INTRODUCTION — Ticks transmit a number of infections to humans and other animals. However, the toxins of various ticks can also cause a disease known as tick paralysis, which can be confused with both infectious and noninfectious conditions.
Tick paralysis was first described by explorers in the Australian outback in 1824 [1]. Eighty-eight years later, the disease was recognized to occur in western Canada [2]. Tick paralysis has subsequently been found to affect humans and domestic and wild animals worldwide, although most cases occur in Australia and North America. Although it is a rare disease in humans, tick paralysis is important to recognize because it can be fatal or nearly fatal [3]. However, if diagnosed promptly, this illness can be cured with the combination of tick removal and supportive care.
EPIDEMIOLOGY — There is no national surveillance system for tick paralysis and reliable information on incidence does not exist. However, the disease appears to be uncommon based upon available literature and clinical experience. Because of its rarity and sporadic nature, the literature on tick paralysis is limited to small case series and substantial reporting biases. As an example, only 33 cases were reported in Washington State in the 50 years between 1946 and 1996, even though tick paralysis was a reportable disease in this state until 1998 [4]. Most cases in North America occur in the western regions of the United States and Canada, but the illness has also been seen in the eastern, southeastern, and south central United States [5]. In addition, cases have been reported in Argentina [6] and in urban settings, among travelers returning from endemic areas such as Australia, Canada, and rural areas in the United States [7,8].
Four cases of tick paralysis were reported in patients from north central Colorado in 2006 [9]. The clustering of these cases was unusual because in previous years, on average, only one case per year was reported from Colorado. Another unusual characteristic of this cluster was the age distribution of cases: only one of four cases occurred in a child. Previous case reports noted that tick paralysis was more likely to occur in children under the age of 10 years because tick toxins are more likely to cause symptoms in individuals with a smaller body mass.
Females were affected more often than males in one case series [10], possibly because ticks are more likely to remain undetected following attachment to individuals with long hair. However, in another case series, the incidence of illness in males and females was similar [11]. As with other tick-borne diseases, most cases occur in the spring and early summer months [11,12].
Tick vectors — There are more than 800 known species of argasid (soft-shelled) and ixodid (hard-shelled) ticks, and over 40 individual tick species are capable of producing salivary toxins that can cause paralysis in humans and animals [13]. The tick species that cause most cases of human tick paralysis in the United States and Canada are Dermacentor andersoni (the Rocky Mountain wood tick), and Dermacentor variabilis (the American dog tick). Other ticks such as Amblyomma americanum (the Lone Star tick) (picture 1), Ixodes scapularis (the black-legged tick) (picture 2), Ixodes pacificus (the western black-legged tick) (picture 3), and Rhipicephalus sanguineus (the brown dog tick) have also been associated with human tick paralysis [11,12]. The primary cause of tick paralysis in Australia is the scrub tick, Ixodes holocyclus.
Studies using a hamster model of tick paralysis suggest that the ability of saliva from some strains of D. andersoni ticks to cause paralysis is a heritable trait [14]. This may explain why tick paralysis is uncommon in some regions (such as Alberta, Canada) despite a high prevalence of tick vectors [15].
PATHOGENESIS — The onset of symptoms of tick paralysis occurs only after a female tick has attached and begun feeding. Symptoms do not typically develop until the tick has fed for four to seven days. The biological effect of tick salivary toxins is, in part, specific to the species of tick. Neurotoxins produced by Dermacentor ticks have a number of pathologic effects including [13]:
●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 precise mechanism for the development of the symptoms of tick paralysis is not fully understood. However, the toxin produced by Dermacentor ticks may interrupt sodium flux across axonal membranes in selected locations such as the nodes of Ranvier and nerve terminals [16]. This in turn may result in weakness through impairment of neural transmission to motor nerve terminals [17].
Neurotoxins produced by I. holocyclus act on presynaptic motor nerve terminals and inhibit the release of acetylcholine. This inhibition can result in total blockade of transmission at myoneural junctions and is temperature-dependent [18]. Low-amplitude compound muscle action potentials can be demonstrated in neurophysiologic testing; motor conduction velocities, sensory studies, and repetitive stimulation are normal [19]. While the toxin of I. holocyclus has not been completely characterized, it bears similarities to botulinum toxin. (See "Botulism".)
CLINICAL FEATURES
Symptoms and signs — Tick paralysis caused by Dermacentor ticks usually begins with paresthesias and a sense of fatigue and weakness, although individual patients may sometimes appear irritable or restless and complain of muscular pain. Fever is characteristically absent, and there is no change in the sensorium or headache unless severe hypoxia or hypercarbia are present. Despite patients' reports of paresthesias, the sensory exam is typically normal. Some patients may develop an unsteady gait that progresses to an ascending complete paralysis. Deep tendon reflexes are characteristically absent. Respiratory paralysis and death can occur in severe cases.
In a review of 332 cases of tick paralysis published almost 60 years ago, the fatality rate was 12 percent [20]. By contrast, a more contemporary series of 33 cases reported a fatality rate of 6 percent [4]. In most fatal cases, the diagnosis of tick paralysis was not considered antemortem.
