Millions of Americans live with uncontrollable shaking of the hands, head, voice or other part of the body. When “the shakes” are not known to be caused by a disease or exposure to a toxic substance, the condition is called essential tremor (ET) because the tremors appear to exist independently from another state. In humans, ET is thought to originate with abnormal signals from the part of the brain called the cerebellum, a region that plays an important role in movement and motor control.
Tremors in animals
Humans are not the only living creatures who develop involuntary rhythmic oscillations (back-and-forth motions) of the limbs or other areas. Tremors have been observed in domesticated animals, zoo animals, and even in wild animals—though these are the hardest to find and study.
Pet veterinarians see and treat animal tremors because their worried owners bring them in for evaluation and treatment when they see trembling and seizure-like motions. The vet interviews the owners, and conducts an examination and such tests as necessary to determine the source of tremors. As in human tremors, lab tests rarely reveal specific abnormalities. Depending on the affected area, especially the hind limbs, imaging may detect injury or deformity affecting the motor nerves.
Cats and dogs can have tremors in their limbs and other body parts, though it’s not considered ET. When animal tremors are classified, there is some overlap with human tremor categories:
- Injury or trauma
- Toxicity induced tremors (certain medications, toxic substances)
- Metabolic conditions such as low blood sugar or nutrient deficiency
- Severe weakness or pain
- Fear or anxiety
- Nervous system disorder
- Muscle spasms related to distemper (dogs)
- Intention tremors in dogs (cerebellar disease in dogs)
- Action tremors in dogs during voluntary movement
- Certain types of tremors in dogs are seen in particular breeds. For example, head tremors or bobbing may occur in boxers, bulldogs, Great Danes, Bassett hounds and others.i
Large domesticated animals can have tremors, too. Horses, cows, sheep and goats can have tremors or muscle twitching/spasms that appear tremor-like but are almost always disease symptoms. In short, animals may have many kinds of tremors and trembling, but there is no evidence that animals have ET as humans do. However, disturbances in the cerebellum of dogs—and possibly other animals—can produce action and intention tremors similar to ET, which begins in the cerebellum.
Learning about ET through animal studies
There have been many clinical studies of humans with essential tremor, including some brain anatomy studies based on autopsy. However, scientists turn to laboratory animals for physical experiments that would not be possible with humans. In fact, animal research has been involved in almost every human medical advance in modern times. Animals help us develop methods for understanding, preventing, treating and curing illness and such disabilities as ET, in which “…full understanding of the relationship between brain circuitry alterations and tremor requires testing in animal models.”ii
Of course, the welfare of animals for research use is strictly regulated by law and ethical practice. Wherever possible, nonanimal research is conducted. However, when animal models are to be used, the animals must be well cared for, as few animals as possible are to be used, and every effort is made to spare them from pain or distress and promote recovery when called for.
Since animals typically don’t develop ET, there are three approaches to create tremor in animals: certain drugs that produce tremors; interventions (surgical or ablative) in small brain regions of animals that create abnormal brain signals; and genetic mutants. Animal models fall somewhat short of human tremor experience because
…no animal model has yet been generated that exactly recreates all features of any of the known tremor disorders in humans. Problems encountered when comparing tremor in animals and humans include differing tremor frequencies and the uncertainty, if specific transmitter abnormalities/central nervous system lesions seen in animal tremor models are characteristic for their human counterparts.iii
New and less harmful ways of inducing tremor in lab animals continue to be developed, even as imaging technologies and other ways of accessing the tremor pathway in human brains make it possible to do direct study without harming people. Meanwhile, we humans owe a great debt of gratitude to the rodents, pigs, monkeys and other creatures that have so far contributed greatly to our information of brain processes and anatomy, and our research into more effective therapies.
iShell, Linda. “The shakyyyyy……doogggg…” CVC Proceedings, Oct. 1, 2008. http://veterinarycalendar.dvm360.com/shakyyyyydoogggg-proceedings
iiPan MK, Ni CL, Wu YC, Li YS, Kuo SH. Animal models of tremor: relevance to human tremor disorders. Tremor Other Hyperkinet Mov (N Y). 2018; 8: 587.
iiiWilms H, Sievers J, Deuschl G. Animal models of tremor. Mov Disord. 1999 Jul;14(4):557-71.