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Diphtheria is caused by a bacterium, Corynebacterium diphtheriae, which typically infects mucous membranes: the nost and throat are favourite places for the infection to take hold, but mucous membranes of the eyes or genitalia can be infected also. The bacteria produce a toxin which causes damage to tissue both at the site of the original infection (the typical sign of diphtheria is a shaggy gray "membrane" on the back of the throat) and in other parts of the body once the toxin is spread via the bloodstream. The most serious effects of diphtheria toxin are on the heart (muscle damage leading to loss of pumping ability), kidneys, and the nervous system. Diphtheria can be treated by giving penicillin or other antibiotics to kill the bacteria, and antitoxin to clear free toxin in the body. However the antitoxin will not clear toxin that has already bound to cells and started to damage them. The better approach is to give toxoid to stimulate immunity to the toxin, thus enabling the body to clear toxin as soon as it appears. (Vaccinating against the bacteria itself is not possible as yet.) The toxoid is given initially at ages 2, 4, and 6 months, again at ages 18 months and 5 years, and regularly every 10 years after that. If this schedule seems familiar, it should: it's the same schedule as that for tetanus immunization, which is also done with a toxoid, and in fact the tetanus and diphtheria toxoids are routinely combined and given together. Thimerosal and Diphtheria Vaccines Several vaccines, including some of the available diphtheria vaccines, contain thimerosal, a preservative that is used in many vaccines and other medicines (including contact-lens storage and cleaning solutions) to prevent bacteria from growing in the solutions. Thimerosal is a mercury-based chemical. Although no one has shown that thimerosal in vaccines poses a danger of mercury poisoning in infants or children, there is a theoretical risk of such poisoning. Diphtheria infection is considered a far greater danger to infants than the (very remote) possibility of toxic effects from thimerosal. Therefore, most public health authorities, including the Centers for Disease Control and Prevention, recommend that infants receive diphtheria vaccine starting at age 2 months whether or not the vaccine contains thimerosal.
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