Frédéric Chiroleu

On a temporal model for the Chikungunya disease: modeling, theory and numerics.

Reunion Island faced two episodes of Chikungunya, a vector-borne disease, in 2005 and in 2006. The latter was of unprecedented magnitude: one third of the population was infected. Until the severe episode of 2006, our knowledge of Chikungunya was very limited. The principal aim of our study is to propose a model, including human and mosquito compartments, that is associated to the time course of the first epidemic of Chikungunya. By computing the basic reproduction number R(0), we show there exists a disease-free equilibrium that is locally asymptotically stable if the basic reproduction number is less than 1. Moreover, we give a necessary condition for global asymptotic stability of the disease-free equilibrium. Then, we propose a numerical scheme that is qualitatively stable and present several simulations as well as numerical estimates of the basic reproduction number for some cities of Reunion Island. For the episode of 2005, R(0) was less than one, which partly explains why no outbreak appeared. Using recent entomological results, we investigate links between the episode of 2005 and the outbreak of 2006. Finally, our work shows that R(0) varied from place to place on the island, indicating that quick and focused interventions, like the destruction of breeding sites, may be effective for controlling the disease.

Vector control for the Chikungunya disease

We previously proposed a compartmental model to explain the outbreak of Chikungunya disease in Reunion Island, a French territory in Indian Ocean, and other countries in 2005 and possible links with the explosive epidemic of 2006. In the present paper, we asked whether it would have been possible to contain or stop the epidemic of 2006 through appropriate mosquito control tools. Based on new results on the Chikungunya virus, its impact on mosquito life-span, and several experiments done by health authorities, we studied several types of control tools used in 2006 to contain the epidemic. We present an analysis of the model, and we develop a new nonstandard finite difference scheme to provide several simulations with and without mosquito control. Our preliminary study shows that an early use of a combination of massive spraying and mechanical control (like the destruction of breeding sites) can be efficient, to stop or contain the propagation of Chikungunya infection, with a low impact on the environment.

Polyphasic characterization of xanthomonads pathogenic to members of the #Anacardiaceae# and their relatedness to species of #Xanthomonas#

We have used amplified fragment length polymorphism (AFLP), multilocus sequence analysis (MLSA) and DNA-DNA hybridization for genotypic classification of Xanthomonas pathovars associated with the plant family Anacardiaceae. AFLP and MLSA results showed congruent phylogenetic relationships of the pathovar mangiferaeindicae (responsible for mango bacterial canker) with strains of Xanthomonas axonopodis subgroup 9.5. This subgroup includes X. axonopodis pv. citri (synonym Xanthomonas citri). Similarly, the pathovar anacardii, which causes cashew bacterial spot in Brazil, was included in X. axonopodis subgroup 9.6 (synonym Xanthomonas fuscans). Based on the thermal stability of DNA reassociation, consistent with the AFLP and MLSA data, the two pathovars share a level of similarity consistent with their being members of the same species. The recent proposal to elevate X. axonopodis pv. citri to species level as X. citri is supported by our data. Therefore, the causal agents of mango bacterial canker and cashew bacterial spot should be classified as pathovars of X. citri, namely X. citri pv. mangiferaeindicae (pathotype strain CFBP 1716) and X. citri pv. anacardii (pathotype strain CFBP 2913), respectively. Xanthomonas fuscans should be considered to be a later heterotypic synonym of Xanthomonas citri.

Survival of Xanthomonas axonopodis pv. citri in Leaf Lesions Under Tropical Environmental Conditions and Simulated Splash Dispersal of Inoculum.

ABSTRACT Asiatic citrus canker (ACC) is a severe disease of several citrus species and hybrids in many tropical and subtropical areas. Populations of Xanthomonas axonopodis pv. citri in leaf and twig lesions are the most important inoculum source for secondary infections. In areas with a marked winter season (e.g., Argentina and Japan), low temperatures induce a decrease of 10(2) to 10(4) in population sizes in lesions, thus creating a discontinuity in the X. axonopodis pv. citri life cycle. The purpose of this study was to evaluate the dynamics of X. axonopodis pv. citri populations in leaf lesions exposed to the mild winter temperatures prevailing in a tropical environment. Internal X. axonopodis pv. citri population levels in Mexican lime leaf lesions reached 10(6) to 10(7) CFU lesion(-1) whatever the lesion size. These densities, however, were not strongly negatively affected by winter temperatures prevailing under experimental conditions. The estimated decrease in internal X. axonopodis pv. citri population sizes was approximately 10-fold. When exposed to 35 mm h(-1) of simulated rainfall, internal population sizes decreased over time by approximately 1 log unit for lesions 1 and 2 months old, but did not for older lesions. A microscopic examination indicated that lignin-like compounds are present in lesions more than 6 months old. The slow decrease over time of X. axonopodis pv. citri population sizes in leaf lesions may be the balanced result of defense reactions by the host at late stages of disease development, and the concomitant multiplication of the pathogen at the margin of old lesions. We conclude that the epidemiological significance of overwintered leaf lesions in the tropics is higher than that reported in other areas.

