La Crosse virus spread within the mosquito population in Knox County, TN.

In Appalachia, La Crosse virus (LACV) is a leading pediatric arbovirus and public health concern for children under 16 years. LACV is transmitted via the bite of an infected Aedes mosquito. Thus, it is imperative to understand the dynamics of the local vector population in order to assess risk and transmission. Using entomological data collected from Knox County, Tennessee in 2013, we formulate an environmentally-driven system of ordinary differential equations to model mosquito population dynamics over a single season. Further, we include infected compartments to represent LACV transmission within the mosquito population. Findings suggest that the model, with dependence on degree days and accumulated precipitation, can closely describe field data. This model confirms the need to include these environmental variables when planning control strategies.

Local persistence of novel regional variants of La Crosse virus in the Northeast USA.

BACKGROUND: La Crosse virus (LACV) (genus Orthobunyavirus, family Peribunyaviridae) is a mosquito-borne virus that causes pediatric encephalitis and accounts for 50-150 human cases annually in the USA. Human cases occur primarily in the Midwest and Appalachian regions whereas documented human cases occur very rarely in the northeastern USA. METHODS: Following detection of a LACV isolate from a field-collected mosquito in Connecticut during 2005, we evaluated the prevalence of LACV infection in local mosquito populations and genetically characterized virus isolates to determine whether the virus is maintained focally in this region. RESULTS: During 2018, we detected LACV in multiple species of mosquitoes, including those not previously associated with the virus. We also evaluated the phylogenetic relationship of LACV strains isolated from 2005-2018 in Connecticut and found that they formed a genetically homogeneous clade that was most similar to strains from New York State. CONCLUSION: Our analysis argues for local isolation and long-term persistence of a genetically distinct lineage of LACV within this region. We highlight the need to determine more about the phenotypic behavior of these isolates, and whether this virus lineage poses a threat to public health.

Double Crossed: A Case of La Crosse Encephalitis.

CASE REPORT: A 10-year-old male with T1DM and recent travel to North Carolina presented to an ED with 1 day of fever, vomiting, and headaches. He was discharged home with the presumptive diagnosis of viral gastroenteritis but returned nine hours later, agitated, and unable to speak. CSF showed pleocytosis. MRI brain was normal, and EEG showed intermittent seizures. He was started on antiepileptics. Antibiotics were discontinued after negative bacterial work-up. Repeat MRI brain one week later showed enhancement in the left cerebral cortex. IVIG was started due to concern for autoimmune encephalitis. Repeat lumbar puncture was positive for La Crosse virus IgM. DISCUSSION: This is the first case of La Crosse encephalitis (LACe) reported in Rhode Island.1 La Crosse virus (LACv) is a ssRNA Bunyavirus transmitted by the eastern tree-hole mosquito typically between July and September. LACv is endemic to the upper Midwestern US and Appalachia. In 2018, 81 of 86 total cases reported by the CDC were pediatric. Children are more likely to present with vomiting, seizures, and focal cortical inflammation or cerebral edema on brain imaging. IgM may be negative early in the disease course. Treatment is antiepileptics and supportive care.

La Crosse Encephalitis: A Persistent Arboviral Threat in North Carolina.

Mosquito-borne diseases remain a significant cause of economic, social, and health burdens in North Carolina. Although recently overshadowed by emerging threats such as chikungunya virus and Zika virus, La Crosse virus and other endemic arboviruses remain persistent environmental health hazards. Indeed, La Crosse virus, West Nile virus, and Eastern equine encephalitis virus accounted for more than 98% of the reported human arboviral diseases acquired in North Carolina in the past decade. Arbovirus infection is increasingly prevalent in Western North Carolina, with La Crosse encephalitis being endemic in this area. While infections are often asymptomatic and seldom fatal, the long-term neurologic sequelae of La Crosse encephalitis represent a significant burden.

Evaluating Surveillance Methods for Arboviral Vectors of La Crosse Virus and West Nile Virus of Southern Appalachia.

