One of the distinguishing features of this research is its long-term scope, spanning both the acute pandemic phase and the post-pandemic transition period. Most wastewater surveillance studies published to date have focused on relatively short windows, often limited to pandemic waves of SARS-CoV-2, which restricts the ability to draw conclusions about how viral circulation behaves under more ordinary epidemiological conditions. By continuing collection through the period when public health interventions were lifted and society returned to pre-pandemic patterns of contact, the researchers were able to observe the re-establishment of seasonal respiratory virus dynamics that had been dramatically suppressed during 2020 and much of 2021. This before-and-after contrast is scientifically precious because it documents, in a single location with consistent methodology, how the near-total interruption of transmission for viruses such as influenza and respiratory syncytial virus was followed by unusual out-of-season resurgences and subsequently by a gradual return to typical winter seasonality.

The concept of a middle-size city is relevant to the broader applicability of wastewater surveillance. Much of the foundational work in this field has been conducted in large metropolitan areas, where sewersheds serve millions of people and dilution effects are substantial but signal magnitude is high. Smaller cities present a different set of conditions: the contributing population is smaller, which can make signals more sensitive to localized outbreaks but also more variable, and the sewer network characteristics, industrial discharges, and demographic composition differ from those of megacities. Demonstrating that a standardized analytical pipeline can produce interpretable, reproducible data in a middle-size urban setting strengthens the case for deploying such systems across heterogeneous municipalities, which is precisely what many national and regional surveillance programs in Europe now aim to do under frameworks supported by the European Commission and coordinated through initiatives involving public health institutes across member states.

Methodologically, long-term wastewater studies of respiratory viruses must contend with several persistent analytical challenges. Viral RNA in sewage degrades over time depending on temperature, pH, and the presence of nucleases, so normalization strategies are needed to distinguish true changes in viral shedding from artifacts introduced by variable wastewater flow, rainfall dilution, or sample processing efficiency. Common approaches include normalizing to fecal indicators such as human adenovirus or pepper mild mottle virus, or to physicochemical parameters like ammonium concentration and flow volume. Recovery controls, typically spiked surrogate viruses, allow laboratories to estimate extraction efficiency for each sample. The choice of concentration method, whether electronegative membrane filtration, ultrafiltration, or polyethylene glycol precipitation, influences sensitivity for different viruses. Studies that maintain the same protocol over years, as this one did, gain an important advantage: temporal comparisons become more reliable because methodological noise is held constant, allowing genuine epidemiological trends to stand out more clearly.

The multipathogen panel typical of such studies generally includes SARS-CoV-2, influenza A and B viruses, respiratory syncytial virus, and often additional targets such as human metapneumovirus, parainfluenza viruses, seasonal coronaviruses, rhinoviruses, and adenoviruses. Quantitative reverse transcription PCR remains the workhorse detection technology because it provides absolute or relative quantification with well-characterized performance. Multiplexing several assays in a single reaction conserves sample volume and reduces cost, which matters when hundreds of samples are processed over multi-year campaigns. The resulting time series can be analyzed for peak timing, peak height, epidemic onset, and the lead time between wastewater signal and clinical indicators such as hospital admissions or sentinel physician reports. Across many studies, wastewater signals for influenza and RSV have tended to lead or coincide with clinical peaks by roughly one to two weeks, a window that can be operationally meaningful for hospital preparedness, staffing decisions, and the timing of public health communications.

The pandemic-to-post-pandemic transition also offers a natural experiment in viral interference and immune landscape dynamics. During the period of intense SARS-CoV-2 circulation and non-pharmaceutical interventions, the near-disappearance of influenza and RSV created a substantial immunity debt, particularly among children born during those years who had never encountered RSV. When restrictions eased, many countries in the Northern Hemisphere, including Spain, experienced an out-of-season RSV wave in the summer of 2021 and an unusually early and intense influenza and RSV season in late 2022. A wastewater time series that spans these events provides an independent record of how quickly viral circulation rebounded and how the relative timing of different pathogens shifted, information that is difficult to reconstruct from clinical data alone because testing practices for non-COVID respiratory viruses were themselves disrupted during the pandemic.

