Ecological correlates of risk and incidence of West Nile virus in the United States

West Nile virus, which was recently introduced to North America, is a mosquito-borne pathogen that infects a wide range of vertebrate hosts, including humans. Several species of birds appear to be the primary reservoir hosts, whereas other bird species, as well as other vertebrate species, can be infected but are less competent reservoirs. One hypothesis regarding the transmission dynamics of West Nile virus suggests that high bird diversity reduces West Nile virus transmission because mosquito blood-meals are distributed across a wide range of bird species, many of which have low reservoir competence. One mechanism by which this hypothesis can operate is that high-diversity bird communities might have lower community-competence, defined as the sum of the product of each species’ abundance and its reservoir competence index value. Additional hypotheses posit that West Nile virus transmission will be reduced when either: (1) abundance of mosquito vectors is low; or (2) human population density is low. We assessed these hypotheses at two spatial scales: a regional scale near Saint Louis, MO, and a national scale (continental USA). We found that prevalence of West Nile virus infection in mosquito vectors and in humans increased with decreasing bird diversity and with increasing reservoir competence of the bird community. Our results suggest that conservation of avian diversity might help ameliorate the current West Nile virus epidemic in the USA     

Long-term prevalence data reveals spillover dynamics in a multi-host (Artemia), multi-parasite (Microsporidia) community

In the study of multi-host parasites, it is often found that host species contribute asymmetrically to parasite transmission. Yet in natural populations, identifying which hosts contribute to parasite transmission and maintenance is a recurring challenge. Here, we approach this issue by taking advantage of natural variation in the composition of a host community. We studied the brine shrimps Artemia franciscana and Artemia parthenogenetica and their microsporidian parasites Anostracospora rigaudi and Enterocytospora artemiae. Previous laboratory experiments had shown that each host can transmit both parasites, but could not predict their actual contributions to the parasites’ maintenance in the field. To resolve this, we gathered long-term prevalence data from a metacommunity of these species. Metacommunity patches could contain either or both of the Artemia host species, so that the presence of the hosts could be linked directly to the persistence of the parasites. First, we show that the microsporidian A. rigaudi is a spillover parasite: it was unable to persist in the absence of its maintenance host A. parthenogenetica. This result was particularly striking, as A. rigaudi displayed both high prevalence (in the field) and high infectivity (when tested in the laboratory) in both hosts. Moreover, the seasonal presence of A. parthenogenetica imposed seasonality on the rate of spillover, causing cyclical pseudo-endemics in the spillover host A. franciscana. Second, while our prevalence data was sufficient to identify E. artemiae as either a spillover or a facultative multi-host parasite, we could not distinguish between the two possibilities. This study supports the importance of studying the community context of multi-host parasites, and demonstrates that in appropriate multi-host systems, sampling across a range of conditions and host communities can lead to clear conclusions about the drivers of parasite persistence.     

Experimental infection of Eurasian collared‐dove (Streptopelia decaocto) with West Nile virus

The Eurasian collared‐dove (Streptopelia decaocto) has recently experienced a population explosion in North America. It is frequently infected with West Nile virus (WNV). To test the hypothesis that the Eurasian collared‐dove is competent to transmit WNV, we experimentally infected two cohorts of doves with two different strains of WNV, CO08, and NY99, respectively. Both virus strains induced a low‐level viremia, capable of infecting a small fraction of vector mosquitoes. We suggest that the Eurasian collared‐dove plays a relatively insignificant role as an amplifying host for WNV, but it may be important where it is locally abundant.     

Diversity and disease: community structure drives parasite transmission and host fitness

Changes in host diversity and community structure have been linked to disease, but the mechanisms underlying such relationships and their applicability to non-vector-borne disease systems remain conjectural. Here we experimentally investigated how changes in host community structure affected the transmission and pathology of the multi-host parasite Ribeiroia ondatrae, which is a widespread cause of amphibian limb deformities. We exposed larval amphibians to parasites in monospecific or heterospecific communities, and varied host number to differentiate between density- and diversity-mediated effects on transmission. In monospecific communities, exposure to Ribeiroia significantly increased mortality (15%), malformations (40%) and time-to-metamorphosis in toads. However, the presence of tree frogs significantly reduced infection in toads, leading to fewer malformations and higher survival than observed in monospecific communities, providing evidence of parasite-mediated facilitation. Our results suggest that interspecific variation in parasite resistance can inhibit parasite transmission in multi-species communities, reducing infection and pathology in sensitive hosts.     

