Models for the dispersal in Australia of the arbovirus vector, Culicoides brevitarsis Kieffer (Diptera: Ceratopogonidae)

Epidemiology and Transmission of Theileria orientalis in Australasia

Oriental theileriosis, a disease primarily impacting cattle is caused by an apicomplexan hemoprotozoan parasite, Theileria orientalis. It has now become established in the Australasia region. The organism was long considered a benign cause of persistent infections; however, an increase in clinical outbreaks since 2006 in the eastern Australian states and New Zealand was associated with the identification of the pathogenic Ikeda (Type 2) and Chitose (Type 1) genotypes. Unlike the pathogenic T. parva and T. annulate, which target leucocytes, clinical manifestation with T. orientalis is due to its effects on erythrocytes, with the infection sometimes designated as Theileria associated bovine anemia (TABA). In Australia and New Zealand, the tick Haemaphysalis longicornis is the principal vector, though other Haemaphysalis species are also likely vectors. The endemic status of infection with pathogenic genotypes in areas with low or absent tick populations is an apparent paradox that may be attributable to alternative modes of transmission, such as mechanical transmission by hematophagous insects (lice, mosquitoes, and biting flies), vertical transmission, and transmission via iatrogenic means. This review addresses the evidence for the different modes of transmission of T. orientalis with particular focus on the reported and potential vectors in Australasia.

Spatial and temporal distribution of Culicoides species in the New England region of New South Wales, Australia between 1990 and 2018

Culicoides are one of the smallest hematophagous flies measuring 1-5 mm in size with only females seeking blood for egg development. The present study investigated spatio-temporal distribution of Culicoides species trapped between 1990 and 2018 at 13 sites in the New England region of NSW, Australia using automated light traps. Trapping locations were divided into three subregions (tablelands, slopes and plains). Nineteen Culicoides species were identified. Culicoides marksi and C. austropalpalis were the most abundant and widespread species. Culicoides brevitarsis, the principal vector of livestock diseases in New South Wales comprised 2.9% of the total catch and was detected in 12 of the 13 locations in the study. Abundance as determined by Log10 Culicoides count per trapping event for the eight most abundant species did not vary significantly with season but trended towards higher counts in summer for C. marksi (P = 0.09) and C. austropalpalis (P = 0.05). Significant geographic variation in abundance was observed for C. marksi, C. austropalpalis and C. dycei with counts decreasing with increasing altitude from the plains to the slopes and tablelands. Culicoides victoriae exhibited the reverse trend in abundance (P = 0.08). Greater abundance during the warmer seasons and at lower altitudes for C. marksi and C. austropalpalis was indicative of temperature and rainfall dependence in this region with moderate summer dominance in rainfall. The Shannon-Wiener diversity index of species was higher on the tablelands (H = 1.59) than the slopes (H = 1.33) and plains (H = 1.08) with evenness indices of 0.62, 0.46 and 0.39 respectively. Culicoides species on the tablelands were more diverse than on the slopes and plains where C. marksi and C. austropalpalis dominated. The temporal and spatial variation in abundance, diversity and evenness of species reported in this diverse region of Australia provides additional insight into Culicoides as pests and disease vectors and may contribute to future modelling studies.

GENETIC RELATEDNESS OF EPIZOOTIC HEMORRHAGIC DISEASE VIRUS SEROTYPE 2 FROM 2012 OUTBREAK IN THE USA

During summer and early fall of 2012, the US experienced the largest outbreak of hemorrhagic disease (HD) on record; deer (both Odocoileus virginianus and Odocoileus hemionus) in 35 states were affected, including many northern states where HD typically does not occur. Epizootic hemorrhagic disease virus (EHDV) was the predominant virus isolated, with serotype 2 (EHDV-2) representing 66% (135/205) of all isolated viruses. Viruses within the EHDV serogroup are genetically similar, but we hypothesized that subtle genetic distinctions between viruses would exist across the geographic range of the outbreak if multiple EHDV-2 strains were responsible. We examined viral relatedness and molecular epidemiology of the outbreak by sequencing the mammalian binding protein (VP2) gene and the insect vector binding protein (VP7) gene of 34 EHDV-2 isolates from 2012 across 21 states. Nucleotide sequences of VP2 had 99.0% pairwise identity; VP7 nucleotide sequences had 99.1% pairwise identity. Very few changes were observed in either protein at the amino acid level. Despite the high genetic similarity between isolates, subtle nucleotide differences existed. Both VP2 and VP7 gene sequences separated into two distinct clades based on patterns of single-nucleotide polymorphisms after phylogenetic analysis. The clades were divided geographically into eastern and western clades, although those divisions were not identical between VP2 and VP7. There was also an association between percent sequence identity and geographic distance between isolates. We concluded that multiple EHDV-2 strains contributed to this outbreak.

