Abstract
We studied the incidence of reported tularaemia by year and region and the prevalence of antibodies against Francisella tularensis in the adult general population in Finland. Moreover, we assessed the correlation between vole population cycles and human tularaemia outbreaks. The seroprevalence study made use of serum samples from a nationwide population-based health survey (Health 2000). The samples of 1,045 randomly selected persons, representative for the Finnish population in each region, were screened with an enzyme-linked immunosorbent assay (ELISA) for the presence of IgG antibodies against F. tularensis, and positive results were further confirmed by immunoblotting. A serological response to F. tularensis was found in 2% (95% confidence interval: 1.1–3.5) of the population. Incidence and seroprevalence were highest in the same areas, and vole population peaks clearly preceded tularaemia outbreaks one year later.
Highlights
Tularaemia is a zoonotic disease caused by the intracellular bacterium Francisella tularensis [1,2]
We report a study conducted in Finland, aiming to determine the incidence of tularaemia and the prevalence of F. tularensis antibodies in the population in 2000 and 2001, to compare the seroprevalence rates with the number of reported cases for the period 1995 to 2013, and to assess for the same period the role of vole population cycles in the temporal and spatial pattern of human tularaemia outbreaks
Diagnostic criteria for reporting include (i) isolation of F. tularensis in a clinical specimen, (ii) a more than four-fold rise in serum antibody titre or a single antibody titre of > 160 when using an agglutination assay or (iii) the presence of specific IgM and IgG antibodies in the serum when an enzyme-linked immunosorbent assay (ELISA) is used
Summary
Tularaemia is a zoonotic disease caused by the intracellular bacterium Francisella tularensis [1,2]. The disease is caused primarily by two of four subspecies: the highly virulent type A strain F. tularensis subsp. The less virulent type B strain F. tularensis subsp. Recurrent outbreaks with hundreds of cases are reported in Finland and Sweden [3,4]; in other European countries, the disease is rare, but many countries report sporadic outbreaks [5,6,7]. In Finland and Sweden, the disease is typically mosquito-transmitted and most cases are reported during August and September in connection with the occurrence of late summer mosquito species [3,4], whereas in Norway, the disease occurs in autumn and winter and the most common source is drinking water contaminated by rodents [11]. Outbreaks in Norway and Kosovo* have been linked to high rodent densities [11,12,13,14]
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