An overview of the evolution of overproduced esterases in the mosquito Culex pipiens. (1/958)

Insecticide resistance genes have developed in a wide variety of insects in response to heavy chemical application. Few of these examples of adaptation in response to rapid environmental change have been studied both at the population level and at the gene level. One of these is the evolution of the overproduced esterases that are involved in resistance to organophosphate insecticides in the mosquito Culex pipiens. At the gene level, two genetic mechanisms are involved in esterase overproduction, namely gene amplification and gene regulation. At the population level, the co-occurrence of the same amplified allele in distinct geographic areas is best explained by the importance of passive transportation at the worldwide scale. The long-term monitoring of a population of mosquitoes in southern France has enabled a detailed study to be made of the evolution of resistance genes on a local scale, and has shown that a resistance gene with a lower cost has replaced a former resistance allele with a higher cost.  (+info)

Replication of dengue type 2 virus in Culex quinquefasciatus (Diptera: Culicidae). (2/958)

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.  (+info)

Can vector control play a useful supplementary role against bancroftian filariasis? (3/958)

A single campaign of mass treatment for bancroftian filariasis with diethylcarbamazine (DEC) in Makunduchi, a town in Zanzibar, United Republic of Tanzania, combined with elimination of mosquito breeding in pit latrines with polystyrene beads was followed by a progressive decline over a 5-year period in the microfilarial rate from 49% to 3%. Evidence that vector control had contributed to this long-term decline was obtained by comparison with another town, Moga, where a DEC campaign was used without vector control and where resurgence of microfilariae could be observed 3-6 years after the campaign. In Zanzibar town, treatment of 3844 wet pit latrines and cesspits with polystyrene beads reduced the adult mosquito population in houses by about 65%. Supplementary treatment of open drains and marshes with Bacillus sphaericus produced little or no additional reduction compared to a sector of the town where only pit treatment with polystyrene was carried out. The cost and effort of achieving the 65% reduction in mosquito population could hardly be justified for its impact on filariasis alone, but its noticeable impact on biting nuisance might help to gain community support for an integrated programme.  (+info)

Bancroftian filariasis on Pemba Island, Zanzibar, Tanzania: an update on the status in urban and semi-urban communities. (4/958)

Cross-sectional clinical, parasitological and entomological surveys for bancroftian filariasis were conducted in Konde, Chake Chake and Kengeja, three urban and semiurban communities on Pemba Island, and the results were compared with similar surveys done 15 years earlier. The overall prevalences of clinical manifestations among males aged 15 years or more (n = 614) was remarkably similar to those recorded 15 years earlier: elephantiasis 1.4% in 1975 and 1.1% in 1990; hydrocele, 22.4% and 21.8%, respectively. However, when the communities were compared individually, there was a reduction in the hydrocele prevalence in Konde from 22.4% to 11.5% and an increase in Kengeja from 27.0% to 35.5%. The overall microfilarial prevalence found during night blood surveys of all individuals aged 1 year or more (n = 2687) was 9.7%, compared to 14.2% recorded in 1975. The reduction was most pronounced in Konde. Of 1052 female mosquitoes caught with CDC light traps, 95% were Culex quinquefasciatus and 5% Anopheles gambiae s.l. Infective larvae of Wuchereria bancrofti were found only in the former. The filariasis situation in urban and semiurban communities on Pemba Island appears not to have changed considerably over the last 15 years.  (+info)

Use of base excision sequence scanning for detection of genetic variations in St. Louis encephalitis virus isolates. (5/958)

Twenty-two isolates of St. Louis encephalitis (SLE) virus of various geographical origins (Brazil, Argentina, Panama, Texas, Missouri, Maryland, California, and Florida) were examined for genetic variation by the base excision sequence scanning (BESS T-scan) method. A fragment was amplified in the envelope gene with the forward primer labeled in the PCR. The BESS T-scan method determined different clusters according to the profiles generated for the isolates and successfully grouped the isolates according to their geographical origins. Two major clusters, the North American cluster (cluster A) and the South and Central American cluster (cluster B), were defined. Two subgroups, the Texas-California subgroup (subgroup A1) and the Missouri-Maryland-Florida subgroup (subgroup A2), were distinguished within group A. Similarly, group B strains were subclustered to a South American subgroup (subgroup B1) and a Central American subgroup (subgroup B2). These results were consistent with those obtained by DNA sequencing analysis. The ability of the BESS T-scan method to discriminate between strains that present with high degrees of nucleotide sequence similarity indicated that this method provides reliable results and multiple applications for other virus families. The method has proven to be suitable for phylogenetic comparison and molecular epidemiology studies and may be an alternative to DNA sequencing.  (+info)

