Comparison of tick resistance of crossbred Senepol × Limousin to purebred Limousin cattle.

The comparison of resistance to natural tick infestation by Rhipicephalus microplus (Canestrini, 1887) of crossbred Senepol × Limousin and purebred Limousin cattle was investigated. The Senepol breed, originated from St Croix Island in the Caribbean is considered as a Bos taurus breed adapted to tropical conditions. Despite its B. taurus genetic background, it is believed to have a good tick resistance, but this resistance has never been assessed previously. Tick counts under natural infestation were carried out to investigate the difference of susceptibility between crossbred Senepol × Limousin and purebred Limousin cattle. Mixed-effect models were used to assess the effect of the breed on the number of ticks. Results show that Senepol × Limousin are five times less infested by ticks than purebred Limousin. These results underline the opportunity to use Senepol cattle for crossing with susceptible B. taurus breeds in tick infested areas, to combine tick resistance with beef production abilities.

Continuous Excretion of Leptospira borgpetersenii Ballum in Mice Assessed by Viability Quantitative Polymerase Chain Reaction.

Rodents are the main reservoir animals of leptospirosis. In this study, we characterized and quantified the urinary excretion dynamics of Leptospira by Mus musculus infected with 2 × 108 virulent Leptospira borgpetersenii serogroup Ballum. Each micturition was collected separately in metabolic cages, at 12 time points from 7 to 117 days post-infection (dpi). We detected Leptospira in all urine samples collected (up to 8 per time point per mouse) proving that Leptospira excretion is continuous with ca. 90% live L. borgpetersenii Ballum, revealed by viability quantitative polymerase chain reaction. Microscopic visualization by Live/Dead fluorescence confirmed this high proportion of live bacteria and demonstrated that L. borgpetersenii Ballum are excreted, at least partly, as bacterial aggregates. We observed two distinct phases in the excretion dynamics, first an increase in Leptospira concentration shed in the urine between 7 and 63 dpi followed by a plateau phase from 63 dpi onward, with up to 3 × 107Leptospira per mL of urine. These two phases seem to correspond to progressive colonization of renal tubules first, then to stable cell survival and maintenance in kidneys. Therefore, chronically infected adult mice are able to contaminate the environment via urine at each micturition event throughout their lifetime. Because Leptospira excretion reached its maximum 2 months after infection, older rodents have a greater risk of contaminating their surrounding environment.

Dengue-1 virus and vector competence of Aedes aegypti (Diptera: Culicidae) populations from New Caledonia.

BACKGROUND: Dengue virus (DENV) is the arbovirus with the highest incidence in New Caledonia and in the South Pacific region. In 2012-2014, a major DENV-1 outbreak occurred in New Caledonia. The only known vector of DENV in New Caledonia is Aedes aegypti but no study has yet evaluated the competence of New Caledonia Ae. aegypti populations to transmit DENV. This study compared the ability of field-collected Ae. aegypti from different locations in New Caledonia to transmit the DENV-1 responsible for the 2012-2014 outbreak. This study also aimed to compare the New Caledonia results with the vector competence of Ae. aegypti from French Polynesia as these two French countries have close links, including arbovirus circulation. METHODS: Three wild Ae. aegypti populations were collected in New Caledonia and one in French Polynesia. Female mosquitoes were orally exposed to DENV-1 (106 FFU/ml). Mosquito bodies (thorax and abdomen), heads and saliva were analyzed to measure infection, dissemination, transmission rates and transmission efficiency, at 7, 14 and 21 days post-infection (dpi), respectively. RESULTS: DENV-1 infection rates were heterogeneous, but dissemination rates were high and homogenous among the three Ae. aegypti populations from New Caledonia. Despite this high DENV-1 dissemination rate, the transmission rate, and therefore the transmission efficiency, observed were low. Aedes aegypti population from New Caledonia was less susceptible to infection and had lower ability to transmit DENV-1 than Ae. aegypti populations from French Polynesia. CONCLUSION: This study suggests that even if susceptible to infection, the New Caledonian Ae. aegypti populations were moderately competent vectors for DENV-1 strain from the 2012-2014 outbreak. These results strongly suggest that other factors might have contributed to the spread of this DENV-1 strain in New Caledonia and in the Pacific region.

