Genetic Health Risks: Equine Herpesvirus Myeloencephalopathy (EHM)

At some point in your equestrian journey, you have or will likely come across a news bulletin announcing that a horse tested positive for Equine Herpes Virus (EHV). There are multiple strains of EHV, but Equine Herpes Virus type 1 (EHV-1) is associated with a neurologic presentation of the virus, Equine Herpesvirus Myeloencephalopathy (EHM), among three other forms. EHM is a more commonly known manifestation of the virus due to its prevalence and sometimes fatal effects. While we know that EHV-1 is highly transmissible and common among horses, we now know that there is a genetic risk factor associated with horses developing EHM that we can test for. In fact, Etalon’s equine DNA database currently reflects that a staggering 3 out of 4 horses have the genetic variant associated with a higher risk of developing EHM.

Key Takeaways

• Equine Herpes Virus type 1 (EHV-1) poses a significant risk to horse health. EHV-1 is associated with the neurologic and potentially fatal manifestation of the virus, Equine Herpesvirus Myeloencephalopathy (EHM). Most horses will not develop EHM due to contracting EHV-1, but those who do can be severely affected.

• EHV-1, the virus that can cause EHM, is highly contagious, spreading through direct horse-to-horse contact, contaminated equipment, and vertical transmission, with factors like asymptomatic shedding and stress contributing to its spread.

• Horses that are heterozygous and homozygous for a genetic variant associated with EHV-1 Induced Myeloencephalopathy have a 1.43x increased risk of developing EHM. Etalon Equine Genetics has testing options available to determine if your horse carries the variant responsible for this increased risk.

Understanding what EHM is, how it spreads, what horses are most at risk, its symptoms, diagnosis, treatment options, prognosis, and prevention methods is crucial for safeguarding the health and well-being of our equine partners. This includes getting a leg up on EHM by identifying their horse’s genetic risk factor with Etalon before it’s too late.

What is EHM?

While there are nine currently identified strains of EHV, EHV type 1 (EHV-1) is the strain associated with four distinct manifestations: neurologic, respiratory, abortion, and neonatal death. EHV-1 is highly contagious, affecting horses worldwide and typically causing respiratory illness or abortion in pregnant mares.

However, in some cases, the virus can lead to EHM, the neurological manifestation of EHV-1 infection. When EHV-1 infects a horse, it initially replicates in the respiratory tract and then spreads throughout the body via the bloodstream. In some horses, the virus can invade the blood vessels that supply the spinal cord and brain, leading to inflammation and damage to the nervous tissues.

How is EHV-1 Spread?

EHV-1, the virus responsible for EHM, is highly contagious among horses. It spreads most commonly through direct horse-to-horse contact but can also be transmitted via contaminated equipment, clothing, or human handlers. Because of its infectious nature, nearly all horses contract EHV-1 by age two, typically without serious clinical effects. Following infection, the virus can enter a dormant phase within the body, leading to a carrier state, known as latent infection, with no outward signs of disease. Current estimates indicate that at least 60% of horses harbor a latent EHV-1 infection. Understanding how EHV-1 spreads is crucial for implementing effective prevention strategies to protect horses from infection that can lead to EHM.

Modes of Transmission:

Direct Horse-to-Horse Contact (Respiratory Secretions): The primary way that EHV-1 is transmitted is through direct contact between horses. EHV-1 can be shed in respiratory secretions, such as nasal discharge or saliva, from infected horses. When an infected horse coughs, sneezes, or even breathes near other horses, the virus can be aerosolized and transmitted to nearby animals. This highly infectious nature is why EHV-1 and other strains of EHV outbreaks are often associated with large equine events and facilities due to the sheer number of horses who can be exposed to the virus.

Fomite Transmission: EHV-1 can also be spread indirectly through contaminated objects, surfaces, or equipment (called fomites). Items such as tack, buckets, grooming tools, stall and stable surfaces, or human clothing can harbor the virus and serve as vehicles for transmission if susceptible horses come into contact with them.

Vertical Transmission: In pregnant mares, EHV-1 can be transmitted to the developing fetus in utero, leading to abortion, stillbirth, or the birth of weak, infected foals. This vertical transmission route perpetuates the spread of the virus within equine populations.

Factors Contributing to Spread:

Asymptomatic Shedding: Horses infected with EHV-1 may shed the virus even without clinical signs, serving as potential sources of infection for other horses. This asymptomatic shedding complicates efforts to identify and isolate infected individuals.

Stress: Stressful conditions, such as transportation, overcrowding, competition, or changes in environment or management, can trigger EHV-1 reactivation in latently infected horses or exacerbate clinical signs in actively infected individuals. Stress management is therefore essential for reducing the risk of EHM outbreaks.

