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.
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, https://doi.org/10.1111/jvim.15540