Showing papers by "Scott C. Weaver published in 2004"

Transmission cycles, host range, evolution and emergence of arboviral disease.

Abstract: Many pandemics have been attributed to the ability of some RNA viruses to change their host range to include humans. Here, we review the mechanisms of disease emergence that are related to the host-range specificity of selected mosquito-borne alphaviruses and flaviviruses. We discuss viruses of medical importance, including Venezuelan equine and Japanese encephalitis viruses, dengue viruses and West Nile viruses.

Venezuelan equine encephalitis emergence: Enhanced vector infection from a single amino acid substitution in the envelope glycoprotein

Abstract: In 1993 and 1996, subtype IE Venezuelan equine encephalitis (VEE) virus caused epizootics in the Mexican states of Chiapas and Oaxaca. Previously, only subtype IAB and IC VEE virus strains had been associated with major outbreaks of equine and human disease. The IAB and IC epizootics are believed to emerge via adaptation of enzootic (sylvatic, equine-avirulent) strains for high titer equine viremia that results in efficient infection of mosquito vectors. However, experimental equine infections with subtype IE equine isolates from the Mexican outbreaks demonstrated neuro-virulence but little viremia, inconsistent with typical VEE emergence mechanisms. Therefore, we hypothesized that changes in the mosquito vector host range might have contributed to the Mexican emergence. To test this hypothesis, we evaluated the susceptibility of the most abundant mosquito in the deforested Pacific coastal locations of the VEE outbreaks and a proven epizootic vector, Ochlerotatus taeniorhynchus. The Mexican epizootic equine isolates exhibited significantly greater infectivity compared with closely related enzootic strains, supporting the hypothesis that adaptation to an efficient epizootic vector contributed to disease emergence. Reverse genetic studies implicated a Ser --> Asn substitution in the E2 envelope glycoprotein as the major determinant of the increased vector infectivity phenotype. Our findings underscore the capacity of RNA viruses to alter their vector host range through minor genetic changes, resulting in the potential for disease emergence.

Dengue emergence and adaptation to peridomestic mosquitoes.

Abstract: Phylogenetic evidence suggests that endemic and epidemic dengue viruses (DENV), transmitted among humans by the anthropophilic mosquitoes Aedes aegypti and Ae albopictus, emerged when ancestral, sylvatic DENV transmitted among nonhuman primates by sylvatic Aedes mosquitoes adapted to these peridomestic vectors We tested this hypothesis by retrospectively examining evidence for adaptation of epidemic and endemic versus sylvatic strains of DENV-2 to Ae albopictus and Ae aegypti First and second-generation offspring of mosquitoes from different geographic regions in the Americas and Southeast Asia were tested for their susceptibility to epidemic/endemic and sylvatic DENV-2 isolates from West Africa, Southeast Asia, and Oceania Both Aedes species were highly susceptible (up to 100% infected) to endemic/epidemic DENV-2 strains after ingesting artificial blood meals but significantly less susceptible (as low as 0%) to sylvatic DENV-2 strains Our findings support the hypothesis that adaptation to peridomestic mosquito vectors mediated dengue emergence from sylvatic progenitor viruses

Genome sequence and attenuating mutations in West Nile virus isolate from Mexico.

Abstract: The complete genome sequence of a Mexican West Nile virus isolate, TM171-03, included 46 nucleotide (0.42%) and 4 amino acid (0.11%) differences from the NY99 prototype. Mouse virulence differences between plaque-purified variants of TM171-03 with mutations at the E protein glycosylation motif suggest the emergence of an attenuating mutation.

Endemic Venezuelan equine encephalitis in northern Peru.

Abstract: Since Venezuelan equine encephalitis virus (VEEV) was isolated in Peru in 1942, >70 isolates have been obtained from mosquitoes, humans, and sylvatic mammals primarily in the Amazon region. To investigate genetic relationships among the Peru VEEV isolates and between the Peru isolates and other VEEV strains, a fragment of the PE2 gene was amplified and analyzed by single-stranded conformation polymorphism. Representatives of seven genotypes underwent sequencing and phylogenetic analysis. The results identified four VEE complex lineages that cocirculate in the Amazon region: subtypes ID (Panama and Colombia/Venezuela genotypes), IIIC, and a new, proposed subtype IIID, which was isolated from a febrile human, mosquitoes, and spiny rats. Both ID lineages and the IIID subtype are associated with febrile human illness. Most of the subtype ID isolates belonged to the Panama genotype, but the Colombia/Venezuela genotype, which is phylogenetically related to epizootic strains, also continues to circulate in the Amazon basin.

Infection patterns of o'nyong nyong virus in the malaria-transmitting mosquito, Anopheles gambiae.

