Don’t assume BTV vector threat is reducing

With another mild autumn being forecast, the midge season looks set to continue long after British Summertime ends – and with it the risk of bluetongue serotype 8 (BTV8) transmission. For that reason, practitioners need to advise farmers why midge control measures should continue at least until the vector-free period is announced.

The efforts required are effectively the same as for fly control, where a two-part protocol is needed to kill adults and prevent larvae from maturing to the next stage. Regular applications of an effective insecticide, removing muck heaps from livestock areas and preventing or draining areas of standing water will help reduce midge numbers, and other nuisance flies. In addition, frequently cleaning out feed troughs and paying attention to hygiene in cow housing should target those difficult-to-clean areas where muck is often trapped.

Employing these techniques should result in risk mitigation, not total elimination of the vector. In isolation, they won't stop transmission of BTV8, warns entomologist Dr Simon Carpenter. As head of entomology and modelling at the Institute for Animal Health (IAH) in Pirbright, which has been at the forefront of midge work since the 1970s and is the European Community reference laboratory for bluetongue, he is well-placed to comment. "Targeting removal of breeding areas can make an impact on reducing numbers of overwintering larvae. But currently BTV vaccination is the most certain way to stop the virus and reduce clinical signs," he says.

Bluetongue affects cattle, sheep, goats, buffalo and deer, causing fever, nasal discharge, swelling and ulceration of the mouth. Direct costs of disease reported in the Netherlands during the 2006 outbreak came to 338,000 Euros; indirect costs totalled 53 million Euros.

At least six of the 47 species of midge in Britain are thought to transmit the virus – all of these species are common on UK farms. Temperature and humidity are critical factors for Culicoides' egg laying, larval and pupal development. Adult midges tend to be vulnerable to cold temperatures and a sharp winter can soon knock down numbers. The vector-free period, when movement restrictions are reduced, relates to low numbers of flying adults. When this is in place, bluetongue is less likely to be transmitted. But Britain's increasingly mild autumn and winter conditions are tending to favour adult survival and may lead to a shorter vector-free period, says Dr Carpenter.

Furthermore, Belgian studies have shown that midges are able to survive a European winter, with temperatures ranging from 5ºC to 10ºC, as Professor Eric Haubruge from Gembloux Agricultural University told an Intervet/Schering-Plough Animal Health BTV symposium. It's thought that midges, like some other insects, need a period of cooler weather to shut down over winter. Dr Carpenter says it's not known whether warm winters lead to lots of midges the next year. "So far, the midge population hasn't changed year on year (we have caught the same numbers this year, as last year), but it is more likely that they can transmit virus – and quicker." This relationship between temperature and survival makes meteorology a key part of midge surveillance.

Both C. obsoletus and C. pulicaris have been found on almost every farm in England, sometimes in huge numbers. Both appear able to transmit BTV8. The problem is that midges thought to transmit BTV8 have different breeding habitats: C. dewulfi likes to breed in dung, whereas C. obsoletus prefers leaf litter and the margins beside stagnant water (they drown quite easily). This means that while some species can be targeted using control techniques, populations of others are much more difficult to remove during mild, wet winters, he says. "C. obsoletus also occurs at lower density, over a wider area than the dung breeders. Our studies will allow us to fully determine the main species involved in transmission. If it is the dung breeders, they can be controlled more effectively."

Dr Carpenter also points out that it's difficult to know how to reduce the risk of midge bites because scientists are still unsure of biting rates. One recent study involving sheep has shown 20 bites/animal over a period of 20 minutes. "We don't know what minimum numbers are needed for transmission to still occur." What is certain, however, is that a large midge population in relatively warm, still conditions will promote the virus. Higher temperatures increase the proportion of a population able to transmit BTV, plus, the rate of BTV replication increases leading to earlier transmission: at 28ºC replication takes just three days, at 15ºC it's three weeks. Unfortunately, midges aren't a major food source for any animal, bat or bird which makes biological control impossible, and midge traps are less effective than animals at attracting midges. "There is no data on their ability to reduce midge numbers effectively. Similarly, light traps are monitoring tools designed to catch very small numbers of midges; they won't control numbers and stop transmission."

Fortunately, there is now a licensed insecticide for cattle which has shown to be effective against Culicoides: Butox® SWISH (deltamethrin). Trials in Germany last summer showed that it will give at least four weeks' control of midges, killing them even after a brief contact time. The standard dose, applied on the neck and topline of the animal delivers full-body coverage of deltamethrin, helped partly by its unique micronised formulation. The product is also highly lipophilic to improve coverage. The trial involved six heifers (each weighing around 400kg) treated with 30ml Butox SWISH (the standard fly dose) and two untreated heifers studied for comparison. During the five-week trial period, contact between midges and treated hair caused immediate paralysis. The midges then died at varying intervals depending on contact time and the number of days post-treatment.

"These compounds act by reducing the overall chance of virus transmission within a herd by cutting the probability of a midge feeding on an infected animal, surviving and then spreading the virus to other herd members," says Dr Carpenter.

Britain's vector-free period is expected to be similar to last year: lasting from November until March. The exact timing, says Dr Carpenter, will depend on the conclusions of the IAH working closely with the Met Office as midge populations in live traps begin to tail off. However, midge monitoring in northern Europe is increasingly likely to use technology to determine risk factors and seasons. According to Dr Laura Rinaldi from the University of Naples, geographical information systems (GIS) and remote sensing technology (RS) could give up to four weeks' advance warning of BTV vector arrival. "Epidemiological surveillance of BTV and the Culicoides vectors, together with GIS and RS, is now being carried out across Europe. Identifying environmental features allows us to be extra vigilant and better allocate resources for cost-effective control," she says.

As the climate continues to warm up – the Met Office predicts 2014 will be 0.3ºC warmer than 2004 – British vets and farmers should be prepared for a longer BTV season, with more transmission of virus and more midge species to transmit virus. While the main preventative measure remains vaccination with Bovilis BTV8, controlling vector numbers can help reduce the risk of transmission.

IN A BOX

MIDGE FACTS

• Optimum temperature for Culicoides is 15ºC to 30ºC.

• Activity rates highest at night but are inhibited at below 8º-10ºC and above 30ºC.

• Temperature affects life cycle, ability to infect livestock and the rate of virogenesis within individuals.

• Adult midges can be dispersed passively on the wind. Wind speeds below 8km/hour are particularly favourable.

• Midges tend to stay close to breeding grounds.

• They can'