Simulations to compare efficacies of tetravalent dengue vaccines and mosquito vector control.

SUMMARY: Infection with dengue, the most prevalent mosquito-borne virus, manifests as dengue fever (DF) or the more fatal dengue haemorrhagic fever (DHF). DHF occurs mainly when an individual who has acquired antibodies to one serotype is inoculated with another serotype. It was reported that mosquito control may have increased the incidence of DF and DHF due to age-dependency in manifesting these illnesses or an immunological mechanism. Tetravalent dengue vaccine is currently being tested in clinical trials. However, seroconversions to all four serotypes were achieved only after three doses. Therefore, vaccines may predispose vaccinees to the risk of developing DHF in future infections. This study employed an individual-based computer simulation, to emulate mosquito control and vaccination, incorporating seroconversion rates reported from actual clinical trials. It was found that mosquito control alone would have increased incidence of DF and DHF in areas of high mosquito density. A vaccination programme with very high coverage, even with a vaccine of suboptimal seroconversion rates, attenuated possible surges in the incidence of DF and DHF which would have been caused by insufficient reduction in mosquito abundance. DHF cases attributable to vaccine-derived enhancement were fewer than DHF cases prevented by a vaccine with considerably high (although not perfect) seroconversion rates. These predictions may justify vaccination programmes, at least in areas of high mosquito abundance. In such areas, mosquito control programmes should be conducted only after the vaccination programme with a high coverage has been initiated.

Geographical gradient of mean age of dengue haemorrhagic fever patients in northern Thailand.

Dengue haemorrhagic fever (DHF) is caused by dengue virus transmitted by Aedes mosquitoes; mean age of patients varies temporally and geographically. Variability in age of patients may be due to differences in transmission intensity or demographic structure. To compare these two hypotheses, the mean age of DHF patients from 90 districts in northern Thailand (1994-1996, 2002-2004) was regressed against (i) Aedes abundance or (ii) demographic variables (birthrate, average age) of the district. We also developed software to quantify direction and strength of geographical gradients of these variables. We found that, after adjusting for socioeconomics, climate, spatial autocorrelation, the mean age of patients was correlated only with Aedes abundance. The geographical gradient of mean age of patients originated from entomological, climate, and socioeconomic gradients. Vector abundance was a stronger determinant of mean age of patients than demographic variables, in northern Thailand.

Effectiveness of citronella preparations in preventing mosquito bites: systematic review of controlled laboratory experimental studies.

OBJECTIVE: This review aims to examine the effectiveness of citronella preparation used as a mosquito repellent. METHODS: Multiple computerized databases such as MEDLINE, EMBASE, Cochrane CENTRAL, CINAHL, and AMED, were searched for controlled laboratory experiments that compared the effectiveness of citronella products to control in repelling Aedes, Anopheles and Culex mosquitoes using the cage or room methods. Outcomes measures were protection time and percentage repellency. The weighted mean difference and 95% confidence interval were calculated comparing the outcomes in the citronella and control groups. Meta-analysis was performed using the DerSimonian and Laird method under a random-effects model. RESULTS: Eleven studies met inclusion criteria. Based on a meta-analysis of studies using the cage method, protection time of the citronella oil for preventing Aedes mosquitoes was less than that in the DEET (N,N-diethyl-m-toluamide) group, with a difference in protection time of 253 min (95% confidence interval: 169-336). The combination of citronella oil and vanillin is likely to have a longer protection time compared with citronella oil alone. In studies using the room method, citronella oil and/or the combination of citronella oil and vanillin provided complete repellency at least 3 h. In Anopheles and Culex mosquitoes, a combination of citronella oil and vanillin product demonstrated a comparable protection time against DEET; however, it remained inconclusive due to a limited number of studies. CONCLUSIONS: Citronella products are less effective than DEET products in terms of duration of protection. Adding vanillin to citronella oil products could prolong the protection time.

Geographical structure of dengue transmission and its determinants in Thailand.

Expansion of dengue has been attributed to urbanization. To test this concept, we examined dengue transmission intensities in Thailand. We used the inverse of mean age of dengue haemorrhagic fever (DHF) cases as a surrogate of dengue transmission intensity (or force of infection). The transmission intensity in Bangkok decreased rapidly since the mid-1990s, to levels that are currently lower than in other regions. Regression analysis revealed that transmission intensity is highest in the Northeastern rural region, mainly due to scarcity of private water wells. Private wells reduce the need for household water containers, the major breeding sites for vectors. Cumulatively, these results show that urbanization is not necessarily associated with intense dengue transmission in Thailand. Paradoxically, the DHF incidence in Bangkok has surpassed other regions despite declines in transmission intensity. This finding implies the existence of endemic stability (i.e. low incidence of a clinical illness in spite of high transmission intensity).