To monitor mosquito-borne diseases, public health departments conduct mosquito and pathogen surveillance. Our objective was to evaluate mosquito monitoring methods for collecting La Crosse virus (LACV) and West Nile virus (WNV) vectors (Aedes and Culex mosquitoes, respectively) in southern Appalachia. Centers for Disease Control and Prevention (CDC) light traps baited with carbon dioxide (CO(2)), CDC light traps baited with CO(2) and BG lure, BG-Sentinel traps baited with CO(2), gravid traps baited with oak (Quercus)-water infusion, and resting traps were compared in eastern Tennessee in 2013. Traps operated at 8 different urban sites throughout Knox County were randomly assigned to and rotated among 6 plots within each site. Results were specific for each vector; the BG-Sentinel trap was the best method for Aedes triseriatus, the CDC trap baited with CO(2) and BG lure was the best method for Ae. albopictus, and the gravid trap was the best method for Ae. japonicus. Culex erraticus collections varied by week and trapping method, indicating no single method was best, but the questing traps collected more mosquitoes. There was no significant trapping difference for Cx. pipiens complex in this region using the methods tested. The results suggest using a combination of trapping methods when sampling for LACV and/or WNV mosquito vectors in southern Appalachia. Effective trapping methods are necessary to enable accurate surveillance, improve control methods, enhance understanding of dispersal, and use for early detection of vectors and pathogens.

La Crosse Encephalitis Virus Infection in Field-Collected Aedes albopictus, Aedes japonicus, and Aedes triseriatus in Tennessee.

La Crosse virus (LACV) is a mosquito-borne virus and a major cause of pediatric encephalitis in the USA. La Crosse virus emerged in Tennessee and other states in the Appalachian region in 1997. We investigated LACV infection rates and seasonal abundances of the native mosquito vector, Aedes triseriatus, and 2 recently introduced mosquito species, Ae. albopictus and Ae. japonicus, in an emerging disease focus in Tennessee. Mosquitoes were collected using multiple trapping methods specific for Aedes mosquitoes at recent human case sites. Mosquito pools were tested via reverse transcriptase-polymerase chain reaction (RT-PCR) of the S segment to detect multiple Bunyamwera and California serogroup viruses, including LACV, as well as real-time RT-PCR of the M segment. A total of 54 mosquito pools were positive, including wild-caught adult females and laboratory-reared adults, demonstrating transovarial transmission in all 3 species. Maximum likelihood estimates (per 1,000 mosquitoes) were 2.72 for Ae. triseriatus, 3.01 for Ae. albopictus, and 0.63 for Ae. japonicus. We conclude that Ae. triseriatus and Ae. albopictus are important LACV vectors and that Ae. japonicus also may be involved in virus maintenance and transmission.

Cemeteries are effective sites for monitoring la crosse virus (LACv) and these environments may play a role in LACv infection.

La Crosse encephalitis (LAC) is the leading arboviral disease among children, and was previously limited to the upper Midwest. In 2012 nine pediatric cases of LAC occurred in eastern Tennessee, including one fatal case. In an attempt to identify sites near an active LACv infection and describe the abundance and distribution of potential LACv vectors near a fatal LAC case in the Appalachian region, we initiated an end of season study using a combination of questing and oviposition mosquito traps placed at forty-nine sites consisting of cemeteries and houses within 16 radial kilometers of two pediatric infections. LACv was isolated from three Aedes triseriatus pools collected from cemeteries and spatial clustering analysis identified clusters of Ae. triseriatus and Ae. albopictus populations that overlapped in the same area as the 2012 LACv cases. Results indicate cemeteries are effective sites for monitoring LACv. The role of cemeteries and specific environmental features will be the focus of future investigations.

Impacts of climate, land use, and biological invasion on the ecology of immature Aedes mosquitoes: implications for La Crosse emergence.

Arthropod-borne viruses (arboviruses) cause many diseases worldwide and their transmission is likely to change with land use and climate changes. La Crosse virus (LACV) is historically transmitted by the native mosquito Aedes triseriatus (Say) in the upper Midwestern US, but the invasive congeners Aedes albopictus (Skuse) and A. japonicus (Theobald), which co-occur with A. triseriatus in water-holding containers, may be important accessory vectors in the Appalachian region where La Crosse encephalitis is an emerging disease. This review focuses on evidence for how climate, land use, and biological invasions may have direct abiotic and indirect community-level impacts on immature developmental stages (eggs and larvae) of Aedes mosquitoes. Because vector-borne diseases usually vary in space and time and are related to the ecology of the vector species, we propose that the ecology of its mosquito vectors, particularly at their immature stages, has played an important role in the emergence of La Crosse encephalitis in the Appalachian region and represents a model for investigating the effects of environmental changes on other vector-borne diseases. We summarize the health effects of LACV and associated socioeconomic costs that make it the most important native mosquito-borne disease in the US. We review of the transmission of LACV, and present evidence for the impacts of climate, land use, and biological invasions on Aedes mosquito communities. Finally, we discuss important questions about the ecology of LACV mosquito vectors that may improve our understanding of the impacts of environmental changes on LACV and other arboviruses.

La Crosse encephalitis surveillance using single versus paired serologic testing.

