Control

Strategies for control of typhoid are divided into three categories:

Elimination of the Reservoir

Identification of carriers in an endemic population is difficult, and eradication of the carrier state costly. This option appears impractical.

Interruption of Transmission Where pure water and food can be assured, typhoid transmission is minimal. Solely by improvement of sanitary conditions in the past century in developed countries, the incidence of typhoid fever has declined from 1 in 200 to 1 in 250,000. Mathematical models suggest that the building of privies in endemic areas would be among the least costly methods of reducing typhoid prevalence. Unfortunately, the pace of progress in making such sanitary improvements is slow.

Imm uniza tion

Exposure to the typhoid bacillus appears to confer some degree of protection against subsequent infection. Volunteer trials in the twentieth century verified this, and the extent of protection was quantified at 75 percent. However, the immunity was relative; it seemed to decay after a number of years, and could be overcome, at any stage, by the administration of a sufficient number of bacilli. Nevertheless, a relative immunity was better than none, and attempts to induce it artificially began almost as soon as the bacillus was isolated in the last decades of the nineteenth century. Since that time, most clinical vaccine trials indicate a protective effect of about 75 percent. At present, three major formulations exist:

Injection of Killed S. typhi Bacilli. This version contributed to the elimination of typhoid from the British and American armies during World War I, and, when administered to Thai schoolchildren in the 1970s, it was apparently instrumental in decreasing endemic typhoid fever. It is cheap and easy to produce, but there is a high rate of adverse reactions (fever, pain at the site of injection), as well as the need for refrigeration, sterile administration, and one to two boosters.

Injection ofVi Antigen. Vi Antigen (the protective polysaccharide envelope of S. typhi) provides, for persons in endemic areas, a degree of protection that is similar to that of the killed vaccine. The efficacy of this vaccine in Western travelers is not known. Only one dose is required, no refrigeration is needed, and no adverse effects have been noted. It requires sterile administration and is relatively expensive.

Oral Vaccine. This vaccine consists of a mutant strain of S. typhi incapable of causing typhoid fever. Studies in Egypt and Chile have documented its efficacy, with no adverse reactions observed. Refrigeration, but not sterile administration, is required. Disadvantages include the problems of storage and administration of a live vaccine, the need for at least three doses, and the higher cost.

Although control of typhoid fever is best accomplished with improvement of sanitary conditions, immunization with the more acceptable oral and polysaccharide vaccines may play an important public health role in developing countries.

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