Effects of tick population dynamics and host densities on the persistence of tick-borne infections

The transmission and the persistence of tick-borne infections are strongly influenced by the densities and the structure of host populations. By extending previous models and analysis, in this paper we analyse how the persistence of ticks and pathogens, is affected by the dynamics of tick populations, and by their host densities. The effect of host densities on infection persistence is explored through the analysis and simulation of a series of models that include different assumptions on tick-host dynamics and consider different routes of infection transmission. Ticks are assumed to feed on two types of host species which vary in their reservoir competence. Too low densities of competent hosts (i.e., hosts where transmission can occur) do not sustain the infection cycle, while too high densities of incompetent hosts may dilute the competent hosts so much to make infection persistence impossible. A dilution effect may occur also for competent hosts as a consequence of reduced tick to host ratio; this is possible only if the regulation of tick populations is such that tick density does not increase linearly with host densities.     

Variation in herbivore density among host plants and its consequences for community structure

Summary The densities of four species of gall-forming sawflies were found to vary significantly among willow host plant clones. Two of the speices varied among host plants at four sites in each of three years. The other two species varied in density among host plants at most of the sites in two of the three years. Total sawfly density also varied significantly among clones. Individual species densities on willow clones were significantly positively correlated between years when all sites were combined and frequently when sites were considered separately. Most pairwise species combinations were independent in density between years, but some negative correlations existed between the stem galler and the leaf galler. Gall-former densities also were largely independent among clones within years with all sites combined and with sites considered separately. The significant correlations were nearly all positive. At all four sites the combination of significant variation in sawfly densities among willow clones in the field and independence of species densities among clones resulted in significantly different communities (relative abundance of species) among willow clones in three years. Although sawfly abundances differed substantially among the four sites, this remained true. It is argued that the pattern of community structure among clones is the result of variation in host plant quality of clones. We propose an hypothesis to account for patterns of herbivore species associations based on intrapopulation host plant variation.     

Host community structure and infestation by ixodid ticks: repeatability,dilution effect and ecological specialization

Abundance of a species in a location results from the interplay between the intrinsic properties of that species and the extrinsic properties, both biotic and abiotic, of the local habitat. Intrinsic factors promote among-population stability in abundance, whereas extrinsic factors generate variation among populations of a species. We studied (a) repeatability and (b) the effect of abundance and species richness of small mammals on the level of their infestation by larvae and nymphs of Ixodes ricinus (ecological generalist) and Ixodes trianguliceps (ecological specialist). We asked if tick infestation parameters are characteristic (=repeatable) for a particular host species or a particular stage of a particular tick species. We also asked how abundance and diversity of hosts affect the level of tick infestation on a particular host species. We predicted that the dilution effect (decrease in tick infestation levels with an increase of host abundance and/or species richness) will be better expressed in an ecological generalist, I. ricinus, than in an ecological specialist, I. trianguliceps. We found that (a) tick abundance, prevalence and aggregation were generally repeatable within tick species/stage; (b) tick abundance and prevalence, but not the aggregation level, were repeatable within host species; (c) the proportion of variance among samples explained by the differences between tick species and stages (as opposed to within-tick species and stage) was higher than that explained by the differences between host species (as opposed to within host species); and (d) the relationship between tick infestation parameters and host abundance and diversity revealed the dilution effect for I. ricinus but not for I. trianguliceps.     

