Chapter 71 - Salmonella Typhi and Salmonella Paratyphi A

Abstract

Salmonella Typhi and Salmonella Paratyphi A, collectively known as typhoidal Salmonella, are causal agents for a serious, invasive (bacteraemic), sometimes fatal disease of humans called typhoid fever or paratyphoid fever (also called enteric fevers). Salmonella Typhi, the lineage causing typhoid fever, is the main group; while Salmonella Paratyphi A, the lineage causing paratyphoid fever, belongs to the second group which comprises a set of three paratyphoid types (the other two being S. Paratyphi C and d-tartrate-negative S. Paratyphi B). All of these lineages are adapted to humans, with S. Typhi and S. Paratyphi A being strictly restricted to growth in humans, and S. Paratyphi C being able to establish infections in experimental animals quite easily (at moderate infection doses); the host-restriction status of d-tartrate-negative S. Paratyphi B is so far unclear. Representing an update on the version published in the first edition of this book that very thoroughly summarized the knowledge available at that time, most of the emphasis in this chapter is on typhoid fever due to S. Typhi in relation to its taxonomy, genomics and genetics, diagnosis, association with disease, mechanisms of invasion and pathogenesis, and antibiotic and vaccine strategies to minimize its impact. The impact of typhoidal Salmonella on human hosts is indeed very large. In 2000, typhoid fever caused over 20 million illnesses and more than 200000 deaths, whereas paratyphoid fever caused an estimated 5.4 million illnesses worldwide. The greatest burden of illness was suffered by infants, children and adolescents in south-central and south-eastern Asia. Typhoid and paratyphoid fever usually present as clinically similar acute febrile illnesses; accurate diagnosis relies on confirmation by laboratory tests. Paratyphoid fever is usually the result of infection by S. Paratyphi A, and recent reports show an increasing incidence of S. Paratyphi A causing enteric fever in developing counties in Asia and show that the earlier notion that paratyphoid fever is less serious than typhoid fever is not correct. The organism must be cultured and identified to make a clear diagnosis; clinical symptoms alone are not adequate. Culture from blood is less sensitive than bone marrow culture and often gives negative results even when bone marrow cultures are positive, but it is usually the practical first choice for patient diagnosis and for epidemiologic studies of the burden of typhoid and paratyphoid fever. Because of a lack of good diagnostic tools, and because the sites of endemic disease are often deficient in clinical and laboratory facilities, the extent of the burden of enteric fever is often poorly characterized in much of the world, especially in sub-Saharan Africa.