Bluetongue South West

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The Disease

Bluetongue is a major disease of sheep, but can infect and affect any ruminant including cattle, deer, camelids (such as alpacas and llamas) and goats.

Click on any headings below to find more information on the disease.

The Vector
Bluetongue is not a contagious disease: it is not spread from animal to animal by direct contact, but needs a vector to move the virus from an infected animal to a susceptible one. The vector is the midge, and several types of midge that are found in the South West are known to be able to transmit the disease.

Fortunately, midges are only able to transmit the disease when the ambient temperature is above 15 degrees centigrade, and so vector transmission ceases through the winter months when average temperatures rarely exceed 15 degrees. Although midges can be seen flying on some warmer winter and spring days, they are unlikely to transmit the disease at this time as the virus has to undergo  replication and some changes whilst in the midge before being passed on to the next animal that the midge bites;  these changes do not occur in colder temperatures.

Midges can live for about three weeks, and so an infected midge has to bit a susceptible animal before it dies to pass the disease on. There is some suggestion that an infected midge can pass the virus to its own offspring, so that they become infectious, but the significance of this is not fully understood.

Transmission from one animal to another is most commonly by the vector midge, but theoretically, transmission can also be by needles, surgical equipment, or other instruments where blood could be moved from one animal to another. However, these routes are insignificant compared to midge transmission.

Bluetongue in the South West region
In the south west region we have special risks of Bluetongue:

  • A significant risk to the biosecurity of the country is the arrival of infected midges from the continent. This occurred in August 2008, when Bluetongue arrived in the Eastern counties by a plume of midges being blown across from Holland. It is unlikely that midges could be blown across the sea from the continent directly into our region. The most likely point of entry for infected midges is along the South and East coasts.
  • All the recent infections of Bluetongue that have occurred since August 2008, both in the South West and other regions, have been due to the importation of infected animals. This trade is quite legal, but wholly undesirable. The imported animals have mostly been cattle without any clinical signs of disease. Because they have not shown signs of disease is not to say that they are not infectious, or that any calves that they may have been carrying are not persistently infected. These animals act as a significant source of infection that is then picked up by the resident midge population. These infected midges can then spread disease to susceptible livestock.
  • Because of the exceptionally high density of susceptible livestock in the South West, infected midges have little difficulty in finding animals to bite and infect. The disease would spread very quickly if it became established in the resident midge population. Once here,  Bluetongue would rapidly become established in the region, if there were enough susceptible livestock to become infected. Vaccinated livestock are not susceptible – they are protected.
  • Although cattle are generally not badly affected, they multiply the virus very efficiently and become major reservoirs of infection. Our large cattle herds and dense cattle population could become heavily infected very quickly. We have over 1.7 million cattle on over 16,000 holdings in the region.
  • The large sheep population in the region is highly susceptible to infection, either from other infected sheep, or infected cattle. We have about 3.25 million sheep on 13,000 holdings in the region.
  • In heavily populated livestock areas of the continent, the disease has spread up to 10 kilometres per day, and it would do the same in the South West if there were enough animals without immunity to sustain the infection.
  • Preventing the spread of the disease is dependent upon minimising the number of susceptible animals available for the virus to infect – vaccination will protect animals and stop them becoming infected and infectious.

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Bluetongue in cattle
In cattle the signs of disease can be quite mild, and may not even be noticed. Animals become vireamic a few days after being bitten by an infected midge carrying the virus, and may have a raised temperature at this time. Some may get ulcers and reddening of the mouth and nose, lameness, and sometimes a dermatitis which may even appear on the teats. High yielding dairy cows will have reduced milk yields, and some cattle will go quite ill. To start with, only a few of the herd may be affected, but as the disease spreads, more and more will become infected.

Most cattle show such mild signs that they are not noticed, even though they are infected and infectious.

Infected cattle carry the virus for several weeks before their own immune system fights the virus and overcomes it. Most recover and become naturally immune to further infection as they develop antibodies which protect them from the virus.

It is now known that if a pregnant cow becomes infected, the virus can cross the placenta into the unborn calf, and the calf is born infected, even though its mother has overcome the infection and recovered. Because the calf had no mature immune system at the time of infection, the calf is persistently infected, and can remain a carrier for life.  Thus, pregnant cattle that are infected may themselves recover, but produce persistently infected calves that are lifelong carriers.

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Bluetongue in sheep
Sheep can be severely affected by Bluetongue infection, with the possibility of high levels of infection and losses. The first signs of disease in a flock will be sickness and depression with high temperatures, salivation and nasal discharge, and some rapid breathing. This develops in to a thicker nasal discharge which can crust around the nose, along with reddening of the lips and ears and tongue. Affected animals often go lame. Pregnant ewes will often abort, and remain infertile.

Many affected sheep will die. Treatment comprises supportive therapy as there is no specific treatment for Bluetongue. Recovered sheep will be unproductive for some time, and indeed, ewes and rams will have reduced fertility for prolonged periods.

Sheep are infectious to biting midges whilst they are sick, and for several weeks afterwards, even if they recover. Thus, infection can build up in a flock as more sheep become infected, and the disease spreads from sheep to sheep by midges.

