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The Equine Viral Arteritis (EVA)
control scheme

Prepared by John O'Flaherty, for the NZ Equine Health Association

EQUINE VIRAL ARTERITIS (EVA) is an acute, contagious disease caused by equine arteritis virus. Although there is only one serotype of EVA recognised, field isolates vary significantly in pathogenicity with the majority of infections being subclinical.

EVA was first isolated in 1953, during an outbreak of respiratory disease and abortion on a standardbred breeding farm near Bucyrus, Ohio, USA, although the disease probably existed for many years prior to that. In 1984 an outbreak in Kentucky, USA, gave EVA global prominence. Seropositive horses have been recorded in many countries. EVA is an OIE list B disease. The virus is relatively fragile and is readily destroyed by sunlight and most disinfectants. However it does survive chilling and freezing so all semen from shedding stallions must be considered infective.

The clinical signs of infection with EVA are extremely variable, ranging from inapparent to relatively mild, through to severe disease. The classical picture of EVA is an incubation period of 1-8 days, followed by fever lasting 1-5 days, leucopenia, and possibly upper respiratory tract inflammation and discharges, weakness, depression, anorexia, limb (and in the stallion, sheath and scrotal) oedema and abortion in the pregnant mare. It has been recently recognised that the virus can cause severe respiratory disease and enteritis in young foals. On the other hand, EVA may present as a transitory inappetance. Severity is greater in old, young or debilitated animals. Mortality from the disease in older horses is uncommon. Abortions can occur from two months of gestation onward.

During outbreaks transmission occurs by the respiratory route via aerosols. Mares are thought to spread the virus for approximately 14 days after infection and to then establish a lifelong immunity. In experimentally infected animals the virus can be recovered from various sites up to 19 days. Carrier stallions play the major role in dissemination and perpetuation of EVA by shedding virus in their semen. Shedding stallions may remain shedders for many years.

Diagnosis is based initially on a blood test and seropositive stallions need to be screened for the semen carrier state. This can be done by test mating the stallion to seronegative mares or more likely by semen virus isolation. For virus isolation, two semen samples are taken containing the sperm rich fraction of the ejaculate.


Equine Viral Arteritis (EVA) is classified in NZ as an unwanted organism under the Biosecurity Act 1993. Under section 52 of the Act, no person shall knowingly spread an unwanted organism such as EVA. Section 53 then imposes a duty on owners of an animal suspected of harbouring EVA not to breed, exhibit or offer that animal for sale. However, a Chief Technical Officer (CTO) MAF may give specific permission to horse owners allowing them to resume horse breeding and associated animal movements under controlled conditions.

As part of the equine industry's voluntary control scheme, the NZ Equine Health Association (NZEHA) has developed, in consultation with the CTO responsible for animal health in the Ministry of Agriculture and Forestry (MAF), suitable conditions under which stallions that are shedding the equine arteritis organism can nevertheless be safely used for breeding. As with the original Control Scheme it is the NZ Equine Health Association's aim to eventually eradicate EVA from NZ.

These controls are intended to enable owners and studmasters to use shedder stallions without undermining the EVA Control Scheme (EVACS) and jeopardising the health status of the equine industry, and without committing an offence under the Biosecurity Act. Owners and studmasters must obtain written permission from the CTO prior to using a shedder stallion for breeding purposes. Such requests must be accompanied by current EVA test results for all stallions (including teasers) standing on the same stud as the EVA shedder(s). They should also be in a position to demonstrate that their actions since the disease was first suspected in their animals were compliant with sections 52 and 53 of the Act. Permission will only be given to those owners or studmasters who agree to the controls listed in this document.

The status of EVA in New Zealand will be constantly under review by the Technical Subcommittee of the NZ Equine Health Association Inc. and they will make recommendations for changes with the EVA Control Scheme Coordinator

Those interested will find a brief outline of the history of EVA in NZ in Appendix 7.


The EVA Control Scheme will be managed by a Scheme Coordinator.

    1. EVA Control Scheme Coordinator (EVACSC)

    1. Information

It is important for the management of the EVACS for various parties to have access to disease information that relates to an individual person's animals or property. As this information is subject to the Privacy Act 1993 it will be necessary as part of the Scheme to have owners/studmasters read the EVA Control Scheme and complete the "Personal Information Consent" form (see appendix 5). Under the Privacy Act 1993 an individual has the right at any time to access and correct any personal information held by any agency.


2.1 Blood testing of Stallions

NZ Animal Health Reference Laboratory

National Centre for Disease Investigation

P O Box 40 742


Ward Street

Upper Hutt

2.2 Semen testing Seropositive Stallions

All seropostive stallions (except those with a fully certified vaccination history and those horses previously proven to be seropositive nonshedders) must be tested to determine if they are carriers of EVA and shedding virus in their semen. (See Appendix 2)

It is imperative that semen samples be collected without the use of any spermicidal chemicals and that samples contain the sperm rich fraction of the ejaculate. The submitting veterinarian and the laboratory should both check the semen sample for sperm.

No restrictions are placed on the breeding of seropositve stallions that are not shedders. They are immune to further infection and are not a risk in terms of spreading EVA



Shedder stallions may still be used for breeding with the written approval of the CTO of MAF. However controls need to be in place so that mares served by a shedder, whether naturally or by artificial insemination, are kept isolated (at least 30 metres) as a group for 21 days after service. This is to prevent mares that are not already immune to EVA spreading the virus through respiratory discharges during the brief viraemic period after their first service, to other non immune mares.

