dispersal of ticks and tick borne diseases by birds - Lista fuglestasjon

order of magnitude (3-10 million
imported nymphs per year). However,
these are estimates with high
uncertainties, and most of the
uncertainties are due to the estimates of
bird populations. For example, nobody
knows if the population of blackbird is
closer to 200,000 or 2,000,000.
Therefore, the uncertainty inherent in the
method of sampling ticks from birds,
with the risk that some of the ticks were
acquired locally, will have a small
impact on these calculations. Although
the import of tick nymphs can be
counted in millions, the number must be
small compared to the number of ticks
that actually exist in Norway. Therefore,
the import of ticks by birds would not
have any mentionable effect on the
population dynamics of an already
existing population. In a long-term
perspective, however, it could have an
impact on the population genetics in
places where migratory birds rest after
crossing the Skagerrak. Unfortunately,
the genetics tools that were developed
for this purpose (Paper I) could not
discriminate foreign from indigenous
tick genes. As an I. ricinus female lays
about 2000 eggs (Randolph 1998), the
multiplication potential is enormous. If
an imported tick brings genes that are in
some way advantageous for the ticks’
survival, transport by birds might
nevertheless have an impact on the gene
pool of the population.
From a quantitative point of view, other
factors will be much more important
than the birds’ import of ticks and tickborne
pathogens. The abundance of ticks
in nature varies naturally from year to
year, from site to site and with
vegetation type (Stafford et al., 1995;
Hubálek et al., 2003 b; Jouda et al.,
2003; Wielinga et al., 2006). Vegetation
type changes with changes in human use
of the nature, and over the longer term,
with climate. Cervid animals (moose, roe
deer and red deer) have had a vastly
increasing population in Norway over
the last 50 years (Andersen et al., 2010).
The distribution range of roe deer and
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red deer is still increasing, and mild winters
may cause even more roe deer to survive.
Apparently, the significance of cervid
animals has increased dramatically since
Thambs-Lyche made his great study on
bovine babesiosis in the 1930s. Thambs-
Lyche wrote: “roe deer hardly play any major
role in the spreading of ticks” and: “ticks
have never been found on red deer”
(Thambs-Lyche, 1943, pp 530 and 531).
Now, hunters regularly find large numbers of
ticks on these animals
(http://www.flattogflue.no/). Ostfeld et al.
(1998 and 2001) showed an interaction
between masting of oak trees and the number
of white-footed mice (Peromyscus leucopus),
the behaviour of white-tailed deer
(Odocoileus virginianus), who feed on
acorns when available, and gypsy moths
(Lymantria dispar). These moths feed on oak
leaves and influence masting, while
parasitoids regulate the population of gypsy
moths, together with the white-footed mouse,
which feed on the moths’ larvae and pupae.
They found that the number of acorns
influenced the number of mice, which in turn
influenced the number of I. scapularis ticks
(Ostfeld et al., 1998 and 2001). The indirect
effect acorns had on the tick population was
stronger than the effect of rainfall and
temperature found by Jones and Kitron
(2000). The incidence of acute human cases
of borreliosis is positively correlated with the
abundance of questing ticks (Stafford et al.,
1998). This illustrates that the epidemiology
of tick-borne diseases has to be viewed in a
wide ecological framework, where climate is
only one of the factors taken into account.
Distribution ranges seem to be largely
limited by vegetation and climate conditions,
except in strictly host specific ticks, like
Amblyomma rhinocerotis, which is limited
by the distribution of its rhinoceros hosts
(Cumming, 1999). Vegetation can be studied
with remote sensing (Randolph, 2000), using
satellite data through the Normalised
Difference Vegetation Index (NDVI), which
basically measures the absorption and
reflection of red light and near infrared light
to discriminate different vegetation types,
such as scrub and grassland, from forest.
These data are obtained through the
Advanced Very High Resolution Radiometer