Repellency of volatile oils from plants against three mosquito vectors.

Volatile oils extracted by steam distillation from four plant species (turmeric (Curcuma longa), kaffir lime (Citrus hystrix), citronella grass (Cymbopogon winterianus) and hairy basil (Ocimum americanum)), were evaluated in mosquito cages and in a large room for their repellency effects against three mosquito vectors, Aedes aegypti, Anopheles dirus and Culex quinquefasciatus. The oils from turmeric, citronella grass and hairy basil, especially with the addition of 5% vanillin, repelled the three species under cage conditions for up to eight hours. The oil from kaffir lime alone, as well as with 5% vanillin added, was effective for up to three hours. With regard to the standard repellent, deet alone provided protection for at least eight hours against Ae. aegypti and Cx. quinquefasciatus, but for six hours against An. dirus. However, deet with the addition of 5% vanillin gave protection against the three mosquito species for at least eight hours. The results of large room evaluations confirmed the responses for each repellent treatment obtained under cage conditions. This study demonstrates the potential of volatile oils extracted from turmeric, citronella grass and hairy basil as topical repellents against both day- and night-biting mosquitoes. The three volatile oils can be formulated with vanillin as mosquito repellents in various forms to replace deet (N,N-diethyl-3-methylbenzamide), the most common chemical repellent currently available.

Larval occurrence, oviposition behavior and biting activity of potential mosquito vectors of dengue on Samui Island, Thailand.

A 1995 outbreak of dengue haemorrhagic fever (DHF) occurred on Samui Island in Thailand with an incidence of almost 500 cases/100,000 population. To find and develop effective strategies to control this disease through cost-effective vector control programs, entomological studies were carried out on the island between 1996 and 1998. There were two species of DHF vectors, Aedes aegypti and Ae. albopictus prevailing on the island, and the population of Ae. aegypti remained relatively constant throughout the year while the abundance of Ae. albopictus increased substantially during the rainy season (May-December) and then declined drastically in the dry season (January-April). The ranges of the three Aedes larval indices, Breteau index (BI), house index (HI) and container index (CI) were 93-310, 43-89 and 16-50 respectively. The ceramic or earthen jars both inside and outside the dwellings and concrete water storage tanks (mostly in toilets and bathrooms) served as the main breeding places of Ae. aegypti whereas coconut husks and coconut floral spathes found outdoors were the major breeding sites of Ae. albopictus. The number of washing water jars, concrete tanks and natural sites infested with Aedes larvae increased significantly in rainy season, with 60% of ovitraps become positive for Ae. albopictus eggs with an average number of 26 eggs/trap in 3 days of setting. There was a complete lack of oviposition by Ae. aegypti in outdoor ovitraps (15 m away from the houses). The indoor biting rate ranged from 1.5 to 8.1 mosquitoes/man-hour, while the outdoor rate was between 5 and 78 mosquitoes/man-hour. Of the indoor biting mosquitoes, 75.4% were identified as Ae. aegypti and 99% of the outdoor ones were Ae. albopictus. The diel biting activity of Aedes during the period from 0800 h to 1700 h in the houses was higher in the morning than in the afternoon period, with a low prevalence between 1300 h and 1400 h.

Mosquito larval control with Bacillus sphaericus: reduction in adult populations in low-income communities in Nonthaburi Province, Thailand.

