Now might be a good time for a text book clear-out.
In 2021, the world’s attention continues to be on covid-19. In the UK, the current focus is on vaccination and a hesitant return to normal life. But the nature of the outbreak means that many countries are still at different stages of the pandemic. Some people have said that the covid pandemic could not have been predicted – but there have been warning signs for many years. These include outbreaks of Ebola, Zika, Influenza, and Middle East Respiratory Syndrome. There have also been warnings that climate change could contribute to infectious disease outbreaks.
Global warming has certainly changed the geographic distribution of mosquitoes and the diseases that they can carry. These include serious infections such as dengue, chikungunya, and malaria. (1) Malaria is a classic case. It is becoming more common in areas where it was rare in the past, simply because of climate change. Temperature change also affects the behaviour of Salmonella, Campylobacter, and Vibrio. It can affect viruses as well, such as Japanese Encephalitis Virus. Climate change can cause drought which in turn results in the concentration of bacteria in fresh waters. But floods cause problems too – they can overwhelm local sewage systems and so result in outbreaks of cholera. Rising global temperatures also mean that diseases that were once concentrated around the equator will now move both north in the Northern Hemisphere and south in the Southern Hemisphere. Certain vectors may also move to areas of higher altitude where lower temperatures might have put them off in the past.
These are examples of change that might be considered predictable. But there is a great deal of unpredictability also. For example, storms and strong winds can prevent mosquitos from biting but can equally extend the distances that they can fly. These are direct effects of climate change on infectious diseases in humans – there are a range of indirect effects also. For instance, climate change can cause crop failure and therefore malnutrition, thus rendering people more vulnerable to infectious diseases.
What, if anything, can we do about this? The obvious answer is to prevent further climate change, but this will take years to happen. In the meantime, we have to ensure that global and local knowledge is current and that we adapt to changing circumstances. For example, malaria control measures that we know work in malarious areas may need to be extended to higher latitudes and sometimes to higher altitudes. The future will also be about adapting our index of clinical suspicion so that we think about new and emerging infectious diseases as part of our differential diagnosis when we see a patient with an unusual illness.
So if you have any old books on tropical infectious diseases, now might be a good time for a clear-out. The reason is that much of what has been taken as received wisdom in the past is now out of date. And the cause of this is climate change. Some of my old textbooks had maps of where certain tropical diseases might strike but these maps are now being rapidly redrawn. There are many examples – Wu and colleagues have outlined some of them in their paper on the impact of climate change on human infectious diseases. (2)
One chapter that I can remember from old medical textbooks is fever in the tropical traveler. It was full of mystery illnesses in patients just returned from exotic locations, and so always made for a good read. But if current trends continue, in the future it will be called fever in the non-tropical traveler.
There is one final reason to think twice before using textbooks. It is not just epidemiology that might be out of date – the core content on diagnosis and treatment might also have been superseded by scientific advances. Which brings us back to covid-19. The pace of scientific discovery has been unprecedented. There are now 175,428 papers on PubMed on covid-19, a disease that it less that two years old. Only digital knowledge resources will work in these circumstances.
KW works for BMJ which produces the clinical decision support resource BMJ Best Practice.
This opinion piece was authored by BMJ Clinical Director, Dr Kieran Walsh. The purpose of this document is to educate and to inform. The content of this document does not constitute medical advice and it is not intended to function as a substitute for a healthcare practitioner’s judgement, patient care or treatment. The views expressed by contributors are those of the author's. BMJ does not endorse any views or recommendations expressed in this document. Readers should also be aware that professionals in the field may have different opinions. Users of this document hereby agree not to use its content as the basis for their own medical treatment or for the medical treatment of others.
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