Study on the transmission threshold value of bancroftian filariasis. (6/958)

OBJECTIVE: To elucidate the transmission dynamic and epidemic trend of bancroftian filariasis occurred under the condition with no control measure taken 5 years after elimination of filariasis. METHODS: A 10-year longitudinal observation (from 1984 to 1994) was made in Huayuan Village in Shengli Township of Tancheng County, which used to be a high bancroftian filariasis-endemic area in southern part of Shandong Province. RESULTS: The microfilarial rate decreased from 0.56% before the study to 0.12% after the study and 8 out of the 9 previous microfilaria-positive cases became negative gradually. During the study period, 6 new microfilaremia cases were detected, 5 of which became negative naturally within 3 to 4 years. Eighty-eight point eight nine per cent of the detected patients with microfilaremia converted into IgG4-negative after 10 years. The natural infective rate of vectors decreased year by year and became zero by the tenth year of the study, the annual transmission potency decreased also from 3.47 to zero by the tenth year. CONCLUSIONS: It showed that under the local natural environment the biting rate representing the vector density which was obtained by capture method was from 24.1 to 52.5 person/night among the residents who did not use mosquito nets, and 13.5 to 21 person/night among the residents who used mosquito nets. The microfilarial rate of 0.56% in population with the average microfilarial density of 6.6 to 20.7 capita/60 microliters ear blood of residual microfilaria-positive patients might be considered as the terminal threshold of transmission.  (+info)

Model ecosystem evaluation of the environmental impacts of the veterinary drugs phenothiazine, sulfamethazine, clopidol, and diethylstilbestrol. (7/958)

Four veterinary drugs of dissimilar chemical structures were evaluated for environmental stability and penchant for bioaccumulation. The techniques used were (1) a model aquatic ecosystem (3 days) and (2) a model feedlot ecosystem (33 days) in which the drugs were introduced via the excreta of chicks or mice. The model feedlot ecosystem was supported by metabolism cage studies to determine the amount and the form of the drug excreted by the chicks or mice. Considerable quantities of all the drugs were excreted intact or as environmentally short-lived conjugates. Diethylstilbestrol (DES) and Clopidol were the most persistent molecules, but only DES bioaccumulated to any appreciable degree. Phenothiazine was very biodegradable; sulfamethazine was relatively biodegradable and only accumulated in the organisms to very low levels. Data from the aquatic model ecosystem demonstrated a good correlation between the partition coefficients of the drugs and their accumulation in the fish.  (+info)

Production of Cry11A and Cry11Ba toxins in Bacillus sphaericus confers toxicity towards Aedes aegypti and resistant Culex populations. (8/958)

Cry11A from Bacillus thuringiensis subsp. israelensis and Cry11Ba from Bacillus thuringiensis subsp. jegathesan were introduced, separately and in combination, into the chromosome of Bacillus sphaericus 2297 by in vivo recombination. Two loci on the B. sphaericus chromosome were chosen as target sites for recombination: the binary toxin locus and the gene encoding the 36-kDa protease that may be responsible for the cleavage of the Mtx protein. Disruption of the protease gene did not increase the larvicidal activity of the recombinant strain against Aedes aegypti and Culex pipiens. Synthesis of the Cry11A and Cry11Ba toxins made the recombinant strains toxic to A. aegypti larvae to which the parental strain was not toxic. The strain containing Cry11Ba was more toxic than strains containing the added Cry11A or both Cry11A and Cry11Ba. The production of the two toxins together with the binary toxin did not significantly increase the toxicity of the recombinant strain to susceptible C. pipiens larvae. However, the production of Cry11A and/or Cry11Ba partially overcame the resistance of C. pipiens SPHAE and Culex quinquefasciatus GeoR to B. sphaericus strain 2297.  (+info)