We evaluated the validity of single versus paired serologic testing for La Crosse virus (LACV) encephalitis surveillance. Compared with paired serology, a single positive IgG or IgM immunoflourescent antibody titre appears useful for LACV encephalitis surveillance with sensitivity, 75%; specificity, 98%; positive predictive value, 95%; and overall test efficiency 92%.

Safety and pharmacokinetics of ribavirin for the treatment of la crosse encephalitis.

BACKGROUND: La Crosse viral encephalitis (LACVE) is associated with residual epilepsy and neurocognitive deficits in survivors. This report summarizes 3 phases of clinical studies of children treated with intravenous (IV) ribavirin (RBV), each one exploring a different phase (I, IIA, IIB) of clinical trial development. METHODS: In phase I, 7 children with life-threatening LACVE were treated with emergency use RBV using a moderate IV dose (8.33 mg/kg/dose q 8 hours day 1, 5 mg/kg/dose q 8 hours days 2-10). In phase IIA, 12 children with severe LACVE were enrolled: 8 treated with RBV (same dose as phase I) and 4 with placebo. In phase IIB an escalated dose was used (33 mg/kg dose 1, then 16 mg/kg/dose q 6 hours for 4 days, and 8 mg/kg/dose q 8 hours for 3 days). RESULTS: In a group of 15 children treated in phase I and phase IIA, RBV appeared safe at moderate dose, but based on steady-state RBV levels of 9.3 µM, estimated cerebrospinal fluid levels were less than 20% of the EC50 of RBV for LACVE. At the escalated dose used in phase IIB, adverse events occurred, likely related to RBV, and therefore the trial was discontinued. Nevertheless, valuable pharmacokinetic (PK) and safety data were obtained at moderate dose, with potential treatment implications for other indications. CONCLUSIONS: Although the results do not support the use of RBV for LACVE, this nevertheless is the largest study of antiviral treatment for LACVE to date and the largest pharmacokinetic analysis of IV RBV in children for any indication.

Interactions between La Crosse virus and bacteria isolated from the digestive tract of Aedes albopictus (Diptera: Culicidae).

Aedes albopictus (Skuse) is a potential vector for many arboviruses, including La Crosse virus (LACV), the leading cause of pediatric encephalitis in North America. Bacteria isolated from the midgut and diverticula of field-caught female Ae. albopictus were cultured and identified using 16S ribosomal RNA gene amplification and sequencing. Members of seven and six bacterial families were identified from the midguts and diverticula, respectively, with nearly half of the isolates identified to the family Enterobacteriaceae. Many are related to bacteria identified in other invertebrates, and several may represent previously unknown species or genera. Of the 24 isolated bacteria, 12 (50%) showed a significant reduction in infectivity of LACV for Vero cells. Inhibition of infectivity ranged from 0 to 44% and was not dependent on bacterial classification. The antiviral activity of these bacteria warrants further investigation as an alternate means to interrupt the LACV transmission cycle.

The spatial epidemiology and clinical features of reported cases of La Crosse virus infection in West Virginia from 2003 to 2007.

BACKGROUND: La Crosse virus (LACV) is a major cause of pediatric encephalitis in the United States. Since the mid-1980s, the number of reported cases of LACV infection in West Virginia has continued to rise and the state currently reports the most cases in the United States. The purpose of this study was to investigate and describe the spatial epidemiology and clinical presentation of LACV infection cases reported in West Virginia, as well as to provide a description of the environmental conditions present at the residences of the LACV infection cases. METHODS: Descriptive and spatial analyses were performed on LACV infection cases reported to the West Virginia Department of Health from 2003 to 2007. Clinical and environmental variables were available for 96 cases and residence data were available for 68 of these cases. Spatial analyses using the global Moran's I and Kulldorff's spatial scan statistic were performed using the population 15 years and younger at both the county and census tract levels to identify those geographic areas at the highest risk of infection. RESULTS: Two statistically significant (p < 0.05) high-risk clusters, involving six counties, were detected at the county level. At the census tract level, one statistically significant high-risk cluster involving 41 census tracts spanning over six counties was identified. The county level cumulative incidence for those counties in the primary high-risk cluster ranged from 100.0 to 189.0 cases per 100,000 persons (median 189.0) and the census tract level cumulative incidence for those counties in the high-risk cluster ranged from 61.7 to 505.9 cases per 100,000 persons (median 99.0). The counties and census tracts within high-risk clusters had a relative risk four to nine times higher when compared to those areas not contained within high-risk clusters. The majority of LACV infection cases were reported during the summer months in children 15 years and younger. Fever, vomiting, photophobia, and nausea were the most commonly reported signs and symptoms. A case fatality rate (CFR) of 3.1% was observed. Wooded areas and containers were present at the majority of case residences. CONCLUSIONS: The cumulative incidences of LACV infection from 2003 to 2007 were considerably higher than previously reported for West Virginia, and statistically significant high-risk clusters for LACV infection were detected at both the county and census tract levels. The finding of a high CFR and the identification of those areas at highest risk for infection will be useful for guiding future research and intervention efforts.