Another dimension of long-term wastewater data is its potential to capture the emergence and replacement of SARS-CoV-2 variants. Variant-specific assays or sequencing of wastewater samples can reveal the rise of Alpha, Delta, Omicron, and subsequent lineages weeks before genomic surveillance of clinical samples detects the same shifts, simply because wastewater aggregates infections across the whole community without the sampling biases introduced by who gets tested. Even when the primary focus of a study is quantitative viral load rather than lineage tracking, the overall SARS-CoV-2 signal reflects the cumulative effect of variant-driven changes in transmissibility, immune evasion, and shedding kinetics. The post-pandemic period, characterized by the evolution of Omicron sublineages and the transition of COVID-19 toward an endemic, wave-like pattern, is particularly interesting in this respect, as wastewater data can help define whether SARS-CoV-2 settles into winter seasonality similar to influenza or retains a distinct periodicity.

From a public health operations standpoint, the value of a multi-year dataset lies in establishing baselines. A single season of data cannot tell decision-makers whether a given viral load measurement represents a normal winter peak or an anomalous surge. After several years of consistent monitoring, thresholds can be defined empirically, for example as multiples of the median off-season concentration, and these thresholds can trigger predefined responses such as enhanced clinical testing, hospital surge planning, or targeted vaccination campaigns. The European Union’s recommendation in 2023 that member states extend wastewater surveillance beyond SARS-CoV-2 to include other pathogens of concern reflects exactly this logic: sustained, standardized monitoring is what converts raw measurements into actionable intelligence. Studies conducted in individual cities, with fully documented protocols and openly reported concentrations, provide the calibration points that such larger programs depend upon.

It is also worth noting the complementary relationship between wastewater surveillance and clinical sentinel systems. Clinical data provide information that wastewater cannot: which individuals are infected, their age distribution, vaccination status, symptom severity, and the identification of specific strains through patient sampling. Wastewater data, conversely, provide population-level coverage without dependence on healthcare-seeking behavior or testing policy, and they are available even when clinical laboratories scale back routine respiratory panels during off-seasons. Integrating the two streams, for instance by correlating wastewater concentrations with hospitalization rates or by using wastewater to trigger more intensive clinical sampling, generally yields better situational awareness than either source alone. The Spanish middle-size city dataset, by covering both pandemic and post-pandemic phases, illustrates how this integration can be evaluated across very different epidemiological regimes, from emergency-driven mass testing to routine seasonal monitoring.

Finally, the scientific community’s interest in studies of this kind reflects a broader shift in how infectious disease surveillance is conceptualized. Rather than reacting to outbreaks after they become clinically visible, public health authorities increasingly seek leading indicators drawn from environmental monitoring, genomic sequencing, and digital data sources. Wastewater surveillance occupies a central place in this vision because it is relatively inexpensive per capita, technologically accessible to regional laboratories, and demonstrably effective across a growing list of pathogens, including not only respiratory viruses but also enteroviruses, hepatitis A, mpox, and antimicrobial resistance genes. Long-term, single-site studies with consistent methodology, such as the one conducted in this Spanish city, serve as the empirical backbone for this transition, demonstrating that the signals are stable, interpretable, and reproducible over years rather than weeks, and that the infrastructure built during the COVID-19 emergency can be repurposed into durable, routine surveillance capacity capable of informing responses to future epidemic threats.

Subject of Research: Pandemic and post-pandemic dynamics of respiratory viruses in a Spanish middle-size city using a long-term wastewater surveillance

Article Title: Pandemic and post-pandemic dynamics of respiratory viruses in a Spanish middle-size city using a long-term wastewater surveillance

Article References: Casado-Martín, L., Hernández, M., Pérez-Alonso, D., Yeramian, N., Alves-Elois, M., Dorighello-Cadamuro, R., Fongaro, G., Eiros, J. M., & Rodríguez-Lázaro, D. (2026). Pandemic and post-pandemic dynamics of respiratory viruses in a Spanish middle-size city using a long-term wastewater surveillance. npj Viruses. https://doi.org/10.1038/s44298-026-00232-2

Image Credits: AI Generated

DOI: 10.1038/s44298-026-00232-2

Keywords: Pandemic, post-pandemic, dynamics, respiratory, viruses, Spanish, middle-size, city, long-term, wastewater, surveillance, scientific research