Socioeconomic and environmental determinants of dengue transmission in an urban setting: An ecological study in Nouméa, New Caledonia.

BACKGROUND: Dengue is a mosquito-borne virus that causes extensive morbidity and economic loss in many tropical and subtropical regions of the world. Often present in cities, dengue virus is rapidly spreading due to urbanization, climate change and increased human movements. Dengue cases are often heterogeneously distributed throughout cities, suggesting that small-scale determinants influence dengue urban transmission. A better understanding of these determinants is crucial to efficiently target prevention measures such as vector control and education. The aim of this study was to determine which socioeconomic and environmental determinants were associated with dengue incidence in an urban setting in the Pacific. METHODOLOGY: An ecological study was performed using data summarized by neighborhood (i.e. the neighborhood is the unit of analysis) from two dengue epidemics (2008-2009 and 2012-2013) in the city of Nouméa, the capital of New Caledonia. Spatial patterns and hotspots of dengue transmission were assessed using global and local Moran's I statistics. Multivariable negative binomial regression models were used to investigate the association between dengue incidence and various socioeconomic and environmental factors throughout the city. PRINCIPAL FINDINGS: The 2008-2009 epidemic was spatially structured, with clusters of high and low incidence neighborhoods. In 2012-2013, dengue incidence rates were more homogeneous throughout the city. In all models tested, higher dengue incidence rates were consistently associated with lower socioeconomic status (higher unemployment, lower revenue or higher percentage of population born in the Pacific, which are interrelated). A higher percentage of apartments was associated with lower dengue incidence rates during both epidemics in all models but one. A link between vegetation coverage and dengue incidence rates was also detected, but the link varied depending on the model used. CONCLUSIONS: This study demonstrates a robust spatial association between dengue incidence rates and socioeconomic status across the different neighborhoods of the city of Nouméa. Our findings provide useful information to guide policy and help target dengue prevention efforts where they are needed most.

Genetic Diversity and Phylogeny of Aedes aegypti, the Main Arbovirus Vector in the Pacific.

BACKGROUND: The Pacific region is an area unique in the world, composed of thousands of islands with differing climates and environments. The spreading and establishment of the mosquito Aedes aegypti in these islands might be linked to human migration. Ae. aegypti is the major vector of arboviruses (dengue, chikungunya and Zika viruses) in the region. The intense circulation of these viruses in the Pacific during the last decade led to an increase of vector control measures by local health authorities. The aim of this study is to analyze the genetic relationships among Ae. aegypti populations in this region. METHODOLOGY/PRINCIPAL FINDING: We studied the genetic variability and population genetics of 270 Ae. aegypti, sampled from 9 locations in New Caledonia, Fiji, Tonga and French Polynesia by analyzing nine microsatellites and two mitochondrial DNA regions (CO1 and ND4). Microsatellite markers revealed heterogeneity in the genetic structure between the western, central and eastern Pacific island countries. The microsatellite markers indicate a statistically moderate differentiation (FST = 0.136; P < = 0.001) in relation to island isolation. A high degree of mixed ancestry can be observed in the most important towns (e.g. Noumea, Suva and Papeete) compared with the most isolated islands (e.g. Ouvea and Vaitahu). Phylogenetic analysis indicated that most of samples are related to Asian and American specimens. CONCLUSIONS/SIGNIFICANCE: Our results suggest a link between human migrations in the Pacific region and the origin of Ae. aegypti populations. The genetic pattern observed might be linked to the island isolation and to the different environmental conditions or ecosystems.

Socio-economic and Climate Factors Associated with Dengue Fever Spatial Heterogeneity: A Worked Example in New Caledonia.

