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  Director : RODHAIN François (frodhain@pasteur.fr)


  abstract

 

The research work deals with various aspects of the relationships between infectious microorganisms and their arthropod vectors. The objectives are : - a better view of epidemiology of vector-borne diseases, a deeper knowledge of parasite-vector relationships with reference to a possible action on vector competence. Some of our programmes involve studies at molecular or cellular levels, while others are dealing with inter-organisms relationships within natural ecosystems. Our research programmes bear upon some particular aspects of Anopheles-Plasmodium relationships, population genetics of domestic Aedes mosquitoes and the variability of their oral susceptibility for dengue and yellow fever viruses, epidemiology of Lyme borreliosis.



  report

cale

I/Molecular analysis of Anopheles gambiae receptivity to Plasmodium falciparum (Catherine BOURGOUIN)

Anopheles gambiae is the main vector of Plasmodium falciparum in Africa. This host-parasite combination is one of the most effective to maintain malaria transmission.

During its sporogonic development in the mosquito, Plasmodium parasite encounters two barriers, successively : the peritrophic matrix and the midgut epithelium. In addition, it suffers major population losses before reaching the oocyst stage on the mosquito midgut wall. The mosquito immune surveillance system might contribute to these losses. On the other hand as few as 10 gametocytes are sufficient for establishing an infection, suggesting the P. falciparum might be able to evade the mosquito immune system.

We analysed the expression pattern of nine genes, belonging to different pathways of the mosquito immunity, during the sporogonic development of P. falciparum in An. gambiae. Mosquitoes were infected by feeding on blood of parasite carriers, in Cameroon. Gene expression was quantified by real time PCR. Ingestion of P. falciparum parasites activates the mosquito immune response in the midgut, and triggers also a systemic response. Each gene exhibits a finely tuned spatio-temporal expression. Only three genes are specifically regulated by ingestion of gametocytes, the infectious stage of the parasite. The expression pattern of the studied genes is substantially different from what has been described in the model system consisting of An. gambiae and the rodent parasite P. berghei, emphasizing the co-adaptation of this host-parasite system (Tahar et al. submitted).

In addition, we pursue the functional analysis of An. gambiae genes previously isolated by a " differential display " screening. Midgut expression of these genes was modified upon exposure to invasive stages of P. falciparum (Bonnet et al., 2001). Antibodies have been produced against the recombinant proteins and are currently used in Western blot and immunocytochemistry experiments.

II/ Population genetics of Aedes mosquitoes and vector competence for dengue viruses (Anna-Bella FAILLOUX)

As the efficiency of disease transmission may depend upon the geographic origin of vector populations, studies on the genetic variation of mosquito populations in relation with transmission of arboviruses have been initiated. Population genetic approaches as well as phylogenetic studies of Ae. aegypti and Ae. albopictus populations are accounted for a better understanding of dengue and yellow fever transmission.

1. Population genetic studies in relation with dengue and yellow fever transmission

Dengue and yellow fever are distributed throughout a large part of tropical countries. The Southeast Asian region is regularly subject to severe dengue epidemics with occurrence of dengue haemorrhagic fever and fatal cases. Changes in the pattern of dengue transmission from epidemicity to hyperendemicity are in part correlated with the replacement of the indigenous mosquito species, Aedes albopictus by Ae. aegypti. A network consisting of medical entomologists from Cambodia (Phnomh Penh), Vietnam (Ho Chi Minh City, Nha Trang, Hanoi) and Thailand (Chiang Mai and Bangkok) has been initiated. Our results showed that : (1) Ae. aegypti genetic differentiation was mainly modulated by human activities (insecticide treatments, water storage practices, small containers accumulated close to human habitations, (2) genetic structure as well as vector competence of Ae aegypti populations were submitted to temporal constraints and vary according to seasons, and (3) oral receptivity of a given mosquito species to dengue and yellow fever viruses (i.e. its ability to get infected and to replicate virus) seems to be subject to variations depending on the competitive interaction between Ae. aegypti and Ae. albopictus. Besides, in the Americas, a co-circulation of yellow fever and dengue serotypes has been also detected. The sequence of events associated with the recent changing epidemiology of dengue was nearly similar to that occurring in Southeast Asia in the past. In the New World, campaigns to control urban epidemics of yellow fever succeeded to nearly eradicate Ae. aegypti from the Americas in the 1950-60s. Since then, with relaxation of treatments in the early 1970s, Ae.aegypti reappeared and Ae. Albopictus has been introduced. Thus, dengue and yellow fever become now a threat in the Americas. To better understand dengue and yellow fever transmission, a network composed of entomologists from Venezuela, French Guiana, Brazil and French West Indies has been set up. At present, entomological studies are focussed on Brazil where dengue and yellow fever viruses co-circulate. Our results on the capacity of Ae. Aegypti and Ae.albopictus to transmit these two arboviruses showed that (1) Ae. aegypti populations are highly susceptible do dengue viruses but showed low infection rates towards yellow fever and (2) Ae. albopictus was less susceptible to dengue viruses and behaved also as a poor vector for the yellow fever virus. These findings were discussed in the view of the changing pattern of yellow fever in Brazil. In the other hand, as dengue is not considered as a public health problem in Africa on contrary to Southeast Asia, and yellow fever epidemics currently observed in Africa have never been reported in Asia, the vectorial efficiency of the two Ae. aegypti forms (the rural formosus form native from Africa and the domestic widespread aegypti form) has been investigated. The formosus form showed low efficiency towards dengue viruses whereas the aegypti form showed high levels of vector competence.

