Equine infectious neurological disease

How are neurological viral diseases diagnosed

• History

• Clinical exam

• CSF analysis and serology

• Medical imaging and functional testing

• Post mortem examination

What history should be asked

• Information about the geographical location of the horse and recent ravel history

• Determine the onset and duration of clinical signs in the affected horse

• Information on health status of other horses in the vicinity of the affected animal

• Vaccination history

What would be seen in a clinical exam

• Normally fever

• Neurological signs depend on neuroanatomical location

What causes fever

• Replication of virus at entry site

• Specific organ infection

• Viraemia

What CSF changes are most common in viral encephalitis

• Mononuclear pleocytosis

• Increased protein concentration

What diagnostic methods can identify the pathogen or antibodies in CSF

• Culture

• PCR

• Antibody detection test

What medical imaging tests use used for CNS disease

• CT

• MRI

• EEG

What is the availability and anatomical reach of CT/MRI in vet patients

Very limited typically only in the head to mid-cervical region

What do CT/MRI usually show in encephalitis

Often non specific intracranial oedema

What post mortem methods are used to diagnose

• Histopathology

• Immunohistochemistry

• PCR

What are the general recommendations for treatment

• Isolate

• Quiet, dark stable

• Deep bedding

• Padded walls

How often should recumbent horses be turned

Every 4-6 hours

What drugs can be given as general treatment

• NSAIDs

• Antiviral therapy

• Ag-specific therapy

• Vitamin E

• Thiamine

How can disease be prevented

• Vacccination

• Minimise exposure to vector or reservoir

What are the main disease syndromes caused by EHV-1

• Respiratory disease

• Abortion

• Myeloencephalopathy

What condition is associated with EHV-2

Keratitis

What condition is associated with EHV-3

Coital exanthema

What disease is linked to EHV-4

Respiratory disease

What condition is associated with EHV-5

Multinodular pulmonary fibrosis

Which equids are affected by EHV-6 to EHV-8

Donkeys

What type of virus is EHV-1 and what are its key structural features

• Alpha herpes virus specific to horses

• Enveloped capsid containing a DNA genome

What are the key features of EHV-1 immunity and infection persistence

• Establishes latent infections

• Humoral immunity is very short lived

• Horses naturally have low antibody levels

How does EHV-1 occur

• Sporadically or as outbreaks

• Outbreaks of myeloencephalopathy

What causes the neurological clinical signs seen in EHV-1 infection

Vasculitis and thrombosis of arterioles in the brain and spinal cord due to endotheliotrophism

What preceding clinical history may be present in horses with EHV-1 neurological disease

• They may or may nit show prior respiratory signs or fever

• There way be a recent history of respiratory disease or abortion on the premises

What are the early stabilisation signs of EHV-1

• Ataxia in hind limbs or all 4 up to recumbency

• Atony of bladder

• Flaccid tail and anus

• Perineal hypoalgesia

• Cranial nerve involvement

How is EHV-1 diagnosed

• Virus isolation - nasal swab, buffy coat, CSF

• Serology - complement fixation, 4 fold rise in titre

• CSF - xanthochromia

What two sources can lead to EHV-1 neurological disease

• A new infection from a respiratory outbreak

• Reactivation of latent virus

What factors can reactivate latent EHV-1, even though this is less likely in the neurological form

• Stress

• Illness

• Overcrowding

What are the key management principles and prognosis indicators for EHV-1 neurological cases