    1. Shedder Stallions Standing Alone

The controls required here are:

3.1.1 Transport of semen from shedder stallions

Semen from "CTO approved" shedder stallions may be transported to other properties which have also received permission from the CTO ("to be in charge of an unwanted organism" as per section 53 Biosecurity Act). The studmaster on the stud receiving the EVA infected semen should apply for this permission to the CTO.

The following conditions will be required to be met:

The owner/agent of the shedder stallion must notify the owner of the recipient property that the semen is from a shedder and get that person to sign a declaration (appendix 4 to be sent to the EVACSC) that:

    1. Shedder Stallions standing with Seropositive Nonshedder Stallion(s)
    2. If the positive titre in the non shedding stallion is due to natural infection then the rules as in 3.1 above apply

      If the titre is due to the stallion having a current and well documented vaccination program then the rules as in 3.3 below apply.

    3. Shedder Stallion(s) standing with Seronegative Stallion(s)

The controls required here are:

There is a very real risk of EVA being transferred indirectly via personnel and fomites. Special care should be taken when handling semen in stud laboratories prior to insemination or preparation for shipping.

3.4.Retiring of shedder stallions

Any shedder stallion must remain in the EVA Control Scheme irrespective of his breed until his owner decides that he should be permanently retired from service. At this time the owner must notify the EVACSC of this decision, and where the horse is to be retired. The owner will undertake to notify the EVACSC of any changes of residence of the retired shedder or if he is brought back into service in which case the stallion will once again have to be used in the manner set out in the EVA Control Scheme of the time.

3.5.Change of Status

Owners of shedder stallions may have the horse's semen checked for the presence of EVA virus on an annual basis. It is known for horses to overcome the infection and become seropositive non shedders


There are live attenuated and inactivated vaccines available. In NZ only the live attenuated vaccine (Arvac - Fort Dodge) has been registered and this is available on the approval of the Chief Technical Officer (CTO). Vaccinated horses should not be used for service within three weeks of vaccination.


The vaccine is administered by intramuscular injection on two occasions at least two weeks apart. Recent work suggests that better immunity is achieved with a longer (up to eight weeks) interval between vaccination shots. This will be clarified as soon as possible. An annual booster is required. Detectable neutralising antibodies develop in the majority of horses within three weeks of injection.


As an additional precaution to reduce the spread of EVA, mares that are to be served by shedder stallions or their transported semen could be vaccinated. This would confer clinical protection, shorten the period of any viraemia and virus shedding, and thus reduce the risk of inadvertent transmission.


As stallions need to be blood tested and shown to be seronegative within three weeks prior to vaccination it may be that those stallions requiring vaccination could be done in the autumn of 2000 as the National EVA Survey of all stallions is undertaken.

Appendix 7


Introduction into NZ

In 1988 MAF became aware that horses with elevated EVA titres had been imported into NZ from USA contrary to our Import Health Standards of the time. Further imports were stopped and a trace back was done on all horses imported from the USA since 1986. Seropositive stallions were semen tested for the virus and 22 shedders were identified.

EVA Control Scheme

The Equine Industry and MAF developed and implemented a voluntary EVA Control Scheme with the commitment to eradicate this disease. In September 1989 EVA was made a notifiable disease under the then Animals Act and this Control Scheme was developed to allow shedding stallions to remain at stud and to avoid the transmission of the disease to new stallions.

The Control Scheme relied on the compulsory EVA testing requirement by both the NZ Harness Racing Conference (now Harness Racing NZ) and the NZ Racing Conference (now NZ Thoroughbred Racing ) of all stallions prior to being registered for breeding. A number of other breeds instituted similar rules

For the owners of shedder stallions a variety of options were available. In general though the easiest option was to keep the shedding stallion(s) on a separate property. This property needed to be inspected by a MAF veterinarian and a Disease Control Place Notice issued. The stud manager was required to keep extensive records for each horse moving onto and off the property and to manage the hygiene of equipment etc. Mares were required to stay on the property for 42 (later reduced to 21) days after mating to a shedder stallion.

With the increasing practice of shipping standardbred semen about the country the EVA Control Scheme was adapted (in 1993) to allow shedder stallion semen to be sent to other properties as long as they had DCP Notices applied and inseminated mares were kept separate for 21 days.

Where a shedding stallion(s) was standing on the same property as a seronegative horse(s) it was necessary to vaccinate the seronegative horse(s) and he and his book of mares, teasers and equipment had to be kept separate (by at least 30 metres) at all times. The vaccine used was a live vaccine and was only available on the authorisation of the Chief Veterinary Officer. The vaccination required annual boosting. An alternative to these measures was to allow only seropositive mares onto the property but this was found too difficult to implement.

The Control Scheme worked well for quite some time, to the point where in early 1997 there was thought to be only one remaining "commercial" shedding stallion at stud in NZ.

Breakdown of the EVA Control Scheme

In 1997 it was identified that a previously seronegative standardbred stallion had become infected and in 1998 it was confirmed that he was a shedder.

Magnitude of the Breakdown

Since the breakdown of the EVA Control Scheme was realised the newly formed New Zealand Equine Health Association (NZEHA) has been working with MAF to identify whether there has been any further spread of the disease. Initially "in contact" stallions standing on studs to which the new shedder's semen was sent were blood tested and any seropositives were semen tested. These tests have identified another shedder. So we now have three known "commercial " shedders and the NZEHA has decided to once again put a Control Scheme in place with the aim of eradicating EVA from NZ.

Since semen from two of these known shedders was sent about NZ in an uncontrolled fashion for two years it is possible that lateral spread, other than via the "in contact" group of stallions, may have occurred. It has been decided to complete a wider survey of all stallions of all breeds to accurately identify the spread of disease. Any new shedders identified will be managed according to the new revised control scheme.



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