What causes ehm in horses

Overview

Equine herpesvirus myeloencephalopathy (EHM) is a serious neurological disease that affects horses. It is caused by certain strains of Equine Herpesvirus (EHV), most notably Equine Herpesvirus type 1 (EHV-1). While EHV infections can manifest in various ways, including respiratory illness and abortion in pregnant mares, EHM specifically refers to the neurological form of the disease. This neurological manifestation occurs when the virus damages the central nervous system, leading to a range of debilitating symptoms.

Understanding Equine Herpesviruses (EHVs)

Equine herpesviruses are a group of viruses that commonly infect horses. There are several types, with EHV-1 and EHV-4 being the most prevalent and clinically significant. Most horses are exposed to EHV at some point in their lives, often experiencing mild or subclinical respiratory infections. However, under certain conditions, particularly stress, these latent viruses can reactivate, or new, more virulent strains can cause severe disease. EHV-1 is particularly concerning because of its ability to cause not only respiratory disease and abortion but also neurological complications like EHM.

The Pathogenesis of EHM

The development of EHM is a complex process initiated by infection with a neuropathic strain of EHV-1. The virus typically enters the horse's body through the respiratory tract. While it can replicate in the nasal passages and cause upper respiratory signs, some strains of EHV-1 have a predilection for infecting endothelial cells, which line blood vessels. When the virus infects the endothelial cells within the blood vessels of the spinal cord and brain, it can trigger an inflammatory response. This inflammation, along with direct viral damage to nerve tissue (neurons and glial cells), leads to the characteristic neurological signs of EHM. The exact mechanisms by which EHV-1 targets the nervous system are still being researched, but it is understood that vascular damage and subsequent ischemia (lack of blood flow) and inflammation play crucial roles in the neurological deficits observed.

Routes of Transmission

EHM is a contagious disease. The primary mode of transmission is through direct contact with infected horses. This includes contact with nasal secretions from horses experiencing respiratory EHV infection, as well as contact with virus shed from aborted fetuses and placental tissues of mares infected with neuropathic EHV-1. The virus can also be spread indirectly through contaminated equipment, such as grooming tools, tack, or even clothing and shoes of handlers. Airborne transmission over short distances (up to 15-20 feet) is also possible, especially in environments with poor ventilation. The incubation period for EHM can vary, but clinical signs typically appear 2 to 10 days after exposure to the virus.

Risk Factors and Contributing Conditions

While any horse infected with a neuropathic strain of EHV-1 is at risk, certain factors can increase the likelihood or severity of EHM. Stress is a significant factor; horses undergoing strenuous training, travel, competition, or experiencing changes in their environment are more susceptible to viral reactivation or developing severe disease. Overcrowding and poor biosecurity practices in stables and at events can facilitate rapid spread. Pregnant mares are at risk for abortion, and the virus shed during abortion can be highly potent in causing neurological disease in susceptible horses. Older horses or those with compromised immune systems may also be more vulnerable.

Clinical Signs of EHM

The symptoms of EHM can range from mild incoordination to complete paralysis. Early signs often include fever, lethargy, and loss of appetite, which can be mistaken for a simple respiratory infection. As the neurological damage progresses, owners and veterinarians may observe:

• Ataxia (incoordination), particularly in the hind limbs.
• Weakness, progressing to inability to stand.
• Muscle tremors.
• A dropped or dragging hind limb.
• Loss of tail tone.
• Urinary incontinence.
• In severe cases, loss of consciousness or seizures.

The severity and progression of signs depend on the extent of damage to the spinal cord and brain. Some horses may recover with supportive care, while others may suffer permanent neurological deficits or require euthanasia due to the severity of their condition.

Diagnosis and Management

Diagnosing EHM typically involves a combination of clinical signs, neurological examination, and laboratory tests. Definitive diagnosis often relies on identifying the virus in samples such as nasal swabs, blood (for viral DNA), or cerebrospinal fluid (CSF). Post-mortem examination of nervous tissue is also a common diagnostic method. Management of EHM is primarily supportive and focuses on reducing inflammation and supporting the horse's bodily functions. This can include anti-inflammatory medications (like DMSO or NSAIDs), corticosteroids (used cautiously), and nursing care to prevent secondary complications like pressure sores or pneumonia. Horses unable to stand may require slings or specialized bedding. Antiviral medications are not typically effective against EHV-1 in the neurological stage.

Prevention and Biosecurity

Preventing EHM is paramount and relies heavily on robust biosecurity measures. Vaccination against EHV-1 is available, but it primarily targets respiratory disease and abortion and does not offer complete protection against neurological EHV-1. However, vaccination may reduce the severity of clinical signs. Key biosecurity practices include:

• Strict isolation protocols for new horses or those returning from events.
• Regular cleaning and disinfection of facilities and equipment.
• Prompt identification and isolation of sick horses.
• Careful management of horse movement, especially during outbreaks.
• Monitoring for signs of illness and reporting suspected cases to a veterinarian immediately.

Understanding the causes and transmission of EHM is crucial for horse owners and caretakers to implement effective prevention strategies and protect their equine populations.