Small Interfering RNA-Mediated Control of Virus Replication in the CNS Is Therapeutic and Enables Natural Immunity to West Nile Virus

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Seasonality and Outbreaks in West Nile Virus Infection

In this paper we analyze the impact of seasonal variations on the dynamics of West Nile Virus infection. We are interested in the generation of new epidemic peaks starting from an endemic state. In many cases, the oscillations generated by seasonality in the dynamics of the infection are too small to be observable. The interplay of this seasonality with the epidemic oscillations can generate new outbreaks starting from the endemic state through a mechanism of parametric resonance. Using experimental data we present specific cases where this phenomenon is numerically observed.     

西尼罗病毒研究进展

西尼罗病毒（West Nile virus,WNV）属黄病毒科,为正单链RNA病毒。它在人类中的感染导致以发热为主要症状的传染性疾病,主要由蚊虫叮咬传播。自20世纪50年代首例报告西尼罗病毒自然感染所致脑炎后的几十年内,西尼罗病毒脑炎在欧洲及中亚地区散在、小规模流行。西尼罗病毒脑炎于1999年在美国的爆发及随后几年在北美的流行引起了极大的关注。这次爆发流行中新出现的种种迹象,如其中间宿主——野生鸟类的大量死亡,人类感染者中中枢神经系统受损比例的增高等,提示近期的遗传变异已使西尼罗病毒感染的病理学与流行病学发生了较显著的变化。另外,随着感染的流行,蚊虫叮咬以外的传播途径,如输血、器官移植、母婴传播等日益受到人们重视。同时,人们对阻止疫情所急需的疫苗的研制也在进行之中。本文就近几年来对西尼罗病毒的感染、免疫与流行病学方面的研究进展进行了综述。     

Threshold Conditions for West Nile Virus Outbreaks

In this paper, we study the stability and saddle-node bifurcation of a model for the West Nile virus transmission dynamics. The existence and classification of the equilibria are presented. By the theory of K-competitive dynamical systems and index theory of dynamical systems on a surface, sufficient and necessary conditions for local stability of equilibria are obtained. We also study the saddle-node bifurcation of the system. Explicit subthreshold conditions in terms of parameters are obtained beyond the basic reproduction number which provides further guidelines for accessing control of the spread of the West Nile virus. Our results suggest that the basic reproductive number itself is not enough to describe whether West Nile virus will prevail or not and suggest that we should pay more attention to the initial state of West Nile virus. The results also partially explained the mechanism of the recurrence of the small scale endemic of the virus in North America. Supported by the Chinese NSF grants 10531030 and 10671143. Supported by the Chinese NSF grants 10801074. Supported by Canada Research Chairs Program, Mathematics for Information Technology and Complex Systems (MITACS), National Microbiology Laboratory, Natural Sciences and Engineering Research Council (NSERC), Canadian Foundation of Innovation (CFI) and Ontario Innovation Trust (OIT), Ontario Ministry of Health and Long-term Care, Peel, Toronto, Chat-Kent Health Units, and Public Health Agency of Canada (PHAC). Supported by NSERC, MITACS, CFI/OIT a new opportunity fund, Early Research Award of Ministry of Research and Innovation (ERA) of Ontario, Infectious Diseases Branch of Ministry of Health and Long Term Care (MOH) of Ontario and PHAC.     

Traveling Waves and Spread Rates for a West Nile Virus Model

A reaction–diffusion model for the spatial spread of West Nile virus is developed and analysed. Infection dynamics are based on a modified version of a model for cross infection between birds and mosquitoes (Wonham et al., 2004, An epidemiological model for West-Nile virus: Invasion analysis and control application. Proc. R. Soc. Lond. B 271), and diffusion terms describe movement of birds and mosquitoes. Working with a simplified version of the model, the cooperative nature of cross-infection dynamics is utilized to prove the existence of traveling waves and to calculate the spatial spread rate of infection. Comparison theorem results are used to show that the spread rate of the simplified model may provide an upper bound for the spread rate of a more realistic and complex version of the model.     