Specialist versus generalist parasites: the interactions between host diversity,environment and geographic barriers in avian malaria

The specialist versus generalist strategies of hemoparasites in relation to their avian host, as well as environmental factors, can influence their prevalence, diversity and distribution. In this paper we investigated the influence of avian host species, as well as the environmental and geographical factors, on the strategies of Haemoproteus and Plasmodium hemoparasites. We determined prevalence and diversity by targeting their cytochrome b (Cytb) in a total of 2,590 passerine samples from 138 localities of Central and South America, and analysed biogeographic patterns and host-parasite relationships. We found a total prevalence of 23.2%. Haemoproteus presented a higher prevalence (15.3%) than Plasmodium (4.3%), as well as a higher diversity and host specificity. We determined that Plasmodium and Haemoproteus prevalences correlated positively with host diversity (Shannon index) and were significantly influenced by bird diversity, demonstrating a possible “amplification effect”. We found an effect of locality and the avian family for prevalences of Haemoproteus and Plasmodium. These results suggest that Haemoproteus is more specialist than Plasmodium and could be mostly influenced by its avian host and the Andes Mountains.     

Determinants of ectoparasite assemblage structure on rodent hosts from South American marshlands: the effect of host species,locality and season

The relative effects of host species identity, locality and season on ectoparasite assemblages (relative abundances and species richness) harboured by four cricetid rodent hosts (Akodon azarae, Oligoryzomys flavescens, Oxymycterus rufus and Scapteromys aquaticus) were assessed across six closely located sites in Buenos Aires province, Argentina. Relative abundances of ectoparasites (14 species including gamasid mites, an ixodid tick, a trombiculid mite, lice and fleas), as well as total ectoparasite abundance and species richness, were determined mainly by host species and to a lesser extent by locality (despite the small spatial scale of the study), whereas seasonal effect was weak, albeit significant. The abundances of some ectoparasites were determined solely by host, whereas those of other ectoparasites (sometimes belonging to the same higher taxon) were also affected by locality and/or season. In gamasids, there was a significant effect of locality for some species, but not for others. In fleas and lice, the effect of locality was similar in different species, suggesting that this effect is related to the characteristic life history strategy.     

熊河水库浮游植物群落结构的周年变化

自2007年5月至2008年4月对湖北省枣阳市熊河水库的浮游植物群落进行调查和分析.调查共发现浮游植物49属74种,绿藻在种类组成上占绝对优势,共40种,其次为蓝藻,有15种,硅藻11种,甲藻3种,金藻和裸藻各2种,隐藻仅1种.蓝藻存在夏季(7月)高峰,优势种为卷曲鱼腥藻(Anabaena circinalis);硅藻、隐藻和甲藻均存在一个春季(3或4月)高峰,优势种分别为双头针杆藻(Synedra amphicephala)、羽纹脆杆藻(Fragilaria pinnata)、卵形隐藻(Cryptomonas ovata)和二角多甲藻(Peridinium bipes).Simpson多样性指数、Shannon-Wiener多样性指数、Pielou均匀度指数和Margalef丰富度指数的年平均值分别是0.60、2.20、0.76和1.15.多样性指数和均匀度指数具有明显的季节变化规律,但无明显的空间分布规律.     

寄主种类、距离和种群密度对烟粉虱扩散的影响

在笼罩条件下,分别以黄瓜、棉花、蕹菜为虫源寄主和诱集寄主,研究寄主植物、空间距离、虫源密度等因子对B型烟粉虱扩散的影响。结果发现,从虫源寄主上扩散的成虫量,与烟粉虱对诱集寄主和虫源寄主的嗜性差异程度有关,诱集寄主相对于虫源寄主的嗜性越强,扩散出来的成虫数量就越多,反之扩散的成虫数量就越少。成虫扩散量与诱集寄主的距离呈负相关,并且诱集寄主的嗜性越强,随着距离的增大,诱集量下降的速率越快。成虫扩散量与虫源种群密度呈正关,但非嗜好寄主上的虫口承载量相对较低,在虫口密度相对较高时,非嗜好寄主上烟粉虱向外扩散的相对量更大。烟粉虱的扩散量与寄主营养状况呈负相关,寄主营养胁迫时,非嗜好寄主上可以获得相对更高的诱集量。     

An attack of Ips amitinus (Coleoptera: Curculionidae: Scolytinae) on arboretum in West Siberia: New host of invasive bark beetle among exotic conifers

