The risk it brings to the UK livestock industry is still uncertain, but bluetongue is a virus that has killed thousands of ruminants, including sheep, cattle, and goats, across Africa and southern Europe in the past decade.
Although impact varies between strains – serotypes – bluetongue can have significant economic effects in terms of losses due to death, sickness and reduced productivity, as well as losses from export revenue. As yet there is no vaccination or effective treatment for the disease strain currently affecting northern Europe (BTV-8).
First recognised in South Africa, the disease has spread north into southern Europe, specifically Turkey, Bulgaria, Croatia and Macedonia.
In autumn 2006, bluetongue spread to The Netherlands, Belgium, Luxembourg, western Germany and parts of north-east France, and identified as a new type – BTV-8 – the same serotype found in the infected Highland cow at Baylham House Farm, Ipswich, Suffolk, last week.
There are only two possible routes for infection introduction, explained Reading University veterinary epidemiologist Nick Taylor. “The most likely route is midges being blown in on prevailing winds and that the Highland cow was bitten by one of these.” Alternatively, infected midges could have travelled in on vehicles travelling to this country.
“Imports of susceptible live animals to the UK from restricted areas in the EU are prohibited and precautionary testing has been carried out on species imported from non-restricted areas and other EU member states, all of which have tested negative. Therefore, there is minimal chance of this being the route of infection.”
How is it transmitted?
Bluetongue virus cannot pass from animal to animal. Although it’s infectious, unlike foot-and-mouth, it’s not contagious. Virus transmission occurs via midges of the culicoides species. However, the likelihood of transmission between and within herds or flocks by unhygienic practices, such as use of contaminated surgical equipment or hypodermic needles, shouldn’t be ruled out either.
|More than 4000 seperate outbreaks on farms across the EU already this year|
DEFRA says initial studies have found that a midge can travel up to 2km (1.25 miles) a day. However, in certain weather conditions, midges can be carried much further, in particular up to 200km (125 miles) over water. When a midge bites an infected animal, virus passes to the midge in the blood meal and the virus then multiplies in the midge.
The main season for transmission is late summer to autumn, with transmission most successful at 27-30C and an incubation period of 5-20 days. However, independent vet consultant Tony Andrews said most outbreaks on the Continent have occurred in October and haven’t died out until December. “Even if we were to get a cold spell, farmers shouldn’t let their guard down as infected midges have proved to outwinter.”
Some symptoms are similar to those associated with foot-and-mouth disease ulcers first appear around the mouth, nose and eyes, before spreading to the rest of the head. Next the animal suffers internal bleeding, lameness and eventually struggles to breathe, hence the association with “blue” tongue.
However, Dr Andrews can’t stress enough that these symptoms rarely occur. “In most instances infected animals won’t show any signs. Sheep and deer are normally the species that show symptoms, with cattle and goats rarely showing signs.
Because bluetongue is spread by midges, compulsory slaughter of infected livestock wouldn’t normally be carried out, reckoned Mr Taylor. “The infected cattle in Suffolk were slaughtered to prevent further transmission of virus to midges. It will not be classed as an outbreak unless other cases are confirmed or an infected midge population is identified.”
Mr Taylor also pointed out that, should a UK outbreak be confirmed, movements and exports of livestock susceptible to bluetongue, including semen, ova and embryos, from the restricted area would also be banned.
Vaccination against bluetongue is possible, but there are 24 serotypes and a vaccine for BTV-8 has yet to be developed, explained Mr Taylor. “In South Africa, where the disease is endemic, a live attenuated (weakened) vaccine has been used with some success, but there is concern that using similar vaccines in European countries could lead to persistence of the vaccine virus and could even cause clinical disease.
“The weakened virus in the vaccine still has the ability to multiply itself. There is then the possibility the virus could get strong again, causing more outbreaks,” he explained.
Work is under way to produce safe, inactivated vaccines, but it’s unlikely a vaccine will be available until next summer. “Both Merial and Intervet are currently working on vaccines.”
With no immediate prevention steps in place, Mr Taylor indicated measures farmers could consider adopting to help prevent a bluetongue outbreak. “Livestock could be kept housed at peak midge biting periods, an hour before sunset and an hour after sunrise.”
Long-term, farmers should evaluate slurry storage and dung heap management to attack the midge habitat. “It may be necessary to consider treating slurry lagoons and dung heaps where the midges breed, and also avoid having standing water in areas around livestock.”
In addition, Schering Plough vet adviser Paul Williams said protecting livestock housing openings with fine mesh netting or coarser material impregnated with insecticide may also help.
“Turning off taps, mending leaks and filling in or draining damp areas will also help dry up breeding areas. Dung heaps and straw bedding should be removed at least weekly to break the immature midge breeding cycle.”
Mr Williams also recommended applying insecticides approved for use on sheep and cattle. “Targeted use of synthetic pyrethroids, such as deltamethrin, applied weekly in and around animal housing and directly onto susceptible animals could act as a defence strategy.”
But, farmers need to speak to their vets about using insecticides, as their use for midge control is off-label and meat withdrawal periods will need modification.
However, pour-on solutions have proved relatively ineffective in northern Europe, according to BVA president David Catlow. “Total protection against bluetongue is difficult to achieve without vaccination and the use of pour-on products is limited by the fact that the quantities required are so high, so there is a risk of toxic poisoning in livestock being treated.”
Even so, Dr Andrews advised farmers in high risk areas to do all they can. “If you farm in areas near ferry ports or airports, then you should be looking at insecticide use as well as evaluation of dung management.
What can you do?
If you are concerned your animals are unwell, inspect stock closely, particularly focusing on the lining of the mouth and coronary band of the foot. Contact your vet immediately to obtain a more precise diagnosis.
An animal suspected of having bluetongue must be reported immediately to your local Animal Health Office. To obtain this number call the DEFRA helpline 08459 335 577 or visit the DEFRA website www.defra.gov.uk .
- For more on bluetongue go to News, p6 and p12.
Clinical signs in sheep
Clinical signs in cattle
NB Some infected animals will not show any of these signs
Other useful sites