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Andrew Pearson

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date: 26 May 2022

Tularaemia is a plague-like bacterial disease of animals (particularly rodents, hares, and rabbits) and man caused by five subspecies of Francisella. Two subspecies predominate: F. tularensis tularensis in North America and F. tularensis holarctica throughout the northern hemisphere. F. tularensis occurs in persistent natural foci causing localized epidemics and sporadic cases in man.

Francisella tularensis subspecies tularensis was described originally as causing a more virulent form of tularaemia than was seen in Europe. More recently recognized are subpopulations of Francisella tularensis subspecies tularensis which have markedly different virulence for man. These have been designated A1a, A1b and A2. Infections resulting from type A1b have been shown to have an attributable mortality of 24% as compared to 4% for tularaemia caused by A1a types.

F. tularensis is one of the most potent bacterial pathogens affecting humans with an infective dose from 1 to 10 organisms. The incubation period is usually 3–5 days (range from 1–21 days). Onset of disease is abrupt, with fever, chills, fatigue, general body aches, and headache. When the bacteria are acquired through skin or mucous membranes, tender regional node enlargement may become conspicuous. When bacteria are inhaled, the infection will result in deep lymph node enlargement.

The clinical epidemiology of human infection is complex since it relates to one of four modes of transmission of the agent harboured in multiple hosts from diverse ecosystems. Clinical presentation of the human disease is indicative of both the mode of transmission and often the source of infection in a specific ecosystem. Tularaemia presenting as ulceroglandular disease results from either vector-borne infection from mosquito or tick bites or occurs as a result of animal contact from bites, hunting or from skinning hares or muskrats. Oropharyhgeal and typhoidal infections predominate in waterborne outbreaks of F. tularensis holarctica. Pulmonary or influenza disease results from airborne transmission associated with either farmers moving rodent contaminated hay or laboratory acquired infection. An intentional aerosol release of F. tularensis tularensis would be expected to result in clinical manifestations similar to those recognized in natural respiratory tularaemia. Both vector-borne and airborne transmission of F. tularensis may both be associated with florid skin manifestations as a presenting symptom of tularaemia. Pulmonary or typhoidal forms of the tularaemia may occur as a complication of localized infection.

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