Diseases in sheep in Ixodes ricinus infested pastures

by

Snorre Stuen 

Department of Sheep and Goat Research Norwegian School of Veterinary Science  

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 (TBE-virus)  

Louping-ill (TBE)  

Sheep, goat, cattle,horse, dog, hare, grouse, human

Detected in Vest-Agder and Sunnhordland (rare)  

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.

From being a disease in ruminants the tick-borne fever bacteria is now detected in connection with disease in several species including human beings (Table 2). Investigations in this country indicate that this bacteria is spread to deer, elk and roe deer in tick areas and that these species can function as a reservoir for the bacteria. In human beings this infection is up till now detected in two cases in Norway, but there is a reason to believe that the infection in human beings is markedly underreported in the country. When it comes to the infection in human beings, the preliminary reports indicate that it generally is of a mild character. Should this infection lead to a reduced immunity, as it is observed in sheep, the disease may open up for serious secondary infections. If one is in areas with ticks and get a high temperature, flu-like symptoms and the blood picture shows a reduced number of white blood cells and blood platelets one should seriously consider the risk of an Ehrlichia infection.

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.

In particular areas that have been investigated, 1/3 of the lambs that were put on tick infested pastures were lost, and this loss is mainly caused by tick-borne fever and complications related to this disease. The main problem of the disease is, as mentioned earlier, that it leads to a debilitated immune defence system and thereby a risk of secondary infections such as blood poisonings (septicemia, pyemia) and inflammations in the joints (joint-ill). The most important bacteria in this connection are Pasteurella hemolytica /trehalosi and Staphylococcus aureus. Apart from the direct losses, there are considerable indirect losses due to lower production, i.e. small lambs in the fall.

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