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Basics ⬇

Outline

BASICS

Overview!!navigator!!

  • EHM is an uncommon CNS disease associated with EHV-1 infection
  • The disease can occur sporadically or in outbreaks and is seen worldwide, mainly in northern Europe and North America
  • There is no breed or sex predilection; however horses >3 years old seem to be more likely to be affected
  • EHV-1 is a species-specific enveloped DNA that typically causes lifelong infection of the host
  • EHM is a rare outcome of an infection that typically causes respiratory disease and abortions
  • EHM is caused by multifocal hemorrhages due to disease of the CNS vasculature, particularly in the lumbosacral spinal cord
  • Infection of the respiratory tract and subsequent local virus multiplication results in peripheral blood mononuclear cell-associated viremia and infection of endothelial cells in the CNS (as well as gonads and the fetal–maternal interface, the latter resulting in abortion). Infection of the endothelial cells results in cell lysis with secondary ischemic and hemorrhagic infarction of neutropils

Signs!!navigator!!

Historical Findings

Index case is often a horse that recently attended a competition. Horses may or may not have a current vaccination record. Respiratory disease or abortion may, or may not, be evident in exposed horses and herd mates.

Physical Examination Findings

  • The latent period before the onset of viremia with pyrexia is approximately 7 days, during which there may be nasal shedding. Neurologic signs occur subsequent to the viremia
  • Pelvic limb ataxia and paresis is usually symmetric but thoracic limb ataxia can rarely be seen, as can brainstem signs more often due to vestibular dysfunction
  • Neurologic signs develop rapidly between 24 and 48 h following the cessation of pyrexia, and will be most severe 24–48 h after onset of clinical signs. Relatively fast improvement of the ataxia and urinary signs might be seen within the next 5–7 days

Diagnosis ⬆ ⬇

DIAGNOSIS

  • A presumptive diagnosis of EHM can be made in horses with peracute spinal cord deficits, dysuria, and bladder distention, particularly if the patient has recently returned from mixing with outside horses
  • Antemortem diagnosis can be made quickly with high sensitivity using real-time quantitative PCR of nasal swabs and EDTA venous blood samples
  • Serum samples should be banked during an outbreak for ELISA or virus neutralization testing on acute and 3–4 week convalescent samples
  • Cerebrospinal fluid, especially from the lumbosacral space, may have xanthochromia and elevated protein content (0.01–0.04 g/L), reflecting the leakage of blood pigments from vasculitis

Differential Diagnosis

Trauma, equine protozoal myeloencephalitis, other viral encephalitis.

Treatment ⬆ ⬇

TREATMENT

  • Once a diagnosis has been made, the entire farm and, separately, individual cases should be strictly quarantined
  • Severely affected clinical cases require excellent nursing care, including bladder catheterization and preferably slinging of paraplegic cases
  • Dexamethasone (0.05–0.1 mg/kg IM BID for 1–3 days) in the early stages of showing clinical signs may be efficacious

Transmission Outbreak Prevention

  • Virus transmitted by contact, especially with affected herd mates
  • Neurologically affected horses can be the source of new transmission of EHM up to a week or more from the onset of clinical neurologic signs and strict isolation protocols must remain in place for 21 days after evidence of active (i.e. new cases) of EHV-1 disease
  • While vaccination decreases viral shedding and is encouraged, vaccination of herd mates is controversial due to the associated immunosuppression
  • Antiviral agents such as valaciclovir (30 mg/kg TID for 48 h then 20 mg/kg PO BID) has been shown to decrease viral transmission in febrile horses before the onset of clinical signs. Gastrointestinal-sparing NSAIDs such as firocoxib (administered at the first detection of pyrexia and continuing for 3–5 days after the fever has ceased) have been shown to prevent viral entry into the endothelial cells

Follow-up ⬆ ⬇

FOLLOW-UP

Expected Course and Prognosis

Mildly affected horses, and rarely even paraplegic patients, can return to full function after several days to more than 1 year.

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Abbreviations!!navigator!!

  • CNS = central nervous system
  • EHM = equine herpesvirus myeloencephalopathy
  • EHV-1 = equine herpesvirus 1
  • ELISA = enzyme-linked immunosorbent assay
  • NSAID = nonsteroidal anti-inflammatory drug
  • PCR = polymerase chain reaction

Suggested Reading ⬆ ⬇

Goehring LS. Equid herpesvirus-associated myeloencephalopathy. In: Furr M, Reed S, eds. Equine Neurology, 2e. Ames, IA: Wiley Blackwell, 2016:225–232.

Mayhew IG. Large Animal Neurology, 2e. Chichester, UK: Wiley Blackwell, 2008.

Walter J, Seeh C, Fey K, et al. Clinical observations and management of a severe equine herpesvirus type 1 outbreak with abortion and encephalomyelitis. Acta Vet Scand 2013;55:19.

Author(s) ⬆

Author: Caroline N. Hahn

Consulting Editor: Caroline N. Hahn