Features
Louping Ill in Sheep
by Dr Hugh Reid
Louping-ill is a virus disease affecting sheep and most other domestic animals as well as man and red grouse. Because it is transmitted by biting ticks the disease is only seen on pastures where ticks are present and at times of the year when ticks are active. The outcome of infection in sheep is however unpredictable; in some infected flocks no clinical disease is observed while in others severe clinical disease and death is frequently seen. However, most wild grouse that become infected will die and thus in some areas the purpose of control of louping-ill is to reduce grouse mortality to enhance the economic return from sporting activities.
An understanding of this disease requires knowledge both of the tick and how the virus infects both the tick and the animal host. The sheep tick has four stages egg → larvae → nymph → adult. After hatching the larvae actively seek a blood meal by climbing up grass stems etc and attaching to any passing animal or bird. They stick their mouth parts through the skin and feed for only 2-3 days. They then fall back into the vegetation and moult into nymphs which will not feed till the following year. After feeding nymphs moult into adults which feed the following year giving rise to the egg masses from which the next generation of larvae emerge. This whole process thus takes at least three years with only about 20 days being spent feeding. Critical to survival of the tick off the host is a moist habitat in a mat of decaying vegetation. Thus ticks tend to be found only on rough upland pasture and will not survive on well drained improved pasture
Ticks feeding on animals with high levels of virus in their blood take in virus which can then be transmitted to other hosts when the tick next feeds after moulting. Thus it is only nymphal and adult stages of tick that can transmit virus as larvae emerging from eggs do not carry virus.
When a sheep becomes infected virus enters the blood stream reaching high levels which ensure that virus can infect other ticks feeding at that time as well as allowing it to enter the brain. It is not until the virus has multiplied in the brain some 6 to 10 days after infection that some sheep will show clinical signs including nervousness, difficulty in walking, paralyses, coma and in some death. Between 5% and 60% of infected sheep may develop clinical signs, but it is not clear why the outcome of infection is so variable. Certainly stress such as gathering and severe weather can increase the probability that disease develops and concurrent infection with tick-borne-fever greatly increases the severity of the disease. Particularly severe outbreaks may occur when susceptible sheep are first put onto a tick infested pasture or where louping-ill is introduced to a ticky pasture for the first time.
Animals that survive infection develop high levels of antibodies in their blood which provide solid protection for the rest of their lives. In addition such protection is passed on very efficiently in colostrum ensuring that lambs can be solidly protected for the first 6 to 8 weeks of life.
Although all ages of sheep are susceptible, it is mainly lambs (not protected by colostrum) and yearlings, retained for breeding that become clinically affected. When disease is suspected, veterinary advice should be sought to confirm the diagnosis.
When losses are confirmed, vaccination should be considered. Though the vaccine is expensive it is highly effective and will provide good protection for at least two years. Repeat vaccination will give life-long protection and help to boost levels of protection in the colostrum. Normally it will only be the ewe lambs retained for breeding that will be vaccinated although whole flock vaccination should be considered when the disease occurs for the first time or when un-acclimatised sheep are introduced to an infected area.
Where total control of the disease is desired to reduce the impact of louping-ill and other tick-borne diseases of livestock (or to improve grouse stocks) it may be necessary to undertake a comprehensive control programme. This will involve determining the level of infection in areas by looking for antibodies in blood samples and embarking on an intensive programme to reduce tick numbers with the use of dips and pour-ons as well as vaccination. In addition it may be necessary to control access of wild species such as hares and deer which may host large numbers of ticks. Such programmes should, however, only be contemplated where the co-operation of farmers and shooting interests can be ensured across a defined area of ground bounded by natural barriers. Otherwise the investment is likely to have limited success and re-emergence of infection is very probable.
Finally although most cases of louping-ill are seen in animals, severe and very rarely, fatal disease can occur when humans become infected. Care should be taken when handling suspect clinical cases or carcases and any ticks that attach should be removed from the person as rapidly as possible to reduce the chances of the tick injecting virus.
Footnote: this article first appeared in the Sheep Farmer magazine and is reproduced with their permission