Gunnar Hasle*, Reidar Mehl**, Gunnar Bjune***, Hans Petter Leinaas****
*Reiseklinikken-Oslo Travel Clinic, St Olavs Plass 3, N-0165 Oslo, Norway
**National
Institute of Public Health, P.O. Box 4404 Nydalen, N-0403 Oslo, Norway
***Gunnar
Bjune, Institute for General Practice and Community Medicine, Faculty of
Medicine, University of Oslo, P.O Box 1130 Blindern, N-0318 Oslo, Norway.
****Hans Petter Leinaas, Dept. of Biology, University
of Oslo, P.O. Box 1050 Blindern, N-0316 Oslo, Norway.
Birds are
known host to nymphs and larvae of ixodid and argasid ticks. Migratory birds
are capable of transporting ticks over large distances and are therefore of
interest in the understanding of tick population dynamics, and spreading of
tick-borne pathogens. Hoogstraal et al. found in 1959-61 ticks in
1040 of 32086 examined birds (3,3%) in autumn migration in Egypt, and between
1956 and 1960 they found ticks in 128 of 786 examined birds (16,3%) in spring
migration in Egypt. Studies on ticks on
migratory birds are done in Norway, Sweden, Finland, Czech Republic,
France and Portugal.
In Norway Mehl et al. performed a study of the
spring migration in 1964 to 1976 on two of the bird
stations in this studyalong the
southern coastline, and they
found a mean tick infestation rate of 4,2%. One problem
in interpreting the results
is that the tick Ixodes ricinus is
abundant around the bird stations, and
may easily infest the birds after arrival. One purpose
of the
present study was to try to discriminate
between resident and alien ticks in order to evaluate the importance
of migratory birds in long distance transport of ticks.
The purpose of this study was to see if there has
been any change in the tick infestation rate on birds. Ticks may change their distribution range,
due to changes in climate and changes in vegetation and host abundance, which
may be results of changes in human activity. GMoreover, global
warming may result in a northward shift in the northern distribution limit of tick species as well as , and birds may introduce new species. Furthermore,
the ticks and the birds themselves may carry pathogens. Tick-borne
encephalitis, which is common in continental Europe, Sweden and Finland, has
newly emerged in Norway. In this perspective it is of great interest to
compare our results with the previous study.
We examined
2874 birds on four different bird ringing stations along the southern coast of
Norway and found ticks on 8,9% of them during the spring migration. Of these, 2165 birds were considered as newly arrived Ixodes ricinus
occurs on
the ground around
the bird stations, and may easily be picked up after arrival. We have therefore
selected the birds that suddenly
occurred in large numbers, and these and thus most
likely carrying ticks from abroad
were considered as newly arrived. Among the( 2165 birds which
were considered as newly arrived 6,1 %
carried ticks). Some of theseMost of these
birds had either had crossed the Skagerak or Kattegat
sea (at least 170 km) the night before, and someor have migrated along the Swedish
coast. Wind
trajectories were obtained to elucidate the probable migration route, but although we know the main migration routes,
the individual route cannot be determined. TFurthermore, the degree of blood
filling was also recorded, to estimate the time the ticks
may have been attached to the birds. Four of the 787 ticks we collected were Hyalomma marginatum nymphs. H. marginatum is common in
Africa and has its, a species with its northern limit in South
Europe.
T, the rest were Ixodes ricinus, which is common at the bird
stationsin coastal areas of southern and western Norway , and is an important vector of
several zoonoses. Ground living birds,
like thrushes, are most likely to carry ticks. In blackbirds examined the average infestation
increased from two ticks per birdxx in
April to
four ticks per bird there were two ticks, in May there were four ticks per bird in average.
Willow warblers have a low tick infestation rate (0,8% among newly arrived
individuals), but their abundance may anyway still make
them important as tick carriers.