Infection can be transmitted between horses in any of the following ways:
• EHV respiratory infections are spread most commonly via the respiratory route (e.g. via droplets from coughing and snorting);
• When mares abort with EHV infection, the fetus, fetal membranes and fluids are particularly dangerous sources of infection, releasing large quantities of infectious virus into the local environment, to be inhaled via the respiratory route (particularly when abortions occur in enclosed shared air space environments) and transmission may occur indirectly via attendants and their implements;
• Mares who have aborted or whose newborn foals have died from EHV infection may transmit infective virus via the respiratory route or genital tract and transmission may occur indirectly via attendants and their implements;
• Older foals with EHV respiratory disease (‘snotty noses’) and sometimes horses with neurological signs are highly contagious and can transmit infection to other horses via the respiratory route and by shedding virus into the environment;
• EHV does not travel long distances (greater than 50 metres) as an aerosol so close contact between horses should be minimised by physical separation into smaller group sizes;
• All these sources of infection are intensified when infected horses are stabled, particularly in shared air space stables, e.g. ‘American-type barns’. Evidence from outbreaks linked to this type of stabling suggests that large quantities of infective virus can be released into the surrounding air following any EHV abortion. When this happens at pasture, there is a greater opportunity for dispersal and dilution of the viral ‘cloud’ than if the abortion occurs when horses are stabled. Breeding stock, particularly pregnant mares and their foals at foot should spend as much time as possible turned out in small groups in adequately sized and well managed paddocks and, when essential, in individually ventilated stabling with provision for heads to be out in the fresh air. It is believed that fresh air has beneficial effects on horses’ natural respiratory and immune defence mechanisms. It may help the horse’s natural respiratory defence system to feed hay from the ground;
• Indirect EHV transmission can occur through the environment because the virus may survive for up to a month, once it has been shed by the horse. Very often the circumstances and handling/management of the first case of abortion is critical to the risk of exposure of other animals on the stud to EHV and ultimately whether there are subsequent abortions due to EHV infection. Consequently, stud farms should develop appropriate biosecurity protocols before any major outbreaks of disease with appropriate protective clothing, equipment, utilities and hand washing facilities for staff specifically allocated when abortions occur, in order to prevent indirect spread of infection to other pregnant mares.
The nature of herpesviruses means that all horses can be ‘carriers’ of EHV in a latent form (meaning that horses are not always infectious to others), which can, under conditions of stress, be reactivated, meaning that they may then transmit infection without showing signs of illness. As EHV is a common endemic infection, it is probable that the vast majority of adult horses are latent carriers and as such have the potential to act as a source of reactivated EHV-1. Currently there is no reliable test for carrier status. In carriers, illness (respiratory, abortion or neurological) may become apparent from time to time, especially after stress (particularly travelling and changing of location and social groups) or after suffering another disease. The virus is potentially contagious at these times and may be transmitted to otherwise healthy but susceptible horses, who may then develop EHV disease.
In late pregnant mares, transport, location, social group change and other types of stress may increase the risk of carrier horses, shedding virus from the nose (often with no accompanying clinical signs of disease in the carrier) as well as the virus crossing the placenta in the pregnant uterus, resulting in fetal infection, leading to abortion. Stud owners and managers should think ahead and group pregnant mares in small group sizes with similar due dates, early in their pregnancies, which can then be maintained without transportation and re-mixing until they foal. Pregnant mares that arrive from sales or from overseas, following associated transportation and social disruption, should always be considered ‘high risk’ for EHV abortion and should be quarantined and managed accordingly. All new arrivals and horses returning from elsewhere should be quarantined and maintained separately from resident horses.
