Tick paralysis may result from a tick bite if the parasite releases a neurotoxin contained in its saliva into the patient's circulation. It is an uncommon disease and up to date, it has only been reported on the Australian and American continents.
TP patients typically present with an acute, ascending paralysis, although upon query a preceding episode of lethargy and irritability may be reported. Muscle weakness and hypotonia starts to affect the lower limbs, aggravates and spreads cranially. Due to reduced motor control while walking, patients may initially claim ataxia. Within one or two days, they may completely lose their ability to maintain themselves on their feet. Deep tendon reflexes are blunted or absent. Eventually, trunk, upper limbs and face may become affected. Striking symptoms are ophthalmoplegia and bulbar palsy, i.e., patients are unable to perform standard eye movements, present dysphagia and dysarthria. Involvement of respiratory muscles results in dyspnea.
Most TP patients are children, preferentially girls. However, adult males account for a significant share of individuals affected by TP, too. Neither gender nor age rule out TP.
If the clinical picture indicates TP, the patient should be thoroughly examined for ticks. It has been suggested that tick bites that occur in close proximity to the base of the skull or the spine are more likely to cause TP. Thus, ticks may be found on the scalp or in areas usually covered by the patient's hair. Also, ticks may remain undetected in skin folds like axillae, groin or perineal region, or in orifices lined with skin, e.g., in nostrils or ear canals. Presumably, holocyclotoxin induces symptoms of paralysis several days after tick attachment. Thus, tick exposure may have occurred several days before symptom onset, while rural areas or even foreign countries were visited .
Clinical findings and confirmed infestation with ticks is diagnostic for TP. Laboratory analyses of blood are not required and would reveal little more than slight elevations of creatine kinase levels.
Important differential diagnoses are:
The mainstays of TP treatment are localization and complete removal of the parasite. It is important to remove the entire tick including its gnathosoma and all mouthparts to avoid dermatological complications. As a routine procedure, the site of attachment should be disinfected afterwards, although injected neurotoxin will not be neutralized by this measure.
Patients presenting with moderate to severe cases of TP may require supportive treatment until complete remission of symptoms. Patients at risk of respiratory failure should be intubated and receive respiratory assistance, possibly for several days. Of note, canine tick antivenom obtained from hyperimmunized dogs is available in Australia and may accelerate recovery in patients bitten by Ixodes holocyclus.
Prognosis is very good. Symptoms generally resolve completely within hours after the tick has been removed. Prolonged muscle weakness may result from retention of the tick's gnathosoma in the patient's skin or from potent neurotoxins released by some Australian tick species. Even in these cases, complete removal of the parasite or supportive therapy until remission occurs a few days later assure recovery without sequelae.
Patients suffering from severe TP who are not attended in a timely manner may die from respiratory failure after paralysis of the respiratory musculature. This is, however, a very rare complication of the disease .
There are almost 1,000 species of ticks described to date and they are distributed throughout the world. However, neither do all these species pose a threat for human health nor do all of them produce neurotoxins that may induce TP. So far, several dozens of tick species have been related to TP, whereby the most important ones seem to be:
Of note, ticks pertaining to other genera, namely Argas, Haemaphysalis, Otobius and Rhipicephalus have been shown to cause paralysis in animals. It cannot be ruled out that they are able to provoke paralysis in humans.
Neurotoxins produced by all these species differ and very few may cause fatalities in men. Because most severe cases are related to Ixodes holocyclus, the general name given to all TP-related neurotoxins is holocyclotoxin .
A tick's life cycle comprises development inside an egg, hatching of larvae, molting into nymphs and finally into adult ticks. All developmental stages except the egg may feed on mammalian blood. According to current knowledge, only adult female ticks are able to induce TP, although this observation may result from the facts that earlier developmental stages primarily feed on smaller hosts and that adult males feed less frequently than adult females.
Most TP patients suffer from mild forms of the disease and do not necessarily seek medical attention, which is why TP is presumably underdiagnosed. Nevertheless, best estimates still consider TP a very rare disease. In Washington State, for instance, TP had to be reported to the local authorities until 1998. Nevertheless, only 33 cases have been notified between 1946 and 1996 . In Australia, 20 fatalities have been registered between 1900 and 1945 .
Prolonged exposure, delayed medical attention and possibly greater susceptibility make TP a much more common diagnosis in veterinary medicine. According to the above cited Australian study, about 100,000 animals die every year from TP in this country. Indeed, human TP has only been described in North America and Australia, but animals on other continents have been diagnosed with this tick-borne disease. Two conclusions may be drawn from this observation: There are ticks that produce TP-related neurotoxins outside of North America and Australia and men may eventually be affected by this disease in other parts of the world. Additionally, travelers who return from endemic regions may develop TP within a few days after exposure .
