Exploring the Hidden Viruses in Mosquitoes with RNA Sequencing

Researchers from Pennsylvania State University have uncovered new insights into the viral world living inside Culex tarsalis mosquitoes. These mosquitoes are common across North America and are well known for transmitting West Nile virus (WNV), a disease that can infect birds, humans, and other mammals. However, what many people don’t realize is that mosquitoes also carry a variety of other viruses that don’t infect humans, known as insect-specific viruses or ISVs.

These ISVs are important because they can interact with disease-causing viruses like WNV inside the mosquito, sometimes helping or hindering their ability to replicate. Understanding these interactions could reveal new ways to limit the spread of mosquito-borne diseases.

To explore this viral community, the Penn State team used a technique called small RNA sequencing (sRNA-seq). This method takes advantage of the mosquito’s natural antiviral defense system, known as RNA interference (RNAi). When a virus infects a mosquito, the RNAi system chops up viral genetic material into small pieces called virus-derived small RNAs (vsRNAs). By sequencing these tiny RNA fragments, scientists can identify which viruses are present in the mosquito,  even without directly isolating the whole virus.

Field Challenges and Powerful Results

Collecting mosquito samples across five western U.S. states during the COVID-19 pandemic presented significant challenges. Many samples were collected under suboptimal conditions, which can lead to degradation of RNA and loss of data. Despite these hurdles, sRNA-seq proved to be a robust and reliable method for virome analysis.

Graphical methods of metagenomics pipeline for virome profiling

Pools of 3 female mosquitoes per location were used for RNA extraction and sRNA-seq. Raw reads underwent quality checks and were aligned to mosquito and bacterial genomes, then unmapped reads were used for contig assembly. Viral and unknown contigs were used for co-occurrence analyses and contig extension. Extended contigs were analyzed by BLAST for virus identification, and available viral reference sequences were used for virus co-occurrence analyses and sRNA profile design.

The researchers analyzed samples from 17 different locations and successfully identified seven insect-specific viruses that have all been associated with Culex tarsalis in previous work. Interestingly, the distribution of these viruses did not follow a clear geographic pattern, but two, Marma virus and Culex narnavirus 1, were consistently found in every state where samples were collected. This widespread presence suggests that these two viruses may be stable and common members of the Culex tarsalis virome.

Significance

This research adds to our growing understanding of how mosquitoes interact with viruses in their environment. By revealing which viruses naturally exist in Culex tarsalis, scientists can start to piece together how these ISVs might influence the mosquito’s ability to spread human pathogens like West Nile virus.

Perhaps even more importantly, the study demonstrates the power of RNA sequencing technologies to produce high-quality viral data from imperfect field samples. In real-world surveillance scenarios, such as mosquito monitoring programs during outbreaks, conditions are rarely ideal. The success of sRNA-seq under these circumstances highlights its value as a practical tool for virus detection and ecological studies.

This work underscores how modern sequencing approaches are helping researchers not only detect known viruses but also uncover hidden viral diversity within insects that play major roles in public health.