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Francisela tularensis subsp. holarctica is the almost exclusive causative agent of tularemia in humans throughout Europe. Although previously considered as relatively rare, recent reports indicate increasing emergence or re-emergence of this disease across Europe. F. tularensis may infect a wide range of hosts including invertebrates, amphibians, birds and mammals but main hosts are considered to be lagomorphs and rodents. However, as these animals are highly susceptible to infection and most infected animals dies rapidly, additional ways of transmission (vectors, fomites) are required for the bacterium to persist over time. Thus, role of arthropods in ecology of tularemia is important, but not fully elucidated. Ticks are considered to be main vectors and likely reservoirs for F. tularensis in most parts of Europe. On the other hand, in Sweden, a country with one of the highest incidence rates of tularemia, mosquito bites are listed as the most frequent way of transmission. As it is case with many other zoonotic agents, F. tularensis is adapted to different physical and biological factors and confined to certain longstanding natural foci. Sporadic (endemic) transmission to domestic animals or humans occurs when such natural foci are disturbed, while larger outbreaks of the disease are result of spillover during massive but time-limited amplification in those areas. Determination of direct and indirect causes of these "massive amplification" is largely complicated by several factors ; i. wide range of hosts ii. different ways of transmission iii. persistence of F. tularensis in aquatic and possible also nonaquatic environments iv. existence of micro-natural foci, each with a predominant variant and mode of transmission. Francisel/a tularensis subsp. holarctica is the almost exclusive causative agent of tularemia in humans throughout Europe. Although previously considered as relatively rare, recent reports indicate increasing emergence or re-emergence of this disease across Europe. F. tularensis may infect a wide range of hosts including invertebrates, amphibians, birds and mammals but main hosts are considered to be lagomorphs and rodents. However, as these animals are highly susceptible to infection and most infected animals dies rapidly, additional ways of transmission (vectors, fomites) are required for the bacterium to persist over time. Thus, role of arthropods in ecology of tularemia is important, but not fully elucidated. Ticks are considered to be main vectors and likely reservoirs for F. tularensis in most parts of Europe. On the other hand, in Sweden, a country with one of the highest incidence rates of tularemia, mosquito bites are listed as the most frequent way of transmission. As it is case with many other zoonotic agents, F tularensis is adapted to different physical and biological factors and confined to certain longstanding natural foci. Sporadic (endemic) transmission to domestic animals or humans occurs when such natural foci are disturbed, while larger outbreaks of the disease are result of spillover during massive but time-limited amplification in those areas. Determination of direct and indirect causes of these "massive amplification" is largely complicated by several factors ; i. wide range of hosts ii. different ways of transmission iii. persistence of F. tularensis in aquatic and possible also nonaquatic environments iv. existence of micro-natural foci, each with a predominant variant and mode of transmission. Due to our geo-ecological and wildlife diversity, all the above factors with an emphasis on the latter should be taken into account when considering the F. tularensis ecology in Croatia. Most of the Croatian research on that subject date from second half of the 20th century when lowland floodplain along the central course of the Sava River and two smaller areas along the Drava riverbanks (Medimurje and the wider area of Koprivnica) were pinpointed as endemic regions. Direct contact with hares during hunting, skinning and food preparation was identified as the main mode of infection. The seasonality of the disease, which has been found to peak during the winter months, also coincided with the hare- hunting season. Nevertheless, the researchers at the time were convinced that small rodents also play a major role in the ecology of F. tularensis, and they were right. Since people generally avoid direct contact with these animals, the transmission route is indirect and epidemiological link is not always clear. There are a number of very different descriptions of outbreaks within Europe that were finally linked to small rodents. Outbreak in Petrinja in the post-war period was associated with food contamination due to the increase in the population of small rodents and poor sanitary conditions. |
