Infection by the bacterium Clostridium tetani causes a person to have lockjaw as part of the disease tetanus, says Mayo Clinic. This bacteria is found in feces, soil and dust and enters the body through injuries such as burns, crushing injuries, puncture wounds, ear infections and infected foot ulcers. It can also enter the body through the infected umbilical stump of a newborn baby if the mother is not immunized.
Tetanus is rare in the United States because of widespread vaccination, explains Mayo Clinic. The bacteria that cause the disease release a toxin that affects the nervous system and causes powerful involuntary muscle contractions. The first symptom is usually spasms and stiffness in the jaw muscles, which is why the disease is often called lockjaw. Stiffness in the neck muscles follows, then difficulty swallowing, tightness in the abdominal muscles and finally painful body spasms. These spasms can last several minutes. Triggers for the spasms include light touches, loud noises or bright lights.
Doctors can administer medications to neutralize the toxins that cause lockjaw, but this is only effective before the toxin bonds to the nerve endings, says Mayo Clinic. Once the toxin affects the nerve endings recovery requires that they regrow, a process that can take months.