Climate Change Leads to Rapid Emergence of Infectious Diseases
That’s the key findings of two new studies that show viruses such as Ebola, H1N1 and TB, as well as dengue and yellow fevers could spread further and become more frequent because of our changing climate.
Mosquito, a disease vector. Photo credit: Creative Commons: Enrique Dans
In one recently published article, zoologists studied climate in two vastly different regions—the tropics and the Arctic—to gain an understanding of how climate change may affect the spread of disease.
In both regions the scientists found that by altering and moving habitat zones of disease-carrying animals, climate change could be making outbreaks of diseases more frequent.
Previously, scientists believed that parasites could not quickly jump from one host to another because of the way parasites and hosts co-evolve. This would, in effect, make new disease more rare as parasites would first have to evolve a genetic mutation in order to move to another species.
However, the new analysis argues that these evolutionary jumps can come quicker then anticipated.
“Even though a parasite might have a very specialized relationship with one particular host in one particular place, there are other hosts that may be as susceptible,”said Daniel Brooks, professor at University of Nebraska-Lincoln.
Newer hosts are more susceptible to infections because they haven’t developed resistances to them, making the hosts more likely to get sicker.
The researchers predict that as humans move deeper in wildlife areas they are more likely to interact with animals affected by new more virulent strains of pathogens. This would increase the rate of human epidemics and could be spread even further through global air travel.
Sometimes the new diseases will come to us more directly.
In another new study, a team of researchers from the U.K. and Germany found that rising temperatures in Europe could bring traditionally tropical diseases such as dengue and yellow fevers to Europe.
The researchers predicted 2.4 billion people could be exposed to the Asian tiger mosquito by the middle of the century, as they emigrate from Africa to Europe’s new warmer climate.
The mosquito can transmit pathogens that spread diseases including dengue fever, chikungunya infection, yellow fever and encephalitis.
The research suggests the chances of the Asian tiger mosquito, hitting the UK and France are higher than previously thought.
Eastern Brazil, the eastern U.S., Western and Central Europe and Eastern China are also likely to provide increasingly suitable habitats for the mosquito between the period 2045 and 2054.
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From the mythical minotaur to the mule, creatures created from merging two or more distinct organisms – hybrids – have played defining roles in human history and culture. However, not all hybrids are as fantastic as the minotaur or as dependable as the mule; in fact, some of them cause human diseases.
When Looking Through a Microscope Isn’t Close Enough.<p>For the last few years, <a href="http://www.rokaslab.org/" target="_blank">our team at Vanderbilt University</a>, <a href="https://www.researchgate.net/lab/Gustavo-Goldman-Lab" target="_blank">Gustavo Goldman's team at São Paulo University in Brazil</a> and many other collaborators around the world have been collecting samples of fungi from patients infected with different species of <em>Aspergillus</em> molds. One of the species we are particularly interested in is <a href="https://doi.org/10.1006/rwgn.2001.0082" target="_blank"><em>Aspergillus nidulans</em>, a relatively common and generally harmless fungus</a>. Clinical laboratories typically identify the species of <em>Aspergillus</em> causing the infection by examining cultures of the fungi under the microscope. The problem with this approach is that very closely related species of <em>Aspergillus</em> tend to look very similar in their broad morphology or physical appearance when viewing them through a microscope.</p><p>Interested in examining the varying abilities of different <em>A. nidulans</em> strains to cause disease, we decided to analyze their total genetic content, or genomes. What we saw came as a total surprise. We had not collected <em>A. nidulans</em> but <em>Aspergillus latus</em>, a close relative of <em>A. nidulans</em> and, as we were to soon find out, <a href="https://doi.org/10.1016/j.cub.2020.04.071" target="_blank">a hybrid species that evolved through the fusion of the genomes</a> of two other <em>Aspergillus</em> species: <em>Aspergillus spinulosporus</em> and an unknown close relative of <em>Aspergillus quadrilineatus</em>. Thus, we realized not only that these patients harbored infections from an entirely different species than we thought they were, but also that this species was the first ever <em>Aspergillus</em> hybrid known to cause human infections.</p>
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(Left) Candida yeasts live on parts of the human body. Imbalance of microbes on the body can allow these yeasts, some of which are hybrids, to grow and cause infection. (Right) Cryptococcus yeasts, including ones that are hybrids, can cause life-threatening infections in primarily immunocompromised people. Centers for Disease Control and Prevention<p><a href="https://doi.org/10.1371/journal.ppat.1008315" target="_blank">Why certain <em>Aspergillus</em> species are so deadly</a> while others are harmless remains unknown. This may in part be because <a href="https://doi.org/10.1016/j.fbr.2007.02.007" target="_blank">combinations of traits, rather than individual traits</a>, underlie organisms' ability to cause disease. So why then are hybrids frequently associated with human disease? Hybrids inherit genetic material from both parents, which may result in new combinations of traits. This may make them more similar to one parent in some of their characteristics, reflect both parents in others or may differ from both in the rest. It is precisely this mix and match of traits that hybrids have inherited from their parental species that <a href="https://www.nytimes.com/2010/09/14/science/14creatures.html" target="_blank">facilitates their evolutionary success</a>, including their ability to cause disease.</p>
The Evolutionary Origin of an Aspergillus Hybrid.<p>Multiple evolutionary paths can lead to the emergence of hybrids. One path is through mating, just as the horse and donkey mate to create a mule. Another path is through the merging or fusion of genetic material from cells of different species.</p><p>It is this second path that appears to have been taken by our fungus. <em>A. latus</em> appears to have two of almost everything compared to its parental species: twice the genome size, twice the total number of genes and so on. But unlike other hybrids, which are often sterile like the mule, we found that <em>A. latus</em> is capable of reproducing both asexually and sexually.</p><p>But how distinct were the parents of <em>A. latus</em>? By comparing the parts contributed by each parent in the <em>A. latus</em> genome, we estimate that its parents are approximately 93% genetically similar, which is about as related as we humans are with lemurs. In other words, <em>A. latus</em>, an agent of infectious disease, is the fungal equivalent of a human-lemur hybrid.</p>
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