Wolbachia modulates Chikungunya replication in Aedes albopictus.

The Aedes albopictus mosquito has been involved as the principal vector of recent major outbreaks due to the chikungunya virus (CHIKV). The species is naturally infected by two strains of Wolbachia (wAlbA and wAlbB). Wolbachia infections are thought to have spread by manipulating the reproduction of their hosts; cytoplasmic incompatibility is the mechanism used by Wolbachia to invade natural populations of many insects including Ae. albopictus. Here, we report a study on the effects of removing Wolbachia from Ae. albopictus on CHIKV replication and examine the consequences of CHIKV infection on some life-history traits (survival and reproduction) of Wolbachia-free Ae. albopictus. We found that Wolbachia-free mosquitoes maintained a highly heterogeneous CHIKV replication compared to Wolbachia-infected individuals. In Wolbachia-infected Ae. albopictus, the regular increase of CHIKV followed by a steady viral load from day 4 post-infection onwards was concomitant with a decline in Wolbachia density. This profile was also detected when examining the two key organs for viral transmission, the midgut and the salivary glands. Moreover, Wolbachia-free Ae. albopictus was not altered in life-history traits such as survival, oviposition and hatching characteristics whether infected or not with CHIKV. We found that Wolbachia is not essential for viral replication, its presence could lead to optimize replication from day 4 post-infection onwards, coinciding with a decrease in Wolbachia density. Wolbachia may regulate viral replication in Ae. albopictus, with consequences on survival and reproduction.

[Poliovirus and apoptosis]. / Poliovirus et apoptose.

Poliovirus is the causal agent of paralytic poliomyelitis. Flaccid paralysis characteritic of poliomyelitis result from the destruction of motor neurons, the specific target cells of poliovirus in the central nervous system (CNS). The development of new animal and cell models has allowed the key steps of the pathogenesis of poliomyelitis to be investigated at the molecular level. In particular, it has been shown that poliovirus-induced apoptosis is an important component of the tissue injury in the CNS of infected mice that leads to paralysis. In this review, the molecular biology of poliovirus and the pathogenesis of poliomyelitis will be briefly described, and then several models of poliovirus-induced apoptosis will be considered ; the role of the cellular receptor of poliovirus, CD155, in the modulation of apoptosis will also be addressed.

Population genetic structure and competence as a vector for dengue type 2 virus of Aedes aegypti and Aedes albopictus from Madagascar.

Starch gel electrophoresis was used to assess the polymorphism of 7 isoenzymes in single mosquitoes (field-collected F0 or F1 generation) for Aedes albopictus (8 strains) from northern Madagascar. Mosquitoes of the F2 generation (3 strains of Aedes aegypti and 10 strains of Ae. albopictus) were tested for oral susceptibility to dengue type 2 virus. Aedes aegypti was less susceptible to viral infection than Ae. albopictus. The genetic differentiation was less high between Ae. albopictus populations collected in agglomerations connected by highly frequented roads, indicating that human ground transportation favors mosquito dispersal. These results have implications for the ecology, pattern of migration, and relative importance in epidemic transmission of dengue viruses between the 2 Aedes species.

Aedes aegypti in French Guiana: susceptibility to a dengue virus.

Twenty-seven samples of Aedes aegypti (F1 generation) from French Guiana were tested for their susceptibility to dengue serotype 2 virus. Very high infection rates were observed by indirect fluorescent antibody (IFA) test. Ae. aegypti samples were pooled according to two groups: the first group (N=10) represented mosquitoes from the urbanized area of Cayenne and surroundings, and the second group (N=17) corresponded to mosquitoes collected in the countryside. Infection rates were found to be similar in these two cases. These findings are discussed in relation with the history of Ae. aegypti in this part of the world.

Aedes aegypti in Tahiti and Moorea (French Polynesia): isoenzyme differentiation in the mosquito population according to human population density.

Genetic differences at five polymorphic isoenzyme loci were analyzed by starch gel electrophoresis for 28 Aedes aegypti samples. Considerable (i.e., high Fst values) and significant (i.e., P values >10(-4)) geographic differences were found. Differences in Ae. aegypti genetic structure were related to human population densities and to particularities in mosquito ecotopes in both Tahiti and Moorea islands. In highly urbanized areas (i.e., the Papeete agglomeration), mosquitoes were highly structured. Recurrent extinction events consecutive to insecticidal treatments during dengue outbreaks tend to differentiate mosquito populations. In less populated zones (i.e., the east coast of Moorea and Tahiti), differences in ecotope characteristics could explain the lack of differentiation among mosquitoes from rural environments such as the east coast of Tahiti where natural breeding sites predominate. When the lowest populated zones such as Tahiti Iti and the west coast of Moorea are compared, mosquito are less differentiated in Moorea. These results will be discussed in relation to the recent findings of variation in mosquito infection rates for dengue-2 virus.