Infection in sheep flocks can be disastrous. Some affected flocks on the continent have had 40% mortality. Those that survive lose condition, can remain ill for days, and become infertile. They are commercially useless.

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Bluetongue in goats
Goats are susceptible to Bluetongue virus. When goats are infected, they generally show mild signs of fever, and then recover. They remain infectious to biting midges for several weeks and so act as a reservoir of infection. There are reports of goats showing more severe signs of infection, including similar signs to infected sheep such as inflamed nose, mouth and skin. Some may become acutely ill, and are severely affected.

Vaccination is a little problematical, and veterinary advice should be sought as the vaccines are not licensed for use in goats. Goats can be effectively protected by vaccination, but the vaccine regime differs with different brands of vaccine, and boosters may be required more frequently than with other animals.

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Blue tongue in camelids
Alpacas and llamas are susceptible to Bluetongue virus infection, but rarely show significant clinical signs. They may become feverish, but generally recover. However, they remain infectious for several weeks, and so act as a reservoir of infection.

Protection by vaccination is possible and practical, but the current vaccines are not licensed for use in camelids. Therefore, seek veterinary advice about vaccination.

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Blue tongue in deer
Bluetongue virus can affect deer, and there are reports of severe disease including sudden death in some species. However, there are no reports of the clinical effects of the current Bluetongue virus serotypes that could affect our common species of deer that exist in the UK.

Deer can be protected by vaccination, but you need to obtain specialist advice.

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Bluetongue risks and control in larger livestock farms
Sheep flocks are most vulnerable to the effects of Bluetongue. The high morbidity and mortality (the proportion of the flock that becomes infected, and the proportion that die of the disease) can be disastrous. The long term effects mean that the flock becomes unviable.

Large cattle herds can become infected insidiously, and the number of infected cattle builds up. Milk yields will drop significantly, and some cattle will become ill. Beef cows will have reduced growth and fertility. Most importantly, the herd becomes a major source of virus to midges, which then spread the disease to others.

As with all infectious diseases, the more susceptible animals there are in a group, the more liable they are to infection. The larger cattle herds and sheep flocks are most susceptible, as they have more susceptible animals present at any one time. The chances of them being infected by a passing carrier midge are higher than with smaller flocks and herds. In the South West, our herds and flocks are often grazed and kept very close together. Disease does not respect ownership, and so in some areas, the density of livestock is such that the population of numerous herds and flocks on contiguous holdings can really be considered to be one large herd or flock.

This makes areas of the South West particularly susceptible to disease should it become established in the region. The risks are higher, and the impact of infection (the effects of infection on our livestock) very much greater than in some other areas.

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Bluetongue control in small livestock farms
Although at lesser risk of infectious diseases, the smaller herds and flocks can be badly affected by the disease. The risks are that in very small farms, the disease may go unnoticed until it has become well established. These smaller populations of animals could then readily act as reservoirs of infection, infecting midges which then pass the disease on and allow it to become established.

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Bluetongue control in smallholdings and individual animals
Smallholdings and keepers of individual animals have particular problems with the prevention and control of Bluetongue. As cattle are often subclinically infected, infection may go unnoticed. Some infected sheep may be mis-diagnosed and go unreported. It is important that smallholders understand the clinical signs of Bluetongue, and ensure that their animals are properly protected, despite the difficulties of vaccinating small numbers of animals.

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The winter period
If the virus can only be transmitted significantly when the temperatures are above 15 degrees, and infected animals rid themselves of the virus after some weeks, it would seem that the infection should die out through our cold winters. Certainly, infections are reduced, but the virus can survive the winter. Researchers are looking for how this can occur, but we know that persistently infected animals that were infected whilst in the womb can carry the virus through the winter, and it may be that some midges can carry it through the winter season as well.

In any case, it is unwise, and very dangerous, to become complacent about Bluetongue.

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The threats to South West Livestock
The most likely ways that our South West animals will become infected with Bluetongue are:

  • Infected animals purchased or introduced from infected areas of Europe and brought in to the region, where they get bitten by midges which then transmit the disease to unprotected livestock in the locality
  • The movement of infected midges that have obtained infection from unidentified or un-noticed infected animals already here in the region.
  • The movement of midges from areas where they have blown in on the wind from infected areas of the continent
  • The movement of infected midges across the country, transmitting infection from animal to animal

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Answers to Bluetongue Quiz

1)  Why is the South West particularly susceptible to Bluetongue?

     d) High density of cattle and sheep

 

2)  Which species is the worst affected by Bluetongue

     c) Sheep

 

3)  What is the first thing you should do if you suspect Bluetongue infection in your animals?

     d) Inform your vet or Animal Health

 

4)  How can the Bluetongue virus survive the winter?

     d) In persistently infected animals

 

5)  What types of Bluetongue virus do the available vaccines protect against?

     c) Btv8

 

6)  How is Bluetongue virus most commonly transmitted from one animal to another?

     a) By Culicoides midges

 

7)  What is the best, most practical method, of preventing and controlling Bluetongue in the South West?

     c) Vaccination of all susceptible animals

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