Pathogenesis and Immunity
F. tularensis is one of the most infectious
bacteria known. It isclassified as a category A agent because of (i) its high infectivity, (ii) ease of dissemination, and (iii) ability to cause substantial
illness and death. Humans become infected after introduction of the bacteria by
intradermal injection, inhalation, or oral ingestion. Clinical manifestations
of disease depend on the mode of infection.
Intradermal injection or inhalations of 10–50 bacilli are required
to cause disease. Nearly, 100 million bacteria are required to cause disease on
oral ingestion. F. tularensis is an
intracellular parasite. In infected animals, the bacteria are found in large
numbers in the reticuloendothelial cells of spleen, liver, etc. These bacteria
remain viable for a lon-ger period in macrophages of the reticuloendothelial
system, because the organisms inhibit phagosome–lysosome fusion. Like other
Gram-negative bacilli, F. tularensis
has endotoxin, but is relatively less biologically active than that found in Escherichiacoli and other Gram-negative
bacilli.
Capsulated strains of F.
tularensis are pathogenic, while non-capsulated strains are nonpathogenic.
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