Mosquito Control Activities during Local Transmission of Zika Virus, Miami-Dade County, Florida, USA, 2016.

In 2016, four clusters of local mosquitoborne Zika virus transmission were identified in Miami-Dade County, Florida, USA, generating "red zones" (areas into which pregnant women were advised against traveling). The Miami-Dade County Mosquito Control Division initiated intensive control activities, including property inspections, community education, and handheld sprayer applications of larvicides and adulticides. For the first time, the Mosquito Control Division used a combination of areawide ultralow-volume adulticide and low-volume larvicide spraying to effectively control Aedes aegypti mosquitoes, the primary Zika virus vector within the county. The number of mosquitoes rapidly decreased, and Zika virus transmission was interrupted within the red zones immediately after the combination of adulticide and larvicide spraying.

Rapid Screening of Aedes aegypti Mosquitoes for Susceptibility to Insecticides as Part of Zika Emergency Response, Puerto Rico.

In response to the 2016 Zika outbreak, Aedes aegypti mosquitoes from 38 locations across Puerto Rico were screened using Centers for Disease Control and Prevention bottle bioassays for sensitivity to insecticides used for mosquito control. All populations were resistant to pyrethroids. Naled, an organophosphate, was the most effective insecticide, killing all mosquitoes tested.

Bourbon Virus in Field-Collected Ticks, Missouri, USA.

Bourbon virus (BRBV) was first isolated in 2014 from a resident of Bourbon County, Kansas, USA, who died of the infection. In 2015, an ill Payne County, Oklahoma, resident tested positive for antibodies to BRBV, before fully recovering. We retrospectively tested for BRBV in 39,096 ticks from northwestern Missouri, located 240 km from Bourbon County, Kansas. We detected BRBV in 3 pools of Amblyomma americanum (L.) ticks: 1 pool of male adults and 2 pools of nymphs. Detection of BRBV in A. americanum, a species that is aggressive, feeds on humans, and is abundant in Kansas and Oklahoma, supports the premise that A. americanum is a vector of BRBV to humans. BRBV has not been detected in nonhuman vertebrates, and its natural history remains largely unknown.

Zika Virus Infection in Patient with No Known Risk Factors, Utah, USA, 2016.

In 2016, Zika virus disease developed in a man (patient A) who had no known risk factors beyond caring for a relative who died of this disease (index patient). We investigated the source of infection for patient A by surveying other family contacts, healthcare personnel, and community members, and testing samples for Zika virus. We identified 19 family contacts who had similar exposures to the index patient; 86 healthcare personnel had contact with the index patient, including 57 (66%) who had contact with body fluids. Of 218 community members interviewed, 28 (13%) reported signs/symptoms and 132 (61%) provided a sample. Except for patient A, no other persons tested had laboratory evidence of recent Zika virus infection. Of 5,875 mosquitoes collected, none were known vectors of Zika virus and all were negative for Zika virus. The mechanism of transmission to patient A remains unknown but was likely person-to-person contact with the index patient.

Co-circulation of West Nile virus variants, Arizona, USA, 2010.

Molecular analysis of West Nile virus (WNV) isolates obtained during a 2010 outbreak in Maricopa County, Arizona, USA, demonstrated co-circulation of 3 distinct genetic variants, including strains with novel envelope protein mutations. These results highlight the continuing evolution of WNV in North America and the current complexity of WNV dispersal and transmission.

Experimental Infection of Amblyomma americanum (Acari: Ixodidae) With Bourbon Virus (Orthomyxoviridae: Thogotovirus).

Following the recent discovery of Bourbon virus (BRBV) as a human pathogen, and the isolation of the virus from Amblyomma americanum (L.) collected near the location of a fatal human case, we undertook a series of experiments to assess the laboratory vector competence of this tick species for BRBV. Larval ticks were infected using an immersion technique, and transstadial transmission of virus to the nymphal and then to the adult stages was demonstrated. Transstadially infected nymphs transmitted virus to adult ticks at very high rates during cofeeding, indicating the presence of infectious virus in the saliva of engorging ticks. Vertical transmission by transstadially infected females to their progeny occurred, but at a low rate. Rabbits fed on by infected ticks of all active life stages developed high titers of antibody to the virus, demonstrating host exposure to BRBV antigens/live virus during tick blood feeding. These results demonstrate that A. americanum is a competent vector of BRBV and indicate that cofeeding could be critical for enzootic maintenance.

