Tick Paralysis in Horses

Summary

Tick paralysis results when a tick’s saliva delivers a paralytic toxin into the bloodstream of its host. Equine tick paralysis is rare, as horses seem to be resistant to most tick toxins. Affected horses typically experience ascending paralysis, with the hindquarters affected prior to the front limbs. Eventually the horse is unable to stand, and often has difficulty breathing and swallowing. Tick paralysis is life-threatening and affected horses may die from respiratory failure or other complications. 

Removal of ticks is crucial to treatment, as they continue to deliver their toxin while feeding. With early and appropriate treatment, most horses survive tick paralysis, but serious complications are common. Prevention of tick paralysis is most effective by employing standard tick control practices such as pasture management and routine application of topical antiparasitic products.

Risk Factors

Horses are considered somewhat resistant to tick paralysis, particularly in North America. There is only one report of tick paralysis in horses in North America to date. It is more common in Australia, due to the potency of the Australian species of tick’s toxin. Although it is rare, tick paralysis is a life-threatening emergency. Any horse that is unable to stand, having difficulty swallowing, or having difficulty breathing requires immediate veterinary care.

Possible Causes

The females of certain species of ticks release toxins into the circulatory system when they feed on an animal. These toxins affect muscle contraction, resulting in widespread paralysis of all muscles. Ticks that are known to produce paralysis toxins include:

• Australian paralysis tick (Ixodes holocyclus) • Rocky Mountain wood tick (Dermacentor andersoni)

• American dog tick (Dermacentor variabilis)

The highest risk for tick paralysis is in the spring and early summer, when female ticks are most active.

Main Symptoms

Horses with tick paralysis are extremely weak, and are unable to contract their muscles properly. Affected horses typically: 

• Are unable to stand (collapse) • Have trouble walking (ataxia) 

• Breathe excessively fast or hard (tachypnea or dyspnea) • Have difficulty swallowing (dysphagia)

Detailed Characterization

Ticks typically seek out places where the skin is thin, so they’re often found on the chest, under the jaw, in the ears and mane, along the flanks, and or under the tail. There is no association between the severity of symptoms and the number of ticks found. Most cases of tick paralysis have only 1-2 ticks found on the horse when symptoms develop. 

Horses that are unable to stand are at a high risk of significant muscle and skin damage. The body weight of the horse compresses these tissues and restricts blood flow, ultimately leading to tissue death. Horses experiencing these problems show symptoms such as:

• Sloughing of the skin • Irritation or rashes in the folds of the skin • Muscle loss

• Damage to the cornea of the eye • Dark red or brown urine

Testing and Diagnosis

Diagnosis is straightforward if ticks are identified on the horse. There is no specific test for tick paralysis. If no ticks are found on the horse, then an investigation into other causes of weakness is carried out. In these cases, administering an antiparasitic treatment to kill any ticks currently feeding on the horse is prudent, as the large size of the horse and extensive hair coat can easily hide the small parasite. If the horse improves after antiparasitic administration, then a presumed diagnosis of tick paralysis is made.

Steps to Recovery

Treatment of tick paralysis depends on the horse’s location. In Australia, an antitoxin is available for paralysis tick bites. There is no antitoxin available for Dermacentor toxins, the causative agent of tick paralysis in North America. Removal of any feeding ticks is a key component of treatment. Symptoms generally do not resolve until the feeding ticks are removed.

Horses require significantly more supportive care when they are unable to stand compared to other species. The massive body weight of a horse easily compresses tissue, preventing appropriate blood flow. Supportive care for recumbent horses includes:

• Placing the horse on padding such as mattresses, blankets, or pillows • Turning the horse over multiple times a day

• Head protection to prevent the horse from scratching its eyes on the ground • Anti-inflammatory medications

• IV fluids, which often includes added nutrition for horses unable to eat • Antibiotics, if infections develop

The symptoms of tick paralysis continue until the tick is removed from the horse’s skin. With treatment, the prognosis of tick paralysis is fair, with about 75% of horses surviving their paralysis episode. The high mortality rate compared to tick paralysis in dogs and cats is primarily due to the complications that arise from horses laying down for long periods of time. Similarly, many surviving horses develop secondary complications such as skin sloughing, eye ulcers, and muscle damage due to prolonged recumbency. Most of these complications are treatable with few long-term effects.

Prevention

Tick paralysis is not directly contagious, however other horses in the same environment as an affected horse are also likely to acquire the ticks that cause paralysis. Due to the relatively high mortality rate, prevention is key in areas where tick paralysis is common. Tick prevention strategies include:

• Keeping pasture grass short and reducing dense vegetation in pastures

• Examining horses daily for ticks, particularly focusing on under the jaw, under the tail, and in the groin

• Using topical antiparasitic products such as sprays or pour-ons before turning horses out into pastures

Is Tick Paralysis Common in Horses?

Tick paralysis is extremely uncommon in horses. There have been occasional reports in Australia. There is only one report in North America to date, affecting two Miniature Horses.

Typical Treatment

• Supportive care •Antitoxin if available

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