In the news this week you may have heard that the Bluetongue BTV3 virus has been detected in the UK again. But how important is it? And what other midge-borne diseases are farmers having to worry about at the moment? Our farm vet Morven Imray has been looking into midges and their diseases…

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What is bluetongue disease and do we need to worry about midge-borne disease?

Bluetongue and Schmallenberg are both viruses that are spread by midges and cause disease in ruminants including cattle, sheep, goats, deer and camelids. There are many similarities between the two diseases but also some key differences, most notably that, in the UK, Bluetongue is a Notifiable Disease, while Schmallenberg is not.

Notifiable animal diseases are those which you are legally obliged to report to the Animal and Plant Health Agency (APHA), even if they are only suspected as a possible cause. This allows movement restrictions to be applied in surrounding areas (known as control zones) to limit any spread if the disease is confirmed by subsequent testing.

It is worth being clear at this stage neither Bluetongue nor Schmallenberg pose a threat to human health or food safety. They give cause for concern in terms of animal health and welfare, due to negative effects on productivity, and because if they become endemic (i.e. constantly present) in the UK, there may be implications for international trade for years to come.

So, where have these diseases come from, can they be prevented or treated, what signs are we looking out for, and what have midges got to do with it? 

History of midge-borne diseases and cattle

Bluetongue (BTV) 

  • First described in 18th Century Africa and didn’t spread to Southern Europe until two outbreaks occurred in the 1950’s and 1970’s. 
  • Since 1998 however, there have been over 20 outbreaks in Southern Europe with the disease becoming endemic (constantly present) in some areas. This pattern is considered to be strongly linked to climate change.
  • 2006 – First big outbreak in Northern Europe, caused by a specific strain (serotype) known as BTV-8. This crossed the channel from Northern Europe to the UK in wind-blown midges resulting in…
  • 2007 – BTV-8 first diagnosed in England but was successfully controlled thanks to a huge effort from stock keepers, vets, and scientists. A combination of movement restrictions, culling, surveillance and vaccination prevented the virus from becoming endemic (constantly present) in the UK.
  • 2023 – second large outbreak in Northern Europe, this time of a different strain – BTV-3. Using weather data and disease knowledge, ongoing UK surveillance testing was targeted to the South and East of England, and in November 2023, BTV-3 was detected in an apparently healthy UK cow. The disease was again controlled through, focused surveillance, movement restrictions, testing and culling. By the end of the 2023-24 midge season, 126 animals were confirmed infected across 73 UK premises, but the virus didn’t make it into the local UK midge population, and endemic disease was avoided.
  • 2024 – BTV-3 was confirmed in a single sheep on 26th August, resulting in a 20km Temporary Control Zone being put in place. There is no vaccine, so hopefully excellent control measures will prove enough in this midge season too.

Schmallenberg (SBV)

  • 2011 – Schmallenberg first detected in Europe, although similar types of viruses had been present in the Middle East for a long time.  Like Bluetongue, Schmallenberg is spread by midges and so …
  • 2012 – Schmallenberg detected in South East England and spread inland and north. At the end of the 2012/13 midge season, Schmallenberg had been detected on 656 holdings, although further blood testing indicated virus exposure in all areas of the UK below the Scottish borders.
  • In 2014 and 2015, surveillance showed that SBV did not circulate, which was great but meant there was very little natural immunity in the UK herd and so … 
  • 2016/17 saw 139 premises in England and Wales with positive cases in lambs, and nine premises in the south of Scotland. True figures are likely to have been much higher as many suspected cases may have been unreported. SBV is NOT notifiable.
  • 2022 saw a small peak of Schmallenberg cases followed by some more in December 2023 and early 2024. 

The role of the midge in Bluetongue disease

Neither Bluetongue nor Schmallenberg virus is transmitted directly from one host animal (cow/sheep/goat etc) to another; even when there are clinical signs of disease. Both viruses rely on replication within the midge, the speed of which varies according to the ambient temperature. Midges exist at the temperature of their surroundings so during colder weather, the virus takes much longer to replicate than in warmer weather.

At ambient temperatures of 28-38C, midges can transfer Bluetongue virus from one animal to the next in just 4 days. Whereas in colder weather, transmission time can be more than 3 weeks. These times are longer for Schmallenberg virus. And during cold weather, the midge may die before it can transmit disease at all.