Identification of super-infected Aedes triseriatus mosquitoes collected as eggs from the field and partial characterization of the infecting La Crosse viruses.

BACKGROUND: La Crosse virus (LACV) is a pathogenic arbovirus that is transovarially transmitted by Aedes triseriatus mosquitoes and overwinters in diapausing eggs. However, previous models predicted transovarial transmission (TOT) to be insufficient to maintain LACV in nature. RESULTS: To investigate this issue, we reared mosquitoes from field-collected eggs and assayed adults individually for LACV antigen, viral RNA by RT-PCR, and infectious virus. The mosquitoes had three distinct infection phenotypes: 1) super infected (SI+) mosquitoes contained infectious virus, large accumulations of viral antigen and RNA and comprised 17 of 17,825 (0.09%) of assayed mosquitoes, 2) infected mosquitoes (I+) contained no detectable infectious virus, lesser amounts of viral antigen and RNA, and comprised 3.7% of mosquitoes, and 3) non-infected mosquitoes (I-) contained no detectable viral antigen, RNA, or infectious virus and comprised 96.21% of mosquitoes. SI+ mosquitoes were recovered in consecutive years at one field site, suggesting that lineages of TOT stably-infected and geographically isolated Ae. triseriatus exist in nature. Analyses of LACV genomes showed that SI+ isolates are not monophyletic nor phylogenetically distinct and that synonymous substitution rates exceed replacement rates in all genes and isolates. Analysis of singleton versus shared mutations (Fu and Li's F*) revealed that the SI+ LACV M segment, with a large and significant excess of intermediate-frequency alleles, evolves through disruptive selection that maintains SI+ alleles at higher frequencies than the average mutation rate. A QTN in the LACV NSm gene was detected in SI+ mosquitoes, but not in I+ mosquitoes. Four amino acid changes were detected in the LACV NSm gene from SI+ but not I+ mosquitoes from one site, and may condition vector super infection. In contrast to NSm, the NSs sequences of LACV from SI+ and I+ mosquitoes were identical. CONCLUSIONS: SI+ mosquitoes may represent stabilized infections of Ae. triseriatus mosquitoes, which could maintain LACV in nature. A gene-for-gene interaction involving the viral NSm gene and a vector innate immune response gene may condition stabilized infection.

La Crosse virus in Aedes albopictus mosquitoes, Texas, USA, 2009.

We report the arthropod-borne pediatric encephalitic agent La Crosse virus in Aedes albopictus mosquitoes collected in Dallas County, Texas, USA, in August 2009. The presence of this virus in an invasive vector species within a region that lies outside the virus's historically recognized geographic range is of public health concern.

Arrival and establishment of Aedes japonicus japonicus (Diptera: Culicidae) in Iowa.

The arrival and establishment of Aedes (Finlaya) japonicus japonicus (Theobald) (Diptera: Culicidae) in Iowa are reported. In total, 518 wild adult specimens were collected through the statewide mosquito and mosquito-borne virus surveillance program in 2007 and 2008. Specimens were collected with New Jersey light traps, CO2-baited CDC light traps, grass infusion-baited gravid traps, and Mosquito Magnet traps located in 12 counties in central and eastern Iowa Specimens were identified morphologically, and identity was further supported by molecular DNA barcoding. Specimens also were tested for infection with West Nile virus (family Flaviviridae, genus Flavivirus, WNV) and La Crosse virus (family Bunyaviridae, genus Bunyavirus, LACV) by reverse transcription-polymerase chain reaction. Although no specimens tested positive for arbovirus infection, the arrival of Ae. j. japonicus in Iowa is a public health concern considering its potential to transmit several arboviruses, particularly WNV and LACV.

The incidence risk, clustering, and clinical presentation of La Crosse virus infections in the eastern United States, 2003-2007.

