Page 200 - TBE_Book_V2_2019

Chapter 11: General epidemiology of TBE

By these classifications, the European TBE foci to the actual importance of any of these
are located in the Central European– mechanisms in the field.
Mediterranean TBE focus region according to
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Korenberg et al. The classification developed A number of models on natural foci of TBEV
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by Rosicky indicates the European TBE foci are now available, but fieldwork is missing. In
are mainly of the theriodic type, while Eastern the early 1960s Austrian researchers were
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European countries have the mixed type or studying TBE foci in Austria. According to the
rarely also the boskematic type. Overall, these authors’ data and estimates, focus size was
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classifications may be helpful in getting an 60,000 m with an estimated 2 million larvae
impression of the focus type in the landscape, and about 500,000 nymphs in the focus. They
but they are not very helpful for describing a estimated that between 500 and 1500 nymphs
TBE natural focus in detail. Also, so far, no (0.1% to 0.3%) are infected at any time in the
clear associations have been identified year and may infect 15 to 30 rodents out of
between genetic profiles or phenotypic char- an estimated total number of 700 rodents in
acteristics of TBEV strains and their respective the focus. They found a total of 4 small
focus types. mammal species with a clear dominance of
Apodemus spp. (Apodemus flavi- collis >

Apodemus sylvaticus > Myodes glareolus >
Microtus agrestis). The focus was highly
The natural cycle
fragmented into old forests, young forests,
and meadows that existed within the forests.
As described above, a natural focus is an area
where the ecological conditions allow the Nosek et al. described the structure of TBE
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presence and transmission of a pathogen. In natural foci in the Czech Republic. Their work
the case of TBEV, a natural focus is an area showed that a focus is maintained by a
where TBEV is circulating among its vectors number of so-called microfoci. The size of the
(ticks of different species and genera) and natural focus is not given. The authors
vertebrates (usually rodents and small mam- estimate that per 10,000 m (1 ha) the number
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mals, which support the transmission of the of ticks ranges from 15,000 to 50,000 nymphs.
TBEV). Details of these transmission cycles and A microfocus is defined as a structure in the
the animal species involved are described in focus area where virus transmission is contin-
Chapter 3. However, at the moment it is not uously active and therefore the virus can be
clear which ecological structures and require- generally detected. The rate of positive ticks in
ments are needed to establish and maintain a the microfocus is approximately 0.5% to 1% in
TBE natural focus. A sufficient number of ticks nymphs and up to 5% in adult ticks.
that are infected or might be susceptible to
infection must be present. Also, a sufficient In a recent study over 4 years in a TBE focus in
number of susceptible small mammals to Hungary, the authors reported that an area of
support virus transmission is required. There 36 ha (3,600,000 m ) was screened and that
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must also be an adequate number of larger only in an area of 0.49 ha (4900 m ) sero-
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animals to support the developmental cycle of positive rodents were detected. They found
the nymphs and adult stages of the tick TBEV in a total of 3 tick pools (2 pools of
vectors, as these rarely found on rodents. The Ixodes ricinus and 1 pool of Haemaphysalis
virus itself is transmitted via viremic concinna) out of 7247 sampled ticks (0.05%).
vertebrates or via co-feeding of TBEV-infected Of note, in an area around 170 m away from
ticks together with non-infected ticks, with the the focus but in the same natural focus area,
latter transmission mechanism being more no TBEV was detected among 2369 sampled
effective. However, so far, no proof exists as

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