Vector-borne diseases: Mosquitoes, ticks, biting flies... how far will they go?

For the past few years, warnings about the dangers of biting insects in spring have raised alarm bells in increasingly northerly regions. Over and above the irritation they cause, hematophagous insects and ticks represent a considerable health risk: in certain conditions, they are capable of hosting and transmitting viruses, parasites and bacteria, becoming disease "vectors."

Some "vector-borne" diseases are already a familiar problem in Europe, like Lyme disease, caused by ticks. Yet most such diseases – malaria, dengue, Zika, Crimean-Congo hemorrhagic fever and yellow fever, for example – may seem like a distant threat. But they are closer than we might care to think: insect vectors are taking advantage of human mobility, rising temperatures and habitat change to widen their range, automatically increasing the risk associated with vector-borne diseases.

As summer and the 2024 Games approach, particular attention is being paid to disease vectors such as mosquitoes, ticks and biting flies.

A global issue

Although vector-borne diseases have long been seen as tropical diseases that are endemic among the world's poorest populations, the situation is changing rapidly: 80% of the global population is now at risk of one or more such diseases. According to the World Health Organization (WHO), they kill more than a million people every year, primarily children, and they often result in debilitating chronic conditions. The most deadly of these diseases is malaria, transmitted by Anopheles mosquitoes: in 2022, 249 million cases were reported worldwide, leading to 608,000 deaths, mostly on the African continent. Dengue is the most widespread vector-borne disease. The dengue virus is transmitted by Aedes mosquitoes, and since 2019 dengue has been present in every world region, putting half the world's population at risk. Outbreaks associated with other pathogens have been on the rise for the past decade, burdening health systems and spreading to new regions. New threats have also emerged, such as the Zika virus, which in 2016 was identified as being a cause of congenital malformations after major outbreaks in South America. Although a number of large-scale programs have been introduced to tackle vector-borne diseases, some are still considered by WHO as neglected diseases, including leishmaniasis and sleeping sickness.

Changing vector and human systems

The mere presence of ticks or insect vectors is not the only condition required for the spread of vector-borne diseases; they need an effective "vector system" involving interactions between a pathogen, a vector and potential human or animal hosts. Understanding this system and the factors that influence it is crucial in combating vector-borne diseases. There are many reasons why these diseases are progressing, but major social and environmental changes are clearly playing a part. The disappearance of malaria from Europe, driven by the rural exodus, has much to teach us. Conversely, the urbanization of human societies can also have a favorable impact on some vector species. Dense urban centers provide them with new habitats and new means of transportation as they follow human mobility patterns, creating new epidemiological situations. Globalization and international travel are also helping to spread pathogens and vectors to all regions of the world. But vectors can only become established in a region if the climate conditions are favorable.

Climate, a key factor

Of all infectious diseases, vector-borne diseases are influenced most strongly by the rise in global temperatures. The impact of climate change may seem straightforward: extended warm seasons enable the majority of insects and ticks to reproduce more successfully and survive for longer. But while this is undoubtedly true for most species, the environmental changes caused by warming are more complex. For example, temperatures can modify a vector's ability to transmit a pathogen, and this interaction can change in response to the environment. Human activities and lifestyles are also affected by rising temperatures. "Blue and green" public policies are leading to the implementation of urban greening plans in towns and cities, and urban expansion is blurring the boundaries between built-up and rural areas, but local populations are not necessarily aware of the problem of ticks and insect vectors. These developments benefit both vectors and animal species that are reservoirs for vector-borne diseases. In France, this is the case for some rodents and birds that can host the bacteria responsible for Lyme disease and transmit them via ticks (Ixodes ricinus) to walkers. This cycle seems to have been amplified in recent years, with the number of annual estimated cases nearly doubling over the past decade.

Vector-borne diseases soon in France?