During 1999 and 2000 several larvicidal treatments of Bacillus sphaericus strain 2362 water dispersible granular (WDG) formulations were made at 50 to 200 mg/m2 in mosquito developmental sites in low-income communities in Nonthaburi Province, Thailand to determine whether larviciding dense populations would results in a noticeable reduction of adult mosquitoes in small treated areas. In the treated area in 1999 (Soi Jumpa), immature populations were suppressed to extremely low levels for extended periods, especially at the higher dosages. This decline in immature populations was followed by a substantial decline in adult mosquitoes. There was a lag of 7 to 14 days post-larval treatments before maximum decline in adults was noted. Adults that emerged prior to treatments survived for 7-14 days or longer, thus no drastic reduction was noted soon after treatments. Despite a slight resurgence in adult mosquitoes during the middle of the experimental period, adult female mosquitoes (over 98% Cx quinquefasciatus), remained low during the 5-month period of trials. During the last 2 weeks (17 days post last treatment) of the experimental period, female populations reached the pre-treatment level. During the 2000 tests at Wat Pikul reduction in larvae was 87-98% for 7 weeks after first treatment at 200 mg/m2, resulting in a reduction of 24 to 73% (2 and 7 days post-treatment respectively) and 87 to 98 (2-6 weeks) in the adults. In the second and third treatments at 50 mg/m2, larval control and subsequent adult reduction were lower and shorter-lived than at the high dosage, and the fourth treatment at 100 mg/m2 did not yield a high level of reduction in the larvae (18 to 33%), but reduction of adults was still 80%. The final fifth treatment at 200 mg/m2 yielded only 18% control of larvae, suggesting tolerance to B. sphaericus at this site. It was shown that at both treated sites repeated treatments with a larvicide such as B. sphaericus could result in substantial reduction in adult mosquitoes. Vigilance for detection of resistance development should be practiced, as resistance could emerge in certain populations following a few treatments.

Cockroach surveys in 14 provinces of Thailand.

Cockroach surveys using sticky traps were conducted in urban areas of 14 Thailand provinces. At least 30 houses in each province were randomly sampled for cockroaches. Each house was trapped in three areas: kitchen, bedroom and outside. A total of 2,648 cockroaches was caught by 550 out of 1,542 traps (35.7%), from 337 of the 514 houses (65.6%). Overall, relative density ranged from 2.6 to 9.1 with an average of 5.2 cockroaches/house. On the average, 47.7% of the cockroaches were caught in the kitchen, 24.4% and 27.9% were caught in the bedroom and outside of dwellings, respectively. There were 10 species of cockroaches caught from the 14 provinces: Periplaneta americana (60.9%), Periplaneta brunnea (15.4%), Neostylopyga rhombiofolia (9.6%), Periplaneta australasiae (9.2%), Pycnoscelis surinamensis (3.3%), Blatella germanica (0.6%), Periplaneta fuliginosa (0.5%), Supella longipalpa (0.3%), Blatella lituricollis (0.15%) and Nauphoeta cinerea (0.05%), belonging to six genera. According to the surveys in this study, Periplaneta americana and Periplaneta brunnea were the most abundant cockroach species in urban Thailand, whereas the kitchen was the major habitat.

Mosquito burden and impact on the poor: measures and costs for personal protection in some communities in Thailand.

To gauge the extent of mosquito problems and their impact on local people in Thailand, a simple questionnaire was designed consisting of 6 questions with multiple choices to be answered in 4 different communities in Thailand in 1998 and 1999. Mosquito biting activity was noted often by respondents. They reported that mosquitoes bit both night and day, and that the insects were abundant both in the dry and the rainy seasons. In all 4 communities, a large proportion of the residents used bed nets, mosquito coils, and aerosol sprays for personal protection; vaporizing mats and repellents were used sparingly. The cost of such measures amounted to dollars 4 to dollars 25 per year per household. For most of the residents, this represented a substantial proportion of their income, and was proportionally greater than the average cost of organized mosquito control in developed countries. This suggests that instituting organized local vector control programs would be cheaper and more effective than the individual use of personal protectants that do not reduce mosquito numbers. An assessment of the available products stocked in neighborhood stores and supermarkets for personal protection was made. A variety of insecticidal aerosols, mosquito coils, liquid sprays, vaporizing mats, and vaporizing liquids was stocked. This ample supply of household insecticides lends support to the preferred methods of protection reported by the respondents. The active ingredients in most of the formulations were synthetic pyrethroids, although a few contained dichlorvos, propoxur, and a few other compounds. Mosquito coils, the most preferred products used by the poor, were evaluated for efficacy, and were found to provide a reduction of 72-96% in landing-biting rates in controlled experiments.

Laboratory and field evaluations of the insect repellent 3535 (ethyl butylacetylaminopropionate) and deet against mosquito vectors in Thailand.