Invasive populations of small spruce bark beetle Ips amitinus were first registered in 2019 in the southeast of Western Siberia. In natural stands of Siberian pine (Pinus sibirica Du Tour), several hundred hectares of outbreak foci of the alien bark beetle were identified. In 2020, a local focus of the bark beetle was found in the conifer collection in the arboretum “Kedr” of the Institute of Monitoring Climatic and Ecological Systems SB RAS, 30 km from Tomsk. The bark beetle caused the main damage to the collection of pines. I. amitinus colonized both host plants Scotch pine (Pinus sylvestris L.) and mountain pine (Pinus mugo Turra), which were previously known to it in the native range in Europe, and the local Siberian species Siberian pine (Pinus sibirica Du Tour), Siberian spruce (Picea obovata Ledeb.) and introduced Far Eastern Korean pine (Pinus koraiensis Sieb. et Zucc.). Demographic characteristics of I. amitinus studied on damaged trees indicate its high reproduction potential in Siberia. The bark beetle outbreak focus was suppressed; however, this plantation requires further annual monitoring of pest abundance and distribution, both to preserve the scientific dendroecological field station and to study the implementation of sential plant conception in relation to the invasion of I. amitinus.     

Evaluation of vector competence for West Nile virus in Italian Stegomyia albopicta (=Aedes albopictus) mosquitoes

West Nile virus (WNV) is a zoonotic arboviral pathogen transmitted by mosquitoes in a cycle that involves wild birds as reservoir hosts. The virus is responsible for outbreaks of viral encephalitis in humans and horses. In Europe, Culex pipiens (Diptera: Culicidae) is considered to be the main vector of WNV, but other species such as Stegomyia albopicta (=Aedes albopictus) (Diptera: Culicidae) may also act as competent vectors of this virus. Since 2008 human cases of WNV disease have been reported in northeast Italy. In 2011, new areas of southern Italy became involved and a first outbreak of WNV lineage 1 occurred on the island of Sardinia. On the assumption that a potential involvement of St. albopicta in WNV transmission cannot be excluded, and in order to evaluate the competence of this species for the virus, an experimental infection of an St. albopicta laboratory colony, established from mosquitoes collected in Sardinia, was carried out. The results were compared with those obtained in a colony of the main vector Cx. pipiens. The study showed St. albopicta collected on Sardinia to be susceptible to WNV infection, which suggests this Italian mosquito species is able to act as a possible secondary vector, particularly in urban areas where the species reaches high levels of seasonal abundance.     

Fine-scale variation in vector host use and force of infection drive localized patterns of West Nile virus transmission

The influence of host diversity on multi-host pathogen transmission and persistence can be confounded by the large number of species and biological interactions that can characterize many transmission systems. For vector-borne pathogens, the composition of host communities has been hypothesized to affect transmission; however, the specific characteristics of host communities that affect transmission remain largely unknown. We tested the hypothesis that vector host use and force of infection (i.e., the summed number of infectious mosquitoes resulting from feeding upon each vertebrate host within a community of hosts), and not simply host diversity or richness, determine local infection rates of West Nile virus (WNV) in mosquito vectors. In suburban Chicago, Illinois, USA, we estimated community force of infection for West Nile virus using data on Culex pipiens mosquito host selection and WNV vertebrate reservoir competence for each host species in multiple residential and semi-natural study sites. We found host community force of infection interacted with avian diversity to influence WNV infection in Culex mosquitoes across the study area. Two avian species, the American robin (Turdus migratorius) and the house sparrow (Passer domesticus), produced 95.8% of the infectious Cx. pipiens mosquitoes and showed a significant positive association with WNV infection in Culex spp. mosquitoes. Therefore, indices of community structure, such as species diversity or richness, may not be reliable indicators of transmission risk at fine spatial scales in vector-borne disease systems. Rather, robust assessment of local transmission risk should incorporate heterogeneity in vector host feeding and variation in vertebrate reservoir competence at the spatial scale of vector-host interaction.     

