Equine viral rhinopneumonitis and EHV types 1 and 4

Two herpes viruses are the cause of disease : EHV-1 (Equine herpes virus type 1) and EHV-4 (Equine herpes virus type 4). Once a horse has been infected, the virus often persists in the organism, in a latent form, and can become active again producing a new bout of the disease. Between 60 % to 70 % of horses are carriers of this latent form.

Herpes viruses are extremely contagious, and can spread due to the following :

• Infected nasal secretions from horses affected by the respiratory form of the disease
• Contact with aborted foetuses, or placental fluids in the abortive form.

The viruses can survive for several days in the environment, horses can therefore become infected without having a direct contact. However the virus is sensitive to common disinfectants.

When considering the the abortive form, the cause of the abortion may lie in :

• The mare being infected by a horse with the respiratory form
• The mare becoming infected by the aborted fœtus or placental fluids of a neighbouring mare
• Infection through contact with contaminated equipment or personnel
• Reactivation of the latent virus, hosted by the mare herself. 

Abortion can occur between 9 days to 4 months after infection. In a group of non vaccinated horses, up to 80 to 90 % of the mares may abort.

Respiratory form

The respiratory form, also called equine viral rhinopneumonitis, is mainly due to EHV-4, but can also be due to EHV-1.

The disease is not unlike equine influenza, but the symptoms are usually more moderate (slight temperature, cough and nasal discharge) for 1 to 2 weeks. These respiratory bouts are more frequently observed in autumn and winter, but can occur all year round. A concentration of equids from different locations, during shows for example, is also a risk factor. Clinical symptoms can be more or less serious or be complicated by secondary bacterial infections, especially in young horses in training

The infection can also go unnoticed, especially when the horse has been vaccinated.

This clinical form is very frequent in France : over a 100 clusters due to EHV-4 were declared to the RESPE in 2019 (12 due to EHV-1), and over 120 in 2018 (more than 30 due to EHV-1)

The abortive form

Herpes virus type 1 is the main cause of infectious abortions.
8 abortions due to EHV1, were declared to the RESPE in 2019, 18 in 2018, as well as 4 abortions due to EHV-4 in 2019, and 2 in 2018.

The abortion occurs withno premonitary symptoms (no vulvar discharge, no milk secretion), typically towards the end of gestation (9th-11th month), but can also occur as early on as the 4th month. The fœtus and placenta are expelled without difficulty, the mare is not actually ill.

In some cases the foal is delivered alive at full term, but has respiratory problems and death ensues, usually withion 3 days. Thses foals are extremly contagious for the other horses present.

Neurological form

The neurological form is mainly due to EHV-1.

8 cases were declared to the RESPE in 2019, 5 in 2018.

Various forms of neurological symptoms can be noted: slight locomotor disorders, ataxia (lack of coordination between the limbs), paresis (slight or partial paralysis), sometimes paralysis, urinary incontinence…

Most frequently, only one horse in a yard is affected, but in some cases several horses can be affected.

Recovery of the horse is variable. Serious neurological forms may require the horse to be euthanized.

• From a naso-pharangeal swab for respiratory and neurological forms
• From tissue samples (liver, lungs of the aborted fœtus, or from the placenta) for the abortive form.

To date, there is no specific treatment against herpes viruses, which are viral diseases. When in presence of a secondary bacterial infection, antibiotics are administered. Symptomatic treatment may be recommended to alleviate clinical symptoms, and in particular to bring the horse’s temperature down (NSAIDs).

With regard to the respiratory form, horses should be rested for at least 3 weeks after the disappearance of the symptoms, so as to promote healing of the damaged respiratory tract.

Concerning the neurological form, additional supportive treatment can be implemented with regard to the symptoms observed : it may for example be necessary to suspend the horse in a harness to avoid falls and problems getting up.

With the abortive form, the mare generally presents no subsequent problems. No treatment is therefore required. There is no « preventative treatment » to avoid abortion once a mare has been infected.

Bonnes pratiques dans les écuries et les élevages en milieu sain

Vaccination should be reasoned as being « collective prophylaxis », and not « individual prophylaxis ». The vaccines available do not prevent the disease entirely, but they reduce the severity and duration of the symptoms. Vaccination does not offer protection against the neurological form. Vaccinating also helps in reducing the amount of virus secreted by an infected horse, and thus reduces the risk of spreading the virus from one horse to another. Consequently, when the virus enters a stud with a proper vaccination protocol (80 % of the equids vaccinated), the percentage of sick horses and abortions is lower. It is therefore important to vaccinate, not only sensitive animals (gestating mares, young horses) and animals at risk (horses frequently moved around), but also horses they are in contact with. It is useless to « overvaccinate » brood mares, if the other horses on the stud are not also vaccinated.

Two vaccines are on the market in France :  Equip EHV 1,4 NDV and Pneumequine NDV.

In the past few years there has been a lack of supply in the vaccines available in France, the batches manufactured could not be marketed as they did not respect the quality criteria. Laboratories can in this case temporarily resort to importing vaccines with a temproray marketing authorization to help breeders out. Evolution in the manufacturing processes leave us to believe better supply in the years to come.

Recommended vaccination protocol

• First time vaccination at the age of 6 months : two injections one month apart, followed by a booster jab 6 months later.
• Boosters every 6 months if possible, without exceeding one year.
• Brood mares : some manufacturers recommend boosters at 5, 7 and 9 months gestation.

Compulsary vaccination against EHV at present only concerns :

• Some stallions
• Race horses in training (flat, steeple chasing, cross country and trotting)

Other preventive measures include :

• In a breeding stud, to protect gestating mares but also the more sensitive young horses, it is important to separate them from horses presenting a high contamination risk (horses traveling to and from shows or competitions, or traveling frequently)
• It is recommended to implement a daily care circuit, starting with the horses most sensitive to disease (gestating mares, young horses) to those most at risk of contracting the disease.
• 3 weeks quarantine for new arrivals in the yard, or ad minima daily temperature monitoring.

Measures for overcoming the disease in a contaminated environment

When in presence of an EHV cluster the measures to be taken are the same as for any contagious disease :

• Isolate sick horses, including those with hyperthermia even if they have not been diagnosed with certainty.
• Implement a daily care circuit, where you first take care of healthy animals before suspected cases, and these before sick animals, alternatively designate a specific person to deal exclusively with the sick animals.
• Use a specific set of equipment for each category of animal (healthy, suspected cases, sick animals) .
• Once the sick horse has left the premises, entirely empty the stable and destroy the contaminated bedding, then high pressure hose down before disinfecting the premises. Leave fallow for a period before reintroducing other horses
• When there is an abortion, disinfect the premises immediately.
• When the abortion takes place in a field, leave fallow for a period of two weeks minimum.