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Epidemiology and Pathology

Typhoid and all other enteric fevers are primarily waterborne diseases, wherein the organisms are transmitted by the excreta of one infected individual to another, usually via polluted drinking water. Flies and contaminated shellfish may also be intermediaries, as can dairy products (including duck eggs), fruits, vegetables and other foods which have been contaminated by carriers of the organisms. Human to human transmission also occurs.

The principal factor in the spread of enteric fever is the individual passing bacilli in feces or urine, or both. This person may be in the acute or convalescent stage of the disease or may be a carrier.

Three types of enteric carriers are recognized:

1. The acute carrier who passes bacilli in his excreta for a short time after an attack of enteric fever.

2. The chronic carrier who deposits bacilli in his excreta for many years. The gallbladder is often the focus of a chronic infection and these patients may develop gallstones and cholecystitis.

3. The passive carrier who passes bacilli in his feces without ever having had clinical symptoms of enteric fever.

Of all the possible modes of infection a contaminated water supply is most common. The explosive onset of epidemics in military or civilian populations can almost always be traced to water contaminated by sewage. The breakdown of sanitation and public health measures which may occur with mass movements of troops or populations during wars or natural disasters, such as floods and earthquakes, favors the outbreak of typhoid epidemics. Throughout history the enteric fevers have plagued armies in the field, even as recently as World War I. Beginning with that war and in more recent conflicts and natural disasters, most populations have been protected by antityphoid inoculation with a dramatic decrease in the number of epidemics.

In tropical countries where the disease is still endemic, sporadic infections are seen throughout the year, but outbreaks are more common towards the end of the hot dry season, when rivers are low and heavily polluted, especially in East and West Africa. Spread then occurs as the waters rise with the rains. Local areas of hyperendemicity may occur in a region. Fecal carriers are the most common source of infection, but chronic urinary carriers are also important, especially in areas with a high incidence of schistosomiasis haematobium and other urinary infections. Recurrent infection is occasionally associated with clonorchiasis, and Salmonella septicemia has been known to occur in patients with schistosomiasis mansoni.

The causative bacterium of typhoid fever, Salmonella typhi (S. typhosa or Eberth bacillus) is a gram negative motile rod 2-4 micra long with 3 antigenic components: the somatic, flagellar and Vi antigens. In common with many intestinal pathogens it does not ferment lactose and grows well in media containing bile salts. The paratyphoid A, B and C bacilli resemble S. typhi in their general morphological characteristics and staining reactions, but differ in their biochemistry and in their antigens.

After ingestion the bacilli multiply in the second part of the duodenum and later in the lymphoid tissue of the Peyer's patches and mesenteric lymph nodes; they eventually enter the blood stream, probably via the thoracic duct, with general dissemination, especially to the liver, spleen and other organs of the reticuloendothelial system. The incubation and severity of typhoid fever depend on the number of organisms ingested and their virulence, the degree of gastric acidity, the motility of the bowel, and the general health of the individual.

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Copyright: Palmer and Reeder

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