Bluetongue has caused devastation in Europe, but so far UK farmers have escaped relatively unscathed. Continued awareness is essential to prevent the disease taking hold here. Paul Roger, from Vet Consultancy Services in North Yorkshire, summarises the threats and controls.
As is widely recognised, threats of exotic disease to our island are increasing through globalisation and the effects of climate change. On the back of these changes, our livestock industry has had to respond pragmatically to new challenges, both economically and at policy level, from direct disease risks that have increased because of a combination of these factors.
Bluetongue virus (BTV) is now a common part of everyday talk and the need to understand what causes this disease and how it spreads and develops has become a focus for many livestock producers. We are well informed on this by our community of scientists, particularly the group at the Institute for Animal Health, Pirbright.
The disease
Bluetongue virus is a vector-borne disease, being spread from animal to animal by a carrier, in this case a midge. To become infected non-diseased animals are bitten by an infected and infective midge.
To become infected non-diseased animals are bitten by an infected and infective midge.
The midge vectors become infected by biting an infected animal, most commonly a cow. The infection in the midge then develops over a number of days and this time relates to the ambient temperature (from 12C up to about 30C). The higher the temperature the faster it occurs and develops over 3–7 days. The virus travels from the gut of the midge into its salivary glands where it reproduces in great numbers.
Once the midge is fully infective, it will pass infection on to every non-infected animal it bites. It takes a similar time for the virus to develop in the infected animal and for the first signs of disease to appear.
The disease that is seen in cattle, sheep, goats and camelids has been severe and although initially only one of the 24 serotypes of BTV, BTV8 was involved in northern Europe, BTV1 has now established a firm footing in France and focal outbreaks of BTV6 and BTV11 have been seen in the Netherlands. But BTV8 remains the present and particular risk to UK livestock.
BTV8, the original infection seen in northern Europe, has had devastating effect on livestock production. Morbidity rates (the proportion of the herd/flock that gets the disease) have varied markedly from 0-100%, but with a mean of about 20% and mortality (the number of infected animals that die from the disease) has been nearly as wide ranging.
Put simply that means that death rates from those getting the disease can be as high as 1 in 7 for cattle and 1 in 5 for sheep for animals showing clinical signs of the disease. The cost of the disease and its after-effects would put most enterprises out of business.
Symptoms
The signs of disease can vary in severity, but are always accompanied by a fever. The animal may show swelling of the head or facial features and there may be nasal erosion and discharge as well as erosions in the mouth. Lameness tends to develop more steadily.
The main diseases showing similar signs are:
Because BTV shows its clinical effects by damage to the blood vessels, it can mimic many diseases and, depending on the system affected and the severity of infection, these signs can be very varied. This means a differential diagnosis by your vet is essential.
Treatment
This is a viral disease and no drugs can combat viruses. Antibiotics may help control secondary infections getting into damaged areas and anti-inflammatory drugs (to help control swelling and discomfort) and analgesics (to help control pain) may help animals overcome some of the depressing symptoms associated with the disease.
But intensive nursing care is needed to allow animals to recover after disease develops. This includes the use of probiotics and vitamin preparations as well as electrolyte drenches and specially prepared fodder as animals are unable to chew or grind their food adequately.
Prevention
Movement controls and avoidance of transporting infected animals into BTV-free areas are a starting point, but midges don’t respect border controls and there are no topical treatments that eliminate the risk completely. Insecticides can help to reduce risk, but not eliminate it. Similarly, common sense preventive measures such as disturbing midge breeding habitats and keeping stock reasonably clean may help reduce challenge.
The success of environmental controls is unknown, as not everything is known about the behaviour of the different midges involved in transmission. But we do have killed vaccines available.
These vaccines are specific to BTV8. A combined vaccine for BTV1 and BTV8 may become available in time, but vaccination should not be delayed by waiting for this as vaccination works best when completed in advance of any challenge.
Experience with vaccine in 2008 has shown marked reduction in cases where widespread mass vaccination of livestock has occurred and less effective control where this approach has not been as well accepted. In Germany, for example, there were 4,500 cases in 2008 compared with more than 20,000 in 2007, whereas France had about 15,000 cases in 2007 and more than 24,000 in 2008. Impressively, the Benelux countries had a handful of cases in 2008 compared with thousands in 2007.
Vaccination recommendations vary as shown in the table, so making an informed vaccine choice is essential. When using vaccine for goats or camelids, you must seek advice on this use from your own vet.