In Europe, diseases like dengue, chikungunya and malaria are often thought of as "travel-related diseases," with cases imported by travelers or caused by infected mosquitoes accidentally introduced on flights from endemic regions. There are just a few hundred such cases each year in France, but local conditions are changing. Vectors are spreading across Europe, and the tiger mosquito, capable of transmitting viruses like Zika, dengue and chikungunya, has now colonized virtually all of mainland France (see map), increasing the probability that a local chain of transmission or even an outbreak may be kick-started by an imported case. While nine episodes of indigenous dengue transmission, in other words cases caused by the local mosquito population, were observed in France in the period from 2010 to 2018, the same number of cases were reported for a single year in 2022. The situation is such that in July 2022 the French Committee for Monitoring and Anticipating Health Risks (COVARS) emphasized that "the emergence and outbreaks of vector-borne diseases in France are inevitable." This observation may seem alarming, but it should be seen in the light of France's specific environmental and health context. Although pathogens have been introduced on multiple occasions, none has become established in the country, and vector activity remains highly seasonal. In short, the conditions are not yet in place to support the maintenance of a year-round vector-based system, but sporadic outbreaks are inevitable. So health authorities are on their guard: in France, a plan to prevent the spread of chikungunya and dengue has been in force since 2006. Preparedness is crucial, and effective means of vector control need to be developed. The considerable diversity and complexity of transmission cycles for vector-borne diseases complicates the task of developing vaccines and therapies. Vector control is currently the most effective weapon, and we can all play our part by using mosquito nets, wearing long trousers and paying particular attention after going for a walk in the woods!

 

Marine Viglietta

Research is just a roundabout way of helping others.

 

Marine Viglietta, 27, a medical entomologist and virologist, completed her PhD in the Arboviruses and Insect Vectors laboratory at the Institut Pasteur. She talks enthusiastically about her research on Aedes aegypti mosquitoes and the viruses they can spread.

When we meet Marine in the laboratory where she works, we find her with pipette in hand, waving at us to wait while she finishes off a procedure. It's a race against the clock for the young scientist – she only has a few weeks left to finish her experiments before she begins writing up her thesis, the culmination of four years of work. She will defend her thesis in September. "I need a holiday!" exclaims Marine, grinning, as she joins us. The smile rarely leaves her face, especially when she is talking about her research.

A few years ago, she began studying medicine, attracted by the idea of being able to "help people" with a job that she finds "admirable." But she soon realized that medicine was not the right path for her – research was where her interests lay. She began by studying epidemiology, then virology at Université de Versailles Saint-Quentin-en-Yvelines. "I wanted to work on epidemic diseases, especially tropical diseases, as I spent a lot of time in the Caribbean when I was little, in Martinique and Guadeloupe, where there were mainly dengue outbreaks. That made me aware of vector-borne diseases transmitted by mosquitoes."

During a Master's internship in the Entomology Unit at the Institut Pasteur de la Guadeloupe, she had a lightbulb moment. "It was the first time that I had handled mosquitoes, and I learned how to rear them. That made me really want to work with these insects."

Head of an Aedes mosquito

For her PhD, Marine answered a call for applications from Anna-Bella Failloux's laboratory at the Institut Pasteur in Paris, which she had "had her eye on for a while." The call asked a simple question: why is there no yellow fever in Asia? "There is no yellow fever despite the fact that the mosquito vector for the disease is present in Asia!" explains Marine. In this research unit that studies mosquito-borne viruses – "arboviruses" –, Marine discovered what it was like to work in a high-security BSL3 laboratory, which required prior training. "I was able to handle arboviruses and work on my own project."

In 2022, she was sent for a few weeks to the Institut Pasteur du Cambodge to learn how to collect mosquitoes in the field. "We caught mosquitoes with traps, in Phnom Penh and in the rural village of Koh Thom. It was very interesting to see what things were really like on the ground – how people live alongside animals and insects. It was an amazing, memorable experience, also from a scientific perspective. But if I had to choose between the lab and the field, I would choose the lab every time. Working in the field is tough and unpredictable – sometimes you don't catch anything."

In the lab, Marine says that she loves seeing her experiments through to the end. "It often takes at least three months before we get any results: we need to rear and infect the mosquitoes, process the samples and then analyze them – it's a lengthy procedure, and I find it fascinating."

On yellow fever in Asia, Marine was able to show that if Aedes aegypti mosquitoes are infected with Japanese encephalitis virus, which is common in this region, it makes them unable to transmit yellow fever virus. The dengue virus also seems to have the same blocking effect. With these first results, Marine has begun to shed light on what was previously "one of the major mysteries of arbovirology."

Once she has defended her thesis and has her PhD, she will set off for a postdoctoral fellowship abroad, but she hasn't yet explored where she might go.

Does she still want to help others? "Research is just a roundabout way of doing so," says the young scientist. "Understanding how things work will help someone some day to find a vaccine or a way of blocking a virus in mosquitoes, and that would prevent hundreds of thousands of people from being infected. It's that prospect that drives me."