Diseases in sheep in Ixodes ricinus infested pastures
by
Snorre Stuen
Department
of Sheep and Goat Research
In
Norway, the sheep (or castor bean) tick, Ixodes
ricinus, is found along the coast from Halden in the south to the central
parts of Helgeland. Lately however, it has been found in several places in the
central parts of the country and in Northern Norway outside its normal area of
distribution. The sheep tick can be moved to many ”new” places with birds
and other animals, but it is still to be seen, whether it will survive in the
new areas. The sheep tick has many local names like ”flått”, ”hantikk”,
”påte”, ”skaumann”, ”skaubjønn”, ”sugar”, ”einerlus” and
”lyng-bobb”.
The tick
sucks blood from host animals, but the direct damage done by the blood sucking
is relatively small in this country, even though some lambs can loose quite a
lot of blood and become anaemic, if they are exposed to strong tick infestation.
The
sheep tick, however, transfers several microorganisms that may course diseases
in small livestock, like Lyme disease, louping-ill, pyemia, pasturellosis and
tick-borne fever (Table 1). The most common disease is tick-borne fever, and I
will therefore concentrate on this disease in the present article.
Table
1. Microorganisms that can be transferred by the sheep tick, Ixodes
ricinus, in Norway
Microorganism |
Disease | Species | Occurrence |
Babesia divergens |
Babesiosis | Cattle, human | In cattle, scattered occurrence along the coast |
Borrelia
burgdorferi s.l. |
Borreliosis (Lyme Disease) | Human,
dog, horse, sheep, cattle |
Not
uncommon in humans in tick areas |
Ehrlichia phagocytophila |
Granulocytic
ehrlichiosis,tick-borne
fever |
Sheep,
goats, cattle, horse, dog, cat, human, deer, small rodents |
Very
common in sheep |
Francisella tularensis | Tularaemia | Hare,
sheep, human |
Not
detected in sheep in Norway |
Louping-ill
virus |
Louping-ill |
Sheep, goat, cattle,horse, dog, hare, grouse, human |
Detected
in Vest-Agder and Sunnhordland |
Staphylococcus aureus |
Pyemia | Lamb
(calf) |
Common in lambs in the spring |
Tick-borne fever
Tick-borne
fever (Granulocytic ehrlichiosis) is no new disease in sheep in Norway - it was
mentioned already in the 18th century in Hardanger. The disease was
detected in sheep in the country in 1940, in large livestock in 1962 and in
goats in 1979. Tick-borne fever is caused by a bacteria (rickettsia) under the
name of Ehrlichia phagocytophila.
Infected animals will get a high temperature (up to > 42°C) that will last
from a few days up to 10-12 days. The incubation period is generally from 5 to
13 days. Young lambs (< 2 weeks old) are less troubled (less fever, shorter
period). In the acute phase, organisms will be found in the white blood cells.
This is simple to examine by making a blood picture, as up to 90% of the
neutrophil blood cells can be infected. Another characteristic feature in the
blood picture is the marked fall in the number of neutrophils beginning by the
end of the fever period and it can last for 1-2 weeks. The neutrophil cells is
the body’s ”first line defence” against infections, and lambs infected
with tick-borne fever will among other things therefore suffer from a reduced
ability to resist other infections. Pregnant sows may miscarry, and rams bought
from tick free areas and put in tick infested pastures may become momentarily
sterile and thereby useless in the mating season for which they are bought.
During the fever period, the lambs show reduced growth, and investigations
suggest that they won’t regain this weight reduction fully later on.
Table 2. Occurrence and
clinical cases of tick-borne fever (Granulocytic ehrlichioses) in the Nordic
countries
|
Cattle |
Sheep |
Horse |
Dog |
Cat |
Game |
Human |
Denmark |
+ |
+ |
+ |
+* |
+* |
|
+* |
Finland |
+ |
+ |
|
|
|
|
+* |
Norway |
+ |
+ |
+ |
+* |
|
+** |
+ |
Sweden |
+ |
+ |
+ |
+ |
+ |
|
+ |
+
detected
* only
serological studies **
elk (from Telemark) and roe deer (from Rogaland)
Occurrence
Tick-borne
fever occurs along the coast from Vestfold to Sør-Trøndelag up to the
Trondheimsfjord. Most cases are detected in Rogaland and Hordaland (fig. 1). The
disease occurs usually in spring, summer and fall, but can in rare cases occur
in the winter. In a study comprising 506 cases of tick-borne fever in sheep,
most of the cases were diagnosed from the middle of May to the middle of June
(49 %) and from September to October (23 %). The diagnose is performed by
detection of the organism in coloured blood pictures or by detection of the DNA
of the bacteria (the PCR method). In performing an autopsy an enlarged spleen (up
to 2-4 times normal size) will be typical for an animal that is infected with
tick-borne fever (Picture 1).
Picture
1. The spleen from 4 weeks old lambs. The spleen on the left side is from a
tick-borne fever infested lamb (can
become up to 2-4 times larger than normal), while the one on the right side is
of a normal size.
|
Bilde 1. Milten fra to 4 ukers gamle lam. Milten til venstre er fra et sjodogg-infisert lam (kan bli 2-4 ganger større enn normalt), mens den til høyre har normal størrelse |
Losses
in tick infested pastures
Tick-borne
fever can in certain areas cause big losses by grazing both in spring and fall.
Studies have shown that the loss of lambs in tick infested pastures is clearly
bigger than in pastures without ticks. There
are tick infested pastures where it has been inadvisable to keep sheep due to
too big losses. The losses are biggest in lambs or animals that graze on tick
infested pastures for the first time.
Figure
1. Map over places where tick-borne fever is diagnosed in large livestock, sheep
and goat from 1948-1994
Studies
have shown that tick-borne fever is very widespread in tick infested pastures in
South Norway and that the disease is underdiagnosed in sheep flocks in the
country. Studies suggest that there are several strains of the tick-borne fever
bacteria in our sheep also within the same flock. There can even be several
types in particular animals. We have shown that particular strains of the
bacteria won’t cause fever in lambs, but it is not clear how ordinary these
strains are. Furthermore, it is not clear whether the different strains can
cause different clinical symptoms and thereby explain the differences in disease
problems that seem to exist between different tick infested pastures. Research
in this field is currently being carried out in our institution.
It takes
very little infectious matter to develop the disease. It seems as if one single
infected neutrophile blood cell is enough to transfer the disease and that both
the fever and the immune debilitation is not dependent on the doses. As the
infection goes on for several months in infected animals, use of the same
syringe needle, for instance in connection with vaccination, can transfer Ehrlichia
from one animal to the other. The infection can go on in a sheep flock
throughout the winter from one grazing season to the next. This means that if
one moves / buys animals that have been on tick infested pastures, one can get
the tick-borne fever infection along therewith. We have also shown that even
very young lambs will be infected on a long term basis with the tick-borne fever
bacteria and can for instance carry the infection from spring to fall pastures.
Studies also suggest that the infection will last in dogs and deer, while it
doesn’t seem to last in human beings and horses.
Presently,
there are no available vaccine against this disease. Control of the tick-borne
fever and the following harmful effects therefore has to take place by
prevention (Table 3).
Table 3. Measures for
reducing lamb losses as a result of tick-borne fever
*
Have the lambs infected as early as possible * Etamine the lambs on a daily basis in the first months on tick infested pastures * Treat the sick lambs and keep them under observation for some weeks * Use tick controlling substances on a regular basis, every 3rd to 4th week, when the lambs are on tick infested pastures * Clear the pastures, drain, change pastures when possible |
This Article is issued in co-operation with “Norsk Sau & Geit