Atishoo, atishoo, we all fall down… : How diseases spread around the world

Science and Technology

Image Credit: CDC Global

We all know that coughs and sneezes spread diseases. The problem is that these days, things are getting a bit more complicated than that. As our lives become more interconnected, and global travel shrinks the world village yet further, it is only inevitable that our diseases will be coming with us wherever we go. When you begin to consider the role of animals, climate change and trade, among others, the old adage is less reductive and more outright misleading. Let’s see how that holiday in Barbados, or the exotic pet from far afield, is just another battlefield in the war against disease.

An Invasive Species is officially defined as a species ‘non-native (or alien) to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health’. While the impact of invasive species is on their own troubling, it is also one of the easier ways for a disease to be spread. Indeed, we ourselves are not above using it, with the Mongols said to have used the bodies of bubonic plague victims as part of the siege of Caffa in Crimea in 1346. This incident has been attributed to bringing plague to Europe and causing the Black Death over the next few years. For the grey squirrels, there is little chance of any backfire like this. Their impact on red squirrels, after being introduced into the UK in the 1870s to add ‘colour’ to the native fauna, has been catastrophic, bringing populations of reds from around 3.5 million to 120,000. Disease has played a key role in decimating their populations in the form of the squirrel parapoxvirus. It causes tumours and lesions to form, which often carries a mortality of 100% in red squirrels, with very few survivors having ever been found. Grey squirrels, meanwhile, are only rarely found suffering its symptoms. They have become immune to the disease, acting as carriers that pass on the virus without contracting the disease. Studies across populations of red squirrels show that only a couple of small populations in Lancashire and Cumbria carried the necessary antibodies to fight a parapoxvirus infection, while the vast majority of individuals within all grey squirrel populations tested did. It is worth pointing out that while the red squirrel had been lost from areas of the country prior to the invasion of the grey squirrel, the latter certainly exploited this lapse to the fullest of its abilities. On colonising the broad-leaved woodlands of much of the south of England, red squirrels found it more difficult to compete with the more voracious greys, which are around twice the weight, driving the reds away into coniferous forests that can support them only. Furthermore, red squirrels experience a change in their behaviour in response to grey squirrels, with less mating also contributing to a declining population. All together, this makes the red squirrel a rare sight in most of the UK, except for a few refuges, such as Brownsea Island (where I myself was privileged to see one), or the Scottish forests where more than three quarters now reside.

Invasive species, as well as being disease vectors themselves like the aforementioned grey squirrel, can also carry them. One example is the Amur Honeysuckle, an invasive plant in North America, which is often infested with lone-star ticks. These ticks carry Ehrlichia chaffeensis, a bacterium which causes the disease human monocytic ehrlichiosis. The symptoms of this disease include anything from fever to meningitis, and in some cases can lead to death. The spread of these ticks is aided by deer, who are attracted to the Amur honeysuckle for feeding, both on the plant itself and also on other plants that the honeysuckle fosters in the understory. These deer can then carry the ticks over great distances, bringing them to new plant populations where they pose a new risk.

While invasive species may be an indirect consequence of our actions, we also have to shoulder some directly for the increasing spread of diseases across the world. Recent epidemics have shown just how quickly a disease can spread through our transport networks, especially international air travel, where we willingly put a dense group of potential hosts into a vehicle capable of moving them over vast distances. After you throw in connecting flights the world is truly a disease’s oyster, with around 90% of the world’s population living within two days of a major city. While more recent epidemics have shown our vulnerabilities, like the Ebola crisis just a few years ago, perhaps the most powerful is the ever-present flu. The Swine-flu outbreak in 2009 showed how our lifestyles, in this case our close interactions with animals, present an opportunity for a disease to spread. In this case, the strain of the influenza virus, after being picked up in Mexico, was found to have spread far and wide, including Australia, South Africa and Japan among many others. A reason it may have been able to do this is because, as a flu virus, it could’ve been mistaken for other forms of the disease. As anyone who plays the game Plague Inc. knows, all it takes is for a disease to be asymptomatic, or have very generic symptoms, for most people to assume it’s a cold and carry on regardless while it silently takes over. Non-human travel also has its part to play, as the farm trade allowed a diverse range of viruses to recombine their DNA and produce unique combinations that can evade vaccination efforts.

Climate change is a further complicating factor for the world of disease. For example, the chytrid fungus has had a devastating impact on amphibian populations across the world over the last decade or so. While human influences like the pet trade in amphibians has helped spread it, the fungus requires damp, cool conditions to prosper. As the world warms, the amount of water evaporating is increasing, contributing to a more humid climate but also a greater amount of cloud cover. The clouds, by blocking the sun’s rays, are therefore causing local cooling, which together gives the chytrid fungus a greater range of locations to grow over. The fungus is then more able to infect the skin of frogs and other amphibians, by forming a cyst within the skin. It uses enzymes to digest its way in, and then feeds off the skin itself. Given that many amphibians use their skin to breathe, using their dense network of capillaries contained within, this has a quick, and often lethal, effect. Vectors too are expanding with climate change, including the mosquito. The incidence of mosquito bites in the run up to 2011 was 2.5 times greater than the same period up to 1996, as rising temperatures enable them to spread beyond their standard range and survive in the UK’s climate. A new arrival to the UK, Aedes albopictus, has recently began to be picked up in Kent, and it seems likely that it will establish and become more common with time.

In addition to these established diseases, new strains and diseases are arising all the time. Emerging infectious diseases are those whose incidence is increasing since its first introduction into a defined host population, and as such when they start out they are relatively unknown. But many are now commonly known, from Ebola to HIV. For example, HIV became well known in the 1970s and 80s, but tracking it back through time reveals that the original jump from primates to humans occurred sometime in the 1920s, probably through the bushmeat trade. Its spread was limited at first until the development of railways near Kinshasa allowed the first infected group to travel to other cities in the Congo, like Brazzaville. Later in the 60s, the disease started to take effect, as one strain of the virus (HIV-M) began to exponentially replicate very fast, and so the infection began to spread worldwide. It was eventually described in medical literature in 1981. The ancestor of HIV, chimpanzee SIV, is likely to have arisen through recombination of other SIV viruses. These rearrangements in the RNA enabled a new form of the disease to develop. Furthermore, RNA has a higher rate of mutation than DNA, making it more likely that a form able to infect humans would soon arise.

With this worrying myriad of ways disease can arise, it seems like we should lock ourselves indoors and never go outside again. It’s worth pointing out, after this doom and gloom, that there are vaccines, and various ways of avoiding disease, to prevent you from contracting it. Furthermore, the changing world may also kill off some diseases, as their hosts are unable to cope with the new conditions. As we move into an increasingly uncertain century, we can be sure that diseases will still be a big part of 21st century life.