A number of clinically important but unusual features of tick paralysis deserve emphasis:
●Paralysis may be bilateral or localized to one arm or leg [5,21]. In one report, a patient with unilateral arm weakness was found to have an engorged tick in the subclavian fossa, suggestive of a tick-toxin-induced brachial plexopathy [22]. Isolated facial paralysis has occurred in patients who have had ticks attached to their external ear canal or to the temporal areas of the scalp [23,24].
●Ataxia of the arms and legs may be present and can lead to initial concern for a cerebellar lesion [5,25].
●Involvement of the facial, ocular, lingual, and pharyngeal muscles may produce diplopia, dysphagia, and/or dysarthria [5,26]. Bulbar muscle involvement may also result in unusual presenting symptoms such as drooling [16].
●Patients with tick paralysis in the United States and Canada typically report going to bed well and awakening later with weakness, paralysis, or ataxia. However, very rarely, patients present with ascending weakness and autonomic dysfunction that may lead to an incorrect diagnosis of Guillain-Barré syndrome [27]. (See 'Differential diagnosis' below.)
●The onset of tick paralysis caused by I. holocyclus in Australia is characteristically slow; patients may have gradually worsening symptoms over 48 to 72 hours until complete paralysis occurs. Patients may simultaneously have internal and external ophthalmoplegia, and pupillary reflexes may be absent or minimally reactive early in the course of illness [3,25].
Laboratory findings — The white blood cell count and analysis of the cerebrospinal fluid (CSF) are characteristically normal. Hypoxia and hypercarbia may be present if respiratory muscles are involved.
Imaging findings — Imaging studies are characteristically normal in patients with tick paralysis, although in one case report an attached tick was first detected on the scalp by magnetic resonance imaging of a child who presented with acute paralysis [28].
DIFFERENTIAL DIAGNOSIS — Tick paralysis can be confused with an array of disorders including Guillain-Barré syndrome (GBS), botulism, cerebellar ataxia, cerebellar stroke, myasthenia gravis, poliomyelitis, acute spinal cord lesions, periodic paralysis due to hypokalemia, insecticide poisonings, exposure to buckthorn, shellfish poisoning, and even hysterical paralysis [29,30]. (See "Guillain-Barré syndrome in adults: Pathogenesis, clinical features, and diagnosis" and "Botulism" and "Clinical manifestations of myasthenia gravis" and "Overview of shellfish, pufferfish, and other marine toxin poisoning", section on 'Paralytic shellfish poisoning'.)
A review of 50 well-documented cases reported in the United States from 1946 to 2006 concluded that the more recently diagnosed cases were more likely to be misdiagnosed as GBS [31]. Misdiagnosis of GBS often leads to expensive and unnecessary therapy such as plasmapheresis and intravenous immunoglobulin infusions.
Differentiating tick paralysis from these conditions should be relatively easy if the following clinical features are carefully considered or sought:
●A meticulous search will usually disclose the presence of a tick in cases of tick paralysis (see 'Suggested approach to diagnosis and management' below).
●Examination of the CSF is typically abnormal in patients with GBS, poliomyelitis, and in those with acute spinal cord lesions, whereas it is normal in patients with tick paralysis.
●Fever is not typically present in patients with tick paralysis. When fever is a prominent part of the prodromal or neurologic illness, tick paralysis is not a likely diagnosis.
●Unlike tick paralysis, botulism usually causes a descending paralysis, and the cranial nerves are affected early in the illness. Pupillary abnormalities are common in botulism and uncommon in tick paralysis.
●Tick paralysis due to the bite of Dermacentor ticks generally progresses over time periods ranging from hours to one or two days, or may be present in the morning after a patient awakens. By contrast, weakness and paralysis in GBS, poliomyelitis or spinal cord lesions progresses more gradually, often over days to weeks.
●Sensation is intact with patients with tick paralysis, whereas it is either mildly or obviously abnormal in patients with Guillain-Barré syndrome or spinal cord lesions.
SUGGESTED APPROACH TO DIAGNOSIS AND MANAGEMENT — A diagnosis of tick paralysis should be suspected when a child or adult presents with sudden or relatively rapid motor weakness, a normal sensory exam and normal vital signs, and a known or possible exposure to a tick bite, particularly in the spring or summer months. In such cases, a careful and meticulous examination should be performed to look for a tick. Special attention should be given to the scalp, the axillae, the ears, labia, buttocks, and the interdigital spaces. The use of a fine-tooth comb can detect ticks embedded in the scalp of people with long hair; such a comb should be used when the diagnosis of tick paralysis is considered likely and a tick cannot be found after a normal examination.
Response following tick removal — Most patients with paralysis due to Dermacentor ticks will recover or substantially improve within a few hours following the removal of the tick. However, weakness and paralysis may worsen for 24 to 48 hours after I. holocyclus ticks are removed. Thus, patients with paralysis or weakness due to I. holocyclus ticks should be observed carefully following tick removal.
