Tetanus infection is rarely a problem in dogs. A bacterium called Clostridium tetani is responsible for this disease. Or rather, the toxin produced by the bacteria. Presence of bacteria alone will not cause clinical signs of disease. C. tetani bacteria produce spores that persist in the environment where they are found in dirt and debris. The spores are resistant to boiling and to many disinfectants. Clinical tetanus develops when C. tetani spores enter wounds where anaerobic conditions favor germination of the spores and promote toxin production. Deep wounds including puncture wounds and nail bed wounds typically seal over quickly creating a low oxygen (anaerobic) environment where the spores can germinate. Patients only develop clinical disease if the toxin is developed and binds to the neurons. Patients may develop localized or generalized tetanus. Of the domesticated animal species, horses are the most sensitive to the neurotoxin. Humans are also very susceptible, while dogs and cats are more resistant to effects of the toxin. This is why dogs do not require tetanus vaccinations like we do.
Clinical signs of disease usually develop 5-21 days after a wound is infected with the bacteria. Signs may affect only one area of the body, such as increased stiffness in a group of muscles or an entire limb. Mildly affected dogs are still ambulatory, but show a stiff gait and have difficulty in standing or lying down in comfortable positions. This stiffness may progress to the point that the limbs are held rigidly extended. Gait changes may vary due to the patient’s degree of excitement. Signs may gradually worsen until they involve the entire nervous system (generalized tetanus).
The classic signs of tetanus include facial muscle spasms and clenching of the jaw to the point that patients cannot eat. This is where tetanus gets the name of “lockjaw.” Other symptoms of generalized tetanus include altered facial expression, protrusion of the third eyelids, increased salivation, difficulty swallowing, extreme muscle stiffness of the legs, elevated body temperature, and altered heart and respiratory rates. The legs are often held straight and are difficult to bend. Extreme rigidity (stiffness) of the legs may give the animal a saw-horse stance, or cause it to fall over and be unable to walk. Spasms of the muscles of the voice box (larynx), chest, and diaphragm may make breathing difficult. Animals may appear to have increased sensitivity to sound and touch. Facial muscle spasms may cause wrinkling of the forehead and retraction of the lips, as if the animal is smiling or grimacing. This facial appearance, known as risus sardonicus, is a classic symptom. Contraction of the masticatory muscles precludes eating. These patients may also have laryngeal spasms, hypersalivation, trouble eating, and difficulty breathing. The chances of surviving infection decreases with the severity of disease.
Neuronal uptake and action of the neurotoxin is irreversible. Thus, recovery requires the outgrowth of new nerve terminals, which explains the long duration of the disease. The prognosis is usually favorable with localized tetanus if progression to generalized tetanus is prevented. Localized rigidity takes about 4 to 8 weeks to resolve.
An injectable antitoxin may be used to neutralize toxin that is not bound to nerves. However, dogs treated with antitoxin are at risk of developing “serum sickness” which is an immune reaction to the equine proteins in the medication. By the time clinical signs are seen, toxin has already bound irreversibly to the nerve-muscle junctions. Because the toxin cannot be removed and its effect cannot be reversed, supportive care must be provided until new nerve-muscle junctions can be generated.
Mildly affected animals may recover with treatment involving wound management and antibiotics that kill Clostridium species. Wounds should be thoroughly cleaned, and debrided of dead tissue to decrease reproduction of the bacteria and production of tetanospasmin toxin. Higher risk of death in cases of generalized tetanus is most due to respiratory compromise, cardiovascular dysfunction, or uncontrollable muscle spasms. Severely affected animals require intensive supportive care and prolonged hospitalization. Treatment typically includes IV fluids, feeding tubes for nutrition, and assisted ventilation for severely affected patients. Animals may be unable to sit up or walk and will require excellent nursing care. Sedatives and muscle relaxants are used to combat the spasms and keep patients comfortable.
Prognosis for recovery from tetanus is variable, depending on the severity of clinical signs at the time treatment is started. Animals with localized tetanus have a better prognosis than those with generalized tetanus. Recovery from generalized tetanus can take weeks to months. Patients with mild disease who receive adequate care will typically recover within 4 weeks.