With regards to gender and age distribution, two risk groups may be defined. On the one hand, TP incidence is higher in female children. In general, pediatric patients who are bitten by a tick are exposed to relatively high doses of neurotoxin and thus, tend to develop paralytic symptoms. It has been proposed that tick bites that occur in close proximity to the base of the skull or the spine are more likely to cause TP. Young girls often have longer hair that covers these regions and ticks may remain undetected for longer periods of time. On the other hand, adult and elder males are overrepresented among TP patients. It has been suggested that this fact results from greater exposure due to typically male professions in the countryside.
Most tick bites occur during spring and summer and thus, TP is most frequently diagnosed during that time of the year.
Ticks pertain to the phylum of arthropods, like insects do. But while the latter form the huge class of insects, ticks are arachnids. Both classes differ in a variety of physiological and morphological properties, the most striking one being that insects have three pairs of legs while arachnids got four pairs of legs. There are few exceptions to that rule.
Mites, ticks, spiders and scorpions are commonly known arachnids, whereby mites and ticks are closely related and both belong to the subclass of acari. Ticks pertain to the order of parasitiformes and the suborder of ixodida. The former taxonomic designation already indicates that most representatives of this group of animals are indeed parasites.
A tick's body is divided into an anterior gnathosoma that carries the animal's mouthparts and the larger idiosoma that supports its legs and fills with blood during feed intake. Evolution brought forward mouthparts that are highly adapted to a tick's hematophagous lifestyle. Ticks dispose of a hypostome that allows them to penetrate human skin, anchor themselves in it and feed on their host's blood. Tick salivary molecules injected into the host facilitate feeding by avoiding blood clotting and modulating host defense mechanisms . Very few of those molecules have been identified and characterized to date.
It is known that the neurotoxins causing TP is produced in the animal's salivary glands. As such, their expression is probably upregulated previous and during feeding. Little is known about their physiological function and only recently, a detailed description of holocyclotoxin-1 has been published . In men, holocyclotoxins presumably interfere with signal transduction at the neuromuscular junction and induction of end-plate potentials, and thus induce muscle weakness and paralysis. However, the molecular target(s) of these toxins have not yet been identified. Moreover, tick salivary molecules - including but not limited to neurotoxins - may act as allergens and cause potentially fatal anaphylaxis .
Prevention of TP consists in prevention of getting bitten by a tick, particularly if living in or traveling to endemic areas. Individuals engaging in outdoor activities like walking, trekking or camping are most often affected by tick bites and should be very careful. In this context, wearing long-sleeved, light-colored clothes and closed shoes reduces the risk of tick bites and facilitates detection of parasites that are looking for food. Certain repellents are effective against ticks and might be used on skin and clothes; others are not recommended for application on the skin. Permethrin, for instance, is very effective against ticks, but may only be used on clothes. Of note, while some veterinarian formulations to keep dogs free from ticks contain permethrin and derivatives, these compounds are highly toxic for cats and should therefore not be used on any clothes, blankets or other surfaces that may come into contact with cats.
While the risk of tick bites is greatest in spring and summer, preventive measures should also be taken during other seasons.
Tick paralysis (TP) is a rare neuromuscular disorder; it is also referred to as tick toxicosis. It is triggered by arachnid neurotoxins that reach human circulation if tick saliva is released into the blood stream while the parasite is feeding. It has to be noted that although TP is a tick-borne disease, it is not an infectious disease.
To date, human cases of TP have only been reported in Australia and North America. They are caused by bites of distinct tick species, mainly those belonging to the genera Ixodes and Dermacentor, respectively. In contrast, in veterinary medicine, TP is recognized as a widely distributed disease that may be provoked by neurotoxins injected into a host's circulation by many different species of ticks.
Girls and adult men seem to be most often affected, but according to current knowledge, individuals pertaining to all races, both genders and all age groups may develop TP. This disease is characterized by an acute onset of progressive, ascending paralysis and blunted deep tendon reflexes. Ophthalmoplegia and bulbar palsy may be observed. Sensory nerve function is not affected . In rare cases, respiratory muscle function may be impaired and TP can be fatal.
Disease progress may be interrupted by removal of the parasite, although prolonged weakness and residual paralysis may require continued supportive treatment. The latter particularly applies for TP cases triggered by Australian ticks .
Tick paralysis (TP) is a rare, neuromuscular disorder caused by toxins that are injected into human circulation while ticks feed on their host's blood. To date, cases of human TP have only been reported in Australia and North America. They are associated with determined tick species, namely those pertaining to the genera of Ixodes (in Australia), Amblyomma and Dermacentor (both in North America).
Patients suffering from TP typically present with an acute, ascending paralysis, i.e., they note increasing weakness in their legs and lose the ability to walk within a few hours or days. Paralysis spreads further and may involve trunk, arms and head. Respiratory failure may result from paralysis of respiratory musculature and may be fatal.
In order to interrupt progression of paralysis, the disease has to be identified and the tick needs to be localized and removed. Generally, symptoms remit completely within very short periods of time after removal of the parasite. In severe cases, mainly in Australia, patients may require supportive therapy or even respiratory assistance for a few days. Nevertheless, the vast majority of TP patients recovers completely.