The case for a more precise approach begins with taxonomy. The Spanish Catalogue of Invasive Alien Species uses O. maxima as its regulatory category, but names found in historical, agricultural and ecological sources do not always refer to the same biological entity. Current taxonomic treatments recognize O. maxima as an accepted species and list names such as Opuntia amyclaea and O. ficus-indica var. amyclaea among its synonyms. Other studies discuss Opuntia dillenii, a name that Plants of the World Online treats as a synonym of Opuntia tuna. These distinctions matter because invasive potential, biological-control responses, ecological interactions and agricultural uses can vary among taxa. A label applied to a group of plants may therefore combine populations with different histories and behaviors. The authors emphasize that species-level identification is particularly important before evidence from one cactus is used to justify control of another. Without that clarification, a single regulatory category can obscure the biological differences that management is supposed to address.

Demographic records further complicate the idea that Spanish Opuntia populations have followed a simple pattern of unchecked expansion. Government agricultural statistics estimated that slightly more than three million bushes recorded as O. maxima grew outside farmland in Spanish territories in 1961. About 1.5 million were in the Canary Islands, while mainland populations included approximately 797,000 in Andalusia and 453,500 in Murcia. By 2011, the total had fallen by nearly 95 percent, to 169,588 bushes, with most remaining in the Canary Islands. In 2019, the records listed 101,445 in the Canary Islands but only about 320 on the mainland—310 in Andalusia and 10 in Murcia. The decline became especially steep after the arrival of the cochineal insect Dactylopius opuntiae in 2007. Estimates in the preceding decade had generally ranged between 200,000 and 300,000 bushes, then dropped below historical levels. These figures do not by themselves measure ecological impact, but they challenge a narrative of uniform demographic expansion and show why current abundance, recent decline and historical distribution all need to be considered.

Population size, however, cannot settle the invasion question. A species may decline overall while still causing serious damage in particular habitats, just as a plant that is harmful in one location may provide resources or structure in another. The Perspective reviews evidence from Spanish islands and mainland sites showing that some Opuntia taxa have become embedded in local ecological networks. Studies of pollination in Tenerife and Menorca found that alien plants, including Opuntia, did not simply displace native plants from pollinator interactions; in some circumstances, they reinforced existing connections between plants and pollinators. Research on seed dispersal likewise found complex interactions involving native and alien animals. These findings do not make the cacti harmless. They show instead that removing them can alter relationships that have developed in transformed landscapes. In Tabarca, removing Opuntia was associated with detrimental effects on local arthropods. On Benidorm, eliminating the plants during an effort to restore native vegetation had unforeseen negative effects on yellow-legged gull colonies. Such examples suggest that eradication should be evaluated as an ecological intervention, not treated as an automatic synonym for restoration.

Other evidence points in the opposite direction and underscores the need for targeted control. The Spanish catalogue reports O. maxima invading stands of Pinus pinea and scrub dominated by Pistacia lentiscus near Doñana, where competition with native vegetation may be a serious concern. In the Canary Islands, research on the lizard Gallotia galloti found that it consumes seeds from both native and alien plants, including O. dillenii. The seeds of Opuntia retained germination potential after passing through the lizard’s gut, while seeds of a plant identified as Scilla cf. haemorrhoidalis, endemic to the islands, were negatively affected. Because the lizard fed according to availability, these interactions could favor some Opuntia taxa while reducing the frequency of vulnerable native plants. The authors therefore reject both extremes: neither alien status alone nor cultural familiarity can establish whether a population should remain. Control is most clearly justified where effects on native or endemic communities are demonstrated or strongly plausible. In those cases, removal should be coupled with restoration capable of replacing lost habitat structure and supporting native communities.

The cactus’s social history makes that balance unusually complicated. European colonizers introduced Opuntia to the Iberian Peninsula during the 1500s, probably first as an ornamental plant. In arid regions, people later cultivated it extensively for fruit, fodder and property boundaries. The plant also supported production of cochineal, a scale insect whose carmine pigment was once an important commercial red dye. In the Canary Islands—especially Tenerife, Lanzarote and Gran Canaria—cultivation expanded for dye production, but high labor costs, synthetic dyes and the shift toward tourism and services reduced its economic importance. On mainland Spain, particularly in Andalusia and Murcia, the fruits known as “chumbos” remained part of local diets. The decline of the bushes after 2007 has consequently been experienced not only as an ecological change but as a loss of food traditions, rural income, landscape identity and local memory. Reports from southern communities have linked the disappearance of chumberas with concern about lost livelihoods and, in some places, increased vulnerability to landslides. These accounts do not override conservation priorities, but they demonstrate that management decisions have consequences beyond species lists and maps.

