Tick paralysis

INTRODUCTION — Ticks transmit a number of infections caused by pathogenic microorganisms. They can also transmit a toxin-mediated illness known as tick paralysis, which can be confused with both infectious and noninfectious conditions.

Although it is a rare disease in humans, tick paralysis is important to recognize because it can be fatal [1]. However, if diagnosed promptly, this illness can be cured by tick removal.

EPIDEMIOLOGY

Historical perspective — Paralysis from tick bites was first described by explorers in the Australian outback in 1824 [2]. Eighty-eight years later, in 1912, the disease was first recognized by medical providers in western Canada [3]. Since then, the illness has been described in countries around the world. (See 'Geographic distribution' below.)

Incidence — Reliable information on incidence of disease is not available because there is no surveillance system for tick paralysis. However, the disease appears to be rare based on available literature, which is limited to small case series.

Geographic distribution — The illness has been described worldwide, and most cases are reported in the United States, southwestern Canada, and eastern Australia [3,4].

In the United States, the majority are reported in the Pacific northwest and western regions, but the illness has also been seen in the eastern, southeastern, and south-central United States [5-7]. In the state of Washington, where tick paralysis was a reportable disease until 1998, 33 cases were reported from 1946 to 1996, highlighting the uncommon nature of the disease [6]. From 2005 to 2016, eleven cases were identified from a single hospital in southern Louisiana, suggesting that the disease may be more common than thought in the southeastern United States [5].

Risk factors — The primary risk factor for tick paralysis is exposure to the outdoors in areas where ticks are prevalent or to animals prone to harbor ticks. As with other tick-borne diseases, most cases occur in the spring and early summer months when ticks are most active [8,9].

In most case series, children under 10 years of age are affected more often than other age groups, presumably because toxin concentrations are higher in individuals with small body mass. Among children, the majority of cases are in girls, possibly because ticks are more likely to remain undetected under long hair [10].

Tick vectors — Multiple different tick species can cause tick paralysis.

The tick species that causes most cases of human tick paralysis in the northwestern United States and Canada is Dermacentor andersoni (the Rocky Mountain wood tick) (picture 1). In the eastern United States, Dermacentor variabilis (the American dog tick) (picture 2) is responsible for most cases. The primary cause of tick paralysis in Australia is the scrub tick, Ixodes holocyclus.

Other ticks such as Amblyomma americanum (the Lone Star tick) (picture 3), Ixodes scapularis (the black-legged tick) (picture 4), Ixodes pacificus (the western black-legged tick) (picture 5), and Rhipicephalus sanguineus (the brown dog tick) have also been associated with human tick paralysis [8,9].

Tick paralysis is a major problem in animal populations, particularly among cattle, sheep, and goats. At least 40 different tick species have been implicated in animal-related cases [3,11].

PATHOGENESIS — The toxin that causes tick paralysis is created in the salivary glands of feeding female ticks; male ticks do not cause tick paralysis.

For transmission to occur, a female tick must attach and begin feeding. As the tick feeds, the toxin concentration in the tick's saliva increases, particularly on day 3 of feeding.

The biological effect of tick salivary toxins depends, in part, on the species of tick:

●Dermacentor ticks' neurotoxin has a number of pathologic effects [11]:

•Slowing of motor nerve conduction velocity

•Lowering of the height of the nerve and muscle action potential

•Impaired propagation of afferent nerve fiber signals

The neurotoxin's precise mechanism of action is not fully understood. The toxin may interrupt sodium flux across axonal membranes in selected locations such as the nodes of Ranvier and nerve terminals, causing weakness through impairment of neural transmission to motor nerve terminals [12,13].

Studies using a hamster model of tick paralysis suggest that the ability of tick saliva to cause paralysis is a heritable trait found only in some strains of D. andersoni ticks [14]. This may explain why tick paralysis is uncommon in some regions (such as Alberta, Canada) despite a high prevalence of known tick vectors [15].

●I. holocyclus neurotoxin acts on presynaptic motor nerve terminals and inhibits the release of acetylcholine, which results in blockade of transmission at myoneural junctions [16]. Low-amplitude compound muscle action potentials can be demonstrated in neurophysiologic testing; motor conduction velocities, sensory studies, and repetitive stimulation are normal [17]. While the toxin of I. holocyclus has not been completely characterized, it bears similarities to botulinum toxin. (See "Botulism".)

CLINICAL MANIFESTATIONS

Classic presentation — Onset of symptoms usually begins once the tick has fed for four to seven days.