How is Europe positioned for a re-emergence of Schmallenberg virus?

Schmallenberg virus (SBV) caused a large scale epidemic in Europe from 2011 to 2013, infecting ruminants and causing foetal deformities after infection of pregnant animals. The main impact of the virus was financial loss due to restrictions on trade of animals, meat and semen. Although effective vaccines were produced, their uptake was never high. Along with the subsequent decline in new SBV infections and natural replacement of previously exposed livestock, this has resulted in a decrease in the number of protected animals. Recent surveillance has shown that a large population of naïve animals is currently present in Europe and that the virus is circulating at a low level. These changes in animal status, in combination with favourable conditions for insect vectors, may open the door to the re-emergence of SBV and another large scale outbreak in Europe. This review details the potential and preparedness for SBV re-emergence in Europe, discusses possible co-ordinated sentinel monitoring programmes for ruminant seroconversion and the presence of SBV in the insect vectors, and provides an overview of the economic impact associated with diagnosis, control and the effects of non-vaccination.

Community analysis of biting midges (Culicoides Latr.) on livestock farms in Denmark

This study presents descriptive statistics and community analysis of adult biting midges trapped at 16 livestock farms by means of light traps on Zealand and Lolland-Falster, Denmark. A total of 9,047 male and female Culicoides divided into 24 species, were caught. Biotic and abiotic factors ranging from presence of different host species (cattle or sheep/goats), presence of small woody areas or wetlands in the surrounding landscape, and agricultural practice (organic or conventional) were included in the community analysis. Only differences in the Culicoides communities between conventional and organic practices were tested significantly different. Total numbers of Culicoides individuals were higher on the organic farms than on the conventional farms. The larger loads of biting midges on the organic farms may be due to free-ranging animals that attracted the midges on pastures and carried them to the stable environment (the cattle of the conventional farms were held inside the stables). Presence of deciduous trees within 500 m of the farms resulted in higher numbers of Culicoides obsoletus s.s., while presence of wetlands increased the numbers of Culicoides punctatus and Culicoides pulicaris. Furthermore, Culicoides riethi and Culicoides puncticollis (subgenus Monoculicoides) were recorded in high numbers on individual farms. C. puncticollis was found for the first time in Denmark and so far only recorded from Zealand.

High seroprevalence of bluetongue virus antibodies in goats in southeast Iran

OBJECTIVE

To describe the seroprevalence rate of bluetongue virus (BTV) in goat flocks in southeast of Iran.

METHODS

The blood samples were collected randomly from herds of southeast of Iran. A total of 93 sera samples were collected between 2011 and 2012. Antibodies to BTV in sera were detected by using a commercial competitive ELISA 3 according to manufacturer's instructions.

RESULTS

The seroprevalence rates were 67.7% for goats. Within a herd, prevalence of BTV seropositive animals ranged from 33.3% to 100.0%. All goat flocks were positive to BTV antibodies.

CONCLUSIONS

This study describes a high seroprevalence rate of BTV in goat flocks in southeast of Iran for the first time.

Thermal limits of two biting midges, Culicoides imicola Kieffer and C. bolitinos Meiswinkel (Diptera: Ceratopogonidae)

Background

Culicoides imicola Kieffer and Culicoides bolitinos Meiswinkel (Diptera: Ceratopogonidae) are both of veterinary importance, being vectors of Schmallenberg, bluetongue and African horse sickness (AHS) viruses. Within South Africa, these Culicoides species show a marked difference in their abundances according to altitude, with C. imicola highly abundant in lower altitudes, but being replaced as the dominant species by C. bolitinos in cooler, high-altitude regions.

Methods

The thermal physiology of field collected adults of each species was determined to evaluate whether it could account for differences in their distribution and abundance. Critical thermal maxima (CT_max) and minima (CT_min), as well as upper and lower lethal temperatures (ULT and LLT) were assessed after acclimation temperatures of 19ˌC, 24ˌC and 29ˌC. Critical thermal limits were determined using an ecologically relevant rate of temperature change of 0.06ˌC.min⁻¹.

