Novel Focus of Sin Nombre Virus in Peromyscus eremicus Mice, Death Valley National Park, California, USA.

The deer mouse (Peromyscus maniculatus) is the primary reservoir for Sin Nombre virus (SNV) in the western United States. Rodent surveillance for hantavirus in Death Valley National Park, California, USA, revealed cactus mice (P. eremicus) as a possible focal reservoir for SNV in this location. We identified SNV antibodies in 40% of cactus mice sampled.

Correction: A forty-year review of Rocky Mountain spotted fever cases in California shows clinical and epidemiologic changes.

[This corrects the article DOI: 10.1371/journal.pntd.0010738.].

Ability of newly emerged adult Culex quinquefasciatus (Diptera: Culicidae) mosquitoes to exit belowground stormwater treatment systems via lateral conveyance pipes.

Culex quinquefasciatus Say mosquitoes flourish in belowground stormwater systems in the southern United States. Recent evidence suggests that oviposition-site-seeking females may have difficulties locating, entering, and ovipositing inside permanent water chambers when surface entry through pickholes in manhole covers are sealed. It remains unknown, however, if newly emerged adults are able to detect cues necessary to exit these partly sealed systems via lateral conveyance pipes or if they perish belowground. Fourth instar Cx. quinquefasciatus were placed within proprietary belowground stormwater treatment systems to determine the percentage of newly emerged adults able to escape treatment chambers via a single lateral conveyance pipe. Overall, 56% of deployed mosquitoes were captured in adult exit traps with an 1:1 male:female ratio. The percentage of adults captured varied significantly among chambers, but was not associated with structural site characteristics such as the chamber depth or the length and course of conveyance pipe to the exit trap. Empirical observations suggested that longbodied cellar spiders, Pholcus phalangioides (Fuesslin), ubiquitous in these structures, may have reduced adult trap capture. Findings demonstrate that newly emerged Cx. quinquefasciatus can exit subterranean chambers under potentially difficult structural conditions but suggest that a portion may perish in the absence of surface exit points in manhole shafts.

Potential Effects of Environmental Conditions on Prairie Dog Flea Development and Implications for Sylvatic Plague Epizootics.

Fleas are common ectoparasites of vertebrates worldwide and vectors of many pathogens causing disease, such as sylvatic plague in prairie dog colonies. Development of fleas is regulated by environmental conditions, especially temperature and relative humidity. Development rates are typically slower at low temperatures and faster at high temperatures, which are bounded by lower and upper thresholds where development is reduced. Prairie dogs and their associated fleas (mostly Oropsylla spp) live in burrows that moderate outside environmental conditions, remaining cooler in summer and warmer in winter. We found burrow microclimates were characterized by stable daily temperatures and high relative humidity, with temperatures increasing from spring through summer. We previously showed temperature increases corresponded with increasing off-host flea abundance. To evaluate how changes in temperature could affect future prairie dog flea development and abundance, we used development rates of O. montana (a species related to prairie dog fleas), determined how prairie dog burrow microclimates are affected by ambient weather, and combined these results to develop a predictive model. Our model predicts burrow temperatures and flea development rates will increase during the twenty-first century, potentially leading to higher flea abundance and an increased probability of plague epizootics if Y. pestis is present.

A forty-year review of Rocky Mountain spotted fever cases in California shows clinical and epidemiologic changes.

Rocky Mountain spotted fever (RMSF) is a life-threatening tick-borne disease documented in North, Central, and South America. In California, RMSF is rare; nonetheless, recent fatal cases highlight ecological cycles of the two genera of ticks, Dermacentor and Rhipicephalus, known to transmit the disease. These ticks occur in completely different habitats (sylvatic and peridomestic, respectively) resulting in different exposure risks for humans. This study summarizes the demographic, exposure, and clinical aspects associated with the last 40 years of reported RMSF cases to the California Department of Public Health (CDPH). Seventy-eight RMSF cases with onsets from 1980 to 2019 were reviewed. The incidence of RMSF has risen in the last 20 years from 0.04 cases per million to 0.07 cases per million (a two-fold increase in reports), though the percentage of cases that were confirmed dropped significantly from 72% to 25% of all reported cases. Notably, Hispanic/Latino populations saw the greatest rise in incidence. Cases of RMSF in California result from autochthonous and out-of-state exposures. During the last 20 years, more cases reported exposure in Southern California or Mexico than in the previous 20 years. The driver of these epidemiologic changes is likely the establishment and expansion of Rhipicephalus sanguineus sensu lato ticks in Southern California and on-going outbreaks of RMSF in northern Mexico. Analysis of available electronically reported clinical data from 2011 to 2019 showed that 57% of reported cases presented with serious illness requiring hospitalization with a 7% mortality. The difficulty in recognizing RMSF is due to a non-specific clinical presentation; however, querying patients on the potential of tick exposure in both sylvatic and peridomestic environments may facilitate appropriate testing and treatment.

