Although recently the news may have been dominated by politics, it doesn’t mean that another rather large issue is not relevant. I am talking about climate change, though it may not only affect the weather!
Climate change can be defined as “a large scale, long-term shift in the planets’ weather patterns or average temperatures” . So I’m sure everyone knows that climate change results in global warming, in fact the global average temperature has risen by 0.89⁰C between 1901 and 2012 . Now this may not sound like a big change but any increase in temperature could have consequences on many natural systems. In fact a temperature increase is just one of many interacting factors that can lead to severe consequences on a global scale such as; a rising of sea levels, more extreme weather events e.g. hurricanes, increased rainfall and changes in nature i.e. seasons appearing earlier/later . The extremity of the changes seen will be linked to the level of greenhouse gases, such as carbon dioxide and methane, that are being released into the atmosphere. Now if these impacts weren’t enough, research has been conducted into how other things may be affected. One of these things are diseases, and one potential interaction could be between climate change and the plague.
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Yersinia pestis is the bacterium that is responsible for causing the plague. The most famous instance of the plague has to be the ‘Black Death’ which between 1347 and 1351 resulted in Europe alone losing a third of its population . Although we may no longer get full blown pandemics, the plague is by no means an old, extinct disease. In fact in endemic areas such as China and Africa there are seasonal outbreaks and thousands of cases are reported to the World Health Organisation (WHO) annually. In November 2014 there were forty deaths and eighty infected in Madagascar .
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This is contracted when a person is bitten by an infected flea. The flea is initially infected by drinking the blood of an infected rodent. The bacteria will multiply within the flea and stick in clumps called a biofilm. This clogs the fleas’ stomach causing it to starve, so it bites a host to feed, but it cannot suppress its hunger as the biofilm is preventing the blood from travelling through the flea. Consequently it vomits the now infected blood back into the bite wound on the host . Once inside the body Y.pestis will multiply in cells and tissues, finally entering the lymphatic system. Inside the lymph nodes it causes inflammation, resulting in the characteristic buboes. From here it may pass into the blood stream causing ‘Septicemic plague’ .
This occurs if the bacteria infect the lungs. It can now be transmitted via airborne droplets if you cough/sneeze, making this variation of the plague more contagious. The initial symptoms are; weakness, headaches, vomiting or spitting blood . Unless this is diagnosed and treated quickly death usually occurs in one to six days, and left untreated the mortality rate is almost 100% .
However, it isn’t all doom and gloom! If you are diagnosed with the plague most variations are responsive to antibiotic therapy and there is a working vaccine .
My main questions in this article are, could climate change impact on plague cases? And if so how does it cause these changes?
I mentioned before that the plague is a vector-borne disease, and it is believed that climate change may have the greatest impacts on these types of diseases. This is because pathogen abundance is temperature dependant and the higher temperatures brought by global warming will promote bacterial replication. This in turn will cause vectors (Fleas), to become infectious faster . In fact Stenseth et al. conducted a study introducing a 1⁰C increase to their Vector-Plague model. The result was a 50% increase in plague prevalence in great gerbils in Kazakhstan . In addition to this, the vector numbers themselves could be affected by climate change impacts. The reason for this is that flea larvae are vulnerable to desiccation so, (to a point), increased rainfall could promote their survival . Furthermore, rodent populations could also be boosted by the increased precipitation. Wetter weather could aid vegetation growth so providing a larger source of food for the rodents. Thus resulting in a greater animal reservoir for the plague . However, if the increase in rainfall is too large it may have the opposite effect as the rodents could die in subsequent floods.
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An increase in extreme weather events is another result of climate change that could impact on plague prevalence. This would have social impacts on prevalence as these extreme events lead to the displacement of individuals (especially in less economically developed countries), and breakdown of infrastructure. Meaning basic sanitation may be lost and individuals could be forced into close proximity with each other and potential animal reservoirs thus promoting the spread of disease . Finally, the warming could result in the alteration of the geographic distribution of diseases such as the plague. The prediction is that they would spread poleward, due to warmer temperatures in the more northern and southern latitudes. Thus meaning that they would be arriving in new areas that may be ill prepared to control/treat them .
However, climatic related changes in plague are hard to determine, not only because we don’t know exactly how the climate is going to change, but also because areas may be affected differently. For example a study by Xu et al. , found that in Northern China (usually an arid climate), increased precipitation raised plague prevalence. Whereas, in Southern China (usually humid), the increase in wetness reduced plague. This demonstrates how plague vectors and reservoirs differ globally and the original climate of an area impacts on the interactions between the plague and climate change . Overall, they predict increased precipitation to increase plague in dry climates but reduce them in wetter climates. In addition, Schotthoefer et al. , found the efficiency of transmission of Y.pestis by fleas was greatly reduced at temperatures between 27 and 30⁰C, (optimum temperature = 23⁰C). Because at higher temperatures Y.pestis can’t effectively form biofilms that are needed to block the flea gut, to prompt regurgitation of the bacteria into a host when fleas feed. Therefore, if climate change results in large temperature rises, plague cases may actually fall .
In conclusion, climate change is occurring and will have its own impacts on the planet. However, what is currently uncertain is the extent of the impact it will have on the planet and on other processes like diseases. As I’ve mentioned there is already evidence that it could affect the plague at the bacterial, vector and host levels thus influencing the prevalence of the disease. But it’s not only the plague that could be impacted… Respiratory diseases such as asthma could be exacerbated due to the increased levels of ozone, dust and pollution that climate change could bring, so reducing air quality . Other vector-borne diseases could also be influenced, for example Lyme disease could be prevalent in more areas as ticks spread due to warmer temperatures .
Until we can better judge the severity of the climate change we are dealing with, we will be unable to generate more accurate predictions and models to determine all the interacting factors and their impacts on diseases. But for now remember to switch off the light as you leave a room and maybe walk to the shops!