This particular disease is caused by a virus that we know quite little about. The CCHF-virus (CCHFV) and we have nothing in our grasp to stop or prevent it but it is one that we our keeping a very, very close eye on. Hence the big interest in the recent case in Glasgow. This virus can have a case-fatality rates of upwards of 30%, however the rate of subclinical infections is largely unknown but might be close to 90%.
Now, CCHFV is one of those awkward viruses, it has a rather complicated lifestyle choice. It is zoonotic as in it comes from animals and is not like measles and mumps. In order to survive it has to infect both ticks (a kind of blood-eating invertebrate arachnid) and vertebrates, like domestic animals or even humans. But the major risk factor for us are tick bites. And to complicate matters even further, the young virus-laden tick particularly likes to live out its youth on the backs of smaller vertebrates like hedgehogs and only once it has matured can it jump to cattle, sheep and goats plus some species of birds (the kind of animals that we like to have around us. The virus is a two-host parasite and so is the tick. The virus can even spread from tick to tick during reproduction (sex and birth) and it can even move from vertebrate to vertebrate, given close contact with infected bodily fluids. It is this final property that makes public health workers so worried: it can really kick off around an ill-prepared hospital.
|CCHFV is a bunyavirus, like schmallenberg virus.|
So to understand why CCHFV is where it is, you have to understand this cycle of infection and given that the same domestic animals are found throughout the world, the major controller of CCHFV presence are the ticks. It is a pretty old and genetically diverse virus which probably spread across its range with the expansion of agriculture from the Middle East across Asia and down in Sub-Sahaharan Africa.
One of the most favourite things of CCHFV are Hyaloma species of ticks, in particularly Hyaloma marginatum. These hard-bodied ticks live for about 3 months and as described above pack that short life full of multiple host changes. These ticks are also pretty widespread across the world but they really prefer dry and open habitats full of their small and large vertebrate hosts. Worryingly these species of tick have been found in areas of South Europe surrounding the mediterranean and has even been found as far north as the UK, carried there by migratory birds. Recently, CCHFV infected ticks were even detected in Spain found on deer.However in both these places, endemic or even epidemic outbreaks of the virus have not been observed. Perhaps the virus is not completing its full life cycle there or maybe infections do occur but are sub-clinical. A recent analysis suggested that the risk to more northerly European regions is low, citing cold springs that would prevent the ticks from surviving. But that doesn't stop Europe from worrying as two years they published a report mentioning that:
"....a rise in temperature and a decrease in rainfall in the Mediterranean region will result in a sharp increase in the suitable habitat areas for H. marginatum and its expansion towards the north, with the highest impact noted at the margins of its current geographic range"
That's right. One of the major issues with tick borne pathogens is the changing environment, whether it is climate change or agricultural growth. Here's a great review on the role of climate change on the distribution of ticks. One of their points regarding CCHFV is that very little is understood about how this virus interacts with ticks (we don't exactly know what species it infects or how) and their vertebrate hosts (what animals are infected in the wild). So to answer the question at the start: will UK ever see CCHFV again? I wouldnt be suprised if another imported case springs up from endemic regions but whether it will establish itself here is not so predictable. Certainly at the minute it cant: our weather is much too harsh for the tick. But what about in the future as climate change alters the environment in and around the Mediterannean? We're going to need more research into the virus to answer the above questions. CCHFV is an important global pathogen and an unmet medical need.