I. holocyclus antitoxin — A hyperimmune serum prepared from dogs has been used widely to treat animals with tick paralysis, but its use in humans has been limited because of the risk of immune-mediated reactions including serum sickness. Thus, cases of human paralysis due to I. holocyclus are usually managed with supportive care that may include mechanical ventilation until recovery occurs [25].
SUMMARY AND RECOMMENDATIONS
●The toxins of various ticks can cause a disease known as tick paralysis, which can be confused with both infectious and noninfectious conditions. Tick paralysis affects humans and domestic and wild animals worldwide, although most cases occur in Australia and North America. Although it is a rare disease in humans, tick paralysis is important to recognize because it can be fatal or nearly fatal. However, if diagnosed promptly, this illness can be cured with the combination of tick removal and supportive care. (See 'Introduction' above.)
●The tick species that cause most cases of human tick paralysis in the United States and Canada are Dermacentor andersoni (the Rocky Mountain wood tick), and Dermacentor variabilis (the American dog tick). Other ticks such as Amblyomma americanum (the Lone Star tick) (picture 1), Ixodes scapularis (the black-legged tick) (picture 2), and Ixodes pacificus (the western black-legged tick) (picture 3) have also been associated with human tick paralysis. The primary cause of tick paralysis in Australia is the scrub tick, Ixodes holocyclus. (See 'Tick vectors' above.)
●The onset of symptoms of tick paralysis occurs only after a female tick has attached and begun feeding. Symptoms do not typically develop until the tick has fed for four to seven days. (See 'Pathogenesis' above.)
●Neurotoxins produced by Dermacentor ticks have a number of pathologic effects including slowing of motor nerve conduction velocity, lowering of the height of the nerve and muscle action potential, and impaired propagation of afferent nerve fiber signals. (See 'Pathogenesis' above.)
●Neurotoxins produced by I. holocyclus act on presynaptic motor nerve terminals and inhibit the release of acetylcholine. (See 'Pathogenesis' above.)
●Tick paralysis caused by Dermacentor ticks usually begins with paresthesias and a sense of fatigue and weakness, although individual patients may sometimes appear irritable or restless and complain of muscular pain. Fever is characteristically absent, and there is no change in the sensorium or headache unless severe hypoxia or hypercarbia are present. Despite patients' reports of paresthesias, the sensory exam is typically normal. Some patients develop an unsteady gait that progresses to an ascending complete paralysis. Deep tendon reflexes are characteristically absent. Respiratory paralysis and death can occur in severe cases. (See 'Symptoms and signs' above.)
●A number of clinically important but unusual features of tick paralysis deserve emphasis:
•Involvement of the facial, ocular, lingual, and pharyngeal muscles may produce diplopia, dysphagia, and/or dysarthria. Bulbar muscle involvement may also result in unusual presenting symptoms such as drooling.
•Paralysis may be localized to one arm or leg, and isolated facial paralysis has occurred in patients who have ticks attached to their external ear canal or to the temporal areas of the scalp. Patients with unilateral arm weakness have also been described, suggesting a brachial plexopathy associated with an engorged tick in the subclavian fossa.
•Patients may present with findings such as ataxia of the arms and legs that spuriously suggest the presence of a cerebellar lesion.
•Patients rarely present with ascending weakness and autonomic dysfunction that may lead to an incorrect diagnosis of Guillain-Barré syndrome. (See 'Symptoms and signs' above.)
●The onset of tick paralysis caused by I. holocyclus is characteristically slow; patients may have gradually worsening symptoms over 48 to 72 hours until complete paralysis occurs. Patients from Australia with tick paralysis may simultaneously have internal and external ophthalmoplegia, and pupillary reflexes may be absent or minimally reactive early in the course of illness. (See 'Symptoms and signs' above.)
●The white blood cell count and analysis of the cerebrospinal fluid are characteristically normal. Hypoxia and hypercarbia may be present if respiratory muscles are involved. (See 'Laboratory findings' above.)
●Tick paralysis can be confused with an array of disorders including Guillain-Barré syndrome, botulism, cerebellar ataxia, myasthenia gravis, poliomyelitis, acute spinal cord lesions, periodic paralysis due to hypokalemia, insecticide poisonings, exposure to buckthorn, shellfish poisoning, and even hysterical paralysis. (See 'Differential diagnosis' above.)
●When the diagnosis of tick paralysis is suspected, a careful and meticulous examination should be performed to look for a tick. Special attention should be given to the scalp, the axillae, the ears, labia, buttocks, and the interdigital spaces. (See 'Suggested approach to diagnosis and management' above.)
●Most patients with paralysis due to Dermacentor ticks will recover or substantially improve within a few hours following the removal of the tick. However, weakness and paralysis may worsen for 24 to 48 hours after I. holocyclus ticks are removed. Thus, patients with paralysis or weakness due to I. holocyclus ticks should be observed carefully following tick removal. (See 'Response following tick removal' above.)
●A hyperimmune serum prepared from dogs has been used widely to treat animals with tick paralysis, but its use in humans has been limited because of the risk of immune-mediated reactions including serum sickness. Thus, cases of human paralysis due to I. holocyclus are usually managed with supportive care that may include mechanical ventilation until recovery occurs. (See 'I. holocyclus antitoxin' above.)
