AETIOLOGY

Francisella tularensis, the aetiological agent of tularaemia, was first isolated and characterized in 1912 during an outbreak of a ‘plague-like’ disease in ground squirrels in Tulare County, California.

The Francisella genus is the sole member of the Fran- cisellaceae family. This genus comprises five species; F his- paniensis, F. noatunensis, F. piscicida, F. philomiragia and F tularensiι. F. tularensis is small (are not considered to have a major role in the maintenance of F. tularensis in nature; their high seropreva­lence indicates former exposure and a probable ability to survive infection¹-⁷). Birds are not considered to be impor­tant components in the ecology of tularaemia¹-⁷). Poten­tially, their most significant roles are in the transport of infected arthropod vectors to new areas, as indicators of tularaemia activity in their prey species, and by contamina­tion of surface waters through body discharges. It was found that a species of protozoan (Acanthamoeba castella- nii) might be an important aquatic-environment reservoir of F. tularensis. This organism can persist for a long time in watercourses, and at low temperatures in the terrestrial environment®.

TRANSMISSION

There are two known cycles of tularaemia: the terrestrial cycle and the aquatic cycle. In the terrestrial cycle, hares and rodents are the most important mammalian hosts, whereas haematophagous arthropods play a role as vectors. Hares and rodents can contaminate the environment through their body discharges. Different routes of trans­mission have been described in hares in Europe. Aeroge- nous infections may be frequent in European brown hares(⁶), whereas the alimentary route in mountain hares in Scandinavia is important in winter(⁸). Stress-related aggression could promote transmission, and cannibalism could be a route of transmission among voles, especially in populations at high density®. In the aquatic cycle, voles and possibly muskrats and beavers serve as the main host species, and could shed live bacteria into the environment. Carcasses of infected animals can further contaminate the water.