Detection and Establishment of Aedes notoscriptus (Diptera: Culicidae) Mosquitoes in Southern California, United States.

Aedes notoscriptus (Skuse), the Australian backyard mosquito, is a pestiferous daytime-biting species native to Australia and the surrounding southwestern Pacific region. It is suspected to play a role in the transmission of several arboviruses and is considered a competent vector of dog heartworm, Dirofilaria immitis (Leidy). This highly adaptable mosquito thrives in natural and artificial water-holding containers in both forested and urbanized areas, from tropical to temperate climates, and has benefitted from a close association with humans, increasing in abundance within its native range. It invaded and successfully established in New Zealand as well as in previously unoccupied temperate and arid regions of Australia. Ae. notoscriptus was discovered in Los Angeles County, CA, in 2014, marking the first time this species had been found outside the southwestern Pacific region. By the end of 2019, immature and adult mosquitoes had been collected from 364 unique locations within 44 cities spanning three southern California counties. The discovery, establishment, and rapid spread of this species in urban areas may signal the global movement and advent of a new invasive container-inhabiting species. The biting nuisance, public health, and veterinary health implications associated with the invasion of southern California by this mosquito are discussed.

Discovery of vector mosquitoes (Diptera: Culicidae) in newly installed above- and belowground stormwater treatment systems in San Diego County, California.

Stormwater treatment systems (STS) intended to mitigate the potentially negative public health and environmental impact caused by urban runoff are highly conducive to mosquito production. Thirty-one STS, 15 aboveground extended detention basins (EDBs) and 16 proprietary belowground systems newly installed along State Route 125 in San Diego County, CA, were inspected monthly between July 2008 and June 2009 for presence of standing water and mosquitoes. During the 12-mo study, standing water was observed in 66% of the 180 total inspections to EDBs and at least once in each of the 15 basins, whereas belowground systems held water year-round in permanent-water sumps. With the exception of one EDB, immature mosquitoes were observed in all STS, during every month of the year in EDBs and all months except December in belowground systems. Cumulatively, mosquitoes were noted in 44% of the 372 total site inspections, with a nearly equal number of positive observations from EDBs and belowground systems. Four species were identified from EDBs, Culex tarsalis Coquillett, Cx. quinquefasciatus Say, Anopheles hermsi Barr and Guptavanij, and Culiseta incidens (Thompson). Cx. quinquefasciatus was the sole species identified from belowground systems. Results derived from this study provide additional evidence for mosquito production in STS because of structural design or persistent inflows, or both, of dry-weather urban runoff. Interagency collaboration is needed to ensure that STS are designed and maintained in a way that minimizes their potential to produce mosquitoes that can negatively affect public health.

Association between Culex quinquefasciatus (Diptera: Culicidae) oviposition and structural features of belowground stormwater treatment devices.

Belowground stormwater treatment devices referred to as Best Management Practices (BMPs) can be persistent sources of Culex quinquefasciatus Say (Diptera: Culicidae) mosquitoes and in urban environments necessitate routine monitoring and insecticide treatment for control. The design of certain structural features of BMPs may discourage mosquito entry and oviposition, potentially reducing the need for control measures. From August 2007 to November 2008, 29 BMPs were intermittently monitored for the presence of mosquitoes and compared against selected structural features. A bucket ovitrap was developed to estimate entry of oviposition-site-seeking females into BMPs via egg raft counts. The number of egg rafts collected was positively associated with larger diameters and shorter lengths of conveyance pipe from the nearest inlet or outlet and with increasing number of pickholes in BMP manhole covers. In contrast, the number of days pickholes were sealed against adult mosquito entry was negatively associated with eggs rafts. Findings suggest that the design of belowground BMPs and their conveyance systems can influence mosquito oviposition inside these structures. Specific design criteria with the greatest potential to minimize mosquito production and provide safe access for monitoring and control activities are discussed.