Aedes albopictus from Albania: a potential vector of dengue viruses.

Aedes albopictus collected in Durazzo, the main port of Albania, were tested for oral susceptibility to dengue type 2 virus and their infection rates were compared to those of an Aedes aegypti strain (Paea) and another strain of Ae. albopictus (Tananarive). Infection rates for the Albanian Ae. albopictus were dose dependent, ranging from 38.9 +/- 13.6% to 85.1% with the titer of the meal increasing from 10 x 8.1 to 10 x 9.1 50% mosquito infectious doses (MID50)/ml. The percentage of infected females was lower for the Ae. albopictus Durazzo strain than for the Ae. aegypti Paea strain: 38.9 +/- 13.6% compared with 92.4 +/- 4.9% for a meal of 10 x 8.1 MID50/ml, respectively. However, the difference was less when the titer of the meal was increased: 85.1% compared with 100% for a meal of 10 x 9.1 MID50/ml, respectively. The infection rate was also lower for the Durazzo strain than for the Tananarive strain of Ae. albopictus. The degree of viral replication in infected females was not significantly different in the 3 strains tested and we were able to demonstrate the ability of females from the Durazzo strain to transmit the virus in the course of a blood meal. Our results lead us to conclude that Ae. albopictus from Albania could serve as a vector for dengue virus.

[Genetic control of vectorial competence in Aedes mosquitoes]. / Contrôle génétique de la compétence vectorielle des moustiques du genre Aedes.

The transmission of pathogens by arthropods is dependent on the relationships that exist between the pathogen, the invertebrate host (the vector) and the vertebrate host, each of which is influenced by environmental variations. Particular attention is given to the knowledge of intrinsic factors and the mechanisms controlling the ability of vectors to transmit pathogens (viruses or parasites). Polymorphism in the expression of susceptibility to oral infection has been shown to occur among geographical samples of mosquitoes. It has been proven that intraspecific variations in vector competence are controlled by one or more genes and expressed in variable proportions within a mosquito population. Recent advances in molecular biology have facilitated accessibility of nucleic acid sequence data. These new techniques allow one to analyse the genotype distribution within and among populations. Population genetic studies are currently used to understand the evolution of species differentiation and provide indications on genetic relationship among field vector populations. Estimations of gene flow with respect to vector capacity have provided rich insight into vector species complexes. Knowledge of intraspecies variation is important for the understanding of vector transmission, disease epidemiology and disease control. In this article, two examples are presented to illustrate the contribution of population genetic studies to the understanding of epidemiology of arthropod-borne diseases: Aedes polyneniensis, a vector of human lymphatic filariasis and Aedes aegypti, the vector of dengue viruses.

Density of deer in relation to the prevalence of Borrelia burgdorferi s.l. in Ixodes ricinus nymphs in Rambouillet forest, France.

The Rambouillet Forest, a Lyme disease-endemic area near Paris, France, was surveyed from September 1994 to October 1995 to determine the risk periods and zones for humans. Firstly, during the period of Ixodes ricinus activity, abundance of nymphs is greater in spring than in autumn. Secondly, we observed significant variation in nymphal abundance between zones according to the density of cervids. The polymerase chain reaction (PCR) was used to detect DNA of Borrelia burgdorferi sensu lato in 461 unfed nymphs. DNA was detected in 38 nymphs (8.2%). By genospecific PCR based on the OspA gene, we detected the three pathogenic spirochetes with occurrences of 10.3, 31.1 and 58.6 for B. burgdorferi s.s., Borrelia garinii and Borrelia afzelii, respectively, indicating that B. afzelii is probably the main Borrelia species in the Rambouillet Forest. Finally, 11.5% of positive nymphs exhibited a double infection. Infection rates of I. ricinus nymphs by B. burgdorferi s.l. were not significantly different throughout the year for a given area, indicating that the risk periods of acquiring Lyme disease are mainly linked to nymph activity and correspond to spring and autumn. Likewise infection rates of nymphs were not significantly different between zones with a high density of deer (more than 100 animals per 100 ha) and zones with lower deer density (less than 20 animals per 100 ha). In addition to the role of deer as an amplifier of tick populations, these data indicate that zones with a high density of cervids should be considered as higher risk areas.

Variation in oral susceptibility to dengue type 2 virus of populations of Aedes aegypti from the islands of Tahiti and Moorea, French Polynesia.

