Common Causes of Equine Neurological Conditions
UC Davis Veterinary Medicine- The Horse Report, Fall 2021
By: Amy Young
Increased understanding of neurologic conditions has enabled earlier diagnosis and improved outcomes, and vaccines have reduced the spread of infectious diseases. However, diagnosis and treatment remain challenging in some cases. Neurologic issues can affect horses at any age, and clinical signs can result from infectious and non-infectious causes.
Infectious
Infectious neurological disorders are well known due to highly publicized outbreaks. Viral neurologic diseases are often vector-borne, commonly spread by mosquitoes. Vaccines are available to prevent infection in many cases. Vector control and biosecurity practices can also help stem the spread of these diseases, which include:
Eastern/Western/Venezuelan equine encephalitis (EEE/WEE/VEE), which cause inflammation of the brain. Although rare, EEE has a high mortality rate in horses and humans. Outbreaks of WEE have resulted in significant numbers of deaths in both species. Venezuelan equine encephalitis occurs in Central and South America, Mexico, and the southern U.S., and is considered a possible bio-warfare agent since it can be transmitted from animals to humans (zoonosis). The EEE/WEE vaccine is recommended annually and the VEE vaccine is risk-based.
Equine herpes myeloencephalopathy (EHM), caused by the neurological form of equine herpesvirus 1 (EHV-1), which attacks the spinal cord and brain, can be fatal. Infected horses can act as carriers and shed the virus. Available EHV-1 vaccines are not labelled as neuroprotectant. However, vaccination is recommended to provide herd immunity against EHV-1 and potentially reduce cases with neurologic disease.
Rabies, which has the highest case fatality rate of any infectious disease. It is a significant threat in South and Central America. Vaccination is recommended annually.
West Nile virus (WNV), which can cause inflammation of the brain and spinal cord, and possibly death. Many infected horses do not show signs of illness, but others develop neurologic disease. Vaccinate against WNV annually.
Aside from viral diseases, horse owners are acutely aware of equine protozoal myeloencephalitis (EPM), caused by infection of the central nervous system with the protozoa Sarcocystis neurona or Neospora hughesi. Clinical signs of S. neurona infection depend on the area of the central nervous system with the parasite and the type of damage caused. Cases due to N. hughesi have a range of clinical signs.
Although bacterial infections of the nervous system, such as meningitis and abscesses, are rare in horses, toxins produced by Clostridium bacteria can result in severe diseases:
Botulism is caused when Clostridium botulinum spores are ingested or infect a wound and release a neurotoxin, resulting in flaccid paralysis. It is highly fatal unless treated with antitoxin. A vaccine against type-B is available for horses in high risk areas, but there is no cross-protection between other types.
Tetanus is a potentially fatal disease that occurs when Clostridium tetani spores enter open wounds where they release a neurotoxin. Clinical signs include stiffness, muscle spasms, and the inability to open the mouth (“lockjaw”). Treatment is challenging and annual vaccination is recommended.
Non-infectious
Non-infectious neurological disorders can be caused by injury, congenital and developmental defects and genetic conditions, as well as by dietary and environmental factors.
Inherited neurological conditions are known to include:
Cerebellar abiotrophy (CA), which is found in Arabians. Foals appear normal at birth, but exhibit neurological deficits within six months of age. Affected horses may be dangerous as they are prone to accidents and injuries. A DNA test is available.
Juvenile idiopathic epilepsy (JIE), which is found in Egyptian Arabian foals during the first year of life. Clinical signs include recurrent seizures characterized by loss of consciousness and rapid muscle contractions. A causative mutation has not been identified. After one year of age, foals no longer experience seizures, but problems related to head trauma can have lasting effects.
Occipitoatlantoaxial malformation (OAAM), which is caused by a developmental defect of the first two vertebrae of the neck and the base of the skull. This causes compression and damage to the spinal cord, resulting in abnormal head and neck movement. A mutation has been identified that causes one form (OAAM1) and a DNA test is available.
Some neurological conditions likely have an inherited component, but modes of inheritance and genetic mutations have not been identified.
Vitamin E deficiencies – Equine neuroaxonal dystrophy (eNAD)/equine degenerative myeloencephalopathy (EDM) result from abnormalities of neurons in the brainstem and spinal cord and affect young horses (6 to 36 months of age). A genetic predisposition is likely, but insufficient dietary vitamin E is required for the onset of clinical signs. There is no effective treatment. Equine motor neuron disease (EMND) occurs in older horses vitamin E deficient for more than 18 months. It affects lower motor neurons, leading to muscle trembling and weakness. Treatment via supplementation has varying success.
Cervical vertebral compressive myelopathy (CVCM), “wobbler syndrome”, is caused by lesions that lead to compression of the spinal cord. Medical management, surgical treatment, and prognosis depend on the age of the horse, and duration and severity of clinical signs.
Other common non-infectious neurological conditions include:
Headshaking, which is characterized by uncontrollable shaking, flicking, or jerking of the head without apparent cause. UC Davis researchers confirmed the involvement of the trigeminal nerve, which runs across the face. In affected horses, it fires too often, causing tingling, itching, or burning sensations.
Neonatal maladjustment syndrome is characterized by foals that appear healthy at birth, but quickly exhibit neurological abnormalities (“dummy foals”). The “Madigan Squeeze Technique” was developed at UC Davis to create pressure that mimics the normal birth canal and reduce symptoms.
The neurological status of any horse varies by day, environment, and situation. When neurological signs are subtle, multiple examinations over several days and under different conditions may be needed for an accurate assessment.
Reprinted with permission by UC Davis.
