Why is there an epidemic of mosquito and tick-borne diseases? Because we are all that’s left for them to feed on

There is an epidemic of mosquito and tick borne diseases spreading among humans. Yellow fever is returning, west nile fever has increased, dengue fever has increased 30-fold between 1960 and 2010 and Zika turned into a brain-damaging disease seemingly out of nowhere. There’s also malaria, which most people are well familiar with.

Most of these diseases occur in tropical countries, but Northern latitudes are witnessing their own problem in the form of an explosion of tick borne diseases. There are sixteen known to us, of which four have only been discovered to exist since 2013. One in particular, is spreading rapidly. Crimean-Congo hemorrhagic fever is spreading in South-Eastern Europe and kills anywhere between 10 and 40% of people it infects. Lyme disease is also spreading very rapidly across the developed world and making it dangerous for humans to venture out into nature.

What causes this problem? There are a few important factors. There is more international travel, at faster paces, than ever before in human history. The natural barriers between human communities that would normally prohibit the spread of disease no longer exist. There are also more humans out there. Diseases that would normally drop below the threshold needed for them to survive due to low population density now manage to find sufficient human hosts to proliferate. One study found a minimum population density of roughly five people per square kilometer for malaria to gain a foothold in a particular part of the Amazon.

Most importantly however, for mosquitoes and ticks to be good transmitters of viruses and other pathogens, they have to be exposed to a limited number of species for them to prey on. Low biodiversity means that it becomes easy for a virus or bacteria to jump from one tick or mosquito to his victim, then from his victim to new ticks or mosquitoes. If the animal that’s bitten is too different from the animals that the pathogen normally replicates in, the tick can continue his life cycle by sucking up blood, but the disease can’t proliferate in the victim. As a result, mosquitoes and ticks are born that have been cleansed from the disease. These animals, that put a stop to the life cycle of the pathogen, are known as “dead end hosts”.

It’s important to have a lot of different types of dead end hosts, if we wish to reign in the spread of vector-borne diseases. Large animals, like deer and carnivores, tend to be relatively poor hosts for these diseases. For Lyme disease, the best host is the white-footed mouse. It’s generally found that places with a lot of different types of rodents have far fewer cases of Lyme disease in humans, than places where species diversity is low.

The king of all dead end hosts however, appears to be the humble lizard. One reason for this is thought to be that many lizards are capable of surviving winter with a below-zero body temperature. During this period, the body of the lizard is cleansed of pathogens like Lyme disease. As a result, studies tend to find that ticks found on lizards are much less likely to carry Lyme disease than ticks found elsewhere. Just 1.4% of ticks on lizards in the Netherlands carry Lyme disease, compared to 24% of ticks found in heath and 19% of ticks found in forests. In the United States, a similar effect is found.

There are different perspectives on biodiversity. Deep ecologists would say that non-human life has a right to survive undisturbed in spite of whatever benefits these organisms may have for humans. Unfortunately, such arguments tend not to be particularly persuasive for politicians like Paul LePage, who argues that “Job creation and investment opportunities are being lost because we do not have a fair balance between our economic interests and the need to protect the environment”.

Because the survival of non-human species needs to deliver economic benefits for most policymakers to show interest, more ecologists these days study the impact of biodiversity on disease burdens in humans. Perhaps the realization that a world completely dominated by humans and a few species that happen to escape our destructive impact is also a world where humans are plagued by a growing number of vector-borne diseases could help a few species escape the threat of extinction that looms over them.

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