Most patients notice progressive weakness in both lower extremities; many complain of an unsteady gait as opposed to weakness. Weakness then ascends to the upper extremities. The cranial nerves are next affected, which can cause diplopia, dysarthria, dysphagia, or drooling [5,12,18]. Finally, respiratory muscles may become weak, which can lead to death if no intervention occurs.

In North American patients, the paralysis typically progresses over 12 to 24 hours before reaching the cranial nerves. In Australian reports, the progression is slower with extremity paralysis progressing over 48 to 72 hours [18].

Before onset of paralysis, some patients experience a prodrome of paresthesias, fatigue, irritability, restlessness, and/or myalgias.

Fever is characteristically absent, and there is no change in sensation or cognition at any point throughout the course of illness.

In most case reports, the patient does not recall a recent tick bite or bites.

Atypical presentations

●Focal paralysis – Uncommonly, paralysis may be localized to one arm or leg, often in the extremity most near the tick bite [5,19]. Tick-toxin-induced brachial plexopathy has been reported in a patient with unilateral arm weakness with an engorged tick in the subclavian fossa [20]. Patients with ticks in their external auditory canal or temporal area of the scalp have experienced isolated facial paralysis. Focal cases are most often reported in Australia [21,22].

●Ataxia – Reports of ataxia are not uncommon in case series, although it is unclear whether patients had true ataxia or gait instability from lower extremity weakness [5,23,24]. In patients with reported ataxia, cerebellar findings (eg, dysmetria, dysdiadochokinesis, nystagmus) are not prominent.

Physical examination — Vital signs and cognition are typically normal in patients with tick paralysis.

Neurologic exam reveals symmetrical muscle weakness in affected muscles, and deep tendon reflexes are typically absent. Sensory examination is normal, even in patients reporting paresthesias.

In advanced cases, cranial nerve abnormalities may be observed, including facial paralysis and abnormal extraocular muscle movements. Pupillary dilation is reported in many Australian case reports but only rarely in North America [1,23].

Thorough skin examination usually reveals an embedded engorged tick. The most common anatomic location of the tick bite is the scalp, but it can be on any part of the body. In some patients, a tick is never found, particularly in patients in Australia.

Laboratory findings — Blood tests are usually normal, including the peripheral white blood cell count and creatine kinase. Cerebrospinal fluid analysis is usually normal as well, including white blood cell count, glucose, and protein.

In patients who have progressed to respiratory muscle weakness, hypercarbia, respiratory acidosis, and hypoxia may be present.

Imaging and electromyography results — Radiologic studies are characteristically normal in patients with tick paralysis. In one reported case, a tick attached to the scalp was discovered on magnetic resonance imaging (MRI) [25].

Electromyography (EMG) findings are nonspecific and typically reveal diffuse reduction in compound muscle action potentials [17,26,27]. In North American patients, sensory and motor conduction may be reduced, whereas they are normal in Australian patients [23,27,28].

DIAGNOSIS

●Clinical suspicion – A diagnosis of tick paralysis should be suspected when a child or adult presents with ascending motor weakness, a normal sensory exam, normal vital signs, and a known or possible exposure to ticks, particularly in the spring or summer months. Geographic locale should be considered, including areas of recent travel; cases have been reported in travelers returning from Australia, Canada, and rural areas of the United States [29-31]. (See 'Geographic distribution' above.)

Because of the rarity of this disease, maintaining a high index of suspicion is necessary. Many cases are misdiagnosed on initial presentation, resulting in critical treatment delays, extensive costly diagnostic testing, unnecessary therapeutic interventions, and increased risk of fatality.

●Establishing the diagnosis – The diagnosis is confirmed by finding an attached engorged tick on a patient with compatible symptoms and signs. Careful and meticulous examination should be performed to look for a tick. Special attention should be given to the scalp, axillae, ears (including the external auditory canals), labia, buttocks, and intertriginous and interdigital spaces. The use of a fine-tooth comb or shaving can detect ticks embedded in the scalp of people with long hair; these interventions should be used when tick paralysis is suspected and a tick cannot be found during examination. Even if one tick is found, examination should be completed to ensure no other ticks are present.

In some individuals, a presumptive diagnosis is made in the absence of finding a tick; in these cases, it is presumed that the tick detached and fell off prior to the patient's physical examination. Other diagnoses should be considered in this circumstance (see 'Differential diagnosis' below). This is more commonly reported in Australia, where onset of symptoms can be delayed and resolution more prolonged.

