Equine Herpesvirus Induced Abortion

The Disease

The events leading to EHV-1 induced abortion may be considered in three phases. Primary EHV-1 infection is acquired via the respiratory route, whereupon the virus infects the epithelial cells lining the naso-pharynx and respiratory tract. Initial infection is rapidly followed by virus transmission to underlying tissues and infection of endothelial cells (vascular and lymphatic) and leucocytes. The infection then enters a 'viraemic' phase, where the virus is carried around the body via infected blood cells (monocytes and T lymphocytes). In this way, the virus can be spread to other sites, including the endothelium of small blood vessels in the endometrial lining of the uterus. Subsequent thrombosis and/or vasculitis of the affected blood vessels compromises the microcirculation of the placenta and allows leakage of virus and/or infected cells into the placental circulation. Where virally-induced thrombosis is particularly extensive this can lead to abrupt placental separation before the fetus has become infected; but the more usual situation in the field is that viral infection spreads to the fetus via the umbilical cord and that further cycles of replication occur in the fetal tissues before abortion occurs. The abortion process is therefore likely to reflect an interplay between vascular compromise in the uterus, and stress following viral infection in the fetus.

As for other herpesviruses, EHV-1 is able to persist lifelong within an infected animal, utilising a quiescent state known as latency. It is believed that up to 75% of the British horse population are carriers of latent EHV-1. Examples of familiar human herpesviruses that exhibit latency are herpes simplex virus (HSV-1 - cold sores) and varicella-zoster virus (VZV - chicken pox). Reactivation from latency occurs when the virus goes from the quiescent state to a state of active replication, thereby spreading to other cells, frequently to sites of virus shedding (enabling the virus to infect other hosts). In the case of HSV-1 and VZV, the virus goes latent in cells associated with nerve ganglia; HSV-1 reactivation results in cold-sores (which can recur periodically) while VZV reactivation results in Shingles (which tends to occur only once). EHV-1 is unusual, with latently infected cells reported in both nerve ganglia and lymphocytes. The relative importance of virus reactivation from latency versus primary infection as a cause for EHV-1 induced abortion is uncertain. In addition to abortion following infection via the respiratory route, it is also possible that infection developed following reactivation of EHV-1 from latency may lead to abortion.

Prevention of abortion

Currently, there is no effective drug treatment of EHV-1 infection. Good stud management and hygiene are therefore the most useful tools in disease control.

The Horse Race Betting Levy Board produces Codes of Practice for the prevention and control of important equine diseases such as Equine Herpesvirus. These codes outline the minimum measures which should be implemented by horse owners, in conjunction with their veterinary surgeons, as a means of limiting and resolving disease outbreaks. Copies of the Codes of Practice are available on the HBLB website and from:

Horserace Betting Levy Board
52 Grosvenor Gardens
Tel: 0171 333 0043
Fax:0171 333 0041
Email: hblb@hblb.org.uk

The main aims of the control measures outlined in the Codes of Practice are to avoid exposure of susceptible horses to infection, and to minimise reactivation of latent virus.

Stress is an important factor in the reactivation of latent virus in carriers. Unnecessary stresses such as prolonged transport should be avoided during late pregnancy.

Any horse or pony can be a source of virus, even if apparently healthy. Pregnant mares should therefore be kept separately from all other stock.

Any mare that aborts or produces a dead or sickly foal must be isolated immediately and veterinary advice should be sought. Other pregnant mares must be prevented from coming into contact with the aborted fetus, placenta and fluids, as these are a major source of infectious virus. Movement on and off the stud should be stopped until EHV-1 has been excluded as the cause of disease.

Thorough cleaning and disinfection of the site is required, as the virus may otherwise survive in the environment for several weeks.

Diagnosis of Herpesvirus

Following immediate control measures, a specific diagnosis is required as quickly as possible. The presence of EHV-1 can only be diagnosed in a laboratory. It is important that appropriate samples are provided for this purpose. A blood sample from the mare is not useful, as serology results following abortion can be misleading.

Diagnosis is currently made by Polymerase Chain Reaction (PCR) amplification of viral DNA from fetal tissues and by the demonstration of characteristic herpesviral lesions in the affected organs by routine histology and immunostaining. Appropriate samples for submission are fetal liver, lung, spleen and thymus in neutral buffered formalin for histopathology, or frozen or pooled in viral transport medium for PCR. AHT staff are happy to advise on sample collection. Ideally, the entire fetus and its membranes may be submitted. PCR has recently replaced virus isolation as a means of EHV-1 abortion diagnosis at the AHT. This means that positive diagnoses can now be made on the same day that the aborted fetus and placenta are examined.


Vaccination against EHV-1 is available and provides a means of restricting the spread of the virus between mares and reducing the chances of abortion in infected mares. It is recommended as part of an overall disease control strategy. However, vaccination does not provide total protection and is not an adequate substitute for good management practices.

Once a programme of EHV vaccination has been started it should be maintained wherever possible. The primary course often involves 2 injections 4-6 weeks apart and booster injections every 6 months thereafter. However, pregnant mares, as a high risk group, are usually vaccinated at 5, 7 and 9 months of gestation.

As with all vaccines, horses should be in good health at the time of vaccination in order that maximum immunological response is achieved. Where there is the possibility of concurrent EHV-related or other disease, it is advisable to delay vaccination until the disease has been investigated. All animals should be considered for inclusion in the programme, including youngstock which are often implicated as the source of infection.

Vaccination should be particularly beneficial in limiting disease within groups of horses, especially where there is movement and mixing within groups (eg competition horses). Vaccination will not necessarily guarantee protection against EHV-1 abortion but will reduce the amount of virus shed, therefore limiting the spread of infection to in-contact animals and subsequently helping to prevent abortion storms.

Researchers at the AHT are actively involved in projects investigating the development of improved vaccines against EHV-1 induced abortion.


To find out more about the AHT why not visit their website www.aht.org.uk


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