The insect repellents 3535 (ethyl butylacetylaminopropionate or IR3535) and deet (N,N-diethyl-3-methylbenzamide) were prepared as 20% solutions in absolute ethanol and evaluated for repellency against many mosquito species in Thailand under laboratory and field conditions using human subjects. In the laboratory, 0.1 ml was applied per 30-cm2 of exposed area on a volunteer's forearm (0.66-0.67 mg active ingredient [AI]/ cm2), whereas in the field, volunteers' legs (from knee to ankle, with a surface area of about 712-782 cm2) were treated with 3 ml per exposed area (0.76-0.84 mg AI/cm2). In the laboratory, both IR3535 and deet showed equal repellency (P > 0.05) for 9.8 and 9.7 h against Aedes aegypti, for 13.7 and 12.7 h against Culex quinquefasciatus, and for 14.8 and 14.5 h against Cx. tritaeniorhynchus, respectively. Anopheles dirus was significantly less sensitive to IR3535 than to deet (P < 0.05), with a mean protection time of 3.8 and 5.8 h, respectively. Under field conditions, both IR3535 and deet provided a high degree of protection against various mosquito vectors ranging from 94 to 100% during the test periods. Both repellents provided a high level of protection for at least 8 h against Ae. albopictus and for at least 5 h against Cx. gelidus, Cx. tritaeniorhynchus, Cx. quinquefasciatus, Mansonia dives, Ma. uniformis, Ma. annulata, Ma. annulifera, Anopheles minimus, and An. maculatus. This study clearly documents the potential of IR3535 for use as a topical treatment against a wide range of mosquito species belonging to several genera.

Efficacy of new formulations of the microbial larvicide Bacillus sphaericus against polluted water mosquitoes in Thailand.

Two new water dispersible granular (WDG) formulations of the microbial control agent Bacillus sphaericus (strain 2362) were extensively evaluated in polluted waters against Culex quinquefasciatus in Thailand. The studies were carried out in stagnant as well as flowing waters during August 1997 to July 1998. The trial period covered both rainy and dry seasons. The two WDG formulations of B. sphaericus tested were low potency (350 ITU/mg) and high potency (630 ITU/mg) products. Both formulations were used at various rates to determine initial efficacy and longevity. The high potency formulations provided excellent control (80 to 90%) of immature mosquitoes at the rates of 50 to 100 mg/ m2, while the less potent formulation yielded similar control at the rates of 89 to 250 mg/m2. Longevity of control was anywhere from one week to four weeks or longer depending on the dosage, habitat, and environmental conditions. Two treatments with low dosages of Bacillus thuringiensis ssp. israelensis WDG provided short-term control lasting for one week. Frequent episodes of heavy rains facilitated long-term suppression of immature mosquitoes in klongs by flushing out the larvae. In the absence of rain, the longevity of treatments in flowing waters was shorter than in the presence of rain. Rain, however, had some but not pronounced effect on longevity in the stagnant water habitats. Operation of floodgates controlling water flow from stagnant water habitats into the Chaophrya River had greater influence on the abundance of mosquito immatures in the ponded polluted waters under dwellings. Precipitation, adding large amounts of water, was probably responsible for diluting the control agents and also resulted in lower counts of immatures per unit volume of water. These variables could influence the efficacy (especially longevity) of treatments employing formulations of microbial agents in operational control programs. The WDG formulations are preferred over other formulations, such as granules, because the former contain larger quantities of toxins per unit mass than the latter and are easily applied as aqueous sprays. Potent formulations with greater content of active ingredients are less costly to ship and transport to distant areas.

Field trials with Bacillus sphaericus formulations against polluted water mosquitoes in a suburban area of Bangkok, Thailand.

Two newly developed Bacillus sphaericus larvicidal formulations, VectoLex CG (corncob granules) and VectoLex WDG (water dispersible granules), were tested against Culex quinquefasciatus larvae in 4 highly polluted breeding sites in Thailand. VectoLex CG, applied at rates of 0.5-2 g/m2, gave satisfactory to complete control of late-instar larvae and pupae for up to 4 wk after treatment. The VectoLex WDG, which had higher potency and was applied at rates of 0.1-0.5 g/m2, gave satisfactory control for 1-4 wk after treatment. Among the factors influencing longevity of control were dosage of a given formulation, precipitation, and flooding of the treated sites; the latter had the greatest impact. Presence of larvivorous fish did not seem to influence larval populations because there were heavy populations of mosquito larvae present in the test sites in the presence of moderate numbers of fish before the application of B. sphaericus treatments.

Dengue vector mosquitos at a tourist attraction, Ko Samui, in 1995.