Amplified fragment length polymorphism and multilocus sequence analysis-based genotypic relatedness among pathogenic variants of Xanthomonas citri pv. citri and Xanthomonas campestris pv. bilvae

Three pathogenic variants (i.e. pathotypes) have been described within Xanthomonas citri pv. citri, the causal agent of Asiatic citrus canker. Pathotype A strains naturally infect a wide range of Citrus species and members of some related genera. In contrast, pathotypes A* and A(w) have narrow host ranges within the genus Citrus and have been isolated from Mexican lime (Citrus aurantifolia L.) and from Mexican lime and alemow (Citrus macrophylla L.), respectively. We used amplified fragment length polymorphism (AFLP) and multilocus sequence analysis (MLSA) based on four partial housekeeping gene sequences (atpD, dnaK, efp and gyrB ) for the genotypic classification of Xanthomonas citri pv. citri and the poorly characterized citrus pathogen Xanthomonas campestris pv. bilvae. A Mantel test showed that genetic distances derived from AFLP and MLSA were highly correlated. X. campestris pv. bilvae showed a close relatedness to the type strain of X. citri, indicating that this pathovar should be reclassified as X. citri pv. bilvae. All pathotype A* and A(w) strains were most closely related to X. citri pv. citri strains with a wide host range (pathotype A), confirming previous DNA-DNA hybridization data. Pathotype A(w) should be considered a junior synonym of pathotype A* on the basis of pathogenicity tests, AFLP, MLSA and PCR using pathovar-specific primers. Evolutionary genome divergences computed from AFLP data suggested that pathotype A* (including A(w) strains) is a group of strains that shows a wider genetic diversity than pathotype A.

Seasonality of Human Leptospirosis in Reunion Island (Indian Ocean) and Its Association with Meteorological Data

Background Leptospirosis is a disease which occurs worldwide but particularly affects tropical areas. Transmission of the disease is dependent on its excretion by reservoir animals and the presence of moist environment which allows the survival of the bacteria. Methods and Findings A retrospective study was undertaken to describe seasonal patterns of human leptospirosis cases reported by the Centre National de Références des Leptospiroses (CNRL, Pasteur Institute, Paris) between 1998 and 2008, to determine if there was an association between the occurrence of diagnosed cases and rainfall, temperature and global solar radiation (GSR). Meteorological data were recorded in the town of Saint-Benoît (Météo France “Beaufonds-Miria” station), located on the windward (East) coast. Time-series analysis was used to identify the variables that best described and predicted the occurrence of cases of leptospirosis on the island. Six hundred and thirteen cases were reported during the 11-year study period, and 359 cases (58.56%) were diagnosed between February and May. A significant correlation was identified between the number of cases in a given month and the associated cumulated rainfall as well as the mean monthly temperature recorded 2 months prior to diagnosis (r = 0.28 and r = 0.23 respectively). The predictive model includes the number of cases of leptospirosis recorded 1 month prior to diagnosis (b = 0.193), the cumulated monthly rainfall recorded 2 months prior to diagnosis (b = 0.145), the average monthly temperature recorded 0 month prior to diagnosis (b = 3.836), and the average monthly GSR recorded 0 month prior to diagnosis (b = −1.293). Conclusions Leptospirosis has a seasonal distribution in Reunion Island. Meteorological data can be used to predict the occurrence of the disease and our statistical model can help to implement seasonal prevention measures.

Pathological Variations Within Xanthomonas campestris pv. mangiferaeindicae Support Its Separation Into Three Distinct Pathovars that Can Be Distinguished by Amplified Fragment Length Polymorphism.

ABSTRACT Bacterial black spot, caused by Xanthomonas campestris pv. mangiferaeindicae, is an important disease of mango (Mangifera indica). Several other plant genera of the family Anacardiaceae were described as host species for xanthomonads. We studied pathological variations among strains in a worldwide collection from several Anacardiaceae genera. Strains were classified into three pathogenicity groups. Group I strains (from the Old World) multiplied markedly in leaf tissue of mango and cashew (Anacardium occidentale). Group II strains (from Brazil) multiplied markedly in cashew leaf tissue, but not in mango. Moreover, mango leaves inoculated with group I and group II strains exhibited lesions with different morphologies, consistent with variations in symptomology previously reported on mango under field conditions. Group I strains produced black, raised lesions, consistent with the original description of the pathovar, whereas group II strains produced brownish, flat lesions. Group III strains produced a unique syndrome on ambarella (Spondias dulcis) and mombin (Spondias mombin). Based on evolutionary genome divergence derived from amplified fragment length polymorphism (AFLP) data, the three groups were genetically distinct and were related to groups 9.5, 9.6, and 9.4 of X. axonopodis identified by Rademaker, respectively. As each group was characterized by unique symptomology and/or host range, we propose that X. campestris pv. mangiferaeindicae be split into three pathovars of X. axonopodis: X. axonopodis pv. mangiferaeindicae, X. axonopodis pv. anacardii, and X. axonopodis pv. spondiae. Within pv. mangiferaeindicae sensu novo, AFLP data were consistent with that previously published for restriction fragment length polymorphism groups and suggested long-distance movement of the pathogen, likely through propagative material.

