Equine Encephalomyelitis: Understanding the “Sleeping Sickness”

Equine encephalomyelitis, often referred to as “sleeping sickness,” is a concerning infectious disease that primarily affects the horse’s brain. This ailment is caused by a virus and is categorized into three distinct strains: Eastern, Western, and Venezuelan, each carrying varying degrees of mortality risk. Among these strains, the Eastern variant is both the most prevalent and the most lethal.

The transmission of encephalomyelitis is facilitated by a network of organisms. The virus finds its home in reservoir hosts, which include birds, reptiles, and rodents. These reservoir hosts are a key part of the virus’s life cycle. Mosquitoes, acting as vectors, play a crucial role by transferring the virus from these reservoir hosts to horses. Consequently, the disease tends to manifest most prominently from midsummer until the arrival of frost. It’s important to note that while horses can transmit the disease to one another through close contact, such as rubbing noses or sharing feed and water containers, they are regarded as “dead-end hosts” due to their low viral count. This makes it highly unlikely for feeding mosquitoes to contract the virus from infected horses.

Clinical Signs of Encephalomyelitis in Horses:

  1. Fever and Depression: Clinical signs typically begin to surface about five days after infection. Fever and a general sense of lethargy are among the initial symptoms.
  2. Gastrointestinal Distress: Some affected horses may exhibit diarrhea.
  3. Altered Mental State: Horses may display a drowsy appearance and a lack of interest in moving.
  4. Behavioral Changes: Behavioral abnormalities such as self-mutilation, hyperexcitability, irritability, and a refusal of food and water can occur.
  5. Central Nervous System Symptoms: The disease may progress to affect the central nervous system (CNS), leading to incoordination, head pressing, circling, paralysis, convulsions, and even coma.
  6. High Mortality Rate: Unfortunately, death typically occurs within two to three days after the initial signs appear.

Treatment and Care

Treatment for encephalomyelitis is primarily supportive in nature and should be administered in a shaded and well-padded area. In severe cases, food and water may need to be provided and evacuated manually. To prevent bedsores, horses lying down for extended periods must be rotated regularly. It’s important to note that the mortality rate for this disease is quite high, and survivors may experience impaired cerebral function, often referred to as being “dummies.”

Preventive Measures

Preventing encephalomyelitis hinges on two key strategies: immunization and mosquito control.

Vaccination: Immunization is a highly recommended preventative measure. The initial vaccine should be administered before the mosquito season begins, typically in the spring or early summer. This should be followed by a second vaccination within two to four weeks. The immunity derived from vaccination lasts for approximately six months, necessitating yearly booster shots. 

Mosquito Control: Reducing mosquito populations around horse facilities is vital in preventing disease transmission. Employing mosquito control measures, such as eliminating breeding sites and using mosquito repellents, can be effective in mitigating the risk.

Understanding and taking appropriate measures against equine encephalomyelitis are essential for the well-being of horses. By vaccinating and implementing mosquito control strategies, horse owners can significantly reduce the risk of this potentially devastating disease, ensuring the health and safety of their equine companions.

By Staff writer