Equine Neurological Disease

Diagnosis is based on a few techniques:

• History- geographical location and recent travel. Determine onset and duration of clinical signs. Information of health status of other horses in the vicinity. Vaccination history. 
• Clinical exam- normally fever coinciding with viraemia, virus replication at entry site or specific organ infection. Symmetrical, multifocal or diffuse lesions. 
• CSF analysis and serology- key in viral encephalitis. Mononuclear pleocytosis and increased protein concentration commonly. Identify ag/ab via culture, PCR and ab detection tests e.g. ELISA. 
• Medical imaging and functional testing- CT, MRI or EEG. Limited. Only head and up to mid cervical region and usually non specific intracranial oedema. 
• Post mortem exam- histopathology, immunohistochem or PCR amplification. 

General treatment Reccomendations:

• Fever and neurological signs- infectious unless proven otherwise so isolate. 
• Quiet, dark stable with deep bedding and padded walls. 
• Recumbent= turn every 4-6 hours.
• NSAIDS, antivirals (valacyclovir), ag specific therapy e.g. WNV and vitamin E/thiamine. 
• Prevention- vaccination/minimise exposure to vector/reservoir. 

Equine Herpes Virus 

EHV-1

• Alpha herpes virus.
• Enveloped DNA virus.
• Latent infection.
• Humoral immunity short lived.
• Naturally low ab levels. 

Can occur sporadically or as outbreaks. 

• 85% morbidity, 10% mortality. Myeloencelopathy. Also associated with respiratory disease and abortion. 
• Clinical signs result of vasculitis and thrombosis of arterioles in the brain and spinal cord. May be a recent history of abortion/respiratory disease on the premises. 
• Sudden onset and early sign stabilization- Ataxia (hindlimbs or all) to recumbency, cauda equina signs atony of bladder, flaccid tail and anus and perineal hypoalgesia. Occasional cranial nerve involvement. 

Diagnosis:

• Virus isolation/PCR-> nasal swab, buffy coat or CSF.
• Serology-> complement fixation/ELISA. 4 fold titre rise.
• CSF-> xanthochromia. 

EHV-1- possible neurovirulent form. New infection from respiratory outbreak or reactivation of latent virus under times of stress (less likely with neurological form). 

Management:

• Isolate.
• Prognosis reasonable with good nursing- recovery day to weeks. Poor prognosis if recumbent more than 24 hours. 
• Recurrence of neurological signs unreported.
• Vasculitis-> antinflammatories, NSAIDS, corticosteroid and aspirin. 

Prevention:

• Vaccination against EHV1 and 4. Ineffective against neurological form.
• Could possibly worsen neurological disease (if immune mediated). 

Rabies

• ssRNA with envelope.
• Transmitted by saliva/contaminated bite wounds. 
• Uncommon.
• Zoonosis.
• Salivary/droplet transmission- carnivore bites to limbs.

Pathogenesis:

• Local inoculation at wound.
• Peripheral nerve access.
• Gradual/slow movement centrally (centripetal axoplasmic flow). 
• Replication in spinal/dorsal root ganglia.
• Rapid CNS spread- spinal cord. 
• Centrifugal spread down nerves to salivary glands.

Incubation period variable. Dependent on virus, strain, and size of inoculum and proximity to CNS.

No pathognomic signs.

• Spinal form-> paralytic.
• Brain stem-> dumb.
• Cerebrum-> furious. 

Spinal/paralytic form:

• Most common.
• Localised hyperaesthesia,- self mutilation and close to inoculum site.
• Progressive ascending- weakness, ataxia and lameness.
• 3-5 day recumbency. 

Dumb/brainstem form or furious/cerebrum form:

• Unusual in horses.

Diagnosis:

• Difficult- vague signs.
• Assumption of rabies in endemic areas.
• Rapid progressive neurological signs.
• PM-> negri bodies within neurones. Flourescent ab. Zoonotic- beware. 

Treat:

• 100% fatal.
• Kill on suspicion- early diagnosis essential.
• Limit human contact. 

Control/Prevention:

• Vaccination very effective (annual in epidemic areas). 
• Horses rarely infect other animals.

Arbovirus- West Nile Virus

• Flavivirus.
• Replicates in birds- reservoir.
• Mosquitoes transfer virus from reservoir to number of species. 
• Horse/human terminal host. 

Pathogenesis:

1. Initial virus replication at inoculum site.
2. Viraemia-> signs of uncomplicated inf-> fever, depression and anorexia. 
3. Some horses-> virus enters the CNS, this results in diffuse or multifocal encephalomyelitis and involvement of the spinal cord is common. 

Neurological Signs

• Muscle fasciculations over entire body.
• Weakness, ataxia and dysmetria.
• Cranial nerves affected sometimes.
• Mentation affected (intracranial oedema).
• Sudden death in some horses.

Treatment:

• Hyperimmune plasma specific to WNV available in USA. 
• Supportive care. NSAIDS, mannitol (decreases oedema) and perhaps corticosteroids. 

Diagnosis:

• ELISA ag specific testing.
• PM-> virus culture of brain material, and immunohistochemistry. 

Prognosis-> few cases gradually resolve-> long term neurological deficits. 

Prevention:

• Reduce vector contact via environment and repellents. 
• USA- vaccine.