Causes of Lower Motor Neuron Disease in Australian Pets

Lower motor neuron disease (LMND) refers to neuropathy resulting from a wide variety of etiologies. Affected animals generally exhibit skeletal muscle weakness displayed as gait abnormalities, lethargy, decreased muscle tone, dysphonia, and/or dysphagia. However, LMND-affected pets typically maintain normal mentation, proprioception, nociception, and spinal reflexes.

A recent review article in the Journal of Veterinary Medicine described some unique causes of LMND in Australian dogs and cats. According to the authors, LMND is a frequently encountered condition in companion animals “largely due to the unique, native fauna of Australia.” In particular, they described LMND due to tick paralysis, snake envenomation, and marine animal intoxication.

Tick Paralysis

Although tick paralysis is a global issue, canine cases are relatively common in Australia, with one recent study describing 3400 canine and feline cases over 2 tick seasons. While tick paralysis in North America is usually caused by bites from Dermacentor spp, Australian cases occur as a result of bites from Ixodes ticks. Tick paralysis is typically restricted to the eastern and southeastern coastal areas inhabited by the ticks’ preferred hosts, the bandicoot and possum, and most pet cases occur from September through December.

Although the mechanism of paralysis is incompletely understood, a holocylotoxin produced by the tick prevents signaling across the neuromuscular junction. The tick must remain attached for at least 3 days in order for paralysis to occur; thus daily inspection and rapid-kill acaricides are common preventive tools in endemic areas.

Snake Envenomation

Several native Australian snakes, including the eastern and western brown snakes, tiger snake, whipsnake, and red-bellied black snake, are venomous and may cause LMND after a bite. The severity of clinical signs typically depends on the species and age of snake, as well as the delivered dose of venom. Also, some species and life stages of snakes may induce a myopathy and/or coagulopathy along with LMND.

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A snake venom detection kit can identify snake venom to the genus level using a serum, blood, urine, or bite swab sample. The authors noted that snake identification is important for selecting the appropriate monovalent antivenin, the use of which significantly improves patient outcome and survival rate.

Tetrodotoxin

Finally, Australian dogs are at risk of LMND due to tetrodotoxin after consuming certain animals. Pufferfish is a common by-catch animal in Australian sport fishing, so dogs may encounter discarded whole or partial fish on the beach. Blue-ringed octopus and certain frogs, newts, sea stars, and sea slugs can also harbor tetrodotoxin.

Tetrodotoxin is not produced by the animal itself, but rather by commensal bacteria consumed or harbored by the animal. This potent neurotoxin acts by blocking fast sodium channels within axons and myocytes. Affected dogs may exhibit vomiting, ataxia, and lethargy, which can rapidly progress to cardiac arrhythmia, respiratory paralysis, and death. As no antitoxin is available for tetrodotoxin exposure, supportive care is the mainstay of treatment.

Dr. Stilwell received her DVM from Auburn University, followed by a MS in fisheries and aquatic sciences and a PhD in veterinary medical sciences from the University of Florida. She provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting.