Tetanus is a disease caused by a toxin secreted by a bacterium known as Clostridium tetani, one of many toxin-secreting Clostridia. 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, meaning that they live in dirt so it should be 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 or bits of wire on the ground, ready to pierce a human or animal foot, but there is nothing special about rusted metal in predisposing to tetanus. The puncture can come from any pointed object. The pointy object provides the puncture (an anaerobic wound) and contamination with dirt provides the Clostridia. A fight involving a bite wound and rolling around in dirt might similarly offer an opportunity for tetanus.

Different animal species have different sensitivity to the tetanus toxin. Horses, humans, and livestock are more sensitive while cats are quite resistant, and almost never get infected. We will be confining our discussion primarily to the dog.

THE TOXIN AND WHAT IT DOES

The tetanus toxin is called “tetanospasmin.” It is produced by the bacterium Clostridium tetani in a wound. The toxin binds to local nerves and migrates up into the central nervous system where it interferes with the release of glycine and GABA, both of which are inhibitory neurotransmitters. The result of this loss of inhibition is painful muscle over-activity, spasms and rigidity. In severe cases, the patient cannot breathe because of the rigid paralysis of the respiratory muscles and a mechanical ventilator is required. As a human disease, tetanus has been known and described since ancient times. In the modern world, human death is unusual and is mostly limited to infants and to the very elderly but even in these groups mortality is only 10-15%.

COURSE OF THE DISEASE

The incubation period of tetanus is 3-21 days with 5-10 days from the time of the original wound infliction being more typical. Classically, the patient first loses his or her ability to blink and must flash the third eyelid to moisten the eye. The patient becomes sensitive to light and sound such that clapping one’s hands can actually create spasms or seizures. Dogs with ears that hang down may actually develop ears that stand up straight, and the facial muscles pull back in such a way as to create what is called the “risus sardonicus” or “sardonic smile.” In more advanced stages, the patient can no longer walk and stands stiffly in what is called a “sawhorse stance.” In an advanced state of disease, the patient cannot walk or even sit up and is at risk for aspiration of saliva or food contents. If the jaws are locked, feeding may be impossible and if the respiratory muscles come to be involved, the patient will not be able to effectively breathe.

In dogs, a localized tetanus is more common than the generalized (whole body form) and only the limb nearest the original wound becomes rigid or will tremor. This is an easier form to treat compared to the more generalized form; however, the localized form can spread and become the generalized form.

In the generalized form, initial signs seem to pertain to the eyes and can easily be mistaken for eye disease in the early stages.

classical human posture with tetanus
(note the rigidity of the entire body) 
(Photocredit: Public Domain image via Wikimedia Commons)

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 a horse’s regular vaccination set (just as it is part of a human’s vaccination set). Because dogs and cats are much more resistant to tetanus than are horses and humans, regular vaccination against tetanus is not recommended for them.

Nursing care requires a darkened room with minimal stimulation even if the pet no longer requires hospitalization. Clenched jaws can be problematic for feeding so expect a liquid diet or slurry to be needed. Soft bedding to prevent bedsores is a must. Improvement is generally noted within the first week of therapy but complete recovery can easily take a month.

Overall, of the 35 dogs included in this study, 77% survived. All the dogs that never progressed beyond severity Group II survived and 50% of the dogs in Groups III and IV survived. Dogs in these latter two Groups required continuous care (intravenous fluids, sedation, nutritional support etc.)

In 27% of cases, the initial wound was a foxtail tract and an additional 50% of wounds were suspicious of foxtail tract. A foxtail, or grass awn, is a sharp plant tuft which easily gets caught in fur and migrates deep into a wound, ending up in an assortment of inopportune areas of the body. An important conclusion of this study was that the wound need not be a bite or traumatic puncture.

Foxtail, a common plant in dry climates
(original graphic by marvistavet.com)