Philippe Sabatier

Horse-, bird-, and human-seeking behavior and seasonal abundance of mosquitoes in a West Nile virus focus of Southern France

Abstract After 35 yr of disease absence, West Nile virus (family Flaviviridae, genus Flavivirus, WNV) circulation has been regularly detected in the Camargue region (southern France) since 2000. WNV was isolated from Culex modestus Ficalbi, which was considered the main vector in southern France after horse outbreaks in the 1960s. Recent WNV transmissions outside of the Cx. modestus distribution suggested the existence of other vectors. To study potential WNV vectors, horse- and bird-baited traps and human landing collections of mosquitoes were carried out weekly from May to October 2004 at two Camargue sites: one site in a wet area and the other site in a dry area, both chosen for their past history of WNV transmission. At the wet site, the most abundant species in bird-baited traps were Culex pipiens L. and Cx. modestus; both species also were found in lower proportions on horses and humans. The most abundant species in horse-baited traps and human landing collections were Aedes caspius (Pallas), Aedes vexans (Meigen), and Anopheles hyrcanus (Pallas) sensu lato; some of these species were occasionally collected with avian blood at the end of the summer. Anopheles maculipennis Meigen sensu lato was an abundant horse feeder, but it was rarely collected landing on human bait and never contained avian blood. At the dry site, Cx. pipiens was the most abundant species in bird- and horse-baited traps. The seasonal and circadian dynamics of these species are analyzed, and their potential in WNV transmission in Camargue discussed.

Rift Valley fever in West Africa: the role of space in endemicity.

Rift Valley fever is an endemic vector‐borne disease in West Africa, which mainly affects domestic ruminants and occasionally humans. The aetiological mechanisms of its endemicity remain under debate. We used a simple spatially explicit model to assess the possibility of endemicity without wild animals providing a permanent virus reservoir. Our model takes into account the vertical transmission in some mosquito species, the rainfall‐driven emergence of their eggs and local and distant contacts because of herd migration. Endemicity without such a permanent virus reservoir would be impossible in a single site except when there is a strictly periodic rainfall pattern; but it would be possible when there are herd movements and sufficient inter‐site variability in rainfall, which drives mosquito emergence.

Spatial clustering of pulmonary tuberculosis and impact of the care factors in Antananarivo City

Objectiveu2002 To analyse the spatial distribution of TB in Antananarivo and investigate risk factors.

Bayesian mapping of pulmonary tuberculosis in Antananarivo, Madagascar

BackgroundTuberculosis (TB), an infectious disease caused by the Mycobacterium tuberculosis is endemic in Madagascar. The capital, Antananarivo is the most seriously affected area. TB had a non-random spatial distribution in this setting, with clustering in the poorer areas. The aim of this study was to explore this pattern further by a Bayesian approach, and to measure the associations between the spatial variation of TB risk and national control program indicators for all neighbourhoods.MethodsCombination of a Bayesian approach and a generalized linear mixed model (GLMM) was developed to produce smooth risk maps of TB and to model relationships between TB new cases and national TB control program indicators. The TB new cases were collected from records of the 16 Tuberculosis Diagnostic and Treatment Centres (DTC) of the city from 2004 to 2006. And five TB indicators were considered in the analysis: number of cases undergoing retreatment, number of patients with treatment failure and those suffering relapse after the completion of treatment, number of households with more than one case, number of patients lost to follow-up, and proximity to a DTC.ResultsIn Antananarivo, 43.23% of the neighbourhoods had a standardized incidence ratio (SIR) above 1, of which 19.28% with a TB risk significantly higher than the average. Identified high TB risk areas were clustered and the distribution of TB was found to be associated mainly with the number of patients lost to follow-up (SIR: 1.10, CI 95%: 1.02-1.19) and the number of households with more than one case (SIR: 1.13, CI 95%: 1.03-1.24).ConclusionThe spatial pattern of TB in Antananarivo and the contribution of national control program indicators to this pattern highlight the importance of the data recorded in the TB registry and the use of spatial approaches for assessing the epidemiological situation for TB. Including these variables into the model increases the reproducibility, as these data are already available for individual DTCs. These findings may also be useful for guiding decisions related to disease control strategies.

A two-patch model of Gambian sleeping sickness: application to vector control strategies in a village and plantations.

A compartmental model is described for the spread of Gambian sleeping sickness in a spatially heterogeneous environment in which vector and human populations migrate between two patches: the village and the plantations. The number of equilibrium points depends on two summary parameters: gr the proportion removed among human infectives, and R0, the basic reproduction number. The origin is stable for R0 <1 and unstable for R0 >1. Control strategies are assessed by studying the mix of vector control between the two patches that bring R0 below 1. The results demonstrate the importance of vector control in the plantations. For example if 20 percent of flies are in the village and the blood meal rate in the village is 10 percent, then a 20 percent added vector mortality in the village must be combined with a 9 percent added mortality in the plantations in order to bring R0 below 1. The results are quite insentive to the blood meal rate in the village. Optimal strategies (that minimize the total number of flies trapped in both patches) are briefly discussed.

