Hidden Viruses: Amoebae as a Water Safety “Trojan Horse”
Karl Landsteiner University team shows gut viruses persisting in water amoebae, urging an evaluation of water safety rules.
Krems (Austria), 18. December 2025 – Human norovirus and adenovirus, two major causes of viral gastroenteritis, can persist for extended periods inside free-living amoebae that are common in natural and engineered water systems. A now internationally published study shows that these gut viruses can “hide” within different stages of the amoebae and remain capable of causing infection after this detour. The work, led by Karl Landsteiner University (KL Krems) together with international partners, indicates that free-living amoebae may act as previously overlooked reservoirs and transport vessels for enteric viruses. This could help explain their surprising persistence in some water systems and provide a basis for more refined risk assessments and, ultimately, safer drinking and recreational waters.
Diarrhoeal diseases caused by contaminated food and water still represent a substantial global health burden, despite marked progress in sanitation and drinking water treatment. Human norovirus is the most common cause of acute viral gastroenteritis worldwide and can cause infections after exposure to only a handful of viruses, while human adenoviruses are also frequently detected in sewage and surface waters impacted by wastewater. Standard water safety concepts largely focus on freely floating viruses in the water column or those attached to particles. However, in recent years free-living amoebae – single-celled organisms ubiquitous in water systems and long known to shelter bacteria and other microbes – have also been recognised as carriers of viruses. Against this backdrop, scientists at the Department of Water Quality and Health of KL Krems, together with colleagues from Canada, Asia and Australia, set out to clarify how norovirus and adenovirus interact with different amoeba species and what this might mean for water-related infection risks.
Amoebae as shelters
“In water microbiology we have long known amoebae as hosts for bacteria such as Legionella, but their role for human gut viruses has been largely overlooked,” says Dr. Mats Leifels, Scientific Staff in the Department of Water Quality and Health at KL Krems and first author of the study. “Our experiments show that common amoebae do not simply digest these viruses – they can protect them, transport them and, in the case of adenovirus, may even support early steps of viral reproductive activity.” In his view, this “cohabitation” between protozoa and enteric viruses adds an important layer to how scientists think about waterborne transmission pathways and the effectiveness of treatment processes.
To explore these interactions, the team co-cultured human norovirus and human adenovirus with three models of free-living amoebae: Vermamoeba vermiformis, Acanthamoeba polyphaga and Willaertia magna, all of which are widespread in water systems. Using quantitative PCR to track viral genetic material and fluorescence microscopy to visualize where the viruses are located within the amoebae, they followed the fate of the viruses over twelve days. Norovirus genomes were found in the cytoplasm and vacuoles of V. vermiformis and A. polyphaga, while adenovirus was observed inside the nucleus of W. magna. Intact virions (i.e. intact and infectious virus particles) of both viruses persisted inside the amoebae in both active and dormant forms as well as in extracellular vesicles throughout the study period. When adenovirus-containing amoebae were transferred to cell cultures, infectious virus could still be recovered, thus indicating that passage through the amoebae did not inactivate it. Moreover, the detection of adenoviral messenger RNA (fragile genetic messenger molecules) linked to fiber protein synthesis pointed to virus-related transcriptional activity within the amoeba host.
Rethinking water protection
Free-living amoebae are highly resilient organisms. In their dormant cyst form they can survive high doses of disinfectants such as chlorine and monochloramine as well as ultraviolet treatment – conditions under which many other microbes are inactivated. If enteric viruses are taken up and sheltered by amoebae, they may therefore bypass some of the barriers that water treatment and sanitation systems rely on. The new study suggests that such amoeba-associated viruses could be relevant not only in drinking water production, but also in sewage-impacted rivers and lakes used for recreational activities, as well as in water reuse schemes. “From a risk assessment perspective, we have to treat free-living amoebae as potential reservoirs and transport vehicles – “viral Trojan Horses” – when we model how long viruses persist in water and how effectively we remove them,” explains Prof. Andreas H. Farnleitner, head of the Department of Water Quality and Health at KL Krems as well as the ICC water & Health* and co-author of the study. “Only then our water safety plans can reflect the full complexity of real systems.”
As climate change, urbanization and water scarcity increase the pressure on surface and groundwater resources, understanding such hidden microbial interactions becomes ever more important. The authors emphasize that their findings are based on laboratory model systems and that further work is needed to quantify amoeba-associated virus loads in real-world waters. Nevertheless, they see a clear case for including free-living amoebae in future water safety and reuse guidelines and for developing monitoring approaches that can distinguish between truly inactivated viruses and those shielded within protozoa. The research fits squarely within the strategic focus of KL Krems: interdisciplinary fields with high relevance to health policy – including exposome research and environmental health with a focus on nutritional science, allergology as well as water quality and health. By contributing experimental evidence on virus–amoeba interactions, KL Krems and its partners aim to support regulators and utilities in designing robust, science-based and “global-change-resistant” measures to protect public health.
Original Publication: Enteric Viruses and Free-Living Amoebae: Protozoa as Potential Reservoirs and Transport Vessels for Human Norovirus and Adenovirus, M Leifels: R Dey: A R Wiedemeyer: D Cheng: C Kolm: F Wu: K Sirikanchana: A H Farnleitner: N J Ashbolt, Water & Ecology 2025, 1(3): 100018, doi:10.1016/j.wateco.2025.100018. https://kris.kl.ac.at/en/publications/enteric-viruses-and-free-living-amoebae-protozoa-as-potential-res-2/
More on KL Krems research: https://www.kl.ac.at/en/research/research-blog
Karl Landsteiner University (12/2025)
The Karl Landsteiner University (KL Krems) is an internationally recognized educational and research institution located on the Campus Krems. KL Krems offers modern, demand-oriented education and continuing education in medicine and psychology as well as a PhD programme in Mental Health and Neuroscience. The flexible educational programme is tailored to the needs of students, the requirements of the labour market and the challenges of science. The three university hospitals in Krems, St. Pölten and Tulln and the MedAustron Ion Therapy and Research Centre in Wiener Neustadt guarantee clinical teaching and research of the highest quality. In its research, KL Krems focuses on interdisciplinary fields with high relevance to health policy – including mental health and neuroscience, molecular oncology as well as the topic of water quality and the associated health aspects. KL Krems was founded in 2013 and accredited by the Austrian Agency for Quality Assurance and Accreditation (AQ Austria).
*The ICC Water & Health is a cooperation between TU Wien, Medical University of Vienna and Karl Landsteiner University of Health Sciences (www.waterandhealth.at). The Inter-University Cooperation Center for Water and Health (ICC Water & Health) sees itself as a scientific platform and competent partner in questions of water quality and its impact on human health. The ICC is dedicated to the development of innovative concepts for assessing water quality, new microbiological and molecular biology methods, efficacy testing of physical and chemical treatment methods, and numerical models for estimating the risk of infection and disease in water use. The lessons learned will be used to derive effective and sustainable health management measures. The ICC was founded by the Vienna University of Technology and the Medical University of Vienna in 2010 and has been sustainably established thanks to the competitive research funding of the Federal Ministry of Science, Research and Economy (BMWFW). In 2017, ICC Water & Health was expanded to include the Karl Landsteiner University of Health Sciences (KL). KL is now an official part of the research platform.
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