A Model for Defining West Nile Virus Risk Perception Based on Ecology and Proximity

West Nile virus (WNV) has made considerable impact as an emerging infectious disease, spreading from coast to coast across North America since 1999. The disease has exhibited great spatial variability in North America, making an ecosystems approach to understanding the local human and vector ecology critical to prevention and control. This study underscores the importance of employing both personal prevention and community participatory approaches to create messages that have been adapted to the local ecology and are designed to reduce the risk of human infection with this mosquito-borne virus. As the virus spreads into new areas, underlying attitudes toward mosquitoes and the local perception of environment/ecology can affect the success of control and prevention measures. This work presents results from focus group discussions conducted in two Colorado counties in 2003, a year of significant WNV activity in the state. Issues addressed include residents’ assessment of risk and how this perception varied by age group and location, use or nonuse of repellents, and community attitudes toward mosquito control in areas with different ecologies and histories of mosquito-borne disease. The need to address individual components of personal prevention, to target prevention to specific audiences, and to disseminate prevention messages through local channels is discussed. The authors propose including aspects of ecology and disease proximity in understanding risk perception and addressing emerging diseases with a prominent ecological component.     

Migrating Birds as Dispersal Vehicles for West Nile Virus

Whereas migrating birds have been implicated in the spread of West Nile virus (WNV), there is no direct evidence of birds actively migrating while infectious. The role of birds in WNV dispersal is difficult to assess in the field. However, this role can be evaluated experimentally because birds in migratory disposition display increased locomotor activity or restlessness under captive conditions. We tested the following hypotheses: (1) migrating passerine birds continue to exhibit migratory activity while infectious with WNV and (2) the migratory state of the individual affects the magnitude of viremia. We examined the migratory activity of two neoarctic-neotropical passerine migrants, Swainson’s thrush (Catharus ustulatus) and gray catbird (Dumetella carolinensis), during acute WNV infection. All gray catbirds and six of nine Swainson’s thrushes exhibited migratory activity while infectious. Moreover, migratory status did not appear to influence viremia titers, as might be expected if individuals were immunosuppressed during migration. Therefore, we demonstrate that migrating passerine birds are potential dispersal vehicles for WNV.     

Multi-species interactions in West Nile virus infection

In this paper, we analyse the interaction of different species of birds and mosquitoes on the dynamics of West Nile virus (WNV) infection. We study the different transmission efficiencies of the vectors and birds and the impact on the possible outbreaks. We show that the basic reproductive number is the weighted mean of the basic reproductive number of each species, weighted by the relative abundance of its population in the location. These results suggest a possible explanation of why there are no outbreaks of WNV in Mexico.     

Serologic survey of West Nile virus in horses from Central-West,Northeast and Southeast Brazil

Since the emergence of West Nile virus (WNV) in North America in 1999, there have been several reports of WNV activity in Central and South American countries. To detect WNV in Brazil, we performed a serological survey of horses from different regions of Brazil using recombinant peptides from domain III of WNV. Positive samples were validated with the neutralisation test. Our results showed that of 79 ELISA-positive horses, nine expressed WNV-specific neutralising antibodies. Eight of the infected horses were from the state of Mato Grosso do Sul and one was from the state of Paraíba. Our results provide additional evidence for the emergence of WNV in Brazil and for its circulation in multiple regions of the country.     

A Comparison of Continuous and Discrete-time West Nile Virus Models

The first recorded North American epidemic of West Nile virus was detected in New York state in 1999, and since then the virus has spread and become established in much of North America. Mathematical models for this vector-transmitted disease with cross-infection between mosquitoes and birds have recently been formulated with the aim of predicting disease dynamics and evaluating possible control methods. We consider discrete and continuous time versions of the West Nile virus models proposed by Wonham et al. [Proc. R. Soc. Lond. B 271:501–507, 2004] and by Thomas and Urena [Math. Comput. Modell. 34:771–781, 2001], and evaluate the basic reproduction number as the spectral radius of the next-generation matrix in each case. The assumptions on mosquito-feeding efficiency are crucial for the basic reproduction number calculation. Differing assumptions lead to the conclusion from one model [Wonham, M.J. et al., [Proc. R. Soc. Lond. B] 271:501–507, 2004] that a reduction in bird density would exacerbate the epidemic, while the other model [Thomas, D.M., Urena, B., Math. Comput. Modell. 34:771–781, 2001] predicts the opposite: a reduction in bird density would help control the epidemic.     

