Horseflies have had a good year: their numbers seem to have exploded as a result of the sustained hot weather, and there has been a corresponding epidemic of bites. Practising in a semi-rural location, I’ve lost count of the number of patients who have been to see me in surgery or out-of-hours. NHS 111 has reported upwards of 9,000 calls per week from probable horsefly victims.
Most blood-feeding flies such as mosquitoes pierce the skin with an ultra-fine proboscis through which they suck up their food. One doesn’t generally notice the bite until later, when the saliva they inject to prevent clotting provokes a localised reaction.
Female horseflies, in contrast, are brutes (it’s only the females that bite animals, needing a protein-rich meal to kick-start egg production). Their proboscis contains several sharp stylets: they stab, slice and saw open a wound. Another sponge-like mouthpart laps up the resultant blood, which is kept flowing by large quantities of anticoagulant saliva.
The reaction can be dramatic – it is not uncommon for a substantial part of a leg or arm to become markedly swollen and inflamed; even blistered. It is so contrary to most people’s experience of an insect bite that they fear infection must have set in, leading to medical consultation.
This poses a dilemma for the clinician. The florid reaction to a horsefly bite can look remarkably like cellulitis – infection of the skin and subcutaneous tissues – which can, to confuse things further, develop on the back of an insect bite. Cellulitis can be serious, even fatal, and no one wants to miss a case. Every summer there is a national uptick in prescriptions for the antibiotics used for suspected cellulitis; this year’s surge is going to be huge. But most of these prescriptions are unnecessary. Doctors’ online forums have been hosting lively discussions with clinicians sharing their ideas for distinguishing insect bite reaction from skin infection, in an effort to reduce antibiotic overuse.
The heatwave is being widely attributed to climate change. With temperatures predicted to rise further over the coming decades, there is concern that diseases more serious than horsefly bites might become more commonplace in the UK. Case reports of Lyme disease are increasing by around 35 per cent per year. While better awareness and detection will be contributing to the rise, it is likely that warmer temperatures are leading to greater numbers of the ticks that transmit the infection to humans.
And while we think of malaria as a tropical disease, as long ago as 2001 the Department of Health was warning that it might become endemic in the UK by mid-century, thanks to global warming. The life cycle of the malaria parasite, Plasmodium, tends to be found in regions where temperatures consistently exceed 16-18°C. One only has to think of the hot, sticky nights we’ve endured this year to see that British summers may soon satisfy this criteria.
Interestingly, though, England was malarial until well into the 19th century. The condition, then known as “the ague”, was associated with wetlands and marshy areas. However, Plasmodium probably died out not because our climate became markedly cooler in the 20th century, but because of other changes that affected its mosquito vector.
While mosquitoes will bite humans, they prefer livestock, and intensive farming provides far greater opportunity to feed on animals in whom Plasmodium cannot complete its life cycle. In addition, the drainage of large areas of marshland for agriculture and settlement deprived mosquitoes of their natural habitat. Several stages of the mosquito life cycle are aquatic, and they cannot breed without stagnant water.
Horseflies require similar conditions, and I wonder whether their numbers will fall next year, as breeding sites dry up in the drought. The effects of climate change are more complex and difficult to predict than temperature rises alone.
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This article appears in the 08 Aug 2018 issue of the New Statesman, The rise and fall of Islamic State