Sporadic detection of dengue virus RNA in California wastewater solids

Dengue virus (DENV) is the most prevalent mosquito-borne virus globally, endemic in over 100 countries and associated with around 40,000 deaths each year. Prior research has shown that DENV RNA can be detected in wastewater, but detection rates vary widely depending on the setting and method used. In this study, we analyzed 1,072 untreated wastewater samples from five treatment plants in California, collected between December 2023 and December 2025, to evaluate wastewater-based surveillance for DENV. We compared wastewater data with reported clinical incidence and Aedes aegypti mosquito abundance measurements over the same period collected by the California Department of Public Health. DENV RNA was found in 1.2% of wastewater samples (13/1,072). DENV-1 was most common (0.6%, 6/1,072), followed by DENV-2 (0.4%, 4/1,072), and DENV-4 (0.3%, 3/1,072). No DENV-3 RNA was detected. DENV RNA was detected only in wastewater from southern California (plants in Los Angeles and San Diego counties), with no detections in wastewater from the Bay Area, including plants located in San Jose and Oakland. There was no correlation between wastewater DENV RNA levels and county-wide reported dengue incidence during the same week. However, counties with a higher incidence over the study period showed greater detection frequency of DENV RNA in wastewater. Weekly Ae. aegypti abundance was correlated with weekly incidence of locally-transmitted cases (Spearman's ρ=0.45, p<0.01). Our results indicate that wastewater surveillance can detect DENV in areas of higher incidence, but the low detection frequency and sporadic nature of positive samples limit its utility as a standalone surveillance approach. Given the low incidence of dengue in California and the presence of robust clinical and mosquito surveillance systems, wastewater monitoring does not currently add significant public health value for dengue surveillance in the state.