Details about the Organism

The discovery of tularemia is attributed to McCoy who reported a plague-like illness in ground squirrels from Tulare county, California, in 1911. The disease is caused by the Gram-negative pleomorphic bacterium, Francisella tularensis. There are two subspecies, type A, (F. tularensis subsp. tularensis) and type B (F. tularensis subsp. holarctica) that cause disease in man and have been considered as potential biological agents. The SCHU S-4 strain of type A is one of the most infectious pathogenic agents known, requiring inoculation or inhalation of fewer than ten organisms to cause disease.

Humans become naturally infected through diverse environmental and animal exposures but there has been no documented person-to-person transmission. Six forms of tularemia are recognised:

• Pneumonic - less than 5% of cases
• Septicaemic - less than 5% of cases
• Typhoidal - up to 25% of cases

Deliberate release of F. tularensis

F. tularensis does not occur naturally in the UK and the very few numbers of cases seen have all been acquired abroad.

The World Health Organisation (WHO) and Centres for Disease Control and Prevention (CDC) in the USA anticipate that the greatest impact in terms of mortality and morbidity following intentional release of F. tularensis would be achieved through aerosolisation of a virulent strain, making inhalation into the lungs the most likely route of infection. Monkeys that inhaled the SCHU S-4 strain developed acute bronchiolitis within 24 hours of exposure to 1-micrometre particles and within 48 hours of exposure to 8-micrometre particles. Bronchopneumonia was most pronounced in animals exposed to the smaller particles.

Epidemiology

F. tularensis infects more than 100 species of wild mammals, birds and insects worldwide. A variety of small mammals, including voles, mice, water rats, squirrels, rabbits and hares, are natural reservoirs of infection. They acquire infection through bites by ticks, flies and mosquitoes, and by contact with contaminated environments. Although enzootic cycles of F. tularensis typically occur without notice, epizootics with extensive 'die-offs' of animal hosts may herald outbreaks of tularemia in humans. The ecosystems depend on the subspecies and locality.

Transmission

Infection with type B strains occurs across northern Europe (including Scandanavia), Russia and Japan and large outbreaks have occurred. Disease due to type B F. tularensis, by comparison with type A infection, is relatively mild, with negligible mortality. Naturally occurring infection with type A is more sporadic and often severe. It is restricted to defined geographical foci in North America, where it accounts for 90% of reported tularemia. People of all ages and both sexes appear to be equally susceptible to tularemia. Person-to-person transmission has not been documented.

Naturally acquired human infection occurs through a variety of mechanisms:

• Bites of infected arthropods including Dermacentor and Ixodes ticks (summer months)
• Contact with infected animals, including cats
• Handling infectious animal tissues or fluids
• Direct contact with or ingestion of contaminated water, food or soil
• Inhalation of infective aerosols e.g. from handling damp hay

Infectious dose

The infectious dose is very low and depends upon the portal of entry and type of F. tularensis. Approximately 10-50 type A organisms can initiate infection by the inhalation route. If SHU S-4 strain is used less than 10 organisms would be required.

Incubation period

Symptoms usually appear between 2-5 days (range: 1-21 days) after exposure.

Period of communicability

Human-to-human spread has not been reported following casual contact. Handling of infectious secretions or tissues may pose a risk to health care workers due to the low infectious dose.

Organism Survival

The survival of F. tularensis in aerosols is short and infective doses are not likely to persist in air for more than a few hours. Survival in non-chlorinated water can occur for up to 90 days. Studies suggest that F. tularensis can persist in the environment perhaps by surviving within protozoa such as Acanthamoeba.

Last reviewed: 9 November 2009