Avian defensive behavior and blood-feeding success of the West Nile vector mosquito, Culex pipiens

Avian defensive behavior against host-seeking arthropods influencestransmission of vector-borne pathogens by affecting mosquitobiting rate, either by preventing vector–host contactor by increasing the rate of multiple host feeding. We exposedhouse sparrows (Passer domesticus L.) and chickens (Gallus gallusdomesticus L.) to Culex pipiens pipiens L. overnight in a largeobservation cage and measured avian defensive behavior ratesand mosquito blood-feeding success. Both bird species exhibiteda range of defensive behaviors, 90% of which were foot stomps,head movements, and wing shakes. Total behavior rates increasedproportionately with mosquito density in both species, increasedafter the first hour of mosquito exposure, and decreased asindividual birds were exposed to mosquitoes multiple times.Mosquito blood-feeding success on house sparrows was high overall(82 ± 5%) and independent of behavior rates. Blood-feedingsuccess on chicks was lower (58 ± 5%) and negativelycorrelated with defensive behavior rate after the first hourof mosquito exposure. Results revealed a higher percentage ofpartial blood meals on chicks (18 ± 3% of all blood mealson chicks) than on house sparrows (4.9 ± 3%). Birds ofboth species ate an average of 9.4 ± 1.2% of mosquitoes,and this percentage was positively correlated with defensivebehavior. High mosquito feeding success on house sparrows supportsits role as a potential amplifying host of West Nile virus.     

Differential activation of human monocyte-derived and plasmacytoid dendritic cells by West Nile virus generated in different host cells

Dendritic cells (DCs) play a central role in innate immunity and antiviral responses. In this study, we investigated the production of alpha interferon (IFN-α) and inducible chemokines by human monocyte-derived dendritic cells (mDCs) and plasmacytoid dendritic cells (pDCs) infected with West Nile virus (WNV), an emergent pathogen whose infection can lead to severe cases of encephalitis in the elderly, children, and immunocompromised individuals. Our experiments demonstrated that WNV grown in mammalian cells (WNV^Vero) was a potent inducer of IFN-α secretion in pDCs and, to a lesser degree, in mDCs. The ability of WNV^Vero to induce IFN-α in pDCs did not require viral replication and was prevented by the treatment of cells with bafilomycin A1 and chloroquine, suggesting that it was dependent on endosomal Toll-like receptor recognition. On the other hand, IFN-α production in mDCs required viral replication and was associated with the nuclear translocation of IRF3 and viral antigen expression. Strikingly, pDCs failed to produce IFN-α when stimulated with WNV grown in mosquito cells (WNV^C7/10), while mDCs responded similarly to WNV^Vero or WNV^C7/10. Moreover, the IFN-dependent chemokine IP-10 was produced in substantial amounts by pDCs in response to WNV^Vero but not WNV^C7/10, while interleukin-8 was produced in greater amounts by mDCs infected with WNV^C7/10 than in those infected with WNV^Vero. These findings suggest that cell-specific mechanisms of WNV recognition leading to the production of type I IFN and inflammatory chemokines by DCs may contribute to both the innate immune response and disease pathogenesis in human infections.     

The ecological differentiation of asexual lineages of cotton aphids: alate behaviour, sensory physiology, and differential host associations

The cotton aphid ( Aphis gossypii Glover) is a cosmopolitan pest species consisting of various host plant-associated lineages that appear to have different patterns of insecticide resistance, electrophoretic banding, and reproductive output across alternative host species. Debate continues as to whether these differences represent intraspecific variation or indicate a complex of cryptic or sibling species. We studied the host plant-related ecology of two asexual lineages of A. gossypii associated with the families Malvaceae and Cucurbitaceae, respectively, in particular the host plant finding and acceptance processes of the alate forms. The two lineages proved to be differentiated across host plants in terms of: (1) their landing and settling behaviour, nymphal production rate, and subsequent colony establishment (i.e. nymphal survival and development) and (2) their sensory abilities in relation to host plant-derived odours. Furthermore, genetic structuring according to host plant type was detected using the gene for elongation factor 1-α. The magnitude of the genetic differentiation across lineages was low, perhaps indicating a relatively recent divergence. The results demonstrate: (1) the presence of (at least) two cryptic species within the current definition of A. gossypii , as anticipated in previous studies, and (2) the practicalities of using a multidisciplinary approach to understanding the 'limits' of asexual species. If sexuals can be generated from each of these lineages, this conclusion will be open to further testing.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 503–519.     