BACKGROUND: Although La Crosse virus (LACV) is one of the most common causes of pediatric arboviral infections in the United States, little has been done to assess its geographic distribution, identify areas of higher risk of disease, and to provide a national picture of its clinical presentation. Therefore, the objective of this study was to investigate the geographic distribution of LACV infections reported in the United States, to identify hot-spots of infection, and to present its clinical picture. METHODS AND FINDINGS: Descriptive and cluster analyses were performed on probable and confirmed cases of LACV infections reported to the Centers for Disease Control and Prevention from 2003-2007. A total of 282 patients had reported confirmed LACV infections during the study period. Of these cases the majority (81 percent) presented during the summer, occurred in children 15 years and younger (83.3 percent), and were found in male children (64.9 percent). Clinically, the infections presented as meningioencephalitis (56.3 percent), encephalitis (20.7 percent), meningitis (17.2 percent), or uncomplicated fever (5 percent). Deaths occurred in 1.9 percent of confirmed cases, and in 8.6 percent of patients suffering from encephalitis. The majority of these deaths were in patients 15 years and younger. The county-level incidence risk among counties (n = 136) reporting both probable and confirmed cases for children 15 years and younger (n = 355) ranged from 0.2 to 228.7 per 100,000 persons. The southern United States experienced a significantly higher (p<0.05) incidence risk during the months of June, July, August, and October then the northern United States. There was significant (p<0.05) clustering of high risk in several geographic regions with three deaths attributed to complications from LAC encephalitis occurring in two of these hot-spots of infections. CONCLUSIONS: Both the incidence risk and case fatality rates were found to be higher than previously reported. We detected clustering in four geographic regions, a shift from the prior geographic distributions, and developed maps identifying high-risk areas. These findings are useful for raising awareness among health care providers regarding areas at a high risk of infections and for guiding targeted multifaceted interventions by public health officials.

Potential for La Crosse virus segment reassortment in nature.

The evolutionary success of La Crosse virus (LACV, family Bunyaviridae) is due to its ability to adapt to changing conditions through intramolecular genetic changes and segment reassortment. Vertical transmission of LACV in mosquitoes increases the potential for segment reassortment. Studies were conducted to determine if segment reassortment was occurring in naturally infected Aedes triseriatus from Wisconsin and Minnesota in 2000, 2004, 2006 and 2007. Mosquito eggs were collected from various sites in Wisconsin and Minnesota. They were reared in the laboratory and adults were tested for LACV antigen by immunofluorescence assay. RNA was isolated from the abdomen of infected mosquitoes and portions of the small (S), medium (M) and large (L) viral genome segments were amplified by RT-PCR and sequenced. Overall, the viral sequences from 40 infected mosquitoes and 5 virus isolates were analyzed. Phylogenetic and linkage disequilibrium analyses revealed that approximately 25% of infected mosquitoes and viruses contained reassorted genome segments, suggesting that LACV segment reassortment is frequent in nature.

[Phylogenetic analysis of complete nucleotide sequence of a snowshoe hare (Lepus americanus) virus genome].

The complete nucleotide consequence of snowshoe hare (Lepus americanus) virus (SSHV) genome was first determined. Phylogenetic analysis indicated the identity of the study strain in the complete S segment and an L-segment with a length of 145 amino acid residues with SSHV sequences from the database GenBank (J02390, AF393325). The homology of an M segment of the study segment was 99.7% with the consequences of the same from the database GenBank (K02539). La Crosse virus is most genetically close to SHV among the representatives of the California encephalitis serocomplex.

Endemic human mosquito-borne disease in Wisconsin residents, 2002-2006.

INTRODUCTION: West Nile virus (WNV) and La Crosse virus (LAC) are the primary mosquito-borne arboviruses associated with human disease in Wisconsin. We examined WNV and LAC human illness surveillance data collected during 2002 through 2006. METHODS: ELISA-based tests developed by the Centers for Disease Control and Prevention (CDC) were used to detect acute WNV and LAC infection in patient sera or cerebral spinal fluid. Public health personnel conducted patient follow-up using standard arbovirus reporting forms. CDC/Council of State and Territorial Epidemiologists definitions were used to determine cases. RESULTS: From 2002 through 2006, 114 confirmed human cases of WNV illness were reported in Wisconsin residents; 82% of illness onsets occurred during August or September. Median age of WNV case patients was 51 years, 49% reported neuroinvasive disease, 56% were hospitalized, and 7 cases were fatal. Confirmed LAC illnesses declined from a high of 27 cases during 2003 to a low of 3 cases during 2005 and 2006. Most LAC illnesses occurred in residents of Western Wisconsin; median age of LAC cases was 9 years. Mean annual incidences of reported confirmed WNV illnesses calculated for high, medium, and low population density groupings were very similar (range: 0.40-0.46 cases/100,000 population). CONCLUSIONS: Humans are at risk for mosquito-borne diseases in Wisconsin. Protection and prevention measures are important statewide, especially during July through September when the risk is greatest.