On Ko Samui, Thailand there were two epidemics of dengue hemorrhagic fever (DHF) in 1966 and 1967, followed by endemics up to 1994. Aedes aegypti and Aedes albopictus were the vectors. From January to July 1995, 51 cases of DHF were reported, out of these were many foreigners who still suffer from dengue fever and return home with negative impression. We carried out an entomological survey around the island and collected the mosquitos to detect dengue virus by digoxigenin-cDNA probe. The data revealed that Aedes aegypti and Aedes albopictus still were abundant and some were infected with dengue virus. Visual larval survey indices (HI, CI and BI) were 90.4, 61.3 and 301.3 respectively. Biting rate (BR) of Aedes mosquitos was high, the average indoor and outdoor BR were 9.7 and 100.8 mosquitos/man-hour. From 13 pools of mosquitos, 8 strains of dengue virus were detected (61.5%). The results may encourage the local authorities to improve vector surveillance and control before the famous island becomes an unpleasant island.

Evaluation of selected anthelmintic compounds for activity against Opisthorchis viverrini.

In vitro and in vivo experiments were employed in the screening of potential anthelmintic agents against Opisthorchis viverrini infection in hamsters. A few selected groups of compounds tested included those that are commercially available as well as those that are still being tested by various pharmaceutical firms. The compounds tested in the present study were praziquantel, amoscanate, albendazole, flubendazole, metrifonate, metronidazole and benzodiazepine derivatives. Results from both in vitro and in vivo experiments showed that at the dosages employed, praziquantel was the only one that gave complete cure, as judged from faecal egg examination and worm recovery at the time of sacrifice. It was therapeutically effective against different developmental stages of O. viverrini including the metacercariae. Moreover, the drug was also effective as a chemoprophylactic agent when given 6 to 12 hr prior to being exposed to infective metacercariae. Other compounds tested were considerably less active although some might have permanently damaged the fluke reproductive capacity, while others were able to suppress egg-laying capacity only temporarily. Together, results suggests that the ineffectiveness of most agents tested in this study is not related to their inability to attain concentrations high enough to kill or damage the flukes in the biliary system but is most likely due the inherent lack of capacity to kill the flukes.

Humoral immune responses in hamsters infected with Opisthorchis viverrini.

The kinetics and nature of humoral immune responses to somatic and excretory-secretory (ES) antigens were investigated in hamsters experimentally infected with different numbers of Opisthorchis viverrini. ES antigens were obtained from the in vitro culture of adult flukes and somatic antigens were aqueous extracts of adult flukes. Antibodies in the serum and bile of infected animals were determined by the microhaemagglutination technique, using glutaraldehyde fixed sheep red blood cells sensitized with these parasite antigens. Antibody responses to both somatic and ES antigens were detected in the serum from the second week of infection onward. The peak response was noted at the end of the second month and declined slowly thereafter. Antibody levels in animals with heavy infections (100 metacercariae) appeared earlier but declined more rapidly than in animals with light infections (25 metacercariae). The serum antibodies were highly sensitive to mercaptoethanol throughout the course of infection (23 weeks). Antibodies also appeared in the bile obtained at the time of sacrifice but their titres were rather low compared with those in the serum. Like serum antibodies, biliary antibodies were reactive with both somatic and ES antigens. Biliary antibodies were of the secondary IgA type. These findings are discussed in relation to pathogenesis of the disease process and to the possible usefulness in immunodiagnosis.

Attempts to induce protective immunity in hamsters against infection by a liver fluke of man (Opisthorchis viverrini).

The development of acquired resistance in opisthorchiasis was studied in hamsters experimentally infected with Opisthorchis viverrini. The induction of protective immunity was attempted by first exposing adult female golden Syrian hamsters to 1, 2 or 3 doses of infective metacercariae obtained from naturally infected cyprinoid fishes and then reinfecting them with 80 metacercariae. In other experiments, animals that were infected with 50 metacercariae were treated with praziquantel prior to being rechallenged in order to eliminate the flukes that had developed from the first infection. The effect of long-term chronic infections was also studied. Faecal egg counts were determined at weekly intervals from 4-5 weeks onwards. The animals were killed 2-3 months after the last infection for worm recovery, and terminal faecal egg output/g faeces/worm was calculated. The data showed that prior infection of animals with O. viverrini did not induce significant protective immunity against reinfection by the same parasite. Lack of protection was also noted in animals reinfected several times with small doses of metacercariae. However, under certain circumstances, prior infection could result in a significant reduction in the faecal egg output due to subsequent infection.