Twenty three samples of Aedes aegypti populations from the islands of Tahiti and Moorea (French Polynesia) were tested for their oral susceptibility to dengue type 2 virus. The high infection rates obtained suggest that the artificial feeding protocol used was more efficient than those previously described. Statistical analysis of the results allowed us to define two distinct geographic areas on Tahiti with respect to the susceptibility of Ae. aegypti: the east coast, with homogeneous infection rates, and the west coast, with heterogeneous infection rates. No geographic differences could be demonstrated on Moorea. The possible mechanisms of this phenomenon are discussed in connection with recent findings on the variability of susceptibility of Ae. aegypti to insecticides.

Replication of dengue type 2 virus in Culex quinquefasciatus (Diptera: Culicidae).

We were able to infect Culex quinquefasciatus by the parenteral route with dengue virus type 2. The percentage of mosquitoes infected was dose dependent and we obtained a rate of 45.6% infected Cx. quinquefasciatus when a 10(5.9) MID50 (mosquito infectious dose for 50% of the individuals as measured in Aedes aegypti) of dengue virus type 2 per mosquito was used. Infection was detected by an immunofluorescent assay performed on mosquito head squashes 14 days after infection. The replication of dengue virus in Cx. quinquefasciatus was either at a very low level of magnitude or generated a large number of noninfectious particles since the triturated bodies of infected Cx. quinquefasciatus did not infect Ae. aegypti mosquitoes when inoculated parenterally. We were unable to infect Cx. quinquefasciatus females orally with an artificial meal that infected 100% of Ae. aegypti females. These findings lead us to agree with the consensus that Cx. quinquefasciatus should not be considered a biological vector of dengue viruses.

Aedes aegypti in Ho Chi Minh City (Viet Nam): susceptibility to dengue 2 virus and genetic differentiation.

Aedes aegypti is the principal vector of dengue viruses, responsible for a viral infection that has become a major public health concern in Asia. In Viet Nam, dengue haemorrhagic fever was first detected in the 1960s and is now a leading cause of death in childhood. We studied the variability in competence of Ae. aegypti as a vector for dengue 2 virus and genetic differentiation in this mosquito species. Twenty mosquito samples collected in 1998 in Ho Chi Minh City were subjected to oral infection and isoenzyme polymorphism analysis by starch gel electrophoresis. Ae. aegypti populations from the centre of Ho Chi Minh City were genetically differentiated and their infection rates differed from those of populations from the commuter belt. These results have implications for insecticidal control during dengue outbreaks.

Short report: microsatellite sequences as markers for population genetic studies of the mosquito Aedes aegypti, the vector of dengue viruses.

We report the isolation of microsatellites from an enriched library of genomic repeated sequences, using a biotin-labeled oligonucleotide bound to streptavidin-coated magnetic particles. Four microsatellites were obtained from a partial library of 120 recombinant clones. This more efficient and rapid method to obtain these specific repeated sequences is preferred to the conventional isolation procedure based on the construction of a genomic library. Microsatellite markers would be promising molecular tools for the study of genetic variability of mosquito populations. Analyses of genetic structure and gene flow would provide information on the distance, direction and rate of dispersal of genes in Aedes aegypti populations. Knowledge on gene dispersal patterns is required to develop vector control strategies.

[Oral receptivity of Aedes aegypti formosus from Franceville (Gabon, central Africa) for type 2 dengue virus]. / Réceptivité orale d'Aedes aegypti formosus de Franceville (Gabon, Afrique centrale) pour le virus de la dengue type 2.

Dengue is widely distributed in the tropics but epidemic activity was rarely reported in Africa before the 1980's. In the past 15 years, increased epidemic dengue fever has been reported both in East and West Africa, raising concern about the ability of local populations of Aedes aegypti to transmit dengue viruses. Ae. aegypti is present in two forms in Africa: Ae. aegypti aegypti and Ae. aegypti formosus. This latter form, much darker, was not originally a local species but is now colonizing artificial breeding sites within cities. We have been able to demonstrate the oral susceptibility for dengue type 2 virus of Ae. aegypti formosus collected in Franceville, Gabon (Central Africa). However, these mosquitoes sampled exhibited lower infection rates than those of a control colony of Ae. aegypti aegypti originating from French Polynesia.

[Lyme borreliosis, emergent disease linked with the environment]. / La borréliose de Lyme, maladie émergente liée à l'environnement.

After a short historical presentation of the discovery of the pathogen and its vector, the authors present the current data on bacterial and acarologic taxonomy. Then they describe their results to assess the mechanisms of circulation of the bacteria in the forests of Ile-de-France, particularly in the forest of Rambouillet. The combined study of abundance and infection frequency of the vectors, small mammals and cervids leads to the characterization of periods and areas of higher risk. The risk periods correlate with high density of I. ricinus nymphs. The risk areas correspond to those of high density of cervids. The role of reservoir of small mammals is confirmed, to the one of large mammals, so debated, is clearly demonstrated.