  1. When a Bluetongue infected midge bites a host animal (e.g. cow/sheep), virus is transmitted from the midge, into the host.
  2. Within a few days this causes viraemia (virus circulating in the host animal) and may also cause clinical disease. 
  3. When an uninfected midge then bites this same animal, it may pick up the virus and become infected itself. 
  4. Virus replicates within the midge before moving to the salivary gland, ready to be transmitted every time the midge bites another host. 
  5. The host animal remains viraemic (having circulating virus) for around 60 days and the midge remains infected for life, continuing to transmit virus with every bite. 
  6. Schmallenberg is transmitted in much the same way.

Clinical signs of Bluetongue and Schmallenberg Disease

Here we come across some major differences between the two viruses.

BluetongueSchmallenberg
Signs more likely in sheep than cattle although more cattle test positive than sheepAdult cattle more likely to show signs than adult sheep
Severe signs in adult sheep, less severe in cattle and less so again in goats.Damage to blood vessels especially around feet, mouth, eyes and teats causes;Haemorrhage (bleeding) Ischaemia (lack of oxygen)Inflammation (swelling, redness)Oedema (fluid accumulation). Mild signs in adults milk dropDiarrhoeapoor appetite weight loss
Signs of disease within days of midge bitingPyrexia (fever)Depression Dyspnoea (difficulty breathing)Redness, swelling and discharge at noseUlceration in mouth causing inappetance and droolingCyanosis (tissue turning blue due to lack of oxygen) within mouth – “blue tongue”Ulceration and redness around coronary band (where hoof joins skin) so lameness. Abortion, stillbirth, weak lambs, brain defects in lambsAlso, milk drop & teat lesions in dairy cattleMost clinical signs seen months after the midge bitesAbortions High barren or low scanning rates due to early pregnancy lossStillbirthsFoetal abnormalities includingBent limbs and fixed jointsTwisted neck or spineDomed skullBrain deformitiesDamage to the spinal cordThese abnormalities can cause much difficulty at lambing/calving so it is important to contact your vet for assistance with difficult deliveries. 
High mortality (death) rate 25-30% in adult sheep with BTV-8>75% mortality reported with BTV-3 in Netherlands 2023/24Low mortality (death) rate in adultsLoss of calves/lambs including those born alive but euthanised on welfare grounds
Disease recurring year on year particularly in endemic areas. Worst in warmer weather years.Disease typically follows a cyclical pattern with peaks every 3-5 years as host animal immunity drops. Worst in warmer weather years
NOTIFIABLE DISEASE IN UKNot notifiable

Diagnosis and testing of midge-borne diseases

If you are concerned about either Bluetongue or Schmallenberg, speak to your vet immediately. There are various other conditions that can cause similar signs such as orf causing mouth, nose, udder and foot lesions; Haemonchus and fluke leading to swelling (oedema); or BVD and border disease causing abortions, still births and abnormalities in newborns.

If you do suspect bluetongue, you must report it immediately by calling:  

03000 200 301 in England  
03003 038 268 in Wales  
Your local Field Services Office in Scotland

Follow this link for more information on What happens when you report a notifiable disease

Where Schmallenberg virus is suspected, the APHA offers free-of-charge testing in 2024 on samples from lambs and calves born with congenital malformations or musculoskeletal deformities. Contact your vet to arrange this.

Treatment of Bluetongue Disease

Sadly, when it comes to Bluetongue, clinically affected animals are often euthanised on welfare grounds. Vets in the Netherlands are reporting much more severe clinical signs and higher death rates from BTV-3 during 2023/24 than was seen as a result of BTV-8. Very early treatment with steroids may reduce some of the initial clinical signs. However in the following days, most sheep go on to lose the shells off their hooves causing extreme pain; so euthanasia is usually warranted to protect welfare.

Mildly affected cases (usually cattle or goats) may survive if treated with anti-inflammatories, pain relief, electrolytes, antibiotics (for secondary infections) and intensive nursing. But it should be stressed again that animal welfare must be at the forefront of our minds during this time; as well as giving consideration to the wider implications of keeping infected animals.  

With Schmallenberg, mild signs in adults may warrant supportive treatment of symptoms. But it is birthing difficulties due to foetal malformations (especially fused limbs) that require the most intervention. It is really important to contact your vet for assistance with these deliveries as they must be handled with knowledge and experience in order to protect the welfare of the mother.

