Humans are inoculated against tetanus, so why aren’t dogs?
The answer is that dogs are relatively resistant to tetanus toxins, and clinical illness is rare in dogs (and even rarer in cats).
Tetanus is caused by a clostridial infection, and small puncture wounds are the most likely source of infection. The clostridial spores take a few days to germinate deep within the tissue, where they release toxins that damage nerves and produce the typical signs of tetanus.
Some dogs are only mildly affected and make a full recovery. Others are more seriously ill and need treatment, including intensive nursing care, sometimes for weeks.
Symptoms
Tetanus also goes by the name “lockjaw” because of the muscle rigidity caused by this infection.
- The first hint of trouble is usually in the leg closest to the injury, where the muscles tense and the limb does not bend as it should. As the toxin spreads, this becomes widespread, with the dog standing in a stiff-legged, sawhorse stance.
- In the later stages, a dog with tetanus has what is described as a “rictus grin.” This unsettling expression is caused by muscle contractions pulling back his lips, and he looks as if he’s grinning in a human way. His forehead is deeply wrinkled and his ears are drawn back.
- Unfortunately, lockjaw also means the dog cannot move his mouth to swallow or eat, which means that in the absence of treatment he quickly becomes dehydrated.
By the time symptoms develop, the toxins are well advanced in the nervous system, and the dog cannot stand. Even small noises can trigger painful muscle contractions. Ultimately, death can occur because of paralysis of the respiratory muscles causing the dog to asphyxiate.
Causes
Tetanus is caused by the bacterium Clostridium tetani.
This bacterium is found in soil and thrives in conditions that lack oxygen. A dog becomes infected when he steps on a sharp object contaminated with C. tetani, and the bacterial spores are inoculated deep into the tissue. In this low-oxygen environment, the spores germinate and produce toxins that cause symptoms.
This toxin has an affinity for nerves supplying muscle and attacks them. It uses outlying nerves as a highway to gain access to the central nervous system, hence the patient’s health progressively worsening over time.
Some dogs are only mildly ill, whereas others are more seriously affected. In severe cases the nerves telling the dog to breathe fail to work, and unless he is artificially ventilated, he may suffocate and die.
Diagnosis
A definitive diagnosis of tetanus is hard to make because general blood tests show nonspecific changes, and antibodies to tetanus do not always show up.
Culture of samples taken from a wound is also tricky because C. tetani hates air exposure and often does not survive the journey to the lab, with subsequent false negative results. Diagnosis is made on the basis of symptoms and by eliminating other conditions that mimic tetanus, including:
Watch this Labradoodle’s amazing recovery from tetanus:
Treatment
In the early stages of tetanus in dogs, the provision of intravenous penicillin antibiotic can help kill C. tetani and stop spores from germinating to produce their deadly toxins.
It is also helpful to clean any wounds and expose as much of the damaged tissue as possible to the air, which C. tetani hates. Tetanus antitoxin is most effective when given before toxins become bound to the nerves, and so it needs to be given early. However, it is not risk-free — some dogs are allergic and may go into shock.
Keeping the dog in a quiet, dark environment helps reduce the stimuli that trigger convulsions, but even so, it is necessary sometimes to sedate the dog to prevent this unpleasant complication. Unfortunately, toxin already bound to nerves cannot be destroyed and must run its course.
Whether the dog survives depends on superb nursing care. This is intensive care medicine and means providing the dog’s fluids and nutrition by tube feeding, artificial ventilation and constant nursing care to avoid pressure sores and urine scald.
Prevention
Prompt cleaning of all wounds with dilute hydrogen peroxide kills C. tetani spores and helps prevent tetanus.
References
- “Thirteen cases of tetanus in dogs.” Adamantos & Boag. Vet Rec, 161: 298–303.
- “Tetanus in a dog.” Matthews & Forbes. 1985. Can Vet Journal, 26: 159–161.
This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed Nov. 2, 2018.
