Parasitic disease spread in a world of anthropogenic climate change

When people hear the word ‘parasite’ it generally comes with a negative connotation. At the start of 2020, Australia was facing the beginning of what would be the most devastating fire season in recorded history. The fire that burnt through the south east coast started when a group of students and lecturers from the ANU were studying ‘what the world would be like without parasites’ in the undergraduate course ‘Appreciating Parasites’. Now we are facing a global pandemic, from a virus that was able to spread because of many of the same reasons that are outlined in the blog post leading on from this introduction. Population density, warming weather, drought, are all things that we may dissociate from parasites but in reality, they are impacted by these stressors, and we are, in turn, by them. Parasites often get such negative opinions because of malaria, Lyme disease, and the general idea of ‘being reliant on another’ for life. Yet often it is humans that have invaded the sphere of the parasite, we have created a warmer climate and they are responding. Humans have a tendency to blame others if things ‘go South’ but we must take responsibility and learn to deal with the consequences.

Climate change is a normal part of the earth’s ecosystem; however, we are currently experiencing an anthropogenic change within this natural phaenomenon. With unusual warming, well beyond the normal rate, many forms of life are unlikely to be able to adapt fast enough, with their environmental niche no longer available. Parasites have a crucial role in the earth’s ecosystem and are often left out of the climate change conversation. Malaria prevalence for example is highly susceptible to changes in climate, increasing five-fold after an El Nino event, due to increased humidity (Bouma et al., 1996). Changes in weather patterns, and an increase in extreme weather events are likely to lead to changes in patterns of parasite transmission, resulting in host populations which have not co-evolved with parasites and their diseases. As humans and our environment are placed under more extreme stressors, we become more susceptible to disease, thus creating a positive feedback loop. A recent projected model has shown that the predicted future climate in southern Europe is likely to be suitable for pathogens transmitted by Aedes albopictus (Caminade et al., 2019). If we then factor in increased urbanisation, we are likely to see further increases in temperature and therefore disease transmission.

A warmer planet creates longer extrinsic incubation rates, extended transmission seasons and expanded distribution for pathogens (Caminade et al., 2019). Higher temperatures can also change the patterns of survival of intermediate hosts and the developmental cycle of the parasite within (Barber et al., 2016).

As water becomes a scarce resource, we will be forced to have larger storage areas for the increased risks of extreme droughts. The likelihood of contamination of large water bodies when it is otherwise limited in the natural environment is elevated, leading to potentially catastrophic disease transmission. An alarming example was the 1998 Sydney water crisis which reported unusually high levels of contaminating parasite species (Cryptosporidium and Giarida, both causing severe diarrhoea and potentially leading to death). This was allegedly due to intake of high amounts of ‘raw water’ into the dams providing drinking water supply to >3 million Sydneysiders after draught-related low water levels in the dam. We are looking at a complex issue, and while parasites may not yet be considered a variable within climate change as a human stressor, we are facing widespread and unpredictable risk. A need to invest time into modelling projected disease spread with temperature increases is paramount to prevent widespread outbreaks. While modelling this spread is relevant, the question of whether humans will be able to survive increased disease spread should also be considered. It is now time and more important than ever to change our behaviour to not give pathogens the upper hand.

Margot SchneiderMelanie Rug

 - Margot Schneider (ANU Undergraduate student) in collaboration with Dr. Melanie Rug


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Read more about Parasites: The Good, The Bad, and The Ugly.