Equis ISSN 2398-2977

Neurology: tick paralysis

Synonym(s): Flaccid paralysis, Foal paralysis

Contributor(s): Graham Munroe, Michael Porter

Introduction

  • Cause: the tick speciesIxodes holocyclus(Shellback tick, Cattle tick, Bush tick).
  • Signs: flaccid paralysis similar to botulism   Botulism  .
  • Diagnosis: isolation and identification ofI. holocyclusinfestation.
  • Treatment: removal of ticks, treatment with parasiticide   Therapeutics: parasiticides  .
  • Prognosis: usually good following removal of ticks.
  • A similar syndrome has been described in North America in which the tick speciesDermacentor andersoniis implicated as a vector.

Pathogenesis

Etiology

  • Ixodes holocyclusbite.

Predisposing factors

General
  • Younger/smaller animals are more susceptible.

Specific

  • Geographical location whereIxodes holocyclusis endemic.

Pathophysiology

  • The tick,Ixodes holocyclus, which usually feeds on native wild animals, can sometimes infest horses.
  • The adult female ticks injects a neurotoxin into the blood stream whilst feeding on the host animal, which can   →    progressive motor paralysis, respiratory depression, and even death in foals and mature horses.
  • The adult female does not inject detectable amounts of toxin until day 3 of attachment to the host, with peak amounts being injected on days 5 and 6.
  • Onset of clinical signs usually occurs within 5-7 days of attachment:
    • Peripheral nerve dysfunction:
      • Neuromuscular blockade    →   an ascending flaccid paralysis (paraparesis   →    quadriplegia).
      • Progressive cranial nerve involvement   →    pupillary dilation, prolapse of the nictitating membrane, depressed gag and swallowing reflex (   →   saliva around the mouth), change or loss of voice (dysphonia).
      • Urinary incontinence   Incontinence  .
      • Localized paresis of musculature at site of tick bite.
    • Esophageal dysfunction: paralysis/paresis esophageal muscles.
    • Respiratory difficulty:
      • Central respiratory depression.
      • Paralysis of diaphragm and intercostal muscles.
    • Cardiovascular effects: cardiotoxic effects of the tick toxin   →    myocardial depression.
  • Severe local skin reaction to tick bites may occur.

Timecourse

  • Eggs on pasture hatch into larvae within 7-9 weeks.
  • Unfed larvae on the ground can remain alive for up to 162 days.
  • Larvae will attach to an available host after about 7 days and feed for 4-6 days before dropping to the ground to molt into nymphs.
  • The nymph stage varies from 19-41 days, but nymphs can survive up to 275 days unfed.
  • Nymphs feed variably for 4-7 days on a host animal before molting into an adult after 3-10 weeks on the ground. 
  • Adults can survive up to 77 days off the host without feeding. 
  • The female adult feeds and engorges for 6-21 days, before she drops to the ground to lay eggs, to begin the cycle again. 
  • The adult female does not inject detectable amounts of toxin until day 3 of attachment to the host, with peak amounts being injected on days 5 and 6.
  • Onset of clinical signs usually occurs within 5-7 days of tick attachment.

Epidemiology

  • Spread between animals via direct infestation with this tick species.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers


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