Abstract: Arthropod-borne alphaviruses transmitted by mosquitoes almost exclusively use culicines; however, the alphavirus o'nyong-nyong (ONNV) has the unusual characteristic of being transmitted primarily by anopheline mosquitoes This unusual attribute makes ONNV a valuable tool in the characterization of mosquito determinants of infection as well as a useful expression system in Anopheles species We developed a series of recombinant alphaviruses, based upon the genome of ONNV, designed for the expression of heterologous genes The backbone genome is a full-length infectious cDNA clone of ONNV from which wild-type virus can be rescued Additional constructs are variants of the primary clone and contain the complete genome plus a duplicated subgenomic promoter element with a multiple cloning site for insertion of heterologous genes We inserted a green fluorescent protein (GFP) gene downstream of this promoter and used it to characterize infection and dissemination patterns of ONNV within An gambiae mosquitoes These experiments allowed us to identify atypical sites of initial infection and dissemination patterns in this mosquito species not frequently observed in comparable culicine infections The utility of these ONNVs for studies in anopheline mosquitoes includes the potential for identification of vector infection determinants and to serve as tools for antimalaria studies Viruses that can express a heterologous gene in a vector and rapidly and efficiently infect numerous tissues in An gambiae mosquitoes will be a valuable asset in parasite-mosquito interaction and interference research

Use of a Recombinant Envelope Protein Subunit Antigen for Specific Serological Diagnosis of West Nile Virus Infection

Abstract: Serological diagnosis of West Nile virus (WNV) infection is complicated by extensive antigenic cross-reactivity with other closely related flaviviruses, such as St. Louis encephalitis virus. Here we describe a recombinant, bacterially expressed antigen equivalent to structural domain III of the WNV envelope protein that has allowed clear discrimination of antibody responses to WNV from those against other related flaviviruses in indirect enzyme-linked immunosorbent assays using standardized control antisera and field-collected samples.

Generation and Characterization of Closely Related Epizootic and Enzootic Infectious cDNA Clones for Studying Interferon Sensitivity and Emergence Mechanisms of Venezuelan Equine Encephalitis Virus

Abstract: Venezuelan equine encephalitis virus (VEEV) is a reemerging pathogen and a continuing threat to humans and equines in the Americas. Identification of the genetic determinants that enable epizootic VEEV strains to arise and exploit equines as amplification hosts to cause widespread human disease is pivotal to understanding VEE emergence. The sensitivity to murine alpha/beta interferon-mediated antiviral activity was previously correlated to the epizootic phenotype of several VEEV strains. Infectious cDNA clones were generated from an epizootic subtype IC VEEV strain (SH3) isolated during the 1992 Venezuelan outbreak and a closely related enzootic, sympatric subtype ID strain (ZPC738). These VEEV strains had low-cell-culture-passage histories and differed by only 12 amino acids in the nonstructural and structural proteins. Rescued viruses showed similar growth kinetics to their parent viruses in several cell lines, and murine infections resulted in comparable viremia and disease. Unlike what was found in other studies of epizootic and enzootic VEEV strains, the sensitivities to murine alpha/beta interferon did not differ appreciably between these epizootic versus enzootic strains, calling into question the reliability of interferon sensitivity as a marker of epizootic potential.

Venezuelan Equine Encephalitis Virus, Southern Mexico

Abstract: Equine epizootics of Venezuelan equine encephalitis (VEE) occurred in the southern Mexican states of Chiapas in 1993 and Oaxaca in 1996. To assess the impact of continuing circulation of VEE virus (VEEV) on human and animal populations, serologic and viral isolation studies were conducted in 2000 to 2001 in Chiapas State. Human serosurveys and risk analyses indicated that long-term endemic transmission of VEEV occurred among villages with seroprevalence levels of 18% to 75% and that medical personnel had a high risk for VEEV exposure. Seroprevalence in wild animals suggested cotton rats as possible reservoir hosts in the region. Virus isolations from sentinel animals and genetic characterizations of these strains indicated continuing circulation of a subtype IE genotype, which was isolated from equines during the recent VEE outbreaks. These data indicate long-term enzootic and endemic VEEV circulation in the region and continued risk for disease in equines and humans.

Genetic determinants of Venezuelan equine encephalitis emergence

Abstract: Following a period of inactivity from 1973-1991, Venezuelan equine encephalitis (VEE) reemerged during the past decade in South America and Mexico. Experimental studies of VEE virus (VEEV) infection of horses with virus strains isolated during these outbreaks have revealed considerable variation in the ability of equine-virulent, epizootic strains to exploit horses as efficient amplification hosts. Subtype IC strains from recent outbreaks in Venezuela and Colombia amplify efficiently in equines, with a correlation between maximum viremia titers and the extent of the outbreak from which the virus strain was isolated. Studies of enzootic VEEV strains that are believed to represent progenitors of the epizootic subtypes support the hypothesis that adaptation to efficient replication in equines is a major determinant of emergence and the ability of VEEV to spread geographically. Correlations between the ability of enzootic and epizootic VEEV strains to infect abundant, equiphilic mosquitoes, and the location and extent of these outbreaks, also suggest that specific adaptation to Ochlerotatus taeniorhynchus mosquitoes is a determinant of some but not all emergence events. Genetic studies imply that mutations in the E2 envelope glycoprotein gene are major determinants of adaptation to both equines and mosquito vectors.

