HLC comparisons to historical records
Each year high volumes of mosquitoes are caught in GTs and LTs in the NWMAD and MMAD, including tens of thousands of potential WNV vectors. Since 2002, many human cases of WNV illness have occurred in the city of Chicago annually, despite rigorous mosquito control campaigns and the efforts of dedicated abatement and local health districts working year-round [6]. Evaluation of mosquito species collections obtained using different trapping methods is not precise in terms of determining abundances in a region due to trap-specific biases, lack of a systematic collection regimen and non-uniform distribution of equally placed trap types in the study region.
Based on a rich body of historical trapping data that cover the past decade, this study provides a reasonable baseline by which Culex species, and Cx. salinarius in particular, are expected to be collected. Mosquitoes of the genera Aedes, Culex, Anopheles, Coquillettidia and Psorophora accounted for 55.6%, 20.6%, 19.7%, 3.6% and 0.4% of the landing catch, respectively. The proportions of Aedes, Anopheles, Coquillettidia and Psorophora in collections from NWMAD and MMAD were 21.1% and 10.2%, 0.6% and 1.0%, 0.5% and 0.07% and 0.01% and 0.2%, respectively. In contrast, proportions of Culex species collected from NWMAD and MMAD were 70.2% and 83.8% in GTs, and 3.7% and 4.8% in LTs, respectively. Overall, HLCs collected higher proportions of non-Culex mosquitoes than GTs or LTs. In a separate study, 785 mosquitoes of various species of genus Culex from the North Shore District and NWMAD of Northern Cook County, Illinois were collected from GTs and LTs between 2017–2021 and submitted for confirmatory genetic species identification (Fritz et al., unpublished). Only two (0.25%) of these specimens were identified as Cx. salinarius, compared to 73.9% of specimens from HLCs. Thus, the expected distribution of species collected will depend on the collection method.
Traditional trapping methods are ideal for the efficient collection and testing of potential WNV mosquito vectors. However, these methods are only effective in identifying potential vectors and controlling virus activity if testing is conducted frequently because Culex populations are known to rebound rapidly after spraying [18]. Conversely, traditional trapping methods appear to collect a limited diversity of mosquito species [36, 37]. One possible explanations for this is that other mosquito species may not have a biological preference or attraction to the simulated baiting conditions (e.g. lights, CO2, lures, etc.), but a more likely reason is resource strain (e.g. limited human time to sort and identify mass quantities of mosquitoes). HLC collections do not have the same proportions of Culex species that GTs and LTs collect. Additionally, the HLC method has two major disadvantages: (i) it is labor intensive and time consuming, requiring a lot of human resources per captured mosquito; and (2) the collector’s risks of acquiring mosquito-borne pathogens, many of which do not have specific treatments or cures, are increased. However, HLCs are better suited than GTs or LTs for estimating the community of female mosquitoes specifically seeking human hosts [24]. The potential benefits in a systematic and targeted design may pinpoint hotspots for the highest concentration of human-seeking mosquitoes. When used in a targeted-style approach, HLCs can narrow the geographic area of interest and provide a more efficient deployment for controlling potential human-seeking WNV mosquito vectors; however, approaches to safeguard the collector’s health must be further developed before widespread adoption of HLCs for monitoring can be considered [24]. Additionally, the HLCs in this study were conducted in small natural areas, embedded within residential areas. Additional studies are needed to ascertain any influence of HLCs on mosquito ecology within these two intersecting habitats [29]. This study analyzed the results of landing rates based on four human collectors. To evaluate any differences in an individual’s attractiveness, we evaluated the rate of mosquito collections per “HLC night,” as a method for standardizing collection values across varying periods of exposure. These values indicate no statistical difference in an individual’s attractiveness, but future studies should address this question with a targeted design evaluating methods for measuring potential human behaviors, characteristics and/or scents that may be interest. Results from this 2-year study have made clear that Cx. salinarius, a less commonly reported WNV vector species in the upper Midwestern USA, was not only present in the Chicago area, but was the most commonly collected WNV vector in HLC collections, accounting for 73.9% of all Culex species landings. Conversely, the two primary vectors of WNV in the Midwestern USA, Cx. pipiens and Cx. restuans, only comprised 4.3% and 2.2% of the total proportion of Culex species that landed on human collectors.