流溪河水库敞水区轮虫多样性与群落的动态特征

流溪河水库是位于北回归线上的大型峡谷型供水水库,于2013—2014年对该水库敞水区轮虫群落及其环境因子进行观测,分析敞水区轮虫多样性与群落动态特征。流溪河水库介于贫营养到中营养水平之间,丰水期(5月—9月)的总磷、透明度均高于枯水期(10月—4月),两年的总磷的平均值为0.019mg/L,透明度为2.55m。两年共检出轮虫40种,单月检出物种数波动范围为8—19种,月均检出轮虫13种。螺形龟甲轮虫、真翅多肢轮虫、沟痕泡轮虫和胶鞘轮虫是主要优势轮虫。2013、2014年轮虫年均总丰度分别为68.9、66.9个/L,两年物种丰富度在丰水期大于枯水期,但枯水期轮虫群落Simpson多样性指数均大于丰水期。采用Bray-Curtis距离测度群落之间的相异性,计算表明两年的枯水期轮虫群落之间的平均距离均大于丰水期,即丰水期时轮虫群落相似性较高,枯水期时轮虫群落变异较大。丰水期的水温和水质参数的变化相对稳定,种类的优势度更为明显,导致丰水期群落之间更为相似。主成分分析表明,无柄轮属、胶鞘轮属、多肢轮属、异尾轮属、泡轮属、晶囊轮属和皱甲轮属的主要种类的全年分布有较大的差异,而其余多数轮虫种类全年分布差异较小,反映了我国南亚热带地区水温全年变幅小,可维持较多的全年性种类共存。群落的方差解析与RDA分析表明,相对于生物与化学变量,物理环境变量主导了流溪河水库敞水区轮虫群落的变异,就单个变量而言,水温和食物是影响流溪河水库敞水区轮虫群落结构的重要因素,降雨则是影响轮虫群落结构变异的宏观因素。绝大多数轮虫为滤食性的,处于食物链底端,个体小、生活史短,轮虫群落在对环境因子变化的响应上与浮游植物具有相似性。     

滇川桂3省暗褐网柄牛肝菌菌腔虫瘿的寄主植物与介壳虫种类调查

暗褐网柄牛肝菌Phlebopus portentosus与介壳虫形成的菌腔虫瘿是该菌营养机制研究的关键环节。本研究先后在云南、四川和广西3省区暗褐网柄牛肝菌产区的16个地点,对菌腔虫瘿的生态和生物学进行了大量的野外调查。发现根部着生菌腔虫瘿的寄主植物有31种,涉及16个科的28个属。与暗褐网柄牛肝菌形成菌腔虫瘿的介壳虫种类有12种,其中10种隶属粉蚧科Pseudococcidae、绵蚧科Monophlebidae、蚧科Coccidae各1种。在不同的寄主植物上菌腔虫瘿的寄生位置和形状会有所不同,与暗褐网柄牛肝菌菌丝形成菌腔虫瘿的寄主植物和介壳虫之间不存在专一性。上述研究结果为暗褐网柄牛肝菌的仿生栽培奠定了基础。     

Experimental parasite community perturbation reveals associations between Sin Nombre virus and gastrointestinal nematodes in a rodent reservoir host

Individuals are often co-infected with several parasite species, yet measuring within-host interactions remains difficult in the wild. Consequently, the impacts of such interactions on host fitness and epidemiology are often unknown. We used anthelmintic drugs to experimentally reduce nematode infection and measured the effects on both nematodes and the important zoonosis Sin Nombre virus (SNV) in its primary reservoir (Peromyscus spp.). Treatment significantly reduced nematode infection, but increased SNV seroprevalence. Furthermore, mice that were co-infected with both nematodes and SNV were in better condition and survived up to four times longer than uninfected or singly infected mice. These results highlight the importance of investigating multiple parasites for understanding interindividual variation and epidemiological dynamics in reservoir populations with zoonotic transmission potential.