A diagnosis of neurotropic equine herpesvirus (nEHV-1) in a horse displaying neurological symptoms has recently been confirmed in Saskatchewan. Prior to diagnosis this horse had travelled to a number of rodeos in Alberta and thus there is the potential for exposure of numerous horses. As a result, horse owners have been alerted and are advised to make informed decisions regarding the health of their horse(s) regarding nEVH-1 diagnosis.
The herpes virus causes respiratory disease, abortions and rarely, neurological (brain and spinal cord) disease. The neurological form of the disease is called Equine herpesvirus myeloencephalopathy (EHM). The virus poses no human health risk.
Equine herpesvirus (EHV) is a common contagious virus of horses. It has been around for a long time and many/most horses are exposed to the EHV at a young age.The most common strains are EHV-1 and EHV-4. EHV-1 can cause respiratory diseases, abortions and neurologic disease. EHV-4 typically causes respiratory disease but has also been known to cause abortions. Expression of clinical signs once infection occurs is influenced by the virulence of the virus, the immune status of the horse affected, and environmental factors. Clinical presentation is markedly exacerbated and amplified by stressors.
The most common expression of viral infection is mild upper respiratory tract disease (rhinopneumonitis) in young horses. Initially infection is characterized by fever with temperatures greater than 38.5 C, lack of appetite and depression. Often but not always, signs of mild upper respiratory tract disease such as serous nasal discharge and coughing will be evident for a few days. Animals may also be exposed to the virus and exhibit no clinical signs. Exposure to the virus can lead to abortions in brood mares.
Equine herpesvirus myeloencephalopathy (EHM) is the third manifestation. In this clinical presentation the virus damages blood vessels that supply the brain and spinal cord which causes loss of neurological function. Although neurological signs from the virus can accompany an outbreak of upper respiratory disease, this is not always the case and neurological signs caused by the virus may appear days, weeks, months or even years after the initial infection. Many horses carry the virus latently throughout their lives without showing signs of illness.
Signs of infection
It is the nature of the herpes virus to periodically become reactivated from its hiding place in the neurological tissue whenever host conditions allow. Often there is a strong link to overwhelming stressors upon the horse, a weakened immune system and reactivation of the virus — thus expression of the neurological form of the disease.
Clinical signs of the infection may include a fever and neurological problems primarily affecting the back half of the body. Symptoms range from mild weakness and lack of co-ordination to complete paralysis. Dog-sitting, decreased tail tone and bladder dysfunctions are hallmark signs. Severely affected horses may become recumbent and unable to rise. Often horses remain bright and alert and continue to eat and drink. Some horses may be mildly affected, merely stumbling, while others are severely affected and unable to rise or urinate.
Diagnosis and treatment
The disease is confirmed by a polymerase chain reaction (PCR) test performed on a nasal swab and/or blood sample. This test identifies horses that are shedding the virus by detecting the genetic material of EHV-1.
Treatment is mostly supportive and may include anti-inflammatory medication, intravenous fluid therapy, and repeated bladder catheterization. Horses that contract nEHV-1 can have a mortality rate up to 40 per cent. Those that become recumbent and are unable to rise are often euthanized due to the complications associated with a downer horse.
Although several vaccines against EHV-1 are currently available none of them state any claim for protection against the neurologic form of EHV-1 infection. In fact recent and/or frequent vaccinations and increased event attendance have been identified as risk factors for EHM development in epidemiological studies.
Although EHM seems to “come out of nowhere,” this is not the case. The principal reservoir for infection is the latently infected horse. It is probable that the majority of horses carry the virus, never showing signs of active infection. It is only under certain conditions of immune compromise that the virus can gain a stronghold and wreak havoc within the horse’s neurological system. The herpes virus is an opportunist.
Since increased event attendance has been identified as one of the risk factors associated with EHM, it is prudent to make reasonable choices regarding a horse’s show, event and travel schedule. Attendants of equine events are advised to familiarize themselves with biosecurity instructions and minimize direct and indirect contact between their horses and others. The natural spread of the EHV-1 virus occurs primarily through the viral shed of infected respiratory secretions. Vigilant biosecurity measures are particularly relevant when horses are travelling distances, commingling, and enduring greater stress. It is advisable that any horse showing signs of respiratory and neurological disease be examined by a veterinarian.