Saturday, 28 September 2013

Bluetongue, Schmallenberg.......Oropouche?
Until relatively recently, Culicoides midges were probably most famed for being an annoyance to people walking, camping, fishing etc. in the Scottish hills. And this is justified - a pub I went to this summer even provided a selection of repellents for their customers. Horse owners most likely associate them with sweet itch, perhaps also that they transmit African Horse Sickness. Many UK and European cattle and sheep farmers would not have thought Cuilcoides were of any significance beyond being a nuisance to their animals.

Next to the whisky and Tennents this Highlands pub provides insect repellents for its customers.

This all changed in 2006, when a severe strain of the Cuicoides transmitted Bluetongue virus (BTV) seemingly parachuted into the middle of Northern Europe and spread wildly, with high levels of morbidity and mortality. The following year BTV arrived into the UK, probably as a result of infected midges being blown across the channel. Fortunately for UK farmers BTV didn't get very far that summer, and by the following year there was a vaccine available. Nevertheless, Culicoides were now very much in the conscience of farmers, which meant that they were fully aware of what may happen when Schmallenberg virus (SBV), another Cuicoides transmitted virus, was discovered in Germany in 2011. Sure enough, a large proportion of the UK sheep and cattle are likely to now be immune to SBV as a result of it sweeping the country in the last couple of years.

The obvious question to ask is, 'what about humans?'. There are several other livestock viruses that are spread by Culicoides, most importantly African Horse Sickness Virus, but if livestock viruses can spread so dramatically, what would happen if a Culicoides-borne virus arrived that could infect humans. The obvious candidate is Oropouche virus, which is from the same family of viruses, the Bunyaviridae, as SBV. This is discussed as an example in a recent paper by Carpenter et al discussing the impact of Culicoides on public health. In the case of OROV, C. paraensis is the primary vector involved in epidemics that occur in south America. An interesting, and potentially important, fact is that the biting patterns of C. paraensis are different from the Culicoides species found in the UK and Northern Europe. Whereas C. paraensis bites at a low rate during both day and night with relatively little impact upon human behaviour, European midges bite so aggressively that people will often seek shelter, effectively reducing exposure levels.

Full-size image (152 K)
Where do midges occur? A) the overlap of livestock and Culicoides,
 and B) the overlap of farmland (i.e. habitats for midges) with urban areas. From Carpenter et al 2013.

If you ignore the aspect of whether there's a virus that fits the requirements, the authors put forward some reasons why an outbreak is perhaps unlikely. Firstly, the biology of the vector may simply mean that not enough survive long enough to transmit an arbovirus. Secondly the habitats of European midges don't tend to overlap with humans as much, thus reducing exposure. And thirdly European midges are seasonal, as opposed to the year-round presence of C. paraensis. All of this suggests that Culucoides may be of relatively limited impact for the transmission of viruses among humans, although they may still facilitate spillover events from animals to humans. For the time being it appears that, where viruses are concerned, Cuicoides appear to be of more importance for livestock diseases. What is for sure though, is that they will remain a pain for anybody wandering the Scottish hills in summer.

Carpenter S, Groschup MH, Garros C, Felippe-Bauer ML, & Purse BV (2013). Culicoides biting midges, arboviruses and public health in Europe. Antiviral research, 100 (1), 102-113 PMID: 23933421