Seasonality and time of host-seeking activity of Culex tarsalis and floodwater Aedes in northern Colorado, 2006-2007.

Effective and economical control of adult vector and pest mosquitoes requires knowledge of their seasonal abundance and host-seeking activity patterns. We conducted research in 2006-2007 to study these variables for Culex tarsalis, Aedes vexans, Ae. melanimon, and Ae. dorsalis in Larimer County, CO. Mosquitoes were collected with traps that segregated catches in 7 consecutive 2-h intervals initiating at 1730 h at 4 sites. Seasonal abundance varied for all species by site and year. Time of host-seeking activity was consistent for all species by site and year. Culex tarsalis counts were significantly higher 1.2-4.5 h after sunset than during the preceding time intervals. Maximum host-seeking activity of the 3 Aedes species occurred from 0.8 h before sunset to 6.5 h after. Host seeking by all species continued throughout the night. For optimal control of Cx. tarsalis adulticide application should start approximately 1 h after sunset, and control of Aedes species should begin soon after sunset, and for all species applications can continue throughout most of the night.

Movement of St. Louis encephalitis virus in the Western United States, 2014- 2018.

St. Louis encephalitis virus (SLEV) is a flavivirus that circulates in an enzootic cycle between birds and mosquitoes and can also infect humans to cause febrile disease and sometimes encephalitis. Although SLEV is endemic to the United States, no activity was detected in California during the years 2004 through 2014, despite continuous surveillance in mosquitoes and sentinel chickens. In 2015, SLEV-positive mosquito pools were detected in Maricopa County, Arizona, concurrent with an outbreak of human SLEV disease. SLEV-positive mosquito pools were also detected in southeastern California and Nevada in summer 2015. From 2016 to 2018, SLEV was detected in mosquito pools throughout southern and central California, Oregon, Idaho, and Texas. To understand genetic relatedness and geographic dispersal of SLEV in the western United States since 2015, we sequenced four historical genomes (3 from California and 1 from Louisiana) and 26 contemporary SLEV genomes from mosquito pools from locations across the western US. Bayesian phylogeographic approaches were then applied to map the recent spread of SLEV. Three routes of SLEV dispersal in the western United States were identified: Arizona to southern California, Arizona to Central California, and Arizona to all locations east of the Sierra Nevada mountains. Given the topography of the Western United States, these routes may have been limited by mountain ranges that influence the movement of avian reservoirs and mosquito vectors, which probably represents the primary mechanism of SLEV dispersal. Our analysis detected repeated SLEV introductions from Arizona into southern California and limited evidence of year-to-year persistence of genomes of the same ancestry. By contrast, genetic tracing suggests that all SLEV activity since 2015 in central California is the result of a single persistent SLEV introduction. The identification of natural barriers that influence SLEV dispersal enhances our understanding of arbovirus ecology in the western United States and may also support regional public health agencies in implementing more targeted vector mitigation efforts to protect their communities more effectively.

Environmental conditions for Jamestown Canyon virus correlated with population-level resource selection by white-tailed deer in a suburban landscape.

Suburban landscapes can alter spatial patterns by white-tailed deer (Odocoileus virginianus) and increase animal contact with vectors, pathogens, and humans. Close-contact relationships at a landscape level can have broad implications for disease epidemiology. From 1995-1999, we captured and radio-collared 41 deer in two suburban forest preserves in Chicago, Illinois. We collected blood to determine if animals were seronegative or seropositive for Jamestown Canyon virus and tracked deer movements within suburban habitats. We developed utilization distributions at the population-level and evaluated resource selection for seronegative and seropositive deer. We used maximum likelihood estimation for model selection via Akaike information criterion and then restricted maximum likelihood estimation to attain unbiased estimates of the parameters in the top-ranking models. The top-ranking model describing the resource selection of seronegative deer received almost the full weight of evidence (Akaike information criterion ωi = 0.93), and included the proportion of wetlands, precipitation in year t, and an interaction of the proportion of wetlands and precipitation in year t. The top-ranking model describing resource selection of seropositive deer received the full weight of evidence (Akaike information criterion ωi = 1.00). The model included distance to nearest populated place, distance to nearest river, length of road in each grid cell, precipitation in year t, and an interaction of the length of road in each grid cell and precipitation in year t. These results are valuable for mapping the spatial configuration of hotspots for Jamestown Canyon virus and could be used to educate local residents and recreationalists to reduce human exposure.

A SURVEY OF THE MOSQUITOES OF KOSRAE STATE, FEDERATED STATES OF MICRONESIA, 2016.