The need for collaboration among government agencies to reduce mosquito production in mandated stormwater treatment structures.

Efforts to improve water quality increasingly rely on structural stormwater best management practices (BMPs) to remove pollutants from urban runoff. These structures can unintentionally produce mosquitoes and may play a role in the spread of mosquito-borne diseases. A questionnaire was distributed to over 300 stormwater and mosquito control agencies in the United States to assess the prevalence of BMPs and associated mosquito production, identify current measures to control mosquitoes within BMPs, and elucidate the extent of collaboration between these agencies. Responses suggest that agencies often lack basic information relevant to minimizing mosquitoes in BMPs, such as the number of structures within an agency's jurisdiction and the frequency of their maintenance, and that greater interagency collaboration could improve control efforts. Approximately 40% of agencies reported regular collaboration to minimize mosquito production in BMPs; however, barriers to such collaborative work included confusion over roles and responsibilities and a lack of interest. The rapid increase of BMPs in urban environments resulting from increasingly stringent water-quality regulations provides justification for increased collaboration between stormwater and mosquito control sectors of government to aid and strengthen public health efforts.

Insecticide Resistance Status of Aedes aegypti (Diptera: Culicidae) in California by Biochemical Assays.

Insecticide resistance in Aedes aegypti mosquitoes poses a major threat to public health worldwide. There are two primary biological mechanisms that can lead to insecticide resistance, target site and metabolic resistance, both of which confer resistance to specific classes of insecticides. Due to the limited number of chemical compounds available for mosquito control, it is important to determine current enzymatic profiles among mosquito populations. This study assessed resistance profiles for three metabolic pathways, α-esterases, ß-esterases, and mixed-function oxidases (MFOs), as well as insensitivity of the acetylcholinesterase (iAChE) enzyme in the presence of propoxur, among Ae. aegypti from the Central Valley and southern California. All field-collected Ae. aegypti demonstrated elevated MFOs and iAChE activity, indicating potential development of pyrethroid and organophosphate resistance, respectively. Although regional variations were found among α-esterase and ß-esterase activity, levels were generally elevated, further suggesting additional mechanisms for developing organophosphate resistance. Furthermore, mosquito samples from southern California exhibited a higher expression level to all three metabolic enzymes and iAChE activity in comparison to mosquitoes from the central region. These results could help guide future mosquito control efforts, directing the effective use of insecticides while limiting the spread of resistance.

Evaluation of manhole inserts as structural barriers to mosquito entry into belowground stormwater systems using a simulated treatment device.

Belowground proprietary stormwater treatment devices can produce mosquitoes, including vectors of West Nile virus. Elimination of vertical entry points such as pick holes in manhole covers may reduce the number of mosquitoes entering and reproducing in these structures. Plastic manhole dish inserts were evaluated as structural barriers against mosquito entry through pick holes in a simulated stormwater treatment device. Inserts were 100% effective at preventing mosquito entry through covers when no other openings existed. In devices configured with an open lateral conveyance pipe, the addition of an insert under the cover reduced mosquito oviposition significantly. Subsequent trials to further elucidate mosquito entry through manhole covers found a significant positive correlation between increasing number of pick holes and mosquito oviposition. Results of the study suggest the potential for manhole dish inserts to decrease the number of mosquitoes entering belowground structures. The different available stormwater treatment systems and site-specific installations may, however, provide a much greater variety of possible alternate entry points for mosquitoes than was addressed in the current study. Further work is needed in field installations to quantify the significance of pick holes to mosquito entry and determine under what conditions, if any, manhole dish inserts would be most effective and appropriate.

Identification of Molecular Determinants of Resistance to Pyrethroid Insecticides in Aedes aegypti (Diptera: Culicidae) Populations in California, USA.

The first breeding populations of Aedes aegypti (Linnaeus) were identified in California in 2013, and have since been detected in 13 counties. Recent studies suggest two introductions likely occurred, with genetically distinct populations in the central and southern regions of the state. Given the threat of dengue, chikungunya, and Zika virus transmission, it is imperative to understand if these populations harbor genes that could confer resistance to pyrethrin-based insecticides, known as pyrethroids, the most commonly used class of adulticides in the state. In 2017, the California Department of Public Health initiated a pesticide resistance screening program for Ae. aegypti to assess the presence of specific mutations on the sodium channel gene (V1016I and F1534C) associated with knockdown resistance to pyrethroids. Mosquitoes collected between 2015 and 2017 from 11 counties were screened for mutations using real-time polymerase chain reaction assays. Results revealed distinctly different resistance profiles between the central and southern regions. The central population displayed nearly fixed resistant mutations at both loci, whereas the southern population was more variable. The relative proportion of resistant alleles observed in sampled mosquitoes collected in southern California increased each year from 2015 through 2017, indicating potential increases in resistance across this region. The presence of these mutations indicates that these mosquitoes may be predisposed to surviving pyrethroid treatments. Additional biological and biochemical assays will help better elucidate the mechanisms underlying insecticide resistance in California Ae. aegypti and prompt the use of pesticides that are most effective at controlling these mosquitoes.

An assessment of mosquito production and nonchemical control measures in structural stormwater best management practices in southern California.

A 3-year study was conducted to assess mosquito production in structural stormwater Best Management Practices (BMPs) installed by the California Department of Transportation in San Diego and Los Angeles Counties. Thirty-seven BMPs were monitored weekly for presence and relative abundance of immature mosquitoes and for conditions conducive to mosquito production. Species identified were Aedes squamiger, Anopheles franciscanus, An. hermsi, Culex quinquefasciatus, Cx. stigmatosoma, Cx. tarsalis, Culiseta incidens, and Cs. inornata. Structures designed with accessible, permanent sources of standing water in sumps, vaults, or basins were observed to support immatures all year. In BMPs intended to drain rapidly and completely, observed larval habitats resulted from design features, component failure, construction flaws, and non-stormwater runoff flows. Specific nonchemical mitigation measures to minimize or eliminate mosquito production were developed, implemented, and evaluated. This study provides a 1st assessment of mosquito production, species composition, and nonchemical control measures in structural stormwater BMPs in southern California.

Effect of conveyance pipe dimension and orientation on mosquito oviposition in a simulated stormwater management device.

Simulated stormwater management devices baited with alfalfa infusion were constructed to test conveyance pipe dimension and orientation as a potential deterrent to mosquito oviposition. Various configurations of pipe diameter, length, and orientation were evaluated based on egg raft counts. Field trials tested pipes of 1.3-, 5-, and 10-cm diam and 0-, 90-, or 270-cm lengths, in both horizontal and vertical orientations. Additional trials of 10-cm-diam horizontal pipe evaluated the effects of a 90 degrees bend, single or dual entry points, and lengths greater than 270 cm. Significantly fewer egg rafts were collected in pipes of smaller diameter and longer length in both horizontal and vertical orientations. A 90 degrees bend or removal of an entry point to pipes of fixed length had no significant effect on oviposition. A maximum tested length of 24.4 m did not preclude oviposition. The results of this study suggest that manipulating diameter and length of conveyance pipe in stormwater management devices may not be an effective strategy to deter oviposition. The need for integrating improved, novel, nonchemical mosquito control measures into designing and operating stormwater management structures is discussed.

Mosquito production in stormwater treatment devices in the Lake Tahoe Basin, California.

In response to increasing evidence of mosquito production in structural stormwater Best Management Practices (BMPs), a collaborative project was developed to document the occurrence, species composition, and seasonal abundance of mosquitoes from selected urban and highway BMPs in the Lake Tahoe Basin, California. Structural and environmental factors associated with mosquito production in highway BMPs were identified and analyzed. Ten species of mosquitoes were collected from 47 BMPs, including Culex tarsalis, Culiseta incidens, Cs. inornata, and 7 species of Aedes. In and around South Lake Tahoe, immatures were most abundant in urban BMPs during the warmer summer and fall months, whereas natural water sources in the surrounding area harbored mosquitoes more often during the colder months of early spring. In BMPs installed along Lake Tahoe's perimeter highways, mosquitoes were observed in 11% of site visits conducted during a single season. Larval presence in highway BMPs was positively associated with water temperature and negatively associated with precipitation, sand, and unspecified organic matter. The significance of mosquito production in BMPs of the Tahoe Basin and the potential for increased transmission of mosquito-borne disease are discussed.