Pseudomonas aeruginosa Outbreak Linked to Mineral Water Bottles in a Neonatal Intensive Care Unit: Fast Typing by Use of High-Resolution Melting Analysis of a Variable-Number Tandem-Repeat Locus

ABSTRACT Pseudomonas aeruginosa is an opportunistic pathogen that causes nosocomial infections in intensive care units. Determining a system of typing that is discriminatory is essential for epidemiological surveillance of P. aeruginosa. We developed a method for the typing of Pseudomonas aeruginosa, namely, multiple-locus variable-number tandem-repeat (VNTR) typing with high-resolution melting analysis (HRMA). The technology was used to genotype a collection of 43 environmental and clinical strains isolated during an outbreak in a neonatal intensive care unit (NICU) that we report. Nineteen strains isolated in other departments or outside the hospital were also tested. The genetic diversity of this collection was determined using VNTR-HRMA, with amplified fragment length polymorphism (AFLP) analysis as a reference. Twenty-five and 28 genotypes were identified, respectively, and both techniques produced congruent data. VNTR-HRMA established clonal relationships between the strains of P. aeruginosa isolated during the outbreak in the NICU and proved, for the first time, the role of mineral water as the inoculum source. VNTR typing with one primer pair in association with HRMA is highly reproducible and discriminative, easily portable among laboratories, fast, and inexpensive, and it demonstrated excellent typeability in this study. VNTR-HRMA represents a promising tool for the molecular surveillance of P. aeruginosa and perhaps for molecular epidemiologic analysis of other hospital infections.

Spatial and Temporal Analyses of Bacterial Blight of Onion Caused by Xanthomonas axonopodis pv. allii

ABSTRACT Bacterial blight of onion is a severe disease, which emerged over the past decade in several onion-producing areas. This disease currently is observed in both the Old and New Worlds. Although the causative agent, Xanthomonas axonopodis pv. allii, is potentially seedborne, seed transmission and significance of seedborne initial inoculum for the development of bacterial blight of onion previously has not been assessed. This article describes experimental work designed to evaluate the biological importance of seedborne X. axonopodis pv. allii as an initial inoculum source, and examine the temporal and spatial dynamics of the disease. Over 3 years, outbreaks of bacterial blight of onion always were induced in experimental plots sown with naturally contaminated seed lots, with a contamination rate determined as 0.04%. Analyses of disease patterns indicated a likely seedborne origin for the inoculum associated with the early stages of epidemics. Spatial analyses performed with several statistical methods indicated aggregated patterns of disease incidence data. Primary foci enlarged over time, and a few distinct secondary foci sometimes were established after occurrence of wind-driven rains (with gusts up to 15 m s(-1)). Distances between primary and secondary foci ranged from less than 1 m (satellite foci) to 25 m. It remains possible that longdistance dispersal of inoculum was at least partly involved in the later stages of epidemics.

Genetic and Pathological Diversity Among Xanthomonas Strains Responsible for Bacterial Spot on Tomato and Pepper in the Southwest Indian Ocean Region

Bacterial spot of tomato and pepper, a major problem in tropical climates, can be caused by several Xanthomonas genospecies. We examined the genetic and pathological diversity of a collection of 72 strains from the southwest Indian Ocean region as part of a regional research and development program to update inventories of agricultural pests and pathogens. Xanthomonas euvesicatoria, X. perforans, X. gardneri, and X. vesicatoria were identified in our strain collection. The identification of strains at the species level was consistently achieved by amplified fragment length polymorphism (AFLP) and multilocus sequence analysis (MLSA). Overall, X. euvesicatoria was the species recovered prevalently. MLSA data based on four housekeeping genes identified two to three sequence types per genospecies. It suggested that sequence variations primarily consisted of synonymous mutations, although a recombination event spanning several hundred nucleotides was detected for some strains of X. euvesicatoria on the atpD gene coding for the F1-F0-ATPase β subunit. The pathogenicity of strains was consistent with data found in the literature. Some pathological variations were primarily observed among strains identified as X. euvesicatoria. This study provides the first ever comprehensive description of the status of Xanthomonas species that cause bacterial spot of tomato and pepper in the southwest Indian Ocean region.