First two cases of tick paralysis in North American horses reported

Embedded and engorged Dermacentor variabilis ticks concentrated at the base of the tail in a 3‐year‐old American Miniature horse.
Embedded and engorged Dermacentor variabilis ticks concentrated at the base of the tail of a three‐year‐old American Miniature horse. Photo: Trumpp et al.

The first two cases of paralysis in horses caused by tick infestation have been reported in the United States.

The cases in American miniature horses are described in the Journal of Veterinary Internal Medicine.

Both horses had been referred together in May last year to the Purdue University Veterinary Teaching Hospital in Indiana.

One, a three‐year‐old filly weighing 93.6kg, had gone down. The owner reported that the horse had experienced 12 hours of difficulty with walking and weakness that progressed to being unable to rise.

The other was a four-year-old mare weighing 108.6kg. She was also suffering from weakness, progressing to recumbency, although was still able to stand unsteadily.

Both horses, who had been bought by the owner nine days earlier from a non-licensed petting zoo, had shown incoordination and weakness 24 hours before referral.

A waterbed for employed for a 3‐year‐old recumbent American Miniature horse presumptively diagnosed with tick paralysis.
A waterbed employed for the three‐year‐old miniature horse presumptively diagnosed with tick paralysis. Photo: Trumpp et al.

Veterinarians checked the animals and found that both had decreased tongue and tail muscle tone, and had normal spinal reflexes.

Cerebrospinal fluid cytology was normal. Testing for equine herpesvirus‐1 proved negative.

However, they found numerous ticks, Dermacentor variabilis, on both horses.

The younger horse had about 150 embedded and engorged ticks, mostly concentrated along the base of the mane and tail.

The older mare had about 100 embedded and engorged ticks, concentrated in the same locations.

The ticks were removed within 24 hours of arrival at the hospital.

Both horses were treated topically with permethrin. Supportive care included fluid therapy, treatment for corneal ulceration, and frequent repositioning during recumbency.

Within 48 hours of tick removal, both horses were neurologically normal.

“Ours is the first reported case of presumptive tick paralysis in horses in North America,” lead authors Kelsey Trumpp and Ashley Parsley, together with their colleagues, reported in the journal.

“Although rare, tick paralysis should be considered in horses presented with acute‐onset weakness progressing to recumbency.”

The authors said further research was needed to learn more about how neurotoxins from Dermacentor tick species affected horses.

Tick paralysis, they noted, is most frequently is reported in dogs. It occurs when an adult female tick attaches to the host and produces salivary neurotoxins that enter the circulatory system of the dog.

These neurotoxins act on presynaptic membranes at the neuromuscular junction and prevent the release of acetylcholine, most commonly resulting in increasing motor paralysis.

The primary tick species implicated in cases of tick paralysis of dogs and cats in North America are Dermacentor andersoni (the Rocky Mountain Wood tick) and D. variabilis (the American Dog tick), whereas tick paralysis in Australia most commonly is caused by Ixodes holocyclus.

“Cases of tick paralysis in large animals have also been reported, but only in Australia,” they noted.

In a retrospective study of 103 horses in Australia with presumptive tick paralysis caused by I. holocyclus, 88% of the horses were recumbent and unable to stand on presentation.

Seventy‐six percent of them were less than a year old, and half were younger than six months old. Thirty-nine percent were miniature horses or ponies.

The case-report team said the two North American horses had also been treated with botulinum antitoxin while in hospital, as botulism had been considered a potential cause of their paralysis.

“However, the rapid improvement in neurologic status observed in these two horses made botulism unlikely because recovery requires regeneration of new motor end plates, which can take up to three weeks.”

“Both horses reported here showed rapid improvement upon tick removal and survived. Neither horse had residual neurologic deficits.

“Given the lack of published reports on tick paralysis in horses in North America, it is likely that horses are relatively resistant to the development of clinical signs.”

The full case-report team comprised Trumpp, Parsley, Melissa Lewis, Joseph Camp Jr and Sandra Taylor, all with Purdue University’s College of Veterinary Medicine.

Presumptive tick paralysis in 2 American Miniature horses in the United States
Kelsey M. Trumpp, Ashley L. Parsley, Melissa J. Lewis, Joseph W. Camp Jr., Sandra D. Taylor.
First published: 03 June 2019, Journal of Veterinary Internal Medicine,

The case report, published under a Creative Commons License, can be read here

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9 thoughts on “First two cases of tick paralysis in North American horses reported

  • June 13, 2019 at 7:07 am

    This is a sad development. Given the global society we live in and all that comes with it ( i.e. trade, increased importation of commodities and livestock) – I think many nations are seeing viruses, bacteria and disease, that has previously never been seen in their area before – both in animals and humans. The answer is better inspection of incoming live stock and other animals and increased funding to try and develop solutions with regard to previously only foreign infestations. Increased knowledge is the most effective strategy to dealing with disease. Viruses and bacteria don’t need a passport or a visa to pay us a visit. The more we can learn, with regard to earlier detection and a concerted effort to develop the appropriate medicines and vaccines to address these illnesses will benefit us tremendously as well as areas in which these diseases have already become entrenched.

  • June 13, 2019 at 8:19 pm

    This “tick paralysis” definitely sounds like Lyme to me…
    I am very sad for these horses and their current owners.

  • June 14, 2019 at 12:59 am

    I am surprised that the owner did not find the ticks especially since there were so many of them. My horses develop a large lump when a tick embeds. Llamas also can get tick paralysis.

  • June 14, 2019 at 1:46 pm

    My filly had this is 2016. Not the first case. It was a long road to recovery for her but she is doing fine now.

  • June 15, 2019 at 7:29 am

    Obviously these horses had not been groomed or looked after for some time, including by the new owner. It seems that whenever there is money to be made on an animal, the animal gets the short end of the stick.

  • June 15, 2019 at 9:47 am

    I commend all of you who worked on this ! Now if any of you could perhaps sway the CDC to see your way in the matters of these awful creatures (ticks) regardless of their origin names (Rocky Mountain Tick or American Dog Tick) that Lyme desiese actually does exist and can become chronic and should be treated by it’s mutation from East coast where it started to the West and North and South.

  • June 17, 2019 at 3:48 pm

    Thees Lyme in Australia an New Zealand but because there is no tick registers it doseent get reported. there are currently around 200 humans with it here in NZ and most unble to get treatment. Its spread mainly by migratory birds, ticks and bird mites..

  • June 21, 2019 at 7:21 am

    I would like to know the state and city these came from. I take great care to monitor our horses and groom them carefully. I have never seen this before.
    Are these ticks indigenous to a certain area and time of year. I know ticks are every where, but these sound special.


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