Development time of Culex mosquitoes in stormwater management structures in California.

A widely recommended strategy to minimize mosquito production in structural stormwater Best Management Practices (BMPs) is to ensure they hold captured water for no more than 72 h. However, this standard may be overly conservative for many mosquito species found in urban environments and may impede or prevent the capacity of BMPs to fulfill more stringent water quality standards in environmentally sensitive areas. Egg-to-pupa development of Culex tarsalis, Cx. pipiens, and Cx. quinquefasciatus were examined during July, August, and September 2006 in stormwater management basins and in water collected from these basins in 3 climatically distinct regions of California: the Lake Tahoe Basin, Sacramento Valley, and Los Angeles Basin. The observed minimum times to pupal development were 6 days for Cx. tarsalis and Cx. quinquefasciatus and 8 days for Cx. pipiens. Multiple linear regression models were used to estimate minimum predicted development times under optimal conditions for each region. The results suggest that water residence times of up to 96 h will not significantly increase the potential for Culex mosquito production in stormwater BMPs in the 3 regions included in this study.

Detection of Rickettsia rickettsii and Bartonella henselae in Rhipicephalus sanguineus ticks from California.

Sixty-two questing adult Rhipicephalus sanguineus (Latreille) ticks were collected by direct removal from blades of turfgrass and adjacent concrete walkways at a suburban home in Riverside County, CA, and tested for the presence of Rickettsia, Bartonella, and Ehrlichia DNA. Polymerase chain reaction (PCR) was used to amplify fragments of the 17-kDa antigen gene and the rOmpA gene of the spotted fever group rickettsiae. One male tick contained R. rickettsii DNA; its genotype differed from R. rickettsii isolates found in Montana and Arizona that cause Rocky Mountain spotted fever and from Hlp#2 and 364D serotypes. One male tick and one female tick contained B. henselae DNA. No Ehrlichia platys or Ehrlichia canis DNAs were detected using nested PCR for their 16S rRNA genes. These findings extend the area where Rickettsia rickettsii may be vectored by Rh. sanguineus. Rh. sanguineus also may be infected with Bartonella henselae, a human pathogen that is typically associated with fleas and causes cat scratch disease.

Limited Evidence for Rickettsia felis as a Cause of Zoonotic Flea-Borne Rickettsiosis in Southern California.

Over 90% of human flea-borne rickettsioses cases in California are reported from suburban communities of Los Angeles and Orange counties and are presumed to be associated with either Rickettsia typhi or Rickettsia felis infection. Ctenocephalides felis (Bouché) is considered the principal vector for both rickettsiae, and R. felis has largely replaced R. typhi as the presumptive etiologic agent based on the widespread incidence of R. felis in cat flea populations. However, with no evidence to confirm R. felis as the cause of human illness in southern California, coupled with recent findings that showed R. felis to be widespread in cat fleas statewide, we propose that this hypothesis should be reconsidered. Evidence of only limited numbers of R. typhi-infected cat fleas in the environment may indicate a very rare infection and explain why so few cases of flea-borne rickettsioses are reported each year in southern California relative to the population.

Detection and Establishment of Aedes aegypti and Aedes albopictus (Diptera: Culicidae) Mosquitoes in California, 2011-2015.

In 2011, a thriving population of Aedes albopictus (Skuse), the Asian tiger mosquito, was discovered within three cities in Los Angeles County over an estimated 52-km2 urban area. Two years later in 2013, Aedes aegypti (L.), the yellow fever mosquito, was detected within several urban areas of Madera, Fresno, and San Mateo counties. State and local vector control agencies responded with an aggressive effort to eradicate or interrupt the spread of these two invasive mosquitoes; however, known populations continued to expand outward and new infestations were identified at an accelerated pace in central and southern California. By the end of 2015, one or both species had been detected within the jurisdictional boundaries of 85 cities and census-designated places in 12 counties. Herein we report on the discovery and widespread establishment of Ae. aegypti and Ae. albopictus in urban areas of coastal, central, and southern California between 2011 and 2015 and discuss the subsequent rapid changes to the activities and priorities of vector control agencies in response to this unprecedented invasion.