Tick paralysis is a progressive paralysis caused by a salivary neurotoxin produced by certain species of ticks. It is caused by over 60 species of ticks worldwide (five in North America, including the deer tick) and can occur in almost any region where ticks are found. It has killed thousands of animals, mainly cows and sheep, in other parts of the world. Although tick paralysis is of concern in domestic animals and livestock in the United States as well, human cases are rare and usually occur in children under the age of 10.

Etiology, Epidemiology, and Pathogenesis:

The potential for inducing paralysis has been demonstrated, described, or suspected in 64 species of ticks worldwide. In North America, D andersoni (the Rocky Mountain wood tick) and D variabilis (the American dog tick) are the most common causes, but D albipictus , I scapularis , Amblyomma americanum , A maculatum , R sanguineus , and O megnini may cause paralysis. In North America, the Rocky Mountain wood tick and the American Dog tick affect dogs most commonly, but sheep, cattle, and humans have also been paralyzed. Cats appear to be resistant to the disease caused by these ticks. Clinical signs of paralysis are seen 5-9 days after tick attachment, and progress from hindlimb weakness to quadriplegia over the next 24-72 hr. If ticks are not removed, death may occur from respiratory paralysis in 1-5 days. Removal of all ticks usually results in improvement within 24 hr and complete recovery within 72 hr.

Tick paralysis occurs when an engorged and gravid (egg-laden) female tick produces a neurotoxin in its salivary glands and transmits it to its host during feeding. Experiments have indicated that the greatest amount of toxin is produced between the fifth and seventh day of attachment (often initiating or increasing the severity of symptoms), although the timing may vary depending on the species of tick. 

Clinical Findings:

Early signs may include change or loss of voice (due to laryngeal paralysis), hindlimb incoordination, change in breathing rate and effort, gagging or coughing, regurgitation or vomiting, and pupillary dilation. Hindlimb paralysis begins as slight to pronounced incoordination and weakness. As paralysis ascends, the animal becomes unable to move hindlimbs and forelimbs, stand, sit, or lift its head. Sensation usually is preserved. Breathing abnormality is of greater prognostic importance than limb paralysis. Respiratory rate may initially increase but, as the disease progresses, becomes slower and obviously labored, especially on expiration. Regurgitation of esophageal contents, saliva pooling, depression of the gag reflex, and attempts to clear the throat may produce a characteristic harsh, groaning respiratory sound. Temperature is normal in the early stages. Paralyzed animals with low body mass (especially cats) may become hypothermic. Conversely, animals kept at high ambient temperatures may need to cool themselves by panting, adding to respiratory difficulty.

Unlike Lyme disease, ehrlichiosis, and babesiosis, which are caused by the systemic proliferation and expansion of parasites in their hosts long after the offending tick is gone, tick paralysis is chemically induced by the tick and can therefore continue only in its presence. Once the tick is removed, symptoms usually diminish rapidly. However, in some cases, profound paralysis can develop and even become fatal before anyone becomes aware of a tick’s presence. 

Diagnosis:

The presence of a tick in conjunction with the sudden (12-24 hr) appearance of leg weakness and/or respiratory impairment is diagnostic. The offending tick may no longer be attached, but a tick “crater” (a small hole surrounded by a slightly raised and inflamed area) in the skin confirms the diagnosis. Because of the inability of laboratory tests to indicate tick paralysis, diagnosis is based on symptoms and the rapid improvement of the patient once the engorged tick is removed.

Treatment:

Removal of the tick(s) is necessary. Frequently, multiple ticks are attached to an animal. The entire animal should be searched diligently and repeatedly, especially on long-haired animals. Most ticks are found around the head or neck, but can be anywhere on the body. Some practitioners prefer to kill the tick before removal. It is important to remove all the mouthparts, since they contain the salivary glands which may continue to infect the patient even after the body of the tick has been removed.

A canine tick hyperimmune serum, also called tick antiserum (TAS), is available for the specific treatment for I holocyclus -induced tick paralysis. This should be given as early in the disease as possible.

About 5% of animals are likely to die despite all treatment efforts, especially those with advanced paralysis and dyspnea. Older animals or those with pre-existing cardiopulmonary disease are at greatest risk.

Prevention and Control:

Owners should not rely wholly on chemical control to prevent tick infestation. They should be advised about when and where their pets will be at risk; encouraged to search the coat daily; keep the coat short if possible; and understand the appropriateness, safety, and limitations of available preventive products. Products recommended for tick control in dogs include fipronil (as a topical spray or spot-on), permethrin (spray or rinse), cythioate (given orally), and amitraz (in an impregnated collar). All have limitations but are useful if used correctly. Fipronil is one of the few products approved for use in cats for tick control.

Prevalent Regions for Tick Paralysis

Ticks that cause tick paralysis are found in almost every region of the world. In the United States, most reported cases have occurred in the Rocky Mountain states, the Pacific Northwest and parts of the South. The five North American species of ticks thought to cause tick paralysis are widely distributed throughout the United States; therefore, the potential for contact with such ticks exists in every state.