Results

Significant differences in CT_min and CT_max were found between acclimation temperatures for C. imicola and C. bolitinos. In C. bolitinos, the LLT of individuals acclimated at 24ˌC was significantly improved (LLT₅₀ = −6.01ˌC) compared with those acclimated at the other temperatures (LLT₅₀ = −4ˌC). Acclimation had a weak (difference in LLT₅₀ of only 1ˌC) but significant effect on the LLT of C. imicola. When CT_min, CT_max, LLT and ULT were superimposed on daily maximum and minimum temperature records from locations where each tested Culicoides species is dominant, it was found that temperatures frequently declined below the CT_min and LLT of C. imicola at the location where C. bolitinos was dominant.

Conclusions

The distribution and abundance of C. imicola is likely directly constrained by their relatively poor tolerance of lower temperatures. Results for C. bolitinos suggest that the adult phase is hardy, and it is hypothesised that the thermal biology of other life stages could determine their range.

Electronic supplementary material

The online version of this article (doi:10.1186/1756-3305-7-384) contains supplementary material, which is available to authorized users.

A spatial simulation model for the dispersal of the bluetongue vector Culicoides brevitarsis in Australia

Background

The spread of Bluetongue virus (BTV) among ruminants is caused by movement of infected host animals or by movement of infected Culicoides midges, the vector of BTV. Biologically plausible models of Culicoides dispersal are necessary for predicting the spread of BTV and are important for planning control and eradication strategies.

Methods

A spatially-explicit simulation model which captures the two underlying population mechanisms, population dynamics and movement, was developed using extensive data from a trapping program for C. brevitarsis on the east coast of Australia. A realistic midge flight sub-model was developed and the annual incursion and population establishment of C. brevitarsis was simulated. Data from the literature was used to parameterise the model.

Results

The model was shown to reproduce the spread of C. brevitarsis southwards along the east Australian coastline in spring, from an endemic population to the north. Such incursions were shown to be reliant on wind-dispersal; Culicoides midge active flight on its own was not capable of achieving known rates of southern spread, nor was re-emergence of southern populations due to overwintering larvae. Data from midge trapping programmes were used to qualitatively validate the resulting simulation model.

Conclusions

The model described in this paper is intended to form the vector component of an extended model that will also include BTV transmission. A model of midge movement and population dynamics has been developed in sufficient detail such that the extended model may be used to evaluate the timing and extent of BTV outbreaks. This extended model could then be used as a platform for addressing the effectiveness of spatially targeted vaccination strategies or animal movement bans as BTV spread mitigation measures, or the impact of climate change on the risk and extent of outbreaks. These questions involving incursive Culicoides spread cannot be simply addressed with non-spatial models.

The challenge of Schmallenberg virus emergence in Europe

The large-scale outbreak of disease across Northern Europe caused by a new orthobunyavirus known as Schmallenberg virus has caused considerable disruption to lambing and calving. Although advances in technology and collaboration between veterinary diagnostic and research institutes have enabled rapid identification of the causative agent and the development and deployment of tests, much remains unknown about this virus and its epidemiology that make predictions of its future impact difficult to assess. This review outlines current knowledge of the virus, drawing comparisons with related viruses, then explores possible scenarios of its impact in the near future, and highlights some of the urgent research questions that need to be addressed to allow the development of appropriate control strategies.

Why did bluetongue spread the way it did? Environmental factors influencing the velocity of bluetongue virus serotype 8 epizootic wave in France

Understanding where and how fast an infectious disease will spread during an epidemic is critical for its control. However, the task is a challenging one as numerous factors may interact and drive the spread of a disease, specifically when vector-borne diseases are involved. We advocate the use of simultaneous autoregressive models to identify environmental features that significantly impact the velocity of disease spread. We illustrate this approach by exploring several environmental factors influencing the velocity of bluetongue (BT) spread in France during the 2007–2008 epizootic wave to determine which ones were the most important drivers. We used velocities of BT spread estimated in 4,495 municipalities and tested sixteen covariates defining five thematic groups of related variables: elevation, meteorological-related variables, landscape-related variables, host availability, and vaccination. We found that ecological factors associated with vector abundance and activity (elevation and meteorological-related variables), as well as with host availability, were important drivers of the spread of the disease. Specifically, the disease spread more slowly in areas with high elevation and when heavy rainfall associated with extreme temperature events occurred one or two months prior to the first clinical case. Moreover, the density of dairy cattle was correlated negatively with the velocity of BT spread. These findings add substantially to our understanding of BT spread in a temperate climate. Finally, the approach presented in this paper can be used with other infectious diseases, and provides a powerful tool to identify environmental features driving the velocity of disease spread.