In response to an outbreak of Zika virus that started in February 2016 on Kosrae Island, Kosrae State, Federated States of Micronesia, we conducted entomological investigations, including a survey to characterize the mosquito fauna on Kosrae, from November 29 to December 8, 2016. Mosquitoes were collected using several surveillance methods in order to sample all stages of the mosquito life cycle. Eggs were collected using ovicups, larvae and pupae were sampled using standard dippers, and adults were collected using aspirators and Biogents-2 Sentinel traps. All species previously recorded from Kosrae State were found in the current survey, confirming their continued presence on the island. Aedes aegypti was detected on Lelu Island, representing a new municipal record. The collection of Ae. vexans nocturnus represents a new species record for Kosrae, increasing the number of known taxa on this island from 6 to 7. The report herein provides updated knowledge of the mosquitoes that occur on Kosrae State, Federated States of Micronesia.

Surveillance for Heartland and Bourbon Viruses in Eastern Kansas, June 2016.

In June 2016, we continued surveillance for tick-borne viruses in eastern Kansas following upon a larger surveillance program initiated in 2015 in response to a fatal human case of Bourbon virus (BRBV) (Family Orthomyxoviridae: Genus Thogotovirus). In 4 d, we collected 14,193 ticks representing four species from four sites. Amblyomma americanum (L.) (Acari: Ixodidae) accounted for nearly all ticks collected (n = 14,116, 99.5%), and the only other species identified were Amblyomma maculatum Koch (Acari: Ixodidae), Dermacentor variabilis (Say) (Acari: Ixodidae) and Ixodes scapularis Say (Acari: Ixodidae). All ticks were tested for both BRBV and Heartland virus (Family Bunyaviridae: Genus Phlebovirus) in 964 pools. Five Heartland virus positive tick pools were detected and confirmed by real-time reverse transcription PCR (rRT-PCR), while all pools tested negative for BRBV. Each Heartland positive pool was composed of 25 A. americanum nymphs with positive pools collected at three different sites in Bourbon County. A. americanum is believed to be the primary vector of both Heartland and BRBVs to humans based upon multiple detections of virus in field-collected ticks, its abundance, and its aggressive feeding behavior on mammals including humans. However, it is possible that A. americanum encounters viremic vertebrate hosts of BRBV less frequently than viremic hosts of Heartland virus, or that BRBV is less efficiently passed among ticks by co-feeding, or less efficiently passed vertically from infected female ticks to their offspring resulting in lower field infection rates.

Surveillance for Tick-Borne Viruses Near the Location of a Fatal Human Case of Bourbon Virus (Family Orthomyxoviridae: Genus Thogotovirus) in Eastern Kansas, 2015.

Bourbon virus (Family Orthomyxoviridae: Genus Thogotovirus) was first isolated from a human case-patient residing in Bourbon County, Kansas, who subsequently died. Before becoming ill in late spring of 2014, the patient reported several tick bites. In response, we initiated tick surveillance in Bourbon County and adjacent southern Linn County during spring and summer of 2015. We collected 20,639 host-seeking ticks representing four species from 12 sites. Amblyomma americanum (L.) (Acari: Ixodidae) and Dermacentor variabilis (Say) (Acari: Ixodidae) accounted for nearly all ticks collected (99.99%). Three tick pools, all composed of adult A. americanum ticks collected in Bourbon County, were virus positive. Two pools were Heartland virus (Family Bunyaviridae: Genus Phlebovirus) positive, and one was Bourbon virus positive. The Bourbon virus positive tick pool was composed of five adult females collected on a private recreational property on June 5. Detection of Bourbon virus in the abundant and aggressive human-biting tick A. americanum in Bourbon County supports the contention that A. americanum is a vector of Bourbon virus to humans. The current data combined with virus detections in Missouri suggest that Bourbon virus is transmitted to humans by A. americanum ticks, including both the nymphal and adult stages, that ticks of this species become infected as either larvae, nymphs or both, perhaps by feeding on viremic vertebrate hosts, by cofeeding with infected ticks, or both, and that Bourbon virus is transstadially transmitted. Multiple detections of Heartland virus and Bourbon virus in A. americanum ticks suggest that these viruses share important components of their transmission cycles.

Land cover variation and West Nile virus prevalence: patterns, processes, and implications for disease control.

Identifying links between environmental variables and infectious disease risk is essential to understanding how human-induced environmental changes will effect the dynamics of human and wildlife diseases. Although land cover change has often been tied to spatial variation in disease occurrence, the underlying factors driving the correlations are often unknown, limiting the applicability of these results for disease prevention and control. In this study, we described associations between land cover composition and West Nile virus (WNV) infection prevalence, and investigated three potential processes accounting for observed patterns: (1) variation in vector density; (2) variation in amplification host abundance; and (3) variation in host community composition. Interestingly, we found that WNV infection rates among Culex mosquitoes declined with increasing wetland cover, but wetland area was not significantly associated with either vector density or amplification host abundance. By contrast, wetland area was strongly correlated with host community composition, and model comparisons suggested that this factor accounted, at least partially, for the observed effect of wetland area on WNV infection risk. Our results suggest that preserving large wetland areas, and by extension, intact wetland bird communities, may represent a valuable ecosystem-based approach for controlling WNV outbreaks.

Avian diversity and West Nile virus: testing associations between biodiversity and infectious disease risk.

The emergence of several high profile infectious diseases in recent years has focused attention on our need to understand the ecological factors contributing to the spread of infectious diseases. West Nile virus (WNV) is a mosquito-borne zoonotic disease that was first detected in the United States in 1999. The factors accounting for variation in the prevalence of WNV are poorly understood, but recentideas suggesting links between high biodiversity and reduced vector-borne disease risk may help account for distribution patterns of this disease. Since wild birds are the primary reservoir hosts for WNV, we tested associations between passerine (Passeriform) bird diversity, non-passerine (all other orders) bird diversity and virus infection rates in mosquitoes and humans to examine the extent to which bird diversity is associated with WNV infection risk. We found t h at non-passerine species richness (number of non-passerine species) was significantly negatively correlated with both mosquito and human infection rates, whereas there was no significant association between passerine species richness and any measure of infection risk. Our findings suggest that non-passerine diversity may play a role in dampening WNV amplification rates in mosquitoes, minimizing human disease risk.

Transmission of Heartland Virus (Bunyaviridae: Phlebovirus) by Experimentally Infected Amblyomma americanum (Acari: Ixodidae).

Heartland virus (HRTV; Bunyaviridae: Phlebovirus) is a recently described cause of human illness in the United States. After field studies conducted in 2012 implicated Amblyomma americanum (L.) as a vector of HRTV, we initiated experiments to assess the vector competence of A. americanum. Larval and nymphal ticks were immersed in high-titered suspensions of HRTV, and then tested for virus at various intervals postimmersion. In a later trial larval ticks were immersed in HRTV, followed by engorgement on a rabbit. A subset of postmolt nymphs was tested for HRTV to document transstadial transmission. Putatively infected nymphs were cofed with uninfected colony larvae to assess nonviremic transmission. In another trial, nymphs were fed on a rabbit and allowed to molt to the adult stage. Male and female ticks fed and mated upon a rabbit, and females were allowed to oviposit. Male and spent female ticks were tested for HRTV, and offspring of infected females were tested to assess vertical transmission. Infection rates of ≤50% were observed in immersed larvae and nymphs tested at intervals following immersion. Transstadial transmission from larvae to nymphs and then to adults was documented. HRTV was detected in a pool of nymphs molted from uninfected larvae cofed with infected nymphs. Vertical transmission of HRTV was observed in progeny of infected females. Infected females took longer to oviposit and produced fewer offspring. Serologic conversions (without viremia) in rabbits fed upon by immersed larvae or transstadially infected ticks indicate horizontal transmission of HRTV.

Surveillance for Heartland Virus (Bunyaviridae: Phlebovirus) in Missouri During 2013: First Detection of Virus in Adults of Amblyomma americanum (Acari: Ixodidae).

During 2013, we collected and tested ticks for Heartland virus (HRTV), a recently described human pathogen in the genus Phlebovirus (Bunyaviridae), from six sites in northwestern Missouri. Five sites were properties owned by HRTV patients, and the sixth was a conservation area that yielded virus in ticks during 2012. We collected 39,096 ticks representing five species; however, two species, Amblyomma americanum (L.) (97.6%) and Dermacentor variabilis (Say) (2.3%), accounted for nearly all ticks collected. We detected 60 HRTV-positive tick pools and all were composed of A americanum: 53 pools of nymphs, six pools of male adults, and one pool of female adults. This is the first record of HRTV in adult ticks. Virus was detected at five properties that yielded A. americanum ticks, including properties owned by four of five patients. Virus was detected at two sites that yielded virus in 2012. Detection of virus in multiple years indicates that the virus persists in ticks within a relatively small geographic area, although infection rates (IR) may vary greatly among sites and between years at a site. IR per 1,000 A. americanum in northwestern Missouri during the April-July 2013 study period were as follows: all adults, IR = 1.13; adult females, IR = 0.33; adult males, IR = 1.90; and nymphs, IR = 1.79. The IR in nymphs, the stage with the largest data set, corresponds to 1/559 infected ticks. Having robust estimates of IR in various stages for A. americanum should lead to more accurate public health messaging and a better understanding of virus transmission.

West Nile virus epizootiology in the southeastern United States, 2001.

We investigated mosquito and bird involvement in West Nile virus (WNV) transmission in July 2001 in Jefferson County, FL, and Lowndes County, GA. We detected 16 WNV-infected pools from Culex quinquefasciatus, Cx. salinarius, Cx. nigripalpus, and Culiseta melanura. In Florida, 11% of 353 bird sera neutralized WNV. Antibody prevalence was greatest in northern cardinal (Cardinalis cardinalis, 75%), northern mockingbird (Mimus polyglottus, 50%), common ground-dove (Columbina passerina, 25%), common grackle (Quiscalus quiscula, 15%), domestic chicken (Gallus gallus, 16%), and house sparrow (Passer domesticus, 11%). Antibody-positive birds were detected in nine of 11 locations, among which prevalence in chickens ranged from 0% to 100%. Seropositive chickens were detected in Georgia as well. The primary transmission cycle of WNV in the southeastern United States apparently involves Culex mosquitoes and passerine birds. Chickens are frequently infected and may serve as effective sentinels in this region.

Detection of West Nile viral RNA from an overwintering pool of Culex pipens pipiens (Diptera: Culicidae) in New Jersey, 2003.

In total, 1,324 Culex pipiens pipiens L. female mosquitoes were collected at Ft. Hancock, Monmouth County, New Jersey, from January to March 2001-2003. Mosquitoes were held in an insectary at 27 degrees C and a photoperiod of 16:8 (L:D) h for 6 to 21 d after which they were tested in 34 pools. West Nile viral RNA was detected in one pool by a TaqMan reverse transcription-polymerase chain reaction assay; however, infectious virus could not be isolated using either Vero cell plaque assay or C6/36 mosquito cells. Twenty females dissected in January and March 2003 confirmed ovarian diapause status. We suggest that the mode of infection in this pool of overwintering females may have been due to vertical (transgenerational) transmission.

Serological investigation of heartland virus (Bunyaviridae: Phlebovirus) exposure in wild and domestic animals adjacent to human case sites in Missouri 2012-2013.

Heartland virus (HRTV; Bunyaviridae: Phlebovirus) has recently emerged as a causative agent of human disease characterized by thrombocytopenia and leukopenia in the United States. The lone star tick (Amblyomma americanum L.) has been implicated as a vector. To identify candidate vertebrate amplification hosts associated with enzootic maintenance of the virus, sera and ticks were sampled from 160 mammals (8 species) and 139 birds (26 species) captured near 2 human case residences in Andrew and Nodaway Counties in northwest Missouri. HRTV-specific neutralizing antibodies were identified in northern raccoons (42.6%), horses (17.4%), white-tailed deer (14.3%), dogs (7.7%), and Virginia opossums (3.8%), but not in birds. Virus isolation attempts from sera and ticks failed to detect HRTV. The high antibody prevalence coupled with local abundance of white-tailed deer and raccoons identifies these species as candidate amplification hosts.

Identification of mosquito avian-derived blood meals by polymerase chain reaction-heteroduplex analysis.

A polymerase chain reaction (PCR) heteroduplex assay (HDA) was developed to identify avian derived mosquito blood meals to the species level. The assay used primers amplifying a fragment of the cytochrome B gene from vertebrate but not invertebrate species. In Culex tarsalis fed on quail, PCR products derived from the quail cytochrome B gene were detected seven days post-engorgement. In an analysis of wild-caught mosquitoes, 85% of blood-fed mosquitoes produced detectable PCR products. Heteroduplex patterns obtained from bird-derived PCR products were found to permit the unambiguous identification of all species examined. No intraspecific variation in HDA patterns was found. The PCR-HDA was used to characterize blood meals in wild caught Cx. tarsalis. Of the 67 blood meals analyzed, 60% were derived from avian sources. Of the avian blood meals, 65% were derived from a single host, the common grackle.