The Asian tiger mosquito (Aedes albopictus) is the most invasive mosquito in the world spreading from its native range to at least 28 identified countries around the globe.
PHOTOGRAPH BY Oliver Spiteri
Species of aggressive mosquitoes are hitchhiking their way within used tyres from their native ranges into Europe, carrying with them deadly diseases such as dengue and chikungunya. How do we stop these mosquitoes’ voyage or is it already too late?
Throughout the 20th and 21st centuries, the sheer quantity of various goods being shipped worldwide has simply expanded beyond individual comprehension. The Taiwanese owned container ship, the Ever Ace, is the world’s largest vessel hauling 23,992 containers across its 400m hull each voyage. Unfortunately, this extensive scale leaves growing opportunistic room for unwanted passengers on deck. Invasive species have commonly become transported worldwide by hitching a free ride on the endless trade routes between global nations, and two aggressive species of mosquitoes are no exception to hopping on board.
Aedes aegypti (the yellow fever mosquito) and Aedes albopictus (the Asian tiger mosquito) originated from Sub-Saharan Africa and Southeast Asia respectively before spreading to South America, North America, and most recently Europe. Both mosquito species actively seek water to lay their eggs. As our urbanised world encroaches further upon the natural, species have adapted to use human creations that replicate the wild, in this case, it’s as simple as an old car tyre. Their concave shape allows for stagnant water to collect, providing the ideal habitat for mosquito larvae. The tyre’s dark colour causes rapid warming in sunlight speeding up larval development and when tyres remain undisturbed over lengthy periods, sequential generations can reach extremely high population densities.
Tyre dealers, oblivious of the threat, ship contaminated second-hand tyres to buyers across the world as the mosquito species voluntarily hitchhike to their new homes. Water isn’t even required to keep the eggs alive. Mosquito eggs are exceptionally tolerant to dry periods surviving months without exposure to moisture, therefore allowing them to be transported to new geographical areas before hatching on arrival when coming into contact with water. This renders the mosquito eggs camouflaged to current biosecurity checks allowing them to freely cross borders to novel environments.
With the spread of these mosquitoes comes the impending threat of epidemic disease outbreaks in new nations. Between the two species, they carry well known harmful diseases such as dengue, yellow fever, chikungunya, and zika. Can the spread of these diseases via the hitchhiking mosquitoes be contained or have we acted too late? What actual solutions do we have in place to detect and prevent the spread of these mosquitoes? And ultimately, how will climate change interact to potentially exacerbate the impacts of these invasive species?
Disposed car tyres provide an ideal rearing environment for mosquito development. leaf detritus is trapped within the curve of the tyre providing food resources for successful larvae development. The tyre trade overall has been identified as the main mechanism for dispersing these two mosquito species to North America, South America, and Europe.
PHOTOGRAPHY BY Maksim Safaniuk
Ae. albopictus alone has hitchhiked from its native range in Southeast Asia to at least 28 other countries across the globe. With its spread to regions such as North America, South America and Europe comes the potential economic burden associated with disease outbreaks. For example, in 2005-2006, La Réunion Island in the Indian Ocean experienced its first ever chikungunya outbreak costing €43.9 million in medical costs and loss of productivity. Of the diseases that Ae. aegypti and Ae. albopictus can transmit, dengue and chikungunya are considered as the main threats to human health. Now we face a situation where both diseases are being locally transmitted on European soil.
Despite the encroachment into Europe, Ae. aegypti is no stranger to the taste of Europe, paying previous visits to the continent on its past travels. Prior outbreaks occurred in the Mediterranean region where the species was established up until the last major outbreak within Athens in 1928. Now Ae. albopictus is growing its range in Europe, just as it has already succeeded in the US.
