The hygiene mythbuster

Infections and how they spread is a gruesomely fascinating subject. For example, the Black Death escaped Asia in the mid-1300s for Europe and beyond when the Mongol army catapulted plague victims into the besieged Crimean city of Caffa. The fleeing survivors then spread the disease to the Mediterranean Basin and onwards to Europe and Africa.

This isn’t just the most spectacular example of biological warfare ever, it has relevance today in a way not immediately apparent. Global warming. Yes, infections really can change global climate.

As a race we’ve always chopped down forests for firewood, for building materials and for agriculture. Fewer trees means less carbon dioxide being removed from the atmosphere, plus burning trees and latterly fossil fuels releases carbon dioxide. This double-whammy is the reason carbon dioxide levels have slowly crept up over millennia. But there have been periodic reversals in that trend.

Plague killed up to two thirds of Europeans and 90% of the population of the Americas after Columbus took it there. While estimates of the death toll vary, it’s been proven that abrupt reversals of this slow CO2 rise all coincided with bubonic plague and other historic pandemics.

Why? High mortality rates led to large-scale abandonment of farms. As forests re-established themselves carbon dioxide was pulled back out of the atmosphere. When plagues subsided normal service was resumed: farms were reoccupied, the new forests cut down and carbon dioxide was returned to the atmosphere once more. 

So, infections can change climate. But climate change – and specifically global warming - changes the spread of infections too. This is happening before our very eyes.

Threats are 'radiating north'

Oceans are getting warmer and spreading infections further. Vibrio parahaemolyticus is usually transmitted by oysters and causes a particularly nasty form of food poisoning. But it doesn’t like the cold. As coastal water temperatures have increased it’s now spread as far north as Alaska because the sea is now warm enough to the growth of V. parahaemolyticus - with resultant outbreaks. 

Another Vibrio – the one that causes Cholera – is also on the move. Its usual hosts are tiny crustaceans that live on phytoplankton. It’s been shown that major cholera outbreaks in coastal regions of India and Bangladesh are preceded – and can be predicted - by climate-driven phytoplankton ‘blooms’. Outbreaks are also occurring in places formerly too cold to support pathogenic vibrios from Africa to Northern Spain.

Zika might be the latest mosquito-borne threat to hit the headlines but there are plenty more to worry about. And as climates get warmer, mozzies are spreading their range further north and to higher altitudes.

Dengue – breakbone fever – is slowly radiating north from the southern states of the USA using the Aedes mosquito; the same vector as Zika. This species had never been seen at altitudes over 1,000 metres above sea level but that range has more than doubled, as have cases of Dengue. 

It’s not just mosquitoes. Climate change will affect the distribution of other disease vectors such as insects and snails which might thrive with increased temperatures or might die off. It’s more likely that like the mosquito, they will move.

Also more rainfall often means more food which means more rodents. We’ve already seen Hantavirus in the UK and Belgium via rats and mice, so Bubonic Plague could be next. Or Legionnaires’ Disease which always spikes after thunderstorms in areas with otherwise hot and humid weather.

It’s not all doom and gloom, though. In the UK Respiratory Syncytial Virus (RSV) is one of the most common causes of coughs and colds and bronchiolitis in children each winter. Since 1990 the RSV season has shortened by two and a half weeks for every 1°C rise in average temperature.

But be careful what you wish for. Just as Zika, West Nile, Dengue and Chikungunya are radiating across the USA on the back of climate change, it also means Europe is at risk of malaria becoming endemic. It only needs a rise of 3°C or so…

Seán Derrig is scientific director at Chemex International