2. Phylogenetic approaches to explain Aedes mosquito distributions

Phylogenetic analyses based on mitochondrial and genomic gene sequences of species in the Ae. albopictus sub-group and in the Aedes scutellaris sub-group have been carried out. This program will give us indications about history of mosquito migrations and the origin of mosquito species introduction such as what has been observed recently with Aedes albopictus in France.

Finally, studies on the molecular basis of Ae. aegypti and arbovirus interactions, and on the virus genome evolution during replication in mosquitoes have been initiated in 2000 through a Transversal Research Program. For such purpose, mosquitoes were infected with one or several viral serotypes produced in our laboratory and virus replication was further followed. Experiments are still in progress.

III/ Eco-epidemiology of Lyme disease (Claudine PEREZ-EID)

Lyme disease, the most common vector-borne disease in Europe as in North America, is our main research activity. We focus our work on two goals : the vector and the transmitted bacteria, and human incidence of the disease in France.

In Europe, Ixodes ricinus, the vector of Lyme disease, transmits the three species of the Borrelia burgdorferi complex responsible for human cases. Each of the three species seems to be associated with specific clinical manifestations depending on the tropism of the Borrelia species for a specific organ. It is therefore of interest to know the respective geographical distribution of each Borrelia species. Such a study needs to be done among the vector and not in humans since (1) the identification of the Borrelia species is easier in the vector, (2) few biological specimens are available in humans, (3) the organ which is most likely to allow the isolation of the Borrelia is not known in human and, finally, (5) because it is very difficult to conclude on the geographical origin of a Borrelia isolated in human. In 2001, we collected ticks on the field in Britany, the most occidental area of France and Europe, using a method allowing statistic analysis of the results. We used GPS maps together with maps showing the vegetation pattern of the area to choose the forests, then cut these forests into 100 ha squares with the help of the " Illustrator " software and, finally, choose randomly a representative number of squares using the " Epi-info " software. In these parcels, ticks were collected every 10 m on a 150 m transect, a methodology previously adjusted by our team, to evaluate their density. The infectious status of ticks for Borrelia was evaluated individually using PCR technique.

The incidence of Lyme disease in France is presently unknown, the results from older studies varying from 16.4 to 40 cases for 100 000 inhabitants. Our goal is to obtain more accurate data in order to be able to better know the incidence and to detect possible differences in the incidence and in the symptoms of the disease in different geographical areas. For this purpose we are performing a retrospective study based on serological analysis requests received by the Pasteur-Cerba laboratory (one of the two major medical analysis laboratories in France) and a prospective study which will start in 2002 with a network of physicians (from private practice and hospital) working as generalists or in different specialties such as dermatologists, neurologists, ophthalmologists, cardiologists, pediatricians.



  publications

puce Publications of the unit on Pasteur's references database


  personnel

  Office staff Researchers Scientific trainees Other personnel
 

DELACOURTIE Claudine cdelac@pasteur.fr

RODHAIN François, I.P., Head of Unit, frodhain@pasteur.fr

BOURGOUIN Catherine, I.P., researcher, cabourg@pasteur.fr

FAILLOUX Anna-Bella, I.P., researcher, afaillou@pasteur.fr

NPEREZ-EID Claudine, I.P., researcher, cperez@pasteur.fr

GARRIGUES Thomas, PhD student

HUBER Karine, PhD student

LAVAZEC Catherine, PhD student

PAUPY Christophe, PhDstudent

TAHAR Rachida, Post-doc

AYAD Nadia, technicienne d’animalerie, nayad@pasteur.fr

FERQUEL Elisabeth, medical doctor

JACQUES Jean-Claude, technician, jjacques@pasteur.fr

MOUSSON Laurence, technician, lmousson@pasteur.fr

THIERY Isabelle, engineer, ithiery@pasteur.fr

VAZEILLE Marie-Christine, engineer, vazeille@pasteur.fr

VILLERET Régine, technician, villeret@pasteur.fr


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