Tick paralysis results from exposure to a neurotoxin released by tick salivary glands during a blood meal; it is the only tick-borne disease not caused by an infectious agent. The toxin appears to be produced exclusively by female, egg-laden ticks. It is most commonly seen in children under 16, and within this population affects girls more than boys, probably because ticks are harder to detect under longer hair. Among adults, men are disproportionately affected.
Worldwide, over 40 tick species have been associated with tick paralysis, but in North America the most common culprits are Dermacentor variabilis (American dog tick) and Dermacentor andersoni (Rocky Mountain wood tick). Bites from Amblyomma and Ixodes ticks can also cause tick paralysis. In the United States, tick paralysis is most common in the Pacific Northwest, Rocky Mountain states and southeastern part of the country. It occurs most frequently in the spring months, from April through June.
Signs and Symptoms
Typically, symptoms in humans appear within 2-6 days of tick attachment. Tick paralysis usually presents as ataxia followed by an ascending paralysis starting in the feet and legs. Fever is rare, and constitutional symptoms, which only sometimes precede the paralysis, are limited to malaise and listlessness. Reflexes in affected areas are reduced or absent. Sensory abnormalities, primarily numbness and tingling in the face and limbs, are frequently reported by patients.
If the tick is not found and removed from the patient, the paralysis can ascend to the trunk and affect respiratory muscles, which can be life-threatening. Cranial nerve involvement has also been described, although it is not common and almost never occurs in the absence of other neurological signs.
Tick paralysis is often confused with Guillain-Barré syndrome, which is clinically similar and much more common than tick paralysis. Unfortunately, electrophysiological tests are not useful in distinguishing between the two. In addition, there are no specific blood tests for tick paralysis, and conventional blood and spinal fluid studies are almost always normal. Thus, correct diagnosis is contingent upon physician awareness. Any case involving sudden-onset ataxia and ascending paralysis, especially in a patient who lives in a tick-endemic area and who fits the demographic profile described above, should be considered suspicious for tick paralysis. Such patients should be searched immediately for ticks, particularly in body areas where the tick might not be immediately apparent, such as the scalp, hairline, ear canals or pubic region.
Once the tick is removed, patients usually recover quickly. Improvement is commonly noted within hours, and further treatment is not required.
Edlow JA, et al. Infect Dis Clin North Am. 2008; 22(3):397-413.
Vedanarayanan V, et al. Semin Neurol. 2004; 24(2):181-4.
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