Poor Biosecurity: Inadequate biosecurity measures, such as lax quarantine protocols, poor hygiene practices, or failure to implement preventive measures during events or gatherings where horses congregate, can facilitate the spread of EHV-1 within and between equine populations.

Symptoms of EHM

After infection, the incubation period for EHV-1 is typically 4-6 days but may be as short as 24 hours. EHM typically presents with minimal respiratory signs, and a fever (rectal temperature greater than 101.5 degrees F) is considered a warning sign. However, horses may develop EHM even without any preceding fever and/or respiratory signs. The neurologic disease progresses rapidly and reaches peak intensity within 24 to 48 hours from the onset of initial neurologic signs.

Recognizing these symptoms is crucial for prompt diagnosis and treatment. Clinical signs of EHM may include:

Neurological Symptoms:

Incoordination (Ataxia): One of the hallmark signs of EHM is ataxia, which is the lack of coordination. Affected horses may have difficulty walking in a straight line, weaving, swaying, stumbling, or exhibiting an unsteady gait.

Weakness: Horses with EHM may demonstrate weakness, particularly in the hind limbs. They may have difficulty rising from a lying position, standing, or supporting their weight on their hind legs.

Muscle Tremors or Twitching: Muscle tremors or twitching may be observed, particularly in the hindquarters or along the spine. These involuntary movements can range from subtle twitches to more pronounced tremors.

Difficulty Urinating or Defecating: Neurological dysfunction can affect the horse's ability to control their bladder and bowel function, leading to difficulty urinating or defecating. Horses may strain to urinate or pass manure, they may exhibit urine dribbling or loss of bowel control.

Leaning or Falling: Horses with severe neurological impairment may lean to one side, tilt their head, or have difficulty maintaining balance. In extreme cases, they may collapse or fall to the ground.

Altered Mental Status: Changes in mental status, such as depression, lethargy, disorientation, or altered responsiveness to stimuli, may occur as a result of neurological damage.

Head Pressing: Some horses may exhibit abnormal behaviors such as pressing their head against walls or objects, which can indicate discomfort or neurological dysfunction.

Seizures: In rare cases, seizures may occur as a result of severe neurological involvement. Seizures can manifest as sudden, uncontrolled movements, loss of consciousness, or convulsions.

Respiratory and Systemic Signs:

In addition to neurological symptoms, horses with EHM may also display general signs of illness, including:

• Fever: EHM typically causes a two-phase fever peaking on day 1 or 2 and again on day 6 or 7.
• Nasal discharge
• Persistent enlargement of the submandibular (under the jaw) lymph nodes
• Loss of appetite

Diagnosis and Treatment

EHM requires a multifaceted approach involving clinical evaluation, diagnostic testing, and supportive care. Prompt recognition of symptoms and early intervention are essential for optimizing the chances of a favorable outcome. Here's an overview of the diagnosis and treatment of EHM:

Diagnosis:

Clinical Evaluation: The diagnostic process typically begins with a thorough clinical evaluation by a veterinarian. This includes a detailed history of the horse's symptoms, physical examination, and neurological assessment to identify signs of ataxia, weakness, muscle tremors, altered cognition, or other neurological abnormalities.

Laboratory Testing: Laboratory tests, such as polymerase chain reaction (PCR) testing, can detect the presence of Equine Herpes Virus type 1 (EHV-1) in nasal swabs, blood samples, or other biological specimens. PCR testing is highly sensitive and specific for detecting viral DNA and confirming active infection. Many veterinarians will collect both a nasal swab and a blood sample to confirm an EHV-1 diagnosis.

Additional Testing: Diagnostic imaging techniques, such as magnetic resonance imaging (MRI) or computed tomography (CT) scans, may be recommended to assess the extent of neurological damage and identify lesions or abnormalities in the spinal cord or brain. In some cases, analysis of cerebrospinal fluid obtained via a spinal tap (lumbar puncture) may be performed to evaluate for signs of inflammation, infection, or other abnormalities suggestive of neurological disease. These imaging and testing modalities can provide valuable information for diagnosing EHM and guiding treatment decisions.

Treatment:

Supportive Care: Treatment for EHM focuses on providing supportive care that aims to alleviate symptoms, stabilize the horse, and prevent further complications. This may include:

• Intravenous fluid therapy to maintain hydration and electrolyte balance.
• Anti-inflammatory medications, such as non-steroidal anti-inflammatory drugs (NSAIDs), may be given to reduce inflammation and pain.
• Gastrointestinal protectants to prevent gastric ulcers or other gastrointestinal complications associated.
• Nutritional support, including feeding soft or easily digestible foods if the horse has difficulty eating or swallowing.
• Antiviral medications may be used to inhibit viral replication and reduce the severity and duration of clinical signs. However, the efficacy of antiviral therapy in treating EHM is debated, and its use may vary depending on the individual case and the veterinarian's discretion.

