trismus: why dogs and cats get lockjaw

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White maltese dog in grassy field | Why dogs get lockjaw
Posted by Dr. Kim Smyth on May 01 2014

Trismus is the medical term for the condition we more commonly call “lockjaw.” Spasms of the muscles that we use for chewing make it difficult to open our mouths, hence the popular nickname for trismus. Today, we’ll discuss common causes for trismus in our pets.

Typically, when we think of lockjaw, our minds go straight to tetanus, which is indeed a cause of trismus. Tetanus is caused by neurotoxins that are formed in the body during the growth of the bacteria Clostridium tetani. Generally, these bacteria enter the body via penetrating injury, but any wound is susceptible.

Thankfully, dogs and cats have a natural resistance to tetanus, so it is relatively rare. However, if disease does occur, it typically shows up about five to ten days after injury and results in either localized or generalized muscle stiffness, including the muscles used for chewing.

Another of the more common causes for trismus in dogs is a condition called masticatory myositis, or inflammation of the muscles of mastication (chewing). Inflammation is caused by an immune mediated attack of the muscles, leading to trismus and wasting away of the muscles at the top of the affected dog’s head.

While we’re on the subject of myositis, or muscle inflammation, we should touch on polymyositis (or inflammation of many muscles) as a potential cause of trismus in cats and dogs. Polymyositis occurs for many reasons—sometimes it is infectious in natures, sometimes it is immune mediated, and sometimes we can’t find an underlying cause at all, so we call it idiopathic. Whatever the cause, if inflammation affects the muscles your pet uses for chewing, you can bet that trismus will result.

Of course, trismus can also be caused by trauma or other problems in the temporomandibular joint (TMJ), which is the joint where the upper and lower jaws meet. Fractures, dislocation, and even arthritis can affect how a dog or cat opens his or her mouth.

Finally, I’d like to touch on one hereditary condition we see in West Highland White Terriers (Westies) and other breeds called craniomandibular osteopathy. This condition is a non-cancerous, non-inflammatory disease that causes proliferation of the bones of the head, including the TMJ. When bony changes affect the TMJ, trismus will certainly be the result.

While there are a lot of causes of lockjaw in pets, thankfully they are all relatively rare. Still, if you notice your pet having trouble picking up food or chewing, or if you see wasting away of the muscles on top of your pet’s head, trismus may be the reason, and it’s worth a trip to the vet.