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Field voles Microtus agrestis as
reservoirs of Bartonella spp.
Huitu, O. , Aaltonen, K. , Henttonen, H. , Hirvelä-Koski, V. , Forbes, K.M. , Perez-Vera, C. ,
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Vapalahti, O. & Sironen, T.
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Background
Methods
Bartonella spp. are intracellular bacteria that cause chronic
infection of the erythrocytes of their mammalian hosts. Typical
reservoir hosts are cats, dogs, ungulates and small rodents,
between which infections are transmitted via fleas, ticks and other
blood-sucking arthropod vectors.
To evaluate the role of field voles as a reservoir
for Bartonella spp. in Finland, and how the risk
of its zoonotic transmission to humans varies
over space and time, we carried out molecular
screening of 679 field voles, trapped from 14
sites across the southern half of Finland over 3
years. For further details on methodology, please
consult Dr. Tarja Sironen.
In recent years, diagnostic advances have led to the identification
of isolated incidents of human Bartonella spp. infection in Finland.
However, as Bartonella spp. are known to colonize a number of
wild rodent species, and they have been detected in questing
ticks, it is plausible that these diagnosed cases are only a realised
fraction of human infections and thus an underrepresentation of
the true zoonotic potential of Bartonella species.
In this study, we aim to determine the role of field voles Microtus
agrestis, periodically the most numerous small rodent in Northern
Europe (Fig 1), as reservoir for Bartonella spp., and how
infection prevalence varies both spatially and temporally with the
population fluctuations of field voles.
Results
We identified four species: Bartonella taylorii, B.
doshiae, B. grahamii and a novel species of the
same genus. Bartonella spp. were found in field
voles throughout the entire study area, excluding
the two northeasternmost locations (Fig. 2). The
prevalence of Bartonella spp. ranged between
14–42 %, and was positively associated with vole
densities six months prior to infection (Fig. 3).
Bartonella spp. prevalence
1.0
0.8
0.6
0.4
0.2
0.0
0
Figure 1. Field vole Microtus agrestis.
Figure 2. Trapping sites in Finland. Circle size
denotes the total number of individuals tested
(N=3-102) for Bartonella spp. Black and white
colors refer to Bartonella spp. positive and negative
individuals, respectively.
10 20 30 40 50 60
Vole density 6 months earlier
70
Figure 3. Prevalence of Bartonella spp. in field vole populations
relative to vole population density ca. 6 months earlier. Each
symbol represents one vole population at one of six trapping
occasions.
Conclusions
Our findings demonstrate that Bartonella spp., also the
human pathogen B. grahamii, are highly prevalent in
natural populations of rodents which frequently come
into contact with humans especially during times of high
population densities. The fact that human cases are only
rarely reported suggests that Bartonella spp. infections in
humans are either underdiagnosed, and/or that Bartonella
spp. are emerging zoonotic pathogens in Northern
Europe.
Natural Resources Institute Finland, Natural Resources and Bioproduction; 2 University of Helsinki, Department of
Virology; 3 Finnish Food Safety Authority Evira
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luke.fi