Influence of larval density or food variation on the geometry of the wing of Aedes (Stegomyia) aegypti.

BACKGROUND AND METHOD: Variation in wing length among natural populations of Aedes (Stegomyia) aegypti (L.) (Diptera: Culicidae) is associated with different vectorial capacities. Geometric morphometrics allowed us to use a more powerful estimator of wing size ('centroid size'), as well as to visualize the variation of wing shape, to describe the effects of density or food variation at larval stage on 20 anatomical landmarks of the wing of A. aegypti. RESULTS: Almost perfect correlations between (centroid) size and larval density or size and larval food were observed in both sexes: a negative correlation with increasing density and a positive one with increasing amount of food. The allometric component of shape change was always highly significant, with stronger contribution of size to shape under food effects. Within each experiment, either food or density effects, and excluding extreme conditions, allometric trends were similar among replicates and sexes. However, they differed between the two experiments, suggesting different axes of wing growth. CONCLUSION: Aedes aegypti size is highly sensible to food concentration or population density acting at larval stages. As larger individuals could be better vectors, and because of the stronger effect of food concentration on size, vector control activities should pay more attention in eliminating containers with rich organic matter. Furthermore, as a simple reduction in larval density could significantly increase the size of the survivors, turning them into potentially better vectors, the control activities should try to obtain a complete elimination of the domestic populations.

Biochemical studies of insecticide resistance in Aedes (Stegomyia) aegypti and Aedes (Stegomyia) albopictus (Diptera: Culicidae) in Thailand.

Biochemical analysis was performed on field caught Aedes (Stegomyia) aegypti and Aedes (Stegomyia) albopictus (Diptera: Culicidae) mosquitoes to determine activities of enzymes including mixed function oxidases (MFO), nonspecific esterases (alpha- and beta-), glutathione-S-transferases (GST), and insensitive acetylcholinesterase (AChE). Biochemical tests were performed on F1 generation of Ae. aegypti field caught mosquitoes, while in Ae. albopictus F2 progenies were used. Twenty-six samples of Ae. aegypti mosquito were collected from areas across different parts of Thailand including Bangkok (central), and the provinces of Chiang Rai (north), Nakhon Sawan (north-central), Nakhon Ratchasrima (northeast), Chonburi (east), Chanthaburi (east), and Songkhla (south). Eight wild caught samples of Ae. albopictus were from Songkhla, Nakhon Sawan, Nakhon Ratchasrima and Kanchanaburi (west) provinces. The susceptibility to pyrethroids (deltamethrin, permethrin), organophosphate (fenitrothion) and carbamate (propoxur) insecticides were revealed in these samples. The biochemical test results were compared with those of the susceptible Bora (French Polynesia) strain. There was significant enhancement of MFO in pyrethroid resistant Ae. aegypti samples, except those from Songkhla and Hauykwang district in Bangkok. Biochemical assay results suggested that nonspecific esterases conferred fenitrothion resistance in Ae. aegypti in Nakhon Sawan, while insensitive AChE and/or nonspecific esterases could play role in fenitrothion resistance in Nakhon Ratchasrima. There was no consistent association of GST with pyrethroid resistance in Ae. aegypti. Low enzyme activities found in Ae. aegypti in Songkhla and in Ae. albopictus corresponded to their insecticide susceptibility status. The increased enzyme activity in field samples reflecting local history of insecticide employment was discussed.

Dynamics of susceptibility and transmissibility of the live, attenuated, candidate vaccines dengue-1 PDK13, dengue-3 PGMK30F3, and dengue-4 PDK48 after oral infection in Aedes aegypti.

Dengue-1 virus PDK13 and isolates from vaccinees (dengue-1 Ib1 and dengue-1 Ib 10), dengue-3 PGMK30F3, and dengue-4 PDK48 were studied for their abilities to infect, disseminate, and replicate in Aedes aegypti mosquitoes by the oral route. In general, infection and dissemination rates were poorer for the vaccine compared with the parent viruses. The transmissibility was also lower for dengue-1 PDK13 than the parent virus, whereas it was not detected for isolates from vaccinees and dengue-3 PGMK30F3. Transmissibility of dengue-4 PDK48 was not determined because no dissemination occurred. Replication rates of vaccine strains were also found to be less efficient than the parent viruses. These imply that vaccination with the candidate vaccine is safe. Moreover, vector attenuation of vaccine viruses was consistent with its phenotypic markers of attenuation, which remained stable after a mosquito passage or after human and mosquito passage.

The use of Toxorhynchites splendens for identification and quantitation of serotypes contained in the tetravalent live attenuated dengue vaccine.

Assurance of identity and quantity is an indispensable part of quality control in the manufacture of vaccines. Dengue-1 PDK13, dengue-2 PDK53, dengue-3 PGMK30F3 and dengue-4 PDK48 in the live attenuated tetravalent dengue vaccine were assayed by identification and quantitation in a mosquito system (Toxorhynchites splendens). Each serotype of dengue virus was identified by dengue specific monoclonal antibodies in the indirect fluorescent antibody test. Virus content was estimated by calculating the 50% mosquito infectious dose (MID50). Differences from 0 to +/-0.5 log10 were observed between the original monovalent titer and that from the blend which showed no significant difference at 95% confidence limit (P < 0.05). This result indicates that there is no interference between dengue serotypes in mosquitoes infected by intrathoracic inoculation with the virus mixture. It can be also concluded that this mosquito system can be used as an effective measure for infectivity titration of each component in the tetravalent dengue vaccine.

The micro-focus reduction neutralization test for determining dengue and Japanese encephalitis neutralizing antibodies in volunteers vaccinated against dengue.

A micro-focus reduction neutralization test (mFRNT) was evaluated as an alternative test to the ordinary plaque reduction neutralization test (PRNT) for the determination of dengue virus and Japanese encephalitis virus neutralizing antibody responses in persons receiving dengue vaccine. The 2 tests were similar in terms of titres and ability to detect seroconversion. Although the neutralizing antibody titres obtained by mFRNT were slightly lower than those given by PRNT, the differences were less than two-fold, indicating than mFRNT was reliable. Reproducibility of mFRNT was confirmed by 10 replicate tests using the same control serum. Therefore, mFRNT may be useful in large-scale investigations of neutralizing antibody levels, for example, in young children forming part of an immunization programme; it can be performed quickly and is economical, requiring only a small volume of sera.

Infection, dissemination, transmission, and biological attributes of dengue-2 PDK53 candidate vaccine virus after oral infection in Aedes aegypti.

The capacity for oral infection, dissemination, and transmission of the dengue-2 candidate vaccine virus DEN-2 PDK53 and an isolate from a vaccinate individual, DEN-2 Ia8, were compared with the parent strain DEN-2 16681. Capacity for oral infection and dissemination to the brain and salivary gland tissues were significantly lower in the first two than in the parent strain (P < 0.001). Replication was more than 100 times higher for the parent strain when compared with the dengue-2 candidate vaccine virus. Transmission was not demonstrated in the mosquitoes orally infected with DEN-2 PDK53 and DEN-2 Ia8, whereas transmission was achieved in 57% (8 of 14) of mosquitoes infected with the parent virus strain. Using immunofluorescence, viral antigen was detected in the mosquitoes infected with DEN-2 PDK53 and DEN-2 Ia8. It was seen mainly in the form of specks scattered in some parts of the tissues, and was strikingly different from that seen in the parent strain, in which major parts of the tissues contained viral antigen in the form of rings and specks. The biological markers of DEN-2 PDK53 and DEN-2 Ia8 retained the biological characteristic of the vaccine after a mosquito passage and a human and mosquito passage, respectively.