Quantifying the spatial dependence of Culicoides midge samples collected by Onderstepoort-type blacklight traps: an experimental approach to infer the range of attraction of light traps

The emergence of bluetongue disease in Europe has led several countries to rapidly establish large-scale entomological surveys of its vectors, which are midges belonging to the genus Culicoides Latreille, 1809 (Diptera: Ceratopogonidae). These surveys have largely been based on the use of Onderstepoort-type blacklight traps. However, the range of attraction of the traps and the spatial dependence of the samples they provide are unknown, which somewhat complicates subsequent analyses. This paper investigates spatial interaction between Onderstepoort-type blacklight traps based on catches at a central trap placed close to two traps set in consecutive on/off modes. The spatial interaction is inferred from the drop in the number of midges collected in the central trap when nearby traps positioned at 50 m, 100 m or 200 m are turned on. The results showed a significant spatial interaction between traps separated by 50 m for female Culicoides obsoletus/Culicoides scoticus and Culicoides dewulfi. No significant interaction was found for female Culicoides of other species, for male Culicoides, or for traps spaced at ≥100 m. Based on the experimental design geometry and on simple assumptions on the distribution of Culicoides midges in the neighbourhood of the traps, the paper also presents a method to infer the range of attraction of the traps.

Bluetongue virus serotype 1 outbreak in the Basque Country (Northern Spain) 2007-2008. Data support a primary vector windborne transport

BACKGROUND

Bluetongue (BT) is a vector-borne disease of ruminants that has expanded its traditional global distribution in the last decade. Recently, BTV-1 emerged in Southern Spain and caused several outbreaks in livestock reaching the north of the country. The aim of this paper was to review the emergence of BTV-1 in the Basque Country (Northern Spain) during 2007 and 2008 analyzing the possibility that infected Culicoides were introduced into Basque Country by winds from the infected areas of Southern Spain.

METHODOLOGY/PRINCIPAL FINDINGS

We use a complex HYSPLIT (Hybrid Single-Particle Lagrangian Integrated Trajectory) model to draw wind roses and backward wind trajectories. The analysis of winds showed September 28 to October 2 as the only period for the introduction of infected midges in the Basque Country. These wind trajectories crossed through the areas affected by serotype 1 on those dates in the South of the Iberian Peninsula. Additionally meteorological data, including wind speed and humidity, and altitude along the trajectories showed suitable conditions for Culicoides survival and dispersion.

CONCLUSIONS/SIGNIFICANCE

An active infection in medium-long distance regions, wind with suitable speed, altitude and trajectory, and appropriate weather can lead to outbreaks of BTV-1 by transport of Culicoides imicola, not only over the sea (as reported previously) but also over the land. This shows that an additional factor has to be taken into account for the control of the disease which is currently essentially based on the assumption that midges will only spread the virus in a series of short hops. Moreover, the epidemiological and serological data cannot rule out the involvement of other Culicoides species in the spread of the infection, especially at a local level.

A new algorithm quantifies the roles of wind and midge flight activity in the bluetongue epizootic in northwest Europe

