What causes ehv in horses

Overview

Equine herpesvirus (EHV) is a common viral infection affecting horses worldwide. Several different strains of EHV exist, but two are primarily responsible for the most significant clinical signs: EHV-1 and EHV-4. These viruses are highly contagious and can lead to a variety of health problems in horses, ranging from mild respiratory symptoms to severe neurological disease and reproductive losses.

What are Equine Herpesviruses?

Equine herpesviruses are DNA viruses belonging to the family Herpesviridae. They are known for their ability to establish latent infections, meaning the virus can remain dormant in a horse's body for extended periods and reactivate under stress. This latency makes complete eradication of the virus from the horse population extremely challenging.

EHV-1 vs. EHV-4

While both EHV-1 and EHV-4 can cause respiratory disease in horses, EHV-1 is the strain most commonly associated with more severe outcomes, including:

• Equine Abortion: EHV-1 is a major cause of contagious abortion in pregnant mares, often leading to late-term fetal loss.
• Neurological Disease (Equine Herpesvirus Myeloencephalopathy - EHM): EHV-1 can infect the nervous system, causing inflammation of the brain and spinal cord. This neurological form, EHM, can result in incoordination, weakness, paralysis, and in severe cases, euthanasia.
• Respiratory Illness: Both EHV-1 and EHV-4 can cause upper respiratory tract infections, characterized by fever, nasal discharge, and coughing, particularly in young horses.

EHV-4 is primarily associated with respiratory disease and is less likely to cause abortion or neurological signs compared to EHV-1.

How is EHV Transmitted?

EHV is highly contagious and spreads through various routes:

• Direct Contact: Horses can contract the virus through direct contact with nasal secretions or saliva of infected horses.
• Aerosol Transmission: The virus can spread through respiratory droplets expelled when an infected horse coughs or sneezes.
• Contaminated Fomites: Objects such as shared tack, water buckets, feeding troughs, and even clothing and hands of handlers can carry the virus.
• In Utero: Infected mares can transmit EHV to their foals before birth.

Horses can shed the virus intermittently, especially during periods of stress, which can lead to new outbreaks even in seemingly healthy populations.

What are the Symptoms of EHV?

The clinical signs of EHV infection can vary depending on the strain of the virus, the age and immune status of the horse, and the specific organs affected. Common symptoms include:

Respiratory Signs:

• Fever (often the first sign)
• Serous nasal discharge (clear, watery)
• Coughing
• Reduced appetite
• Swollen lymph nodes in the throat area
• Lethargy

Abortion and Neonatal Foal Disease:

• Sudden abortion, often with no prior signs of illness in the mare.
• Stillbirths or weak foals that may die shortly after birth.

Neurological Signs (EHM):

Ataxia (incoordination), particularly in the hind limbs

Weakness or stumbling

Loss of tail tone

Difficulty urinating

Recumbency (lying down and unable to rise)

In severe cases, paralysis and death

It's important to note that some horses may be infected with EHV but show no clinical signs, acting as silent carriers and spreaders of the virus.

Diagnosis and Treatment

Diagnosis of EHV typically involves laboratory testing, which can include:

• PCR (Polymerase Chain Reaction): Detects viral DNA in nasal swabs, blood, or tissue samples.
• Virus Isolation: Growing the virus in cell cultures from samples.
• Serology: Measuring antibody levels in the blood, though this is less useful for diagnosing acute infections as antibodies can persist from past infections.

There is no cure for EHV. Treatment focuses on supportive care to manage symptoms and prevent secondary bacterial infections. This may include rest, anti-inflammatory medications, and in cases of neurological disease, intensive nursing care and medications to reduce inflammation in the nervous system.

Prevention and Biosecurity

Preventing EHV outbreaks requires a multi-faceted approach:

• Vaccination: Vaccines are available for EHV, primarily targeting respiratory disease and abortion. While they can reduce the severity and shedding of the virus, they do not guarantee complete protection against infection or disease, especially neurological forms. Consult with your veterinarian about appropriate vaccination protocols for your horses.
• Biosecurity Measures: Strict biosecurity protocols are crucial, especially in environments with multiple horses (e.g., boarding stables, training facilities, show grounds). This includes:
  - Isolating new horses for at least 2-3 weeks.
  - Avoiding sharing equipment between horses.
  - Practicing good hygiene (hand washing, disinfecting surfaces and equipment).
  - Promptly isolating any horse showing signs of illness.
• Management of Pregnant Mares: Implementing specific biosecurity and vaccination programs for pregnant mares is essential to minimize the risk of abortion.
• Stress Reduction: Minimizing stress for horses, as stress can trigger viral shedding and reactivation of latent infections.

Early detection, prompt isolation of sick horses, and rigorous biosecurity practices are key to controlling the spread of EHV in equine populations.