Control of Bluetongue Disease

Euthanasia (or culling) and movement restrictions will likely form part of the control strategy early on in this new UK Bluetongue outbreak. This is because removing infected animals and restricting movements helps to limit onward spread of the disease; and works to prevent the disease becoming endemic. 

The situation with Schmallenberg is different because disease is often only seen months after infection occurred; so movement restrictions and culling would not affect control. Likewise, if Bluetongue virus becomes circulating within the midge population, these control measures are likely to be suspended as they will no longer have the same impact in controlling the disease. 

Vaccination against BTV

The good news is that vaccination is the most effective control measure for BTV. And is the reason why so little bluetongue disease was seen in the UK during the BTV-8 outbreak of 2007. Vaccination causes the production of antibodies which neutralise virus; reducing the amount circulating within the host as well as the length of time that a host animal is viraemic. Both these effects minimise the number of midges that become infected from that one host animal. 

Bad news time…

There are no approved vaccines for either BTV-3 or for Schmallenberg at the moment in the UK and there is no cross-protection from BTV-8 vaccine against BTV-3.

Back to the good news! 

At the time of writing, three BTV-3 vaccines are being used and carefully studied in Europe. Early findings from the Netherlands suggest vaccinated animals are more mildly affected and recover faster than unvaccinated animals.

But a little bit more bad news…

On many vaccinated premises in the Netherlands, over half the animals on the holding are still becoming ill within a 2-week period. So, vaccination does not appear to protect from disease altogether. 

The situation in Europe is being closely monitored by the UK so that careful consideration can be given to the use of vaccines against BTV-3 if the virus cannot otherwise be controlled if it is becoming established in the UK. 

Prevention of Bluetongue Disease

If midges could be excluded from an area, the west coast of Scotland would have been leading the charge years ago! Sadly, this is not the case. Products such as herbal licks unfortunately have very little evidence of being able to deter the mighty midge. And it cannot be relied upon when it comes to preventing Bluetongue and Schmallenberg. Similarly, insecticides for animal treatment would need to kill midges instantly upon contact (before they could bite) to prevent transmission of disease. Furthermore, overuse of insecticides would have disastrous effects on insects such as bees and other pollinators. 

So, other than the hope of future vaccines, what else can be done to reduce the risk of midge borne diseases in your stock.

General actions:

  • Most importantly, stock keepers should monitor their animals frequently for clinical signs and contact their vet promptly with any concerns. 
  • Develop clear biosecurity policies with your vet to avoid buying in disease, especially in pregnant animals
  • Move the muck heap away from farm sheds to avoid attracting midges closer to animals.
  • Where small numbers or very valuable stock (e.g. rare breed tups) are concerned, housing animals from mid-summer to the end of the year may be considered. This would involve hanging insecticide impregnated midge netting and using fans to circulate the air.

Bluetongue

  • Ensure all animals and land are registered with APHA – this information is vital in the event of a disease outbreak.
  • If you live in or buy livestock from high-risk counties (currently Norfolk, Suffolk, Essex, Kent and East Sussex), take advantage of ongoing free testing available for animals moving from these areas to elsewhere in Britain. 
  • Understand what it means to be in a control zone and check where your nearest designated abattoir is incase this should be needed.
  • Now movement restrictions are being introduced, look ahead at your resources. If animals require to be moved for grazing/housing for example, apply for movement licenses well in advance of needing them. 

Schmallenberg

  • Peak midge season is usually August/September, and it is during the second month of pregnancy that sheep are most susceptible to SBV, therefore early mating/lambing flocks are most at risk of foetal deformities.
  • Delaying tupping until midge activity is reduced in October/November may be a good strategy although consideration must be given to the effect a later lambing could have in terms of resources and profitability. 
  • Avoiding synchronising large groups of ewes may be advisable in some circumstances, so as to avoid having all the ewes at the same stage of pregnancy at the same time.

Support available

Coping with the threat of incoming diseases such as Bluetongue and Schmallenberg can seem overwhelming when it comes on top of the day-to-day ups and downs of keeping livestock. In the UK we are fortunate to have robust disease surveillance in place. And continue to benefit from working closely with colleagues particularly in the Netherlands who are sharing their experience of dealing with these diseases. If you are concerned about Bluetongue and Schmallenberg on your farm, contact your vet to talk things over or call the Bluetongue Hotline on 024 7771 0386. If you’re feeling overwhelmed, reach out to friends, neighbours or some of the organisations linked below who will be happy to offer support.

For more disease information click on the links below;

Further reading: