She spent her childhood on the island of Borneo, completed her PhD in Australia and arrived in Paris without speaking a word of French. Cassandra Koh, a virologist at the Institut Pasteur, has got mosquitoes under her skin – scientifically speaking.
As a specialist in the mosquito virome, she is exploring largely uncharted territory, namely the billions of viruses hosted by these insects, with the aim of understanding how the next disease outbreaks might emerge.
We meet a scientist who pairs curiosity with rigor and a passion for science with a deep appreciation for French cuisine.
Borneo, aged 16 – a genetics class that changed everything
The story begins in Pontianak, an Indonesian city on the island of Borneo, with a biology class that sparked a lifelong interest. When Cassandra Koh was 16, her teacher asked the class to prepare a presentation on a topic of their choice. She chose genetics, and specifically genetic modification, and took to her research with a passion that surprised her teacher.
"I was always a curious child. Why is the sky blue, why do caterpillars turn into butterflies... The day I discovered the concept of genes, I said to myself that genetics and DNA were the blueprint for all forms of life," she remembers.
Her teacher was so impressed by the quality of her work that she asked Cassandra if she could keep her slides – an early sign of what might lie ahead. But in Indonesia in the 2000s, wanting to become a scientist rather than a doctor was seen as a somewhat eccentric option. "Careers in research were practically unheard of in Indonesia. My parents didn't understand my choice. I might as well have said I wanted to become a musician."
Cassandra was the daughter of entrepreneurs who had studied in New Zealand, and while her family valued education, they did not see a stable future in scientific research. But Cassandra's conviction never wavered, and she ended up heading to Australia to complete a degree and then a PhD.
Billions of secrets in every mosquito
For her thesis, she explored the dengue virus and the mosquito that transports it. "The aim was to use genetics to understand why some mosquitoes are able to transmit dengue and others are not," she summarizes. That quest has been a key thread running through her scientific career to date.
Seven years after her PhD, Cassandra joined the team led by Argentinian scientist Carla Saleh at the Institut Pasteur in Paris, and since then the scope of her research has widened considerably. While her PhD could be represented by a relatively simple triangle – dengue virus, mosquito vector, infected humans –, her current research has opened up a world of dizzying complexity. The technique of metagenomic sequencing, used by scientists to sequence all the genetic material contained in a sample in just a few days, has revealed that mosquitoes do not just carry a few known viruses; they play host to billions of largely unexplored viral sequences. Cassandra's team wants to shed light on how this viral universe is influencing the diseases transmitted by mosquitoes via an "ecoepidemiology" approach, in other words examining communicable diseases based on multiple and varied criteria within a complex ecosystem.
"We collect mosquitoes in the wild and we ask: What viruses are they hosting? Why those viruses in particular? This is a field in which exciting new discoveries are constantly being made," she explains.
The viruses can be divided into two broad categories:
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arboviruses, like dengue, Zika or West Nile virus, capable of infecting vertebrates and humans; and
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"mosquito-specific" viruses, which seem to be limited to mosquitoes and are transmitted vertically from one generation to the next, without ever crossing the species barrier to other hosts.
We know that these viruses exist because we detect their genes. But their biological role, where they come from and where they go next – those are things we don't yet know.
A new frontier of knowledge has been opened up by the development of sequencing technologies, now quick to use and easily accessible, forcing the scientific community to rewrite some of what it thought it knew about viruses and mosquitoes.
Understanding, anticipating and discovering
Cassandra Koh's curiosity is also driven by a question that is both obvious and alarming – what if some of these unknown viruses are actually beneficial for mosquitoes?
"If these viruses are actually strengthening mosquitoes, helping them to survive or reproduce in the current context of climate change, then that spells bad news for us! A healthier mosquito is potentially a more effective vector," she explains, with clinical lucidity.
With funding from the European Research Council, Cassandra and her team are determined to elucidate the role of these viruses in their environment within the biology of the insect, comparing them with arboviruses that are pathogenic for humans. This is not just about satisfying academic curiosity – genomic analyses have already shown that the ancestors of arboviruses were originally mosquito-specific viruses that subsequently acquired the ability to infect vertebrates.
Understanding this evolutionary journey will help us understand how an epidemic threat might emerge – before it happens.
For this scientist who admits a preference for the lab rather than the tropical jungle, exploring and understanding how the world of insects and viruses could tip over into a global health crisis is all just part of her day-to-day reality.
Cassandra Koh: Key Dates
2013: Bachelor’s degree in Biotechnology, summa cum laude, Monash University, Australia
2019: Ph.D. in Biological Sciences, Monash University, Australia
2019: Began postdoctoral research at the Virus and RNA Interference Unit, Institut Pasteur, France
2022: Awarded Springboard to Independence (S2I) funding from LabEx IBEID
2024: Awarded the ERC Starting Grant “MULTITUDES”
2025: Launched the Mosquito and Virus Ecology Team, Virus and RNA Interference Unit, Institut Pasteur, France
