Salmonella typhi Infection Severity (Typhoid Fever)

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Human Uniqueness Compared to "Great Apes": 
Likely Difference
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Typhoid fever caused by Salmonella Typhi infection can occur in endemic or epidemic forms in human populations, following oral ingestion of these bacteria. When chimpanzees were experimentally given this organism, they required larger doses to obtain an infection and had a reduced severity of the symptoms and pathological responses. The disease in adult chimpanzees was said to closely resemble the mild form of human typhoid fever frequently seen in human children. The complicated and sometimes lethal form of the disease seen in human adults was not seen in the chimpanzee. The reason for this difference is unclear, but it may be that an over-reactive human immune response is responsible for some of the most severe symptoms.



Timing of appearance of the difference in the Hominin Lineage as a defined date or a lineage separation event. The point in time associated with lineage separation events may change in the future as the scientific community agrees upon better time estimates. Lineage separation events are defined in 2017 as:

  • the Last Common Ancestor (LCA) of humans and old world monkeys was 25,000 - 30,000 thousand (25 - 30 million) years ago
  • the Last Common Ancestor (LCA) of humans and chimpanzees was 6,000 - 8,000 thousand (6 - 8 million) years ago
  • the emergence of the genus Homo was 2,000 thousand (2 million) years ago
  • the Last Common Ancestor (LCA) of humans and neanderthals was 500 thousand years ago
  • the common ancestor of modern humans was 100 - 300 thousand years ago

Possible Appearance: 
6,000 thousand years ago
Probable Appearance: 
2,000 thousand years ago
Definite Appearance: 
100 thousand years ago
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  1. Studies on infection and immunity in experimental typhoid fever. I. Typhoid fever in chimpanzees orally infected with Salmonella typhosa., EDSALL, G, GAINES S, LANDY M, TIGERTT W D., SPRINZ H, TRAPANI R J., MANDEL A D., and BENENSON A S. , J Exp Med, 07/1960, Volume 112, p.143-66, (1960)