At the end of the Second World War military equipment from Asia was shipped back to the US, including tyres. Within the imported cargoes A.albopictus was found to be present alerting the authorities of this imminent invasion. However, despite this foreshadowing discovery the tyre industry rolled on with the US subsequently importing millions of tyres annually from all over the world to destinations throughout the US. Tyre imports from Japan, Taiwan, Indonesia and Hong Kong all went unchecked although being native habitats for Ae. albopictus. Unsurprisingly in 1987, Ae. albopictus was discovered to be established in Houston, Texas and has now gone on to spread to 25 states across the country. Laws were introduced in 1988 to dry out, clean and fumigate all tyres arriving from ports of previous infestation but by then the invasion was already complete. By skipping the time and labour required to unload and inspect containers of used tyres, the US lost the war on defending their borders to Ae. albopictus: a story of a true trojan horse.
In 1988 the U.S Public Health Service decided that all tyre imports from ports known to have Ae. albopictus presence must be dry, clean and fumigated. By the time this policy was implemented the species had established populations in 15 US states.
PHOTOGRAPH BY Shutterstock
Now armed with established populations, US mosquitoes are seen to be globetrotting to Europe still using tyres as a free ride. Tyre exports from Miami, Florida in 2010 introduced Ae. aegypti into two separate locations in the Netherlands. However, Ae. aegypti lack established populations in Europe whereas Ae. albopictus is confidently asserting itself throughout the southern regions of the continent including Spain, France, and Italy.
The distribution of Ae. albopictus recorded in march 2021 in Europe shows populations in numerous areas. Ae. albopictus has become established across multiple countries in southern Europe and has been spreading northwards since 1995.
FIGURE BY European Centre for Disease Prevention and Control
Populations of Ae. albopictus are already present in Europe. Now the question to address is how climate change will allow these to spread. Multiple studies projecting future climate trends predict that the southern UK, the Balkans and central Europe will become highly suitable for Ae. albopictus survival by the middle of this century. Additionally, by the end of the 21st century, the risk of seasonal dengue outbreaks could become a reality for many nations in Europe if these Aedes vectors become fully established. Is cutting our emissions to reduce atmospheric warming the primary hope to decrease mosquito range in Europe, or can detection and prevention of these species in Europe play a role in controlling the spread?
Initially, tyre traders believed that simply drying out tyres before exportation would be enough to kill the larvae. However, once learning of the egg’s resistance to dry periods, there needed to be a way to ensure tyres didn’t contain them. Suggestions such as cutting tyres in half, covering them up and drilling holes in the side were put forward to prevent water pooling, but both are labour intensive and unconvincing in stopping water collecting. Instead, treatment with larvicides such as spinosad has been found to be highly effective against Ae. aegypti and Ae. albopictus larvae whilst being minimally disruptive to non-target species. But what happens if the larvae and eggs are still present, how will we detect them?
Insecticides are commonly applied in situations where insects negatively interact with humans such as crop yields. Spinosad is an insecticide based on chemical compounds found in the soil-dwelling bacterial species Saccharopolyspora spinosa.
PHOTOGRAPH BY Dan Susa
Researcher Clare Strode of Edge Hill University, UK puzzled over this question with her fellow researchers on a tea break before suggesting a simple but convincing idea. By using adhesive tape along the inner of the tyre, mosquito eggs would stick, allowing their presence to be confirmed directly by sight. The DNA from the eggs can then be screened in the lab to understand which exact species is present within the tyre. Applying this method to global borders would allow national authorities to stop the unwanted mosquitoes from crossing borders before the tyres are fully treated, stopping the hitchhikers in their tracks.
Adult mosquitoes, larvae and eggs found within tyres can be taken to laboratories to determine the species and presence of diseases in the vectors. This will need to become a vital part of biosecurity measures to detect and prevent mosquito spread in the future.
PHOTOGRAPH BY Shutterstock
The introductions of invasive species worldwide show no indication of halting whilst biosecurity checks are glanced past, and trade continues to exponentially expand. By successfully investing in appropriate biosecurity the consequences of a mosquito invasion especially in Europe could have been avoided. Instead, the early shortcomings in biosecurity may leave the continent to bear the risk of future epidemic battles. It appears that these invasive mosquitoes are here to stay, and they aren’t planning on travelling off anytime soon.
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