A hitchhiker’s guide to invasive alien species and disease transmission

Invasive alien species (IAS) are named thus because they end up stepping on other animals’ (or plants’) toes and are very much unwelcome. Whether they are introduced on purpose or by accident, IAS quickly become pests to the local ecosystem and can have effects from drastic alterations of the environment to simply being a better predator than the natives. Increasingly, in a world where industrialisation, globalisation and migration are on the exponential rise, IAS can be transported out of their natural habitats to wreak havoc in places from neighbouring locations to halfway across the world. With climate change becoming slowly more evident in day-to-day life, from erratic weather to rising sea levels and habitat change, IAS are becoming a worry not only in the sense of endangering native wildlife, but also with disease transmission to humans and other animals.

Rapid industrialisation, in the beginning without any limits, whilst causing an economic boom has also increased greenhouse gas emissions around the world. But while the ozone layer has been recovering from wearing thin on account of these emissions, the gas pollutants have been trapped inside the earth’s atmosphere, slowly heating it. This rise in temperature might not feel like a bad thing down on Earth, but it could pave the way for the introduction of tropical diseases never before found in more temperate climates.

Mosquitoes are a well-known vector for disease, infecting humans through their bite and blood contamination. Native to tropical and subtropical areas, mosquitoes thrive in the warm and wet conditions. But as conditions grow warmer in geographically adjacent places the mosquito habitat can expand. Without land bridges, humans can transfer mosquitoes unknowingly to novel places where with the right conditions they can continue disease transmission.

The Zika virus outbreak from 2015 to 2016 can be explained by a variety of reasons; rising temperatures in South America complemented by the El Niño weather pattern created ideal conditions for Aedes albopictus and Aedes aegypti to transmit the disease. With a mortality rate of around 8.3%, according to a study in Brazil at the time, Zika can cause mild symptoms and muscle pain or, at the other extreme, microcephaly. During El Niño, sea temperatures rose, which affected pressure levels and wind patterns and, in this case, resulted in increased rainfall and temperature changes on land as well. These temperature changes can alter the frequency at which mosquitoes bite humans, increasing transmission of the Zika virus and also dengue virus, which is carried by Aedes aegypti mosquitoes. Increasingly erratic weather patterns caused by climate change as well as rising temperatures could see the expansion of mosquito habitats and facilitate the spread of deadly viruses with greater intensity.

The effect of rising temperatures can perhaps be mitigated with responses designed to help control and eradicate disease, by health organisations where education and preventative measures like mosquito nets can be employed; an important aspect to consider is the availability of habitats for these insects. For example, the Asian tiger mosquito (A. albopictus), native to tropical climates in southeast Asia, can transmit dengue, West Nile, Japanese encephalitis and Zika viruses, all of which currently have no cure and treatment that has limited effects. However, this mosquito has disseminated widely via the tyre trade, illustrating the human effects on dispersion of IAS. In the past 20 years the Asian tiger mosquito has spread to around 28 countries outside its native habitat and survives in rainwater collected by tyres left outside in between transportation. Not only does it transmit deadly human diseases that are spreading geographically, but it can also feed 24 hours a day. This is rare for mosquitoes, which usually only feed at dusk and dawn, thus presenting a threat to other species of mosquito, which may not transmit disease, as well as a direct threat to human health.

Closer to home are the pests that we might be more accustomed to in the form of rats and mice, which haunt the underbellies of cities and towns. These creatures have co-existed alongside humans for centuries, facilitating their spread to new habitats. Inadvertently distributed by humans, the black rat (Rattus rattus) originated in Asia but now thrives globally and has adapted to our environments, both rural and urban. The success of both rats and mice lies in their behavioural plasticity giving them the ability to adapt to new habitats quickly.

Thus, as globalisation has increased, and habitat destruction through deforestation and urban expansion has occurred, the vermin have comfortably rolled with the changes at the expense of other small vertebrates, birds and reptile species worldwide. Moreover, they act as vectors for disease including the plague bacterium Yersinia pestis, which can be transmitted by the rat species Rattus norvegicus and R. rattus via fleas – and caused human outbreaks of the plague as recently as 2002 in Madagascar. Although these events may be rare, spillover could occur at any time with the presence of such a large disease reservoir.

Perhaps one of the biggest threats to human health comes in the form of viruses capable of causing pandemics. In 2019 the coronavirus SARS-CoV-2, which originated in bats and was discovered in the Wuhan province in China, was transmitted to humans and is thought to be carried and transmitted by other animals too. One of the reasons postulated to contribute to this global emergency was the rapid expansion of urban areas in China and loss of natural habitats for a plethora of wildlife including bats around these cities and towns, thus bringing bats into closer contact with humans and allowing spillover of disease.

Nipah virus, a henipavirus that can cause acute respiratory failure and encephalitis, also has a large reservoir in bats and has existed there for centuries. However, spillover events have been seen in recent years, with transmission to humans and pigs, and from pigs to humans in countries including Malaysia, Singapore and Bangladesh – where the fruit bat vector is native. This may be in part due to deforestation across Malaysia and alteration of habitats caused by weather patterns such as El Niño, which caused mass bat migration southward towards towns and cities in the late 1990s.

The mass deforestation released huge amounts of greenhouse gases, contributing to the rising temperatures of the Earth, which in turn causes sea temperatures to slowly increase, creating a drop in pressure. Knock-on effects included droughts and flooding as wind patterns were altered by the change, which is thought to have caused the fruit bat migration and perpetuated Nipah virus outbreaks in 1998–1999 in Malaysia. Whilst small outbreaks can be controlled due to the virus’ diminished capacity for transmission, with a mortality rate of between 40% and 75%, there have been further outbreaks spattered through the years since the first major outbreak in 1998–1999. Habitat loss and climate change are assisting the migration of these bat species and as bat habitats decrease there is potential for heightened human–bat interactions and further spread of Nipah virus.

Plant IAS ring a familiar bell, with Himalayan balsam thriving here in the UK, brought in during the Victorian era by plant collectors abroad. But whilst plants are at risk of competition for resources, they are also vulnerable to disease. The common whitefly measures up to around a millimetre in length yet can devour over 900 different species of plants globally, in the process being capable of transmitting over 100 different plant viruses. Begomoviruses, criniviruses and torradoviruses are among the more deadly viruses that whiteflies transmit, and they can have huge economic effects due to their propensity for crop damage. In huge numbers, whiteflies have travelled far and wide from their native India and are thought to be accidentally introduced by human travel via infected plant material; they are now being found on every continent except Antarctica, with the capacity for huge effects on ecosystems and biodiversity.

IAS can be accidentally or deliberately introduced to new environments and many of these occasions occur through human causes, be it control measures or on the back of increased globalisation and travel. Whilst some IAS may not interact with humans directly, such as the Nipah-carrying fruit bats, their proximity could prove dangerous to human health, in addition to compromising the life cycles of native flora and fauna. Restrictions on IAS are in place to curb their spread, but many IAS are wily and adapt quickly to new environments – a trait that will be necessary as climate change accelerates and habitats and weather patterns alter.