Implications for human WNV transmission
Culex salinarius Coquillett mosquitoes, commonly known as the unbanded saltmarsh mosquito, are among the most competent vectors of WNV in laboratory transmission studies [38,39,40]. The virus has also been frequently isolated from wild-caught specimens [41,42,43,44,45]. The etymology of the name salinarius pertains to salt but is slightly misleading given that the species appears to tolerate low to moderate levels of salt (3–11 ppt) in freshwater/brackish environments, particularly in ponds and other small bodies of water along coastlines [46,47,48]. Historically, the range of this species stretches from Maine to Florida, and around the Gulf of Mexico to Texas [49]. Although specimens of this species have been collected as far north as Ontario, Canada, reports pertaining to its overall abundances in the upper Midwestern USA have remained low, although its presence has been reported from various locations in the Midwest USA [50,51,52,53,54]. Despite Cx salinarius and Cx. pipiens belonging to the same genus, these two species do not contain high quantities of fat bodies and do not enter a reproductive diapause [55, 56]. Instead, overwintering is thought to occur in natural shelters and animal burrows, and females have been known to seek blood meals at the first signs of mild weather [49]. In addition to not being treated by traditional control methods, the natural habitats that enable the winter survival of Cx. salinarius emphasize the importance of incorporating broader vector control measures in different landscapes within urban and suburban environments.
Perhaps the most startling difference between Cx. salinarius and Cx. pipiens is the host blood meal preference. Culex pipiens highly prefers avian hosts, and blood meal analyses have revealed that the proportion of mammalian hosts rarely exceeded 15–20% [2, 57]. In contrast, the host blood meal preference of Cx. salinarius exceeded 60% mammalian hosts and between 35 and 40% avian hosts [12, 58,59,60]. When potential implications for WNV spillover in the Midwestern USA are factored in, Cx. salinarius provides three potentially critical points of concern: (i) this species is more commonly observed in its northern range, leading to new questions regarding the species’ northern geographic limit [61], and it is either not commonly trapped with GT and LT methods, or it is not being correctly identified (via morphological methods); (ii) efforts to mitigate potential WNV vectors are not designed for targeting Cx. salinarius, given its tendency to breed in natural habitats; and (iii) the high mammalian host preference, aggressive biting nature and high vector competence provide the key ingredients for a mosquito to potentially become a highly efficient bridge vector of WNV and other encephalitis viruses [58, 59, 62].
Upon discovering the high ratio of Cx. salinarius landing rates, in comparison to those of any other Culex species in Chicago, additional data inquiries were made from prior research conducted in the region. Colleagues from the city of Chicago’s vector control program and a collaborative team working in various sites of Southern Cook County shared their abundance data for comparison (Table 1; Additional file 1: Table S1, Table S2). In summary, out of a total of 1,476,411 collected Culex specimens, only 1665 (0.11%) were identified as Cx. salinarius. HLC efforts resulted in 34 Cx. salinarius collections out of a total of 46 Culex specimens (73.9%). HLC efforts totaled 7.87 collections per trap night versus 0.022 collections per trap night for all other collection efforts. Taken together, this result equates to HLC collections producing over 364-fold the number of Cx. salinarius mosquitoes than any other effort combined. One caveat to this finding, however, is the potential for misclassification bias; HLC collections were speciated genetically, while other trapping data is generally based on high-throughput morphologic identification, which may fail to identify a species that is not expected in high numbers and is morphologically similar to a common species.
Interventions and future research
The surprising abundance of Cx. salinarius in the HLC collections described here in Chicago is a testament to the effectiveness of the HLC collection method [63]. This study provides compelling evidence in support of alternative trapping methods as a useful tool to provide updates on the overall abundance and diversity of potential disease vectors that are targeting human hosts. As a standard public health measure, these types of environmental health “checkups” may be needed more frequently due to the rapidly changing forces of the present day, such as new introductions of vectors and pathogens.
Public health and mosquito control agencies should consider adding a supplemental plan to their current monitoring and mitigation strategies, occasionally conducting surveillance of and/or targeting natural breeding areas where Cx. salinarius may reside (see Additional file 1: Text S1 & Figure S5 for brief habitat analysis). While HLC collection methods are not practical for routine surveillance, sparing usage implemented at strategic time periods (e.g. for early season WNV “sentinel” use, at or around historical peak human transmission, in known “hot spot” human and/or bird transmission locations, etc.) could provide added utility and breadth to existing strategies, providing important information that may increase the efficiency and knowledge in targeting potential mosquito vectors. With the aid of human scented lures and/or CO2, traps like BG-sentinels or CDC LTs may also be a viable alternative method for capturing large quantities of Cx. salinarius and other human-host seeking mosquitoes in natural habitats [64,65,66].
While these incidental findings may have implications for the enzootic and zoonotic transmission potential of WNV, suggestions from this study remain as hypotheses and should be interpreted as a guide for improving targeted surveillance and WNV mitigation efforts. Onward research demands the frequent and repeated detection of WNV RNA in Cx. salinarius females captured in their environment to substantiate the hypotheses provided in this manuscript. Additional HLC collections should be repeated and include a deeper evaluation of micro-scale factors that may influence a mosquito’s landing preferences, including preferred feeding locations on the human body (Additional file 1: Figure S6), microclimate (Additional file 1: Figure S7), and the addition of sampling residential areas.