MODELLING SYNERGISTIC EFFECTS OF TWO TOXIC AGENTS IN THE HONEYBEE

In previous experiments, a synergistic lethal effect on the honeybee has been demonstrated with two toxic agents: deltamethrin and prochloraz. The toxic agents were applied simultaneously or sequentially. Deltamethrin is metabolized by esterase and by cytochrome P-450-dependent monooxygenase. Prochloraz is a potent competitive inhibitor of cytochrome P-450. An attractive hypothesis explaining this synergic toxicity is that the persistence of deltamethrin is due to inhibition of the oxidative metabolism of deltamethrin by prochloraz. We tested this hypothesis using a mathematical model. A compartmental model in continuous time was constructed to represent this synergy by analyzing the degradation of toxic agents. This pharmacokinetic model was based on differential equations, using numerical resolution and Runge-Kutta’s integration method. When applied to a mixture of toxic agents, this model implies that the cumulative mortality of both compounds might be delayed compared to the mortality from treatment with toxic agents used separately. The hypothesis of metabolic synergy is compatible with experimental data only if the half-life of deltamethrin is short. The data from the model and the experimental data, diverged when deltamethrin was administered before prochloraz, which suggests that the phenomenon is more complex than expected.

Qualitative interaction between the observer and the observed in veterinary epidemiology.

The phenomenon of qualitative interaction between the research activity and the studied object exists in veterinary epidemiology, as in all observation-based disciplines. Observational studies carried out under farm conditions strongly involve the farmers and the farms usual advisors in the data collection. This leads to various biological and sociological interactions between the observer and the observed that can modify the results of the study. Some methodological considerations are proposed in order to consider such qualitative interaction, from the designing of the study through to the interpretation of the results. Examples are presented to illustrate these ideas.

Modelling and determination of the transmission contact rate for contagious bovine pleuropneumonia.

Contagious bovine pleuropneumonia (CBPP) is a cattle respiratory disease that represents one of the major threats to cattle health and production in sub-Saharan Africa. The transmission contact rate of CBPP plays a key role in the spreading dynamics of the disease. We have developed an approach based on the combination of a SEIR model describing the spread of CBPP with the dynamic of seroconversion to determine the transmission contact rate for CBPP. This method has been subsequently applied to serological diagnostic data obtained from an experimental vaccine trial. As a result, we find that the transmission contact rates for subclinical, clinical and chronic infective states are respectively, 0.084/N, 0.45 and 0.14/N per animal per day, where N is the herd population size, and the basic reproductive number corresponding to this trial (N=28) is R0=27.

Time-delay dynamics for contagious bovine pleuropneumonia.

Modelling of contagious disease usually employs compartmental SEIR-like models where the waiting times in respective compartments are exponentially distributed. In this paper, we are interested in investigating how the distributions of sojourn times in infective compartments affect the dynamics and persistence of the contagious bovine pleuropneumonia, a chronic respiratory disease of cattle. Two kinds of extreme distributions of the sojourn times are considered: a Dirac delta-function and truncated Gaussian function leading to a model with (non-constant) delay and the classical exponential distribution that stands for a model without delay. Expressions of the basic reproductive numbers are derived and dynamical behaviours are discussed for the three models. It is found that the spreading of disease exhibits wave-like oscillations for the time-delay dynamics. In contrast, the disease appears to last longer when the spreading is described by the classical dynamics without delay. Subsequently, the time-delay dynamics turns out to be more appropriate for the description of an experimental epidemic of CBPP.

Modelling based method for pharmacokinetic hypotheses test

The aim of this work is to propose methods to test mechanism of synergy of toxic agents in bees. A synergy between prochloraz, an imidazole fungicide, and deltamethrin, a pyrethroid insecticide, was demonstrated experimentally. The hypothesis is that prochloraz modifies the penetration or the metabolism of deltamethrin. This hypothesis is tested using a pharmacokinetic box model. A previous experimental work showed that bee instantaneous mortalities were higher, from the time t1 to the time t2 after spraying, in groups sprayed with deltamethrin at dose D0 in the presence of prochloraz (Δ+P) than in those sprayed with deltamethrin alone at a dose α time as high (αΔ). We postulate that accrued mortality is proportional to the cumulated internal deltamethrin (ID2). ID2 of treatment (Δ+P) had to be greater than ID2 of treatment (αΔ) during the period from t1 to t2 so that the hypothesis would be consistent with the experimental data. The limit, for which the hypothesis is conceivable, is the ID2(αΔ) = ID2(Δ+P) curve. We study, in particular, the asymptotic behaviour of the limit curve when different parameters of the kinetic model tend to 0 or ∞. These limits allow to verify quickly and easily whether a mechanism is conceivable or not As the limits are calculated with algebraic values, the test can be used for other synergies.