Aridification raises the stakes because the cactus is now being evaluated in landscapes where water scarcity is reshaping agriculture and restoration. Models place much of the plant’s potential distribution in coastal and peri-coastal areas, where mild climates overlap with dense settlement, tourism, infrastructure, irrigated farming and intense water demand. In places such as La Axarquía, southern Spain’s expanding irrigated agriculture has contributed to severe water stress, creating pressure to reconsider land uses that depend less heavily on irrigation. The authors suggest that carefully identified and managed Opuntia could, in suitable settings, contribute to a more diversified dryland economy. Its fruits have food value, its pads can serve as fodder, and its drought tolerance may support production where water-intensive crops are becoming harder to sustain. Prickly pear stands have also been associated in other Mediterranean research with soil protection and the natural regeneration of woody plants in areas at risk of desertification. These potential benefits remain conditional: a cactus that helps stabilize degraded land in one setting could threaten an endemic community in another. Climate adaptation therefore requires site-specific evidence, not a general presumption that drought tolerance makes any introduced population desirable.

The proposed alternative is a nature-positive management system that treats risk and value as variables to be measured at population level. Authorities would first clarify taxonomic identity, then assess ecological interactions, habitat sensitivity, landscape history and realistic restoration goals. Where native or endemic species are threatened, intervention could include control or eradication, followed by restoration rather than simple removal. In degraded, arid or historically transformed landscapes where ecological risk is low or manageable, containment, tolerance or carefully regulated use could be considered instead of blanket eradication. This framework also asks managers to account for food security, rural livelihoods, local knowledge, cultural value and the relationship between production and water demand. Revived cultivation might support low-water agriculture or niche markets, while cochineal production could benefit from renewed interest in natural dyes and improved extraction methods. None of these possibilities is presented as a guaranteed solution, and the Perspective does not report a new experiment, systematic review or taxonomic revision. It is a synthesis intended to improve decisions. Its broader warning is that conservation baselines are shifting: as climate change and land-use transformation make some historical ecosystems increasingly difficult to restore, effective policy may need to protect biodiversity while acknowledging the ecological functions and human meanings that long-established alien populations can acquire.

The authors also caution that the sharp demographic decline should not be interpreted as evidence that ecological questions have disappeared. The spread of Dactylopius opuntiae changed the abundance of plants recorded under the O. maxima category, but it does not resolve whether surviving populations belong to the same taxa or whether their effects are benign across all habitats. Biological control can therefore alter the visibility and distribution of a population without eliminating the need for ecological assessment. Monitoring should track recruitment, vegetative spread, interactions with native species and changes in habitat condition, rather than relying only on counts of established bushes.

This approach also places restoration objectives at the center of management. Removing a long-established cactus may be appropriate where it threatens endemic flora or sensitive communities, but removal alone does not recreate the former ecological state. Decisions should specify what habitat or function is intended to replace the population, how success will be measured and whether restoration is feasible under increasing aridity and water scarcity. By linking these choices to the Sustainable Development Goals, the Perspective frames invasive-species policy as a coordination problem involving biodiversity, land degradation, water use and rural well-being. Its emphasis on transparent evidence and site-level evaluation is intended to make regulation more proportionate without weakening protection where risks are substantial.

Subject of Research: Context-dependent management of naturalized Opuntia populations in Spain under aridification

Article Title: Rebalancing the management of naturalized Opuntia in Spanish territories under aridification: taxonomy, invasion status, cultural value, and climate adaptation

Article References: Perea-García, J. O., & Medina-Lorenzo, E. (2026). Rebalancing the management of naturalized Opuntia in Spanish territories under aridification: taxonomy, invasion status, cultural value, and climate adaptation. Discover Ecology, 2(1), Article 20. https://doi.org/10.1007/s44396-026-00039-8

Image Credits: AI Generated

DOI: 10.1007/s44396-026-00039-8

Keywords: Opuntia, Spain, invasive species, taxonomic uncertainty, aridification, water scarcity, restoration ecology, climate adaptation, Rebalancing, management, naturalized, Spanish