Tick Paralysis

Etiology

Over 40 species of ticks have been associated with tick paralysis. In North America, most cases are associated with Dermacentor species. Dermacentor variabilis, the American dog tick, and Dermacentor andersoni, the Rocky Mountain wood tick, are the most common species associated with tick paralysis. Other ticks such as Amblyomma americanum, the Lone Star tick, and Ixodes scapularis, the black-legged tick, are also associated with this disease. In Australia, Ixodes holocyclus, the scrub tick, is most commonly implicated. A salivary neurotoxin only produced by an engorged female tick during feeding induces paralysis. Clinical presentation varies depending on the species of tick.[4][5][6]

Epidemiology

Tick paralysis was first described in Australia in the 19th century, but clusters of cases have been described in Argentina, Canada, and in several regions of the United States. It has been reported in both humans and domesticated animals. Like most tick-borne illnesses, the peak incidence is in the spring and early summer. It is reported more commonly in children, perhaps accounting for greater toxin effects given smaller body mass. It appears to be more common in females, possibly because long hair makes an engorged tick more likely to escape early detection. Reporting is not universally required, and reliable data on incidence and distribution do not exist.

Pathophysiology

Tick paralysis primarily affects motor pathways. Symptoms typically develop after the tick has been attached for 3 to 7 days and may vary depending on the species of tick. The precise mechanism is not fully understood, but with Dermacentor species, it most likely involves interruption of sodium flux across axonal membranes resulting in weakness due to impairment in transmission to motor nerve terminals. The neurotoxins produced by Ixodes holocyclus act on presynaptic motor neuron terminals to inhibit the release of acetylcholine and may produce clinical findings similar to botulism. Onset is typically slower than other forms of tick paralysis and may be associated with ophthalmoplegia.[7][8][9]

History and Physical

Most patients present with fatigue and weakness which progress to ataxia and then ascending paralysis. Some also report irritability, muscle pain or paresthesias or other subjective sensory complaints as early symptoms. Fever is absent, and there is no associated rash, headache or change in mental status. Sensory exam, despite paresthesias, is usually normal. Weakness starts in the legs and ascends and progresses rapidly. Deep tendon reflexes are absent. Muscles innervated by cranial nerves may be involved and may occasionally include pupillary dilatation.  Involvement of respiratory muscles may lead to respiratory failure or death. Patients rarely present with a history of tick bites, and a thorough physical exam with careful attention to the scalp, axilla, interdigital spaces and perineum is critical in making the diagnosis. Providers frequently miss ticks, and they are often found by caregivers or other members of the healthcare team. Patients may have more than one tick attached. Guillain-Barre syndrome is the most common misdiagnosis, and a thorough search for a tick in any patient where this diagnosis is considered is critical to avoid unnecessary testing and expensive therapies such as plasmapheresis or immune globulin, which are not helpful in this setting. Tick paralysis usually progresses more rapidly than Guillain-Barre or Miller-Fisher syndrome.

Evaluation

Neuroimaging studies are normal unless they coincidentally reveal an attached tick missed on the physical exam. Serum white blood cell count and cerebrospinal fluid analysis are normal. In cases with respiratory involvement, pulmonary function testing and blood gasses may guide the need for intubation and respiratory support. Electromyography shows the reduced amplitude of compound muscle action potentials. Repetitive nerve stimulation studies are normal.

Treatment / Management

Treatment involves removal of the tick. This is best accomplished using fine forceps applied close to the skin with gentle, steady, upward and outward traction, taking care to avoid leaving mouth parts embedded in the wound. Care is otherwise supportive but may require intubation for respiratory support in severe cases.[10][11]

Prevention is the best way to avoid tick-borne illness. Except for tick-borne encephalitis, there is no vaccine available to prevent tick-borne disease. Protective clothing, such as long pants, long sleeves, and closed shoes should be worn in tick-infested areas, particularly in the late spring in summer when most cases occur. Pant legs should be tucked into socks when walking through high grass and brush. Permethrin, which is an insecticide, may be applied to clothing and is quite effective in repelling ticks. Other tick repellents such as diethyl-m-toluamide (DEET) may be applied to skin or clothing, with variable effectiveness. DEET can be quite toxic, with effects ranging from local skin irritation to seizures. DEET should be avoided in infants.

Most patients with paralysis caused by Dermacentor species will fully recover within hours of tick removal. However, in cases caused by the Australian Ixodes holocyclus tick, weakness and paralysis may initially worsen in the first 24 to 48 hours after the tick is removed necessitating inpatient observation for respiratory compromise. 

Differential Diagnosis

The differential diagnosis of tick paralysis includes other causes of paralysis such as Guillain-Barre syndrome, botulism, poliomyelitis, myasthenia gravis, and spinal cord lesions.

Prognosis

Most patients with tick paralysis will fully recover with supportive care and removal of the tick. While tick paralysis has no long-term sequelae when identified and treated, mortality may be as high as 12% in untreated or misdiagnosed cases.