DIFFERENTIAL DIAGNOSIS — Tick paralysis can be confused with an array of neurologic disorders.

The presence of a tick embedded on an individual with compatible symptoms is a key differentiating factor.

●Guillain-Barré syndrome (GBS) – This is the most frequent misdiagnosis in patients with tick paralysis [32]. Both syndromes present with ascending paralysis and may have cranial nerve involvement and prodromal paresthesias. CSF analysis in GBS usually reveals elevated protein, whereas CSF protein is normal in tick paralysis. GBS is not associated with pupillary abnormalities, in contrast to Australian tick paralysis, which commonly presents with dilated pupils.

Some experts believe GBS should not be diagnosed until the presence of a tick has been ruled out by thorough physical examination. (See "Guillain-Barré syndrome in children: Epidemiology, clinical features, and diagnosis" and "Guillain-Barré syndrome in adults: Pathogenesis, clinical features, and diagnosis".)

●Acute cerebellar ataxia in children – This syndrome typically occurs in children under the age of six and occurs two to three weeks after a febrile prodromal (usually viral) illness. Acute onset of gait instability is the primary symptom, and nystagmus, dysmetria, dysdiadochokinesia, dysarthria, emesis, irritability, and headache may be present. Unlike tick paralysis, motor weakness and areflexia are uncommon. (See "Acute cerebellar ataxia in children".)

●Poliomyelitis and polio-like illnesses – Poliomyelitis is characterized by asymmetrical flaccid paralysis, whereas tick paralysis is usually symmetrical. Due to widespread vaccination, poliovirus infection is extremely rare in countries where tick paralysis primarily occurs.

Other polio-like illnesses known to cause asymmetrical paralysis include West Nile virus infections as well as acute flaccid myelitis, a polio-like syndrome in children that has been associated with enterovirus 68. (See "Acute flaccid myelitis".)

Patients with these conditions often have abnormal spinal MRIs, pleocytosis in cerebrospinal fluid, and fever. (See "Poliomyelitis and post-polio syndrome" and "Clinical manifestations and diagnosis of West Nile virus infection" and "Acute flaccid myelitis".)

●Transverse myelitis and spinal cord lesions or infarct – These conditions typically present with extremity weakness that extends to the level of the injury. Unlike tick paralysis, however, numbness, paresthesias, pain, bowel and bladder dysfunction, hyperactive deep tendon reflexes, and abnormal MRI findings are common. (See "Transverse myelitis: Etiology, clinical features, and diagnosis" and "Disorders affecting the spinal cord".)

●Botulism – Both tick paralysis and botulism present with progressive symmetrical paralysis. However, the paralysis from botulism begins with the cranial nerves and then descends to cause motor weakness, whereas tick paralysis causes ascending paralysis beginning in the lower extremities. (See "Botulism".)

●Organophosphate (pesticide), shellfish, pufferfish tetrodotoxin, or buckthorn poisoning – Specific organophosphate pesticides can cause ascending symmetrical flaccid paralysis, as can ingestion of the fruit of the buckthorn plant in Mexico [33]. Shellfish and pufferfish poisoning cause flaccid paralysis and perioral paresthesias that begin within a few hours of ingestion. Exposure history can help differentiate these conditions from tick paralysis. (See "Organophosphate and carbamate poisoning", section on 'Clinical features' and "Overview of shellfish, pufferfish, and other marine toxin poisoning", section on 'Paralytic shellfish poisoning'.)

●Myasthenia gravis – Unlike tick paralysis, this condition is usually slowly progressive and manifests as ocular symptoms (eg, ptosis, diplopia) and proximal motor weakness, and rarely includes distal extremity weakness. (See "Clinical manifestations of myasthenia gravis".)

MANAGEMENT

Tick removal as definitive treatment — Definitive therapy for tick paralysis is removal of the tick from the patient's body. Techniques for removing ticks are discussed in detail separately. (See "Evaluation of a tick bite for possible Lyme disease", section on 'Technique for tick removal'.)

In North American patients, symptoms rapidly improve within hours of tick removal and completely resolve by 24 hours. In Australian patients, symptoms often continue to worsen for 24 to 48 hours after tick removal, before subsiding over days or sometimes weeks [3].