The 2006 bluetongue (BT) outbreak in northwestern Europe had devastating effects on cattle and sheep in that intensively farmed area. The role of wind in disease spread, through its effect on Culicoides dispersal, is still uncertain, and remains unquantified. We examine here the relationship between farm-level infection dates and wind speed and direction within the framework of a novel model involving both mechanistic and stochastic steps. We consider wind as both a carrier of host semio-chemicals, to which midges might respond by upwind flight, and as a transporter of the midges themselves, in a more or less downwind direction. For completeness, we also consider midge movement independent of wind and various combinations of upwind, downwind and random movements. Using stochastic simulation, we are able to explain infection onset at 94 per cent of the 2025 affected farms. We conclude that 54 per cent of outbreaks occurred through (presumably midge) movement of infections over distances of no more than 5 km, 92 per cent over distances of no more than 31 km and only 2 per cent over any greater distances. The modal value for all infections combined is less than 1 km. Our analysis suggests that previous claims for a higher frequency of long-distance infections are unfounded. We suggest that many apparent long-distance infections resulted from sequences of shorter-range infections; a 'stepping stone' effect. Our analysis also found that downwind movement (the only sort so far considered in explanations of BT epidemics) is responsible for only 39 per cent of all infections, and highlights the effective contribution to disease spread of upwind midge movement, which accounted for 38 per cent of all infections. The importance of midge flight speed is also investigated. Within the same model framework, lower midge active flight speed (of 0.13 rather than 0.5 m s(-1)) reduced virtually to zero the role of upwind movement, mainly because modelled wind speeds in the area concerned were usually greater than such flight speed. Our analysis, therefore, highlights the need to improve our knowledge of midge flight speed in field situations, which is still very poorly understood. Finally, the model returned an intrinsic incubation period of 8 days, in accordance with the values reported in the literature. We argue that better understanding of the movement of infected insect vectors is an important ingredient in the management of future outbreaks of BT in Europe, and other devastating vector-borne diseases elsewhere.

Estimating front-wave velocity of infectious diseases: a simple, efficient method applied to bluetongue

Understanding the spatial dynamics of an infectious disease is critical when attempting to predict where and how fast the disease will spread. We illustrate an approach using a trend-surface analysis (TSA) model combined with a spatial error simultaneous autoregressive model (SAR_err model) to estimate the speed of diffusion of bluetongue (BT), an infectious disease of ruminants caused by bluetongue virus (BTV) and transmitted by Culicoides. In a first step to gain further insight into the spatial transmission characteristics of BTV serotype 8, we used 2007-2008 clinical case reports in France and TSA modelling to identify the major directions and speed of disease diffusion. We accounted for spatial autocorrelation by combining TSA with a SAR_err model, which led to a trend SAR_err model. Overall, BT spread from north-eastern to south-western France. The average trend SAR_err-estimated velocity across the country was 5.6 km/day. However, velocities differed between areas and time periods, varying between 2.1 and 9.3 km/day. For more than 83% of the contaminated municipalities, the trend SAR_err-estimated velocity was less than 7 km/day. Our study was a first step in describing the diffusion process for BT in France. To our knowledge, it is the first to show that BT spread in France was primarily local and consistent with the active flight of Culicoides and local movements of farm animals. Models such as the trend SAR_err models are powerful tools to provide information on direction and speed of disease diffusion when the only data available are date and location of cases.

A wind density model to quantify the airborne spread of Culicoides species during north-western Europe bluetongue epidemic, 2006

Increased transport and trade as well as climate shifts play an important role in the introduction, establishment and spread of new pathogens. Arguably, the introduction of bluetongue virus (BTV) serotype 8 in Benelux, Germany and France in 2006 is such an example. After its establishment in receptive local vector and host populations the continued spread of such a disease in a suitable environment will mainly depend on movement of infected vectors and animals. In this paper we explore how wind models can contribute to explain the spread of BTV in a temperate eco-climatic setting. Based on previous work in Greece and Bulgaria filtered wind density maps were computed using data from the European Centre for Medium-Range Weather Forecasts (ECMWF). Six hourly forward wind trajectories were computed at pressure levels of 850 hPa for each infected farm as from the recorded onset of symptoms. The trajectories were filtered to remove wind events that do not contribute to possible spread of the vector. The suitable wind events were rastered and aggregated on a weekly basis to obtain weekly wind density maps. Next to this, cumulated wind density maps were also calculated to assess the overall impact of wind dispersal of vectors. A strong positive correlation was established between wind density data and the horizontal asymmetrical spread pattern of the 2006 BTV8 epidemic. It was shown that short (<5 km), medium (5-31 km) and long (>31 km) distance spread had a different impact on disease spread. Computed wind densities were linked to the medium/long-distance spread whilst short range spread was mainly driven by active Culicoides flight. Whilst previous work in the Mediterranean basin showed that wind driven spread of Culicoides over sea occurred over distances of up to 700 km, this phenomenon was not observed over land. Long-distance spread over land followed a hopping pattern, i.e. with intermediary stops and establishment of local virus circulation clusters at distances of 35-85 km. Despite suitable wind densities, no long range spread was recorded over distances of 300-400 km. Factors preventing spread Eastwards to the UK and Northwards to Denmark during the 2006 epidemic are discussed. Towards the east both elevation and terrain roughness, causing air turbulences and drop down of Culicoides, were major factors restricting spread. It is concluded that the proposed approach opens new avenues for understanding the spread of vector-borne viruses in Europe. Future developments should take into consideration both physical and biological factors affecting spread.

