disease

Abortions: Abortion due to EHV-1 usually occurs late in pregnancy between 8-11 months of gestation but can be as early as 4 months. The incubation period is highly variable with abortions occurring from 10 days to 4 months after initial infection. Most abortions occur within 30 days of the presumed initial exposure. Abortion is usually spontaneous with the mare showing no signs of the impending abortion and the placenta is usually passed covering the foal. Foals that are aborted before 6 months of gestation may be severely autolyzed. At full term infected foals may be born alive but are often abnormal from birth with signs of weakness, jaundice and difficulty in breathing. These foals usually die rapidly within one to three days, but occasionally some foals may survive longer.

Respiratory Disease: EHV-1 can cause respiratory disease, but in Australia respiratory disease has not been a feature associated with abortion storms. EHV4 causes respiratory disease and in rare instances has been responsible for single abortions.

Neurological Disease: EHV-1 may cause neurological disease manifesting itself as ataxia and paralysis. Horses affected by EHV-1 CNS disease that become recumbent have a very poor prognosis for survival.

Epidemiology

EHV-1 is endemic in the horse population of Australia. Serological evidence indicates that approximately 30% of the adult horse population in some studies have previous exposure to

EHV-1. As with all herpes viruses, once infected with EHV-1 horses will be latently infected for life. Studies in Australia have demonstrated that foals can be infected very early in life, presumably by their dam. This mare to foal spread of EHV-1 has been shown to occur in both vaccinated and unvaccinated groups of animals. EHV-1 is then spread from foal to foal both before and after weaning. This silent cycle of EHV-1 spread often occurs in the absence of any discernable clinical disease, which emphasises the importance of maintaining pregnant mares in isolation from other groups of horses on the farm. Pregnant mares are the group of horses at most risk from EHV-1 disease and management should ensure that these animals are subjected to as little stress as possible to avoid possible reactivation of latent infection.

There are only two sources of EHV-1 on a farm; either:

• a resident animal that reactivates a previously latent infection or,
• the introduction of EHV-1 from outside the group.

To minimise reactivation it is important to reduce stress, particularly in groups of mares in late gestation. To minimise the chances of introducing EHV-1 from outside the group it is important to segregate the pregnant mares from weanlings/yearlings as these young horses may be a source of EHV-1. It is also advisable to divide mares into small groups (to minimise the extent of exposure in the event of an abortion) based on their foaling date and to maintain these groups until foaling. Introducing new mares into these established foaling groups may directly introduce EHV-1 (especially if the mare is straight off a truck or from the sales etc where she may have been stressed and reactivated a latent EHV-1 infection). This introduction of a new mare may also indirectly by a source of EHV-1 as the pecking order needs to be re-established after the introduction and the associated stress may cause reactivation. To this end it is advisable not to introduce new mares into established groups of pregnant mares.

Natural infection of EHV-1 occurs via the respiratory tract from contact with:

1. Aborted foetuses, foetal membranes, foetal fluids and uterine discharges. This is the most important source of virus in the event of an abortion storm. Prompt attention to the index case and other in contact mares is a critical issue in reducing the impact of EHV-1 abortion.
2. Nasal and ocular discharges and aerosols from the respiratory tracts of horses with an active EHV-1 respiratory infection. This may include recently aborted mares or clinically infected young horses as well as asymptomatic carriers.
3. Contaminated material such a pasture, feed, feeding utensils, grooming equipment, halters, rugs, bedding, vehicles and staff. This route of infection is of most concern after there has been a case of abortion on the farm. It is always good hygienic practice to limit sharing of equipment, and this is especially important when dealing with the massive amount ofEHV-1 around the aborted foetus.

After EHV-1 abortion or the birth of an infected foal, the virus clears rapidly from the genital tract. Mares generally recover well after abortion with no compromise to their reproductive efficiency and can be bred to the stallion on the second oestrus post abortion. However, aborting mares may shed the virus via the respiratory route for up to 2 weeks.

Horses infected with EHV-1 develop latent infections. Stress is thought to play a role in reactivation of the latent infection with resultant shedding of the virus into the environment via the respiratory tract.

The virus is readily destroyed outside the body by heat, ultraviolet radiation and a wide range of commonly used disinfectants including those containing surfactants, iodophors, hexachlorophenes or phenols. The virus can survive in the environment for one to two weeks in cool and moist conditions if cleaning and disinfection are not adequate

Institution of Preventive Management:

Useful recommendations to reduce the chances of EHV-1 abortion occurring:

• Segregate pregnant broodmares from non-pregnant mares and maintain pregnant mares in as small a group as possible (about 10) subdivided according to stage of gestation. This should be done as soon as possible after the end of the breeding season and at least by weaning time. These groups should be maintained without additions until mares have foaled.
• Run weanlings and yearlings in paddocks well away from pregnant broodmares and avoid co-mingling or use of the same facilities e.g. yards. Arrange staff work schedules so that pregnant mares are handled and fed first in the day.
• Avoid co-mingling resident mares with non-resident mares. Isolate mares in small groups of 2-3 on arrival at stud. After an initial quarantine period of about 3 weeks these could be amalgamated to groups of ideally about 10.
• Minimise stress factors for pregnant mares. Avoid overcrowding, poor feed conditions and avoid travel later than 8 weeks prior to foaling.
• Establish isolation facilities and emergency abortion protocols.
• Keep records of paddock movements of horses and introductions

Vaccination

An inactivated EHV-1 / EHV-4 vaccine is commercially available in Australia. This vaccine is registered as an aid in the control of EHV-1 abortion when used in conjunction with appropriate management practices. Mares should be vaccinated in the 5th, 7th and 9th months of gestation. Vaccination is required by some studs before mares can be transported to the farm. Vaccination, however is not 100% protective and is not a substitute for effective management. Vaccinated horses have been known to abort and outbreaks of EHV-1 abortion storms have been reported in vaccinated horses.

Management of an EHV-1 Abortion:

The actions that are most important in minimising the spread of EHV-1 abortion on a farm are the actions taken when the foetus is first found. The first action should be to isolate the mare that has aborted and if possible remove her from the paddock to an isolation area. The area in which the abortion occurred should be identified, fenced off and disinfected. The aborted foetus should be sent to the laboratory for diagnostic testing. Mares should be considered as in-contact if they are in the same paddock as the aborted foetus, or if they are able to have direct contact with mares in the same paddock as the abortion (i.e. if they share a water trough or have direct contact over as single fence line). All in-contact mares should be placed in isolation until they abort or until they deliver a live, healthy foal.

Diagnosis:

VIRUS ISOLATION AND PCR: Virus detection by culture and/or PCR tests are required for a definitive diagnosis. Laboratory time for EHV-1 PCR is one day after receipt of sample. A positive EHV-1 culture takes 1 - 3 days, but culture is only done on samples that are PCR positive (Centre for Equine Virology CEV, University of Melbourne). Virus detection should be considered essential to confirm a diagnosis and should ideally by performed on all aborted foetuses.

SEROLOGY: An EHV1 specific ELISA is available at the CEV. Serology on a single sample is not useful for diagnostic purposes as a positive result may merely indicate previous infection experience or vaccination. It may be of value in some cases where EHV1 is suspected but the foetus has not been found. If a serum sample can be collected soon after abortion and again 2 weeks later, a rising antibody titre may be demonstrated.