• Isolate affected horses

• Prognosis is reasonable with good nursing care

• Better if not recumbent

• Recovery can take days to weeks

• Poor prognosis if recumbent for more than 24 hours

Which treatments are used to address vasculitis in EHV-1 neurological disease

• NSAIDs

• Costicosteroids

• Aspirin

What EHVs are there vaccinations for

1 and 4 but ineffective against neurological form

What type of virus causes rabies and how is it transmitted

• Lyssavirus

• Single stranded enveloped RNA virus

• Transmitted via saliva contaminated bite wounds

What is the tissue tropism of the rabies virus

It is neurotrophic targeting nervous tissue

Where is rabies most commonly found

• Africa

• Middle and Far East

• South and North America

How common is rabies in horses, and why is it significant

It is uncommon in horses but is an important zoonotic disease

How is rabies typically transmitted to horses

• Via saliva droplets

• Canid or carnivore bites on the limbs

Describe the early pathogenesis of rabies from inoculation to CNS entry

1. Virus locally incoluated at the wound

2. Enters peripheral nerves

3. Moves centrally via slow centripetal axoplasmic flow

4. Replicates in spinal and dorsal root ganglia

How does rabies spread after reaching the CNS

1. Spreads rapidly through CNS

2. Moves centrifugally down nerves to tissues including salivary glands

What factors influence the variable incubation period of rabies (9 days–1 year)

• Virus strain

• Host species

• Amount of inoculum

• Proximity of the inoculation site to CNS

Why does proximity of inoculation to the CNS affect rabies incubation time

Shorted distance to nerves and CNS allows faster centripetal neural spread reducing incubation duration

How do clinical signs of rabies vary and why are there no pathognomonic signs

• Signs range from mild hindlimb lameness to sudden death

• Varies because they depend entirely on neuro-anatomic location of pathology

What clinical forms of rabies occur based on lesion location and how do they present

• Spinal cord - paralytic form

• Brainstem - dumb form

• Cerebrum - furious form

What are the early signs of the spinal/paralytic form of rabies and where do they occur

• Localised hyperaesthesia

• Often with self mutilation

• Typically at the extremity or inoculation site

How does the spinal/paralytic form of rabies progress clinically

It shows progressive ascending ataxia, weakness, lameness leading to recumbency within 3-5 days

Is the dumb strain common in horses

No it is unusual

What are the signs of the dumb form

• Depression

• ANorexia

• Head tilt

• Ataxia

• Dementia

• Blindness

• Salvation and dysohagia

• Tail, penis and bladder paralysis

• Self mutilation

What are the signs of the furious form

• Photo and hydrophobia

• Agression

• Hyperaesthesia

• Tenesmus

• Muscle tremors

• Seizures

Why is diagnosing rabies difficult in live animals, and what clinical suspicion is used in endemic areas

• Signs are vague and non specific

• In endemic areas rapidly progressive neurological signs are assumed to be rabies

How is rabies confirmed post-mortem

• By histology showing oesinophilic Negri bodies within neurons

• By flourescent antibody testing

How does rabies effect humans

• Almost always fatal

• Vaccinated humans receiving post exposure boosters will survive

How does rabies effect horses

• 100% fatal

• Best to kill on suspicion

• Limit human contact

• Early diagnosis essential

How do you control and prevent rabies

• Vaccination very effective

• Horses rarely infect other animals

• International controls

Where is West Nile Virus found

• Africa 

• Along Nile river

• Middle east

• Appearance in North America

• Occasional outbreaks in Europe

What is the main reservoir for West Nile Virus and how is it transmitted to other species

• Birds act as a reservoir

• Mosquitoes transfer the virus from birds to other hosts

Why are humans and horses considered terminal hosts for West Nile Virus

Because they do not develop sufficient viraemia to transmit the virus further

What are the early events in West Nile Virus pathogenesis after inoculation

• Initial viral replication at the inoculation site

• Followed by viraemia causing fever, depression and anorexia

What happens when West Nile Virus enters the CNS in horses

It causes diffuse or multifocal encephalomyelitis commonly involving the spinal cord

What are the neurological signs of the West Nile Virus

• Muscle fasciculations over entire body

• Weakness, ataxia and dysmetria

• Cranial nerves may be affected

• Mentation may be affected

• Sudden death in some horses

How is West Nile Virus treated

• hyperimmune plasma specific to WNV available in USA

• NSAIDs

• Mannitol to decrease oedema

• Corticosteroids

How is West Nile Virus daignosed

• Ag specific testing

• Virus culture of brain material post mortem

• Immunohistochemistry post ortem

What is the prognosis of West Nile Virus

Few cases gradually resolve leaving long term neurological deficits

How is West Nile Virus prevented

• Reduce vector contact

• Vaccination