Clostridium tetani is a gram-positive anaerobic, spore-forming bacillus. There are different strains of this bacteria worldwide, but the neurotoxin (tetanospasmin) that they produce is antigenically identical. Resistant spores are found in the environment - especially soil and the faeces of domesticated animals - including cats and dogs. When disease occurs spores have usually been introduced into a wound (often a deep puncture wound), they germinate locally and the toxin is absorbed into the body from the wound site where it is produced.
The toxin binds with nerve endings (at the presynaptic site) of inhibitory neurons and their effect is irreversible.
Clinical signs usually occur within 10 days of a wound being inflicted but can take 3 weeks.
Tetanus can be localised - and affect just one limb or region of the body , for example - or it can be generalised.
Early signs include over reaction to sounds, noises, movement and other stimuli (hyperaesthesia). This is followed by stiffness tetanic (rigid) paralysis and eventually tonic convulsions .
The muscle rigidity that the neurotoxin causes results in a stiff gait and often the tail is carried outstretched pointing backwards or curled up dorsally. The animal finds it difficult to stand or lie down. Ears are held pricked up, the third eyelid protrudes, the lips are drawn back and other facial muscles may go into spasm. The animal may have difficulty opening it's mouth because of involvement of the masticatory muscles ("lock-jaw"), and this causes difficulty eating (dysphagia).. There is increased salivation, increased heart rate and respiratory rate, and sometimes laryngeal spasm.
Megaoesophagus is a common complication.
Eventually the animal dies from respiratory arrest.
Diagnosis is made from the presenting history of a wound and the progressive clinical signs. Occasionally no wound can be found in which case it often will have been a small, deep puncture wound that has healed over on the surface of the skin.
Leucocytosis and neutrophilia may be present and increased muscle enzyme concentrations may be present in the blood (creatine kinase CK and aspartate aminotransferase AST).
Serum antibody titres to tetanus toxin can be measured to confirm exposure, but isolation of the organism C.tetani from wounds is difficult to achieve, and though direct smears with Gram-stain may show evidence of round endospores these are similar to spores produced by other bacteria.
Severely affected animals require long, intensive treatment which may not be rewarding as many will die despite the care and attention.
Initial treatment involves the administration of tetanus antitoxin intravenously to neutralize circulating toxin as soon as possible. There is a risk of anaphylactic shock developing when administered by this route, but it is much more rapid in having an effect. A test dose of 0.1ml of antitoxin should be given subcutaneously or intradermally and the site re-examined after 30 minutes. If a weal or urticarial rash has developed an anaphylactic episode should be anticipated. In any event a glucocorticoid, epinephrine (0.1ml diluted 1:10,000) given intravenously, and an antihistamine should be available on hand in case of such a reaction.
Penicillin G (20,000-100,000 Units/kg four times daily) or tetracycline (22mg/kg four times daily) are the antibiotics of choice and should be given intravenously, or intramuscularly in the case of Penicillin G to kill any C.tetani bacteria. Metronidazole (10mg/kg every 8 hours for dogs; 250mg total twice daily for cats) given by mouth for a total of 10 days may be more effective - but it is more toxic.
A combination of phenothiazine (chlorpromazine) and barbiturates can been used to control the spasms and convulsions.
Surgical debridement or excision of necrotic or abscess tissue may be needed, and flushing with hydrogen peroxide may be effective increasing oxygen tension to inhibit anaerobic activity.
Vaccination against tetanus is routinely advised for horses and people, but is not advised for cats and dogs.
Long term problems
Updated October 2013