The incubation period of tetanus is variable ranging from few days to several weeks, but commonly 6–12 days. Duration of incubation period depends on: (a) distance of primary wound infection from the CNS, (b) the inoculating dosage of bacteria, (c) toxigenicity of bacteria, and finally (d) immune status of the host.
C. tetani causes tetanus, which can be of the following types:(a) generalized tetanus, (b) neonatal tetanus, (c) localized teta-nus, and (d) cephalic tetanus.
Generalized tetanus is the most common form of tetanus. It occurs when the toxin produced at the wound site spreads through the lymphatics and blood to multiple nerve terminals. This is because the blood–brain barrier prevents direct entry of toxin to the CNS. The extent of the trauma varies from minor injury to contaminated crush injury. The incubation period varies from 7 to 21 days and depends on the distance of the site of wound from the CNS. Trismus or lock jaw is due to involve-ment of masseter muscle and is the most common and early sign of the disease. Difficulty in swallowing, irritability, and restlessness are the other early signs.
As the condition progresses, patients have generalized muscle rigidity with intermittent reflex spasms in response to stimuli, such as noise or touch. Tonic contractions cause opisthotonus, a condition characterized by flexion and adduction of the arms, clenching of the fists, and extension of the lower extremities. During these episodes, patients have intact sensorium and feel severe pain. The spasms can cause fractures, tendon ruptures, and acute respiratory failure. Risus sardonicus or characteristic sardonic smile is a typical feature, which occurs due to contin-ued contraction of facial muscles. The condition may progress for 2 weeks even with administration of antitoxin because of the time needed for intra-axonal antitoxin transport.
Prognosis of tetanus is dependent on (a) incubation period, (b) time from spore inoculation to first symptom, and (c) time from first symptom to first tetanic spasm. Tetanus with short incubation period is graver than tetanus with long incubation period. Recovery is slow and usually takes as long as 2–4 months.
Neonatal tetanus is a generalized tetanus, resulting from infection of a neonate. It occurs primarily in underdeveloped countries and is a major cause of infant mortality. The infec-tion occurs due to the use of contaminated blade, knife, or other materials to cut or dress the umbilical cord in newborns, particularly of unimmunized mothers.
Usually at the end of the first week of life, infected infants become irritable, feed poorly, and develop rigidity with severe spasms. The condition has a very poor prognosis for survival. The mortality rate exceeds 70%.
Localized tetanus is an unusual form of tetanus. The disease is confined to the extremity with a contaminated wound and occurs when only the nerves supplying the affected muscle are involved. The condition is characterized by rigidity of mus-cles, caused by a dysfunction in the interneurons that inhibit the alpha-motor neurons of the affected muscles. No CNS involvement occurs in localized tetanus, and it has very low mortality rates.
Cephalic tetanus is a variation of localized tetanus. The con-dition usually occurs following head injury or occurs with infection of the middle ear. Incubation period is very short (1–2 days).
Symptoms may be localized or may become generalized and include isolated or combined dysfunction of the cranial motor nerves, most frequently the seventh cranial nerve. This particu-lar condition has a poor prognosis.