Supportive care — Patients often require care in intensive care units for close monitoring and respiratory support, sometimes including mechanical ventilation. Hospitalization for observation is necessary after tick removal, until symptoms clearly improve and respiratory function normalizes. Patients in the United States are often discharged the day after tick removal, whereas patients in Australia typically require longer hospital stays [23].

Limited role of antitoxin — Use of hyperimmune serum anti-toxin prepared from dogs has been reported in some severely ill patients in Australia [3,17,21]. However, this intervention is typically avoided because of high incidence of acute allergic reactions and serum sickness in humans. Although there are a few case reports of rapid improvement after anti-toxin administration, other case series do not support its use due to improvement in patients without antitoxin and antitoxin-associated adverse effects.

PROGNOSIS — Prognosis has improved as hospital and intensive care have evolved. Prior to the advent of modern intensive care units and respiratory care, mortality rates of 10 to 12 percent were reported among confirmed cases [34]. By contrast, more contemporary series suggest that fatalities are rare, except among patients in whom the diagnosis of tick paralysis was not considered antemortem [5,6].

PREVENTION — Tick paralysis is prevented by avoiding tick bites and performing frequent tick checks after outdoor exposures. Prevention of tick bites is discussed in detail separately. (See "Prevention of Lyme disease", section on 'Personal protection' and "Prevention of Lyme disease", section on 'Environmental interventions'.)

SUMMARY AND RECOMMENDATIONS

●Epidemiology – Tick paralysis is a rare toxin-mediated illness that has been found in countries around the world, especially the western United States, southwestern Canada, and eastern Australia. (See 'Incidence' above and 'Geographic distribution' above.)

●Risk factors – The primary risk factor for tick paralysis is exposure to the outdoors in areas where ticks are prevalent, particularly in spring and early summer months.

Children under 10 years of age appear to be at highest risk. (See 'Risk factors' above.)

●Tick vectors – The tick species that causes most cases vary by geography: Dermacentor andersoni in the western United States and Canada, Dermacentor variabilis in the eastern United States, and Ixodes holocyclus in Australia. (See 'Tick vectors' above.)

●Clinical manifestations (see 'Clinical manifestations' above):

•Classic presentation – Most patients present with symmetrical ascending motor weakness that begins in the lower extremities and moves to the upper extremities, then to the cranial nerves, and finally the respiratory muscles. Respiratory paralysis may lead to death.

In North American patients, extremity paralysis progresses over 12 to 24 hours, compared with 48 to 72 hours for Australian patients. Some patients experience prodromal paresthesias, fatigue, and other nonspecific symptoms.

Fever is absent, and there is no change in sensation or cognition. Most patients do not recall a recent tick bite. (See 'Classic presentation' above.)

•Atypical presentations – Some patients have focal motor weakness in the nerves adjacent or distal to the tick bite. Ataxia has been reported as well. (See 'Atypical presentations' above.)

•Ancillary test results – Lab tests, cerebrospinal fluid analysis, and imaging (including magnetic resonance imaging [MRI]) are typically normal. Electromyography (EMG) findings are nonspecific. (See 'Laboratory findings' above and 'Imaging and electromyography results' above.)

●Diagnosis – A diagnosis of tick paralysis should be suspected when a patient presents with ascending motor weakness, a normal sensory exam, normal vital signs, and a possible exposure to ticks, especially in spring or summer months in the western United States, southwestern Canada, or eastern Australia.

The diagnosis is confirmed by finding an attached engorged tick on a patient with compatible symptoms and signs and resolution of illness after removal. Meticulous examination should be performed to look for a tick. (See 'Diagnosis' above.)

●Differential diagnosis – History and neurologic examination can differentiate neurologic disorders that mimic tick paralysis. The presence of an embedded engorged tick on an individual with compatible symptoms is a key differentiating factor. (See 'Differential diagnosis' above.)

●Management – Tick removal is the definitive treatment. After tick removal, North American patients typically have complete resolution within 24 hours, whereas Australian patients may worsen for 24 to 48 hours before improving.

Supportive care, including mechanical ventilation, is necessary in some patients. Because the disease resolves with tick removal, antitoxin is generally unnecessary although it has been used in a few case reports in Australia. (See 'Management' above.)

●Prognosis – Fatalities are rare and occur when the diagnosis is not considered antemortem. (See 'Prognosis' above.)

●Prevention – This consists of avoiding tick bites and performing frequent tick checks after outdoor exposure. (See "Prevention of Lyme disease", section on 'Personal protection' and "Prevention of Lyme disease", section on 'Environmental interventions'.)