Survival analysis on aggregate data to assess time to sero-conversion after experimental infection with Bovine Leukemia virus

Bovine Leukemia virus (BLV) is a ubiquitous retrovirus that affects mainly cattle. Knowledge of the precise moment of infection is fundamental for identification and evaluation of factors related to BLV transmission. Systematic reviews and meta-analyses provide good evidence on the effects of medical interventions. The objectives were to estimate time to sero-conversion after experimental infection using data from retrieved literature and to detect factors that may influence the length of that interval using survival analysis on pooled data. An analysis using aggregate data from 36 studies totalling 438 observations was performed. From this, four sets were created and analysed by interval-censored accelerated failure time models (AFT) with different distributions (exponential, Weibull, log-logistic, lognormal and generalized gamma), and some variants of the Cox model (Andersen-Gill, smoothing splines) with and without a frailty effect. The AFT gamma model fit best and the estimated median time to sero-conversion in the null model was 57 days (95% confidence interval (CI): 49; 75) using all data and 47 days (95% CI: 39; 55) when only studies using experimental inoculation were considered. Some factors were consistently associated with time to sero-conversion. These included exposure by animal-to-animal contact (resulting in a seven-fold increase in time to sero-conversion compared to direct inoculation), diagnostic method to detect sero-conversion (time to sero-conversion was 1.4 times shorter when AGID was used compared to ELISA), and transmission by insect bites (biological media) delayed sero-conversion 2.3 times compared transmission via needles or other inanimate media. After fitting a frailty Cox model, results showed that sero-conversion in susceptible animals after infection using donors, in which presence of virus before the experiment started was confirmed, increased the hazard of sero-conversion two times in comparison with donors in which virus presence was not confirmed before start of the experiment. Inoculation with blood decreased the hazard 2.5 times in comparison with lymphocyte suspensions. Heterogeneity due to different research groups was also present. Finally, a Cox model with smoothing splines contained three variables: research group, route of inoculation and a non-linear spline for infective dose. In conclusion, it can be stated some factors that influence the time to sero-conversion were identified and quantified and that a moderate influence of research centre existed. These results may contribute to the estimation of the most probable times of infection in field conditions and in a better evaluation of control measures.

Climate change and the recent emergence of bluetongue in Europe

Bluetongue, a devastating disease of ruminants, has historically made only brief, sporadic incursions into the fringes of Europe. However, since 1998, six strains of bluetongue virus have spread across 12 countries and 800 km further north in Europe than has previously been reported. We suggest that this spread has been driven by recent changes in European climate that have allowed increased virus persistence during winter, the northward expansion of Culicoides imicola, the main bluetongue virus vector, and, beyond this vector's range, transmission by indigenous European Culicoides species - thereby expanding the risk of transmission over larger geographical regions. Understanding this sequence of events may help us predict the emergence of other vector-borne pathogens.

Effects of altitude, distance and waves of movement on the dispersal in Australia of the arbovirus vector, Culicoides brevitarsis Kieffer (Diptera: Ceratopogonidae)

The dispersal of the biting midge and arbovirus vector Culicoides brevitarsis in the Bellinger, Macleay and Hastings river valleys and up the escarpment of the great dividing range (GDR) of mid-northern coastal New South Wales, Australia, from 1995 to 2003 was studied. The midge moved up these valleys from the endemic coastal plain in at least two waves between October and May, and both waves were modelled. Dispersal time can be explained by direct distance from the coast and the altitude of the sites. Dispersal times due to distance were similar at 18.2 +/- 2.2 (S.D.) and 15.9 +/- 2.6 weeks per 100 km for first- and second-occurrences at fixed altitude. Time of the first wave was extended 0.48 +/- 0.22 weeks for every 100-m rise in altitude and the second by 1.14 +/- 0.24 weeks for every 100-m rise for a set distance. Although C. brevitarsis can move up the escarpment of the GDR (and possibly transmit virus), vector dispersal, survival and establishment at and beyond the top of the range are limited. A third model showed that previously described slower movement of C. brevitarsis up the more-southerly Hunter valley relative to movements down the coastal plain also was related to increasing altitude.