Evidence of vertical transmission of West Nile virus in field‐collected mosquitoes

Male and nulliparous female mosquitoes were surveyed for evidence of vertical WNV infection in East Baton Rouge Parish, Louisiana. Adult male mosquitoes collected by trapping and aspiration, and adult male and nulliparous female mosquitoes reared from field‐collected larvae were tested. Adult male Culex spp., female Aedes albopictus (Skuse), and female Culex quinquifasciatus Say mosquitoes that were collected as larvae were test‐positive for WNV RNA. Infectious WNV was detected using virus isolation in field‐collected male Aedes triseriatus Say and Culex salinarius Coquillett; these data represent the first field evidence of vertical transmission of WNV in Ae. triseriatus and Cx. salinarius.     

Climate change impacts on West Nile virus transmission in a global context

West Nile virus (WNV), the most widely distributed virus of the encephalitic flaviviruses, is a vector-borne pathogen of global importance. The transmission cycle exists in rural and urban areas where the virus infects birds, humans, horses and other mammals. Multiple factors impact the transmission and distribution of WNV, related to the dynamics and interactions between pathogen, vector, vertebrate hosts and environment. Hence, among other drivers, weather conditions have direct and indirect influences on vector competence (the ability to acquire, maintain and transmit the virus), on the vector population dynamic and on the virus replication rate within the mosquito, which are mostly weather dependent. The importance of climatic factors (temperature, precipitation, relative humidity and winds) as drivers in WNV epidemiology is increasing under conditions of climate change. Indeed, recent changes in climatic conditions, particularly increased ambient temperature and fluctuations in rainfall amounts, contributed to the maintenance (endemization process) of WNV in various locations in southern Europe, western Asia, the eastern Mediterranean, the Canadian Prairies, parts of the USA and Australia. As predictions show that the current trends are expected to continue, for better preparedness, any assessment of future transmission of WNV should take into consideration the impacts of climate change.     

Apoptosis in mosquito midgut epithelia associated with West Nile virus infection

The mosquito Culex pipiens pipiens is a documented vector of West Nile virus (WNV, Flaviviridae, Flavivirus). Our laboratory colony of C. p. pipiens, however, was repeatedly refractory to experimental transmission of WNV. Our goal was to identify if a cellular process was inhibiting virus infection of the midgut. We examined midguts of mosquitoes fed control and WNV-infected blood meals. Three days after feeding, epithelial cells from abdominal midguts of mosquitoes fed on WNV fluoresced under an FITC filter following Acridine Orange staining, indicating apoptosis in this region. Epithelial cells from experimental samples examined by TEM exhibited ultrastructural changes consistent with apoptosis, including shrinkage and detachment from neighbors, heterochromatin condensation, nuclear degranulation, and engulfment of apoptotic bodies by adjacent cells. Virions were present in cytoplasm and within cytoplasmic vacuoles of apoptotic cells. No apoptosis was detected by TEM in control samples. In parallel, we used Vero cell plaque assays to quantify infection after 7 and 10 day extrinsic incubation periods and found that none of the mosquitoes (0/55 and 0/10) which imbibed infective blood were infected. We propose that programmed cell death limits the number of WNV-infected epithelial cells and inhibits disseminated viral infections from the mosquito midgut.     

The use of early summer mosquito surveillance to predict late summer West Nile virus activity

Utility of early‐season mosquito surveillance to predict West Nile virus activity in late summer was assessed in Suffolk County, NY. Dry ice‐baited CDC miniature light traps paired with gravid traps were set weekly. Maximum‐likelihood estimates of WNV positivity, minimum infection rates, and % positive pools were generally well correlated. However, positivity in gravid traps was not correlated with positivity in CDC light traps. The best early‐season predictors of WNV activity in late summer (estimated using maximum‐likelihood estimates of Culex positivity in August and September) were early date of first positive pool, low numbers of mosquitoes in July, and low numbers of mosquito species in July. These results suggest that early‐season entomological samples can be used to predict WNV activity later in the summer, when most human cases are acquired. Additional research is needed to establish which surveillance variables are most predictive and to characterize the reliability of the predictions.     