Isolation and Quarantine: Infected horses should be isolated from healthy animals to prevent the spread of EHV-1 to susceptible individuals. Strict quarantine protocols, including separate housing, dedicated equipment, and enhanced biosecurity measures, are necessary to minimize the risk of transmission within the equine population.

Prognosis

The prognosis for horses with EHM varies widely, with an estimated mortality rate of 20% to 80% depending on factors such as the severity of neurological damage, the promptness of treatment, and the individual horse's overall health and response to therapy. While some horses may make a full recovery with appropriate care, others may experience long-term neurological deficits or succumb to the disease, especially if they lay down and are unable to get back up. Early detection, aggressive supportive care, and diligent management are critical for improving outcomes in horses affected by EHM.

Genetic Risk Factors

Think of genes as the blueprints that tell our bodies how to function. In horses, there is a gene called tetraspanin 9 (TSPAN9), which plays a role in building the lining of blood vessels. This lining, which is made up of endothelial cells, is crucial for keeping blood vessels healthy and functioning properly.

Within the TSPAN9 gene, there's a specific variant called BIEC2_946397. Research has found that horses with one or two copies of this risk allele are 1.43x more likely to develop EHM if they contract EHV-1.¹ Essentially, having this specific variant of the gene can increase a horse's chances of developing EHM if they get infected with EHV-1.

Knowing whether a horse carries this genetic risk factor can help horse owners and veterinarians protect a horse's health by taking extra precautions to avoid exposure to EHV-1 or taking additional measures if they are exposed.

Testing and Understanding EHM Risk

Testing for this genetic variant is now available in our Pro Package and Health Panel DNA tests, providing horse owners with valuable information about their horse's susceptibility to EHM. Results may indicate higher or lower risk based on the presence or absence of the EHM risk variants.

EHM/EHM = homozygous for EHV-1 Induced Myeloencephalopathy Risk, 1.43x increased risk

EHM/n = heterozygous for EHV-1 Induced Myeloencephalopathy Risk, 1.43x increased risk

n/n = no variant detected

It is important to note that if your horse carries one or two copies of the EHM risk variant, it does not mean that your horse has or will definitely develop EHM! What it does mean, however, is that they are more likely to develop EHM should they become infected with EHV-1. Knowing your horse’s genetic EHM risk level lets you make proactive management decisions well before the next EHV-1 outbreak.

Prevention

Preventing EHM relies on strict biosecurity measures to limit the spread of EHV-1. This includes quarantine protocols for new arrivals, regular disinfection of facilities and equipment, and minimizing contact with potentially infected horses.

Biosecurity Protocols: Implementing strict biosecurity protocols is essential for preventing the spread of EHV-1. This includes quarantining new arrivals, segregating sick or exposed horses, regularly disinfecting facilities and equipment, and minimizing direct and indirect (i.e. avoiding shared water and feed buckets) horse-to-horse contact.

Vaccination: While vaccination against EHV-1 may not prevent infection entirely, it can reduce the severity and duration of clinical signs and decrease viral shedding, thereby helping to control the spread of the virus within equine populations.

Stress Management: Minimizing stressors and providing optimal species-appropriate care, nutrition, and management can help support horses' immune function and reduce the risk of EHV-1 reactivation or clinical disease.

Monitoring and Surveillance: Regular monitoring for clinical signs of EHV-1 infection, prompt isolation of suspected cases, and communication with veterinarians and other horse owners can facilitate early detection and containment of outbreaks.

For horses with a known genetic predisposition for a higher EHM risk, owners can proactively implement targeted management strategies to reduce the environmental risk in susceptible individuals. This may include using a combination of all the previously mentioned methods of enhanced biosecurity measures, vaccination protocols, stress management techniques, and close monitoring for clinical signs of infection for the most comprehensive risk management.

Conclusion

EHM represents a significant threat to horse health and welfare, but advances in genetic testing offer new opportunities for understanding and mitigating this risk. Understanding your horse's genetic predisposition to EHM can empower you to make informed decisions for their health and well-being. By combining a knowledge of EHM with proactive prevention strategies, every horse owner can take meaningful steps to protect their equine companions from this devastating disease.

References ¹ Brosnahan, M. M., Al Abri, M. A., Brooks, S. A., Antczak, D. F., & Osterrieder, N. (2019). Genome-wide association study of equine herpesvirus type 1-induced myeloencephalopathy identifies a significant single nucleotide polymorphism in a platelet-related gene. Veterinary journal (London, England : 1997), 245, 49–54. https://doi.org/10.1016/j.tvjl.2018.12.013