Phylogenetic status and matrilineal structure of the biting midge, Culicoides imicola, in Portugal, Rhodes and Israel

The biting midge Culicoides imicola Kieffer (Diptera: Ceratopogonidae) is the most important Old World vector of African horse sickness (AHS) and bluetongue (BT). Recent increases of BT incidence in the Mediterranean basin are attributed to its increased abundance and distribution. The phylogenetic status and genetic structure of C. imicola in this region are unknown, despite the importance of these aspects for BT epidemiology in the North American BT vector. In this study, analyses of partial mitochondrial cytochrome oxidase subunit I gene (COI) sequences were used to infer phylogenetic relationships among 50 C. imicola from Portugal, Rhodes, Israel, and South Africa and four other species of the Imicola Complex from southern Africa, and to estimate levels of matrilineal subdivision in C. imicola between Portugal and Israel. Eleven haplotypes were detected in C. imicola, and these formed one well-supported clade in maximum likelihood and Bayesian trees implying that the C. imicola samples comprise one phylogenetic species. Molecular variance was distributed mainly between Portugal and Israel, with no haplotypes shared between these countries, suggesting that female-mediated gene flow at this scale has been either limited or non-existent. Our results provide phylogenetic evidence that C. imicola in the study areas are potentially competent AHS and BT vectors. The geographical structure of the C. imicola COI haplotypes was concordant with that of BT virus serotypes in recent BT outbreaks in the Mediterranean basin, suggesting that population subdivision in its vector can impose spatial constraints on BT virus transmission.

Geographical and seasonal distribution of the bluetongue virus vector, Culicoides imicola, in central Italy

Following the first incursion of bluetongue virus (BTV) into Italy, the geographical and seasonal distribution of the biting midge Culicoides imicola Kieffer (Diptera: Ceratopogonidae), the main vector of BTV and African horse sickness virus, was investigated in two regions of central Italy (Lazio and Tuscany). Surveillance of Culicoides was carried out between July 2001 and December 2002 using light traps: 1917 collections were made in 381 trap sites, well distributed across both regions. During the survey, bluetongue outbreaks were recorded in both regions. Culicoides imicola was found in 89 (23%) trap sites, distributed fairly continuously along the whole western coastline, between 41.2697 degrees N and 44.05724 degrees N. It was found only occasionally inland and usually in low abundance, with catches of more than 1000 specimens per night found in only two sample sites and 74% of catches numbering fewer than 10 specimens. Adults were caught from March to mid December, with peaks ranging from the end of August to mid November. The coastal distribution and the presence of only few sites with year-round records of adult vectors suggests that colonization may have occurred recently, by passive wind-dispersal from external source areas (Sardinia and Corsica). Alternatively, the species may occur in established, previously undetected, autochthonous populations that are limited from extension inland and northern-ward within Lazio and Tuscany by cool winter temperatures.

A demonstration of interval-censored survival analysis

Interval-censoring occurs in survival analysis when the time until an event of interest is not known precisely (and instead, only is known to fall into a particular interval). Such censoring commonly is produced when periodic assessments (usually clinical or laboratory examinations) are used to assess if the event has occurred. My objectives were to raise awareness about interval-censoring including its existence, the potential ramifications of ignoring its existence, the different types of interval-censored data, and the analytical methods to analyze such data (including availability in standard statistical software). Asynchronous interval-censored survival analysis was demonstrated by parametric evaluation of risk factors for the time to first detected shedding of Salmonella muenster (identified by repeated periodic fecal cultures) for a herd of dairy cows. These results were compared with those from survival analyses which ignored or approximated the interval-censoring. Ignoring or approximating the asynchronous interval-censoring in the survival analysis generally resulted in the risk factors' regression coefficients having the same signs and a decrease (often >50%) in their absolute size. All the standard errors from the three methods of approximating the interval-censoring were <40% of their interval-censored counterparts. The conclusions drawn from the asynchronous interval-censored analysis versus those from the approximations varied dramatically. (The general conclusion from the approximations was that none of the risk factors for this example warranted further consideration.) That ignoring or approximating the left- and interval-censored nature of the dependent variable resulted in biased results was consistent with the literature. In the currently available asynchronous interval-censored models, the inclusion of time-dependent covariates that vary continuously is awkward. Statistical models for the semi-parametric estimation of asynchronous interval-censored survival analysis are not generally available in standard statistical software.