BACKGROUND/OBJECTIVES: Understanding the factors underlying the spatio-temporal distribution of infectious diseases provides useful information regarding their prevention and control. Dengue fever spatio-temporal patterns result from complex interactions between the virus, the host, and the vector. These interactions can be influenced by environmental conditions. Our objectives were to analyse dengue fever spatial distribution over New Caledonia during epidemic years, to identify some of the main underlying factors, and to predict the spatial evolution of dengue fever under changing climatic conditions, at the 2100 horizon. METHODS: We used principal component analysis and support vector machines to analyse and model the influence of climate and socio-economic variables on the mean spatial distribution of 24,272 dengue cases reported from 1995 to 2012 in thirty-three communes of New Caledonia. We then modelled and estimated the future evolution of dengue incidence rates using a regional downscaling of future climate projections. RESULTS: The spatial distribution of dengue fever cases is highly heterogeneous. The variables most associated with this observed heterogeneity are the mean temperature, the mean number of people per premise, and the mean percentage of unemployed people, a variable highly correlated with people's way of life. Rainfall does not seem to play an important role in the spatial distribution of dengue cases during epidemics. By the end of the 21st century, if temperature increases by approximately 3 °C, mean incidence rates during epidemics could double. CONCLUSION: In New Caledonia, a subtropical insular environment, both temperature and socio-economic conditions are influencing the spatial spread of dengue fever. Extension of this study to other countries worldwide should improve the knowledge about climate influence on dengue burden and about the complex interplay between different factors. This study presents a methodology that can be used as a step by step guide to model dengue spatial heterogeneity in other countries.

Can human movements explain heterogeneous propagation of dengue fever in Cambodia?

BACKGROUND: Determining the factors underlying the long-range spatial spread of infectious diseases is a key issue regarding their control. Dengue is the most important arboviral disease worldwide and a major public health problem in tropical areas. However the determinants shaping its dynamics at a national scale remain poorly understood. Here we describe the spatial-temporal pattern of propagation of annual epidemics in Cambodia and discuss the role that human movements play in the observed pattern. METHODS AND FINDINGS: We used wavelet phase analysis to analyse time-series data of 105,598 hospitalized cases reported between 2002 and 2008 in the 135 (/180) most populous districts in Cambodia. We reveal spatial heterogeneity in the propagation of the annual epidemic. Each year, epidemics are highly synchronous over a large geographic area along the busiest national road of the country whereas travelling waves emanate from a few rural areas and move slowly along the Mekong River at a speed of ~11 km per week (95% confidence interval 3-18 km per week) towards the capital, Phnom Penh. CONCLUSIONS: We suggest human movements - using roads as a surrogate - play a major role in the spread of dengue fever at a national scale. These findings constitute a new starting point in the understanding of the processes driving dengue spread.

Climate-based models for understanding and forecasting dengue epidemics.

BACKGROUND: Dengue dynamics are driven by complex interactions between human-hosts, mosquito-vectors and viruses that are influenced by environmental and climatic factors. The objectives of this study were to analyze and model the relationships between climate, Aedes aegypti vectors and dengue outbreaks in Noumea (New Caledonia), and to provide an early warning system. METHODOLOGY/PRINCIPAL FINDINGS: Epidemiological and meteorological data were analyzed from 1971 to 2010 in Noumea. Entomological surveillance indices were available from March 2000 to December 2009. During epidemic years, the distribution of dengue cases was highly seasonal. The epidemic peak (March-April) lagged the warmest temperature by 1-2 months and was in phase with maximum precipitations, relative humidity and entomological indices. Significant inter-annual correlations were observed between the risk of outbreak and summertime temperature, precipitations or relative humidity but not ENSO. Climate-based multivariate non-linear models were developed to estimate the yearly risk of dengue outbreak in Noumea. The best explicative meteorological variables were the number of days with maximal temperature exceeding 32°C during January-February-March and the number of days with maximal relative humidity exceeding 95% during January. The best predictive variables were the maximal temperature in December and maximal relative humidity during October-November-December of the previous year. For a probability of dengue outbreak above 65% in leave-one-out cross validation, the explicative model predicted 94% of the epidemic years and 79% of the non epidemic years, and the predictive model 79% and 65%, respectively. CONCLUSIONS/SIGNIFICANCE: The epidemic dynamics of dengue in Noumea were essentially driven by climate during the last forty years. Specific conditions based on maximal temperature and relative humidity thresholds were determinant in outbreaks occurrence. Their persistence was also crucial. An operational model that will enable health authorities to anticipate the outbreak risk was successfully developed. Similar models may be developed to improve dengue management in other countries.