The Hamster as an Animal Model for Eastern Equine Encephalitis—and Its Use in Studies of Virus Entrance into the Brain

Abstract: Eastern equine encephalitis virus (EEEV) produces the most severe human arboviral diseases in the United States, with mortality rates of 30%-70%. Vasculitis associated with microhemorrhages in the brain dominates the pathological picture in fatal human eastern equine encephalitis, and neuronal cell death is detectable during the late stage of the disease. We describe use of the golden hamster to study EEEV-induced acute vasculitis and encephalitis. In hamsters, EEEV replicates in visceral organs, produces viremia, and penetrates the brain. The pathological manifestations and antigen distribution in the brain of a hamster are similar to those described in human cases of EEEV.

Experimental Everglades virus infection of cotton rats (Sigmodon hispidus).

Abstract: Everglades virus (EVEV), an alphavirus in the Venezuelan equine encephalitis (VEE) serocomplex, circulates among rodents and vector mosquitoes and infects humans, causing a febrile disease sometimes accompanied by neurologic manifestations. EVEV circulates near metropolitan Miami, which indicates the potential for substantial human disease, should outbreaks arise. We characterized EVEV infection of cotton rats in South Florida, USA to validate their role in enzootic transmission. To evaluate whether the viremia induced in cotton rat populations regulates EVEV distribution, we also infected rats from a non–EVEV-endemic area. Viremia levels developed in rats from both localities that exceeded the threshold for infection of the vector. Most animals survived infection with no signs of illness, despite virus invasion of the brain and the development of mild encephalitis. Understanding the mechanisms by which EVEV-infected cotton rats resist clinical disease may be useful in developing VEE therapeutics for equines and humans.

Molecular Phylogeny of the Vomerifer and Pedroi Groups in the Spissipes Section of the Subgenus Culex (Melanoconion)

Abstract: Members of the New World mosquito subgenus Melanoconion of the genus Culex are important vectors of many alphaviruses including eastern and Venezuelan equine encephalitis viruses (VEEV). We investigated the phylogenetic relationships among nine putative species of the Vomerifer and Pedroi Groups of the Neotropical Spissipes Section by sequencing the internal transcribed spacer 2 (ITS-2) region of ribosomal DNA and using phylogenetic analyses. Results demonstrated that, within the Spissipes Section, the Vomerifer and Pedroi Groups are monophyletic sister groups. The clade comprised by Culex adamesi and Culex ribeirensis showed a sister group relationship to the group consisting of Culex pedroi sequences. The monophyly of the Vomerifer Group corroborated previous suggestions that it is a natural group. However, our topology showed that there are two well-supported, divergent groups within a major clade consisting of Cx. pedroi sequences, suggesting the possibility of a cryptic Cx. pedroi-like species. This finding could have important epidemiological implications for VEEV transmission in Central and South America.

Susceptibility of Ochlerotatus taeniorhynchus (Diptera: Culicidae) to infection with epizootic (subtype IC) and enzootic (subtype ID) Venezuelan equine encephalitis viruses: evidence for epizootic strain adaptation.

Abstract: To test the hypothesis that adaptation to epizootic mosquito vectors mediates emergence of Venezuelan equine encephalitis virus (VEEV) from enzootic progenitors, experimental infection studies were conducted to determine the susceptibility of Ochlerotatus taeniorhynchus (Wiedemann) to epizootic and enzootic strains. Artificial blood meals containing epizootic subtype IC strains isolated during the 1962–1964, 1992–1993, and 1995 Venezuelan/Colombian epizootics and closely related Venezuelan enzootic subtype ID strains were used to compare infectivity and transmission potential. Their greater infectivity and replication suggested that adaptation of epizootic strains to Oc. taeniorhynchus may have enhanced epizootic transmission during the 1962–1964 and 1995 IC coastal epizootics. However, strains from the small 1992–1993 Venezuelan outbreak that did not extend to coastal regions do not seem to infect this species better than closely related subtype ID strains. Adaptation of VEEV to epizootic vectors such as Oc. taeniorhynchus mosquitoes may be a determinant of some but not all VEE emergence events and may influence spread into coastal regions.

Susceptibility of Urban and Rural Populations of Aedes albopictus from São Paulo State, Brazil, to Infection by Dengue-1 and -2 Viruses

Abstract: In Brazil, dengue viruses (DENV) are transmitted by Aedes aegypti (L.). However, the recent introduction of Aedes albopictus (Skuse) has caused concern, because some populations of this species also are capable of DENV transmission. Ae. albopictus from rural and urban localities of Sao Paulo State, Brazil, were artificially exposed to infection with DENV-1 and -2 to evaluate their susceptibility. The rates of infection (6.2 and 5.7% for DENV-1; 5.3 and 12% for DENV-2) and dissemination (22.2 and 40% for DENV-1; 0 and 20% for DENV-2) were low in both urban and rural populations, with no significant differences detected between these mosquito populations.