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Shea Cox
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Tetanus: Not Just from Bites and Rusty Nails
Foxtails are another source of the toxin
Dog Yawning

We have all heard of tetanus shots and have some sense that we are supposed to periodically get them, especially after a dirty cut, scratch with a piece of metal or some sort of bite wound. Some of us may even know that tetanus is often referred to as lockjaw, but the general knowledge of tetanus generally does not extend much beyond that, and many people are not aware that tetanus can be a problem for animals as well as people.

Different animal species have different sensitivity to the tetanus toxin. On the spectrum of tetanus sensitivity, horses, humans and livestock are most sensitive and dogs are less sensitive. And then there are cats: They are quite resistant and almost never get infected (as we all know, cats have a different rule book than the rest of us). We will, of course, focus on our favorite, dogs.

How does exposure occur?

Tetanus is a disease caused by a toxin that is secreted by a bacterium known as Clostridium tetani. These bacteria are anaerobic, meaning that they grow in conditions where there is no oxygen, such as a deep bite wound or puncture. Clostridia are soil bacteria and they live in dirt, so it is easy to see how a puncture contaminated with dirt would be the classical tetanus-yielding wound. Such wounds are particularly common on farms where there might be nails on the ground, ready to pierce a pet in the foot. A fight involving a bite wound and rolling around in dirt might also offer an opportunity for tetanus.

Another interesting source of exposure are foxtails. A study performed at UC Davis School of Veterinary Medicine looked at 35 cases of canine tetanus and found the initial wound for 27 percent of the dogs treated was a foxtail tract, and an additional 50 percent of wounds were suspicious of foxtail tract! An important take away of this study: The wound does not need to be a bite or traumatic puncture. Yet one more reason to fear the foxtail! [See Protecting Your Dog Against Foxtails by Nancy Kay, DVM.]

What is the toxin and how does it work?

The tetanus toxin is called tetanospasmin and it is produced by the bacterium Clostridium tetani in a wound. Here comes the nerdy part: The toxin binds to local nerves and moves up into the central nervous system where it interferes with the release of glycine, an amino acid that also acts as an inhibitory neurotransmitter. The result of this loss of inhibition is painful muscle over-activity, spasms and rigidity. In severe cases, the pet cannot breathe because of the rigid paralysis of the respiratory muscles and a mechanical ventilator is required.

Initial signs seem to pertain to the eyes and can easily be mistaken for eye disease in the early stages. Classically, the pet loses the ability to blink and must flash the third eyelid to moisten the eye. The patient becomes so sensitive to light and sound that clapping your hands can create spasms or seizures. In this phase of disease, these signs may be attributed to other toxins, such as snail bait or moldy toxins, resulting in a misdiagnosis.

Dogs with ears that hang down may develop ears that stand up straight, and the facial muscles pull back in such a way as to create what is called risus sardonicus, or the sardonic grin. In more advanced stages, the patient can no longer walk and will stand stiffly in what is called a sawhorse stance.

This video on tetanus shows the characteristics symptoms (don’t worry, happy ending!).

How is this diagnosed?

Unfortunately, there is not an easy diagnostic test that can be performed to give us the “ah-ha!” answer. The diagnosis is generally made based on the appearance of the pet and history of a wound. Classically, there is a history of a wound in a tetanus case (generally in the preceding 1 to 2 weeks) but sometimes the wound has gone unnoticed by the owner and this important clue is not available.

It is possible to measure antibody levels against the tetanus toxin, but this has not been widely used in the clinical setting. Also, attempting to culture Clostridium tetani from the wound, as a way to support a diagnosis, is generally not successful.

How is it treated?

The first step in treatment is antibiotics to kill the Clostridia. Happily, exotic antibiotics are not needed: Good ol’ fashioned penicillin does the trick. Sedation and anti-seizure medications are necessary to control the muscle spasms and/or seizures. Nursing care is a cornerstone of treatment and requires a darkened room with minimal stimulation.

Clenched jaws can be problematic for feeding, and a liquid diet or slurry is needed. Soft bedding to prevent bedsores is a must. The decision on whether or not to include tetanus antitoxin is more controversial. Antitoxin is an antibody solution (a blood product) generated by either a horse or human to bind and destroy the tetanus toxin.

Improvement is generally noted within the first week of therapy but complete recovery can easily take a month.

What about a tetanus shot?

Tetanus toxoid is the tetanus shot most of us have had at one time or another. It is a vaccine against the tetanus toxin and it is part of our own human vaccination set. Because dogs are much more resistant to tetanus than humans, regular vaccination against tetanus is not recommended for them.

I’ll end this topic by saying that luckily this is not a common occurrence; I have only witnessed one case of this in my 10 years of practice. However, the potential does exist and it is always good to have this kind of knowledge sitting on the back burners of our brains… you never know what kind of medical mischief our pets will get in to!

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Veterinarian Shea Cox has enjoyed an indirect path through her professional life, initially obtaining degrees in fine arts and nursing. She later obtained her veterinary medical degree from Michigan State University in 2001 and has been practicing emergency and critical care medicine solely since that time. In 2006, she joined the ER staff at PETS Referral Center in Berkeley and cannot imagine a more rewarding and fulfilling place to spend her working hours. In her spare time, she loves to paint, wield her green thumb, cook up a storm and sail. Her days are shared with the three loves of her life: her husband Scott and their two Doberman children that curiously occupy opposite ends of the personality spectrum.

CommentsPost a Comment
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Submitted by Anonymous | October 19 2011 |

Interesting story, but I'm still waiting for you to show a picture of a foxtail plant close up! The article you link to includes a photo, and I can only assume that they are foxtails in the photo, but I have no idea! Please, do us a favor and show us a close-up photo of this weed!

Submitted by Lisa Wogan | October 20 2011 |

You'll find photos of foxtails here and here for a start.

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