Polymorphic microsatellite loci from the West Nile virus vector Culex tarsalis

Since its introduction in 1999, West Nile virus (WNV) has spread across North America. Culex tarsalis is a highly efficient WNV vector species. Many traits such as virus susceptibility, autogeny and host preference vary geographically and temporally in C. tarsalis. Culex tarsalis genomic libraries were developed and were highly enriched for microsatellite inserts (42–96%). We identified 12 loci that were polymorphic in wild C. tarsalis populations. These microsatellites are the first DNA‐based genetic markers developed for C. tarsalis and will be useful for investigating population structure and constructing genetic maps in this mosquito.     

Human Exposure to Malathion during a Possible Vector-Control Intervention against West Nile Virus. I: Methodological Framework for Exposure Assessment

The arrival of West Nile Virus in North America prompted public health authorities to develop intervention plans in order to prevent the propagation of this mosquito-transmitted pathogen. The last-resort measure proposed by the Government of Quebec (Canada) is the large-scale application of insecticides by aerial or ground Ultra Low Volume (ULV) treatment. This article presents an assessment of the exposure to malathion and its metabolite, malaoxon, for a population where an eventual application of malathion would occur. Each exposure pathway is detailed by describing the equation and every conservative assumption. This methodological framework was then used with the aim of assessing the toxicological risk based on a probabilistic approach (see companion article, this issue). In the current study, a daily absorbed dose of the mixture of malathion and malaoxon was estimated in terms of “malathion equivalent dose.” Each exposure pathway following a single event of ground or aerial ULV spraying of malathion was investigated for the population, classified into five age groups. Dermal exposure to dislodgeable residues on turf, foliage, and hard surfaces was estimated to be the most important source of exposure compared to any other pathways; it accounts for 63% to 98% of the estimated cumulative absorbed doses. The hand-to-mouth behavior of toddlers may also contribute considerably to their “malathion-equivalent” exposure (i.e., approximately 15% of the cumulative dose). Otherwise, the current study shows that ground ULV spraying is the type of treatment that may induce lower exposure, because the predicted concentration of malathion on turf and foliar surfaces is less than the one predicted for aerial ULV spraying.     

西尼罗病毒的RT-PCR检测与鉴定

建立西尼罗病毒敏感、特异、快速的RT-PCR检测方法用于实验室诊断和流行病学监测。采用一步RT-PCR和套式PCR法对西尼罗病毒感染的乳鼠脑和细胞培养上清进行扩增,并对扩增产物进行序列测定。两种方法均可分别从两种组织中扩增出与预期大小相一致的片段,套式PCR法比一步RT-PCR法更为敏感,该扩增片段与西尼罗病毒埃及Eg101株相应序列的同源性为99％。     

Spatial spreading of West Nile Virus described by traveling waves

In this work, we propose a spatial model to analyze the West Nile Virus propagation across the USA, from east to west. West Nile Virus is an arthropod-borne flavivirus that appeared for the first time in New York City in the summer of 1999 and then spread prolifically among birds. Mammals, such as humans and horses, do not develop sufficiently high bloodstream titers to play a significant role in the transmission, which is the reason to consider the mosquito-bird cycle. The model aims to study this propagation based on a system of partial differential reaction-diffusion equations taking the mosquito and the avian populations into account. Diffusion and advection movements are allowed for both populations, being greater in the avian than in the mosquito population. The traveling wave solutions of the model are studied to determine the speed of disease dissemination. This wave speed is obtained as a function of the model's parameters, in order to assess the control strategies. The propagation of West Nile Virus from New York City to California state is established as a consequence of the diffusion and advection movements of birds. Mosquito movements do not play an important role in the disease dissemination, while bird advection becomes an important factor for lower mosquito biting rates.     

The backward bifurcation in compartmental models for West Nile virus

In all of the West Nile virus (WNV) compartmental models in the literature, the basic reproduction number serves as a crucial control threshold for the eradication of the virus. However, our study suggests that backward bifurcation is a common property shared by the available compartmental models with a logistic type of growth for the population of host birds. There exists a subthreshold condition for the outbreak of the virus due to the existence of backward bifurcation. In this paper, we first review and give a comparison study of the four available compartmental models for the virus, and focus on the analysis of the model proposed by Cruz-Pacheco et al. to explore the backward bifurcation in the model. Our comparison study suggests that the mosquito population dynamics itself cannot explain the occurrence of the backward bifurcation, it is the higher mortality rate of the avian host due to the infection that determines the existence of backward bifurcation.