Field investigations of tularemia in Norway.

In Norway, tularemia is a common disease in small rodent and hare populations, where large outbreaks can be observed. In humans, the yearly number of cases is low, usually less than ten, with peaks up to 44 recorded in recent years. Serological investigations on hunters and healthy school children nevertheless indicate, with up to 4.7% positivity in the latter group, that Francisella tularensis low-grade infection is widespread. F. tularensis in co-culture with amoebae, e.g. Achantamoeba castellanii, may grow after internalization and kill the amoeba. As with Legionella, Francisella virulence may be enhanced after protozoan ingestion. This suggests a mechanism that can explain the pattern of dissemination and infection in our region.