Dengue: a continuing global threat.

Dengue fever and dengue haemorrhagic fever are important arthropod-borne viral diseases. Each year, there are ∼50 million dengue infections and ∼500,000 individuals are hospitalized with dengue haemorrhagic fever, mainly in Southeast Asia, the Pacific and the Americas. Illness is produced by any of the four dengue virus serotypes. A global strategy aimed at increasing the capacity for surveillance and outbreak response, changing behaviours and reducing the disease burden using integrated vector management in conjunction with early and accurate diagnosis has been advocated. Antiviral drugs and vaccines that are currently under development could also make an important contribution to dengue control in the future.

Dengue vector-control services: how do they work? A systematic literature review and country case studies.

The increasing incidence and geographic expansion of dengue suggest limitations of vector-control operations. We undertook an analysis of services with two methods: a systematic literature review; and case studies (stakeholder interviews, questionnaires) in Brazil, Guatemala, The Philippines and Viet Nam. In the systematic literature review there were only a few studies (strict criteria, 9 studies; less strict criteria, a further 16 studies and 3 guidelines). Of the 9 studies, 3 showed little change of control operations over time but did show strategic changes (decentralisation, intersectoral collaboration). Staffing levels, capacity building, management and organisation, funding and community engagement were insufficient. The case studies confirmed most of this information: (1) a lack of personnel (entomologists, social scientists, operational vector-control staff); (2) a lack of technical expertise at decentralised levels of services; (3) insufficient budgets; (4) inadequate geographical coverage; (5) interventions relying mostly on insecticides; (6) difficulties in engaging communities; (7) little capacity building; (8) almost no monitoring and evaluation. Stakeholders' doubts about service effectiveness were widespread, but interventions were assumed to be effective with increased resources. The analysis underlined the need for: operational standards; evidence-based selection/delivery of combinations of interventions; development/application of monitoring and evaluation tools; needs-driven capacity building.

Six months of Aedes aegypti control with a novel controlled-release formulation of pyriproxyfen in domestic water storage containers in Cambodia.

A field evaluation of a novel, 5% controlled-release formulation of pyriproxyfen was carried out in 400-500 liter concrete water storage jars, the most common and important larval habitat of the dengue vector, Aedes aegypti, in Cambodia. The formulation consisted of cylindrical resin strands, 3 mm in diameter and 40 mm in length. Pyriproxyfen was applied to 100 jars at a target dose of 0.03-0.04 mg of active ingredient (a.i.) per liter (30-40 ppb) in households in Phum Thmei, a village near the capital city of Phnom Penh, in April 2005. Inhibition of adult emergence (IE) in field populations ofAe. aegypti was measured every 2 weeks for 34 weeks. IE in treated jars exceeded 90% for 20 weeks, and remained above 80% until the end of the study. In 25 untreated jars, failure of pupae to metamorphose into viable adults remained below 4%. No alteration of taste or other undesirable effects of the treatment were reported by householders. This single treatment provided control of Ae. aegypti in water jars for the length of the main dengue transmission season in Cambodia that normally extends from May to November.

Community-based use of the larvivorous fish Poecilia reticulata to control the dengue vector Aedes aegypti in domestic water storage containers in rural Cambodia.

A community-based study of the distribution of larvivorous fish, Poecilia reticulata (common name: guppy), in water storage containers for dengue control was undertaken in 14 villages and approximately 1,000 households in Cambodia. Community volunteers reared guppies and distributed them in water jars and tanks in households for which they were responsible. A nearby control area received no intervention. One year after project commencement, 56.9% of eligible containers contained guppies and there was a 79.0% reduction in Aedes infestation in the intervention community compared with the control. Smaller or discarded containers unsuitable for guppy distribution in the intervention area also had 51% less infestation than those in the control area, suggesting a "community-wide" protective effect. In addition, there was less infestation in villages with higher rates of fish uptake, suggesting that the presence of fish was responsible for a reduction in Aedes infestation. This applied vector control model was well-accepted, effective, efficient, and shows promise as a sustainable community-based, non-insecticidal intervention for dengue vector control in large domestic water storage containers in rural Cambodia and elsewhere.

Resurgence of chikungunya.

Chikungunya, an arboviral disease transmitted by Aedes mosquitoes, has recently increased dramatically in incidence and geographic extent. Large outbreaks have affected islands of the Indian Ocean, India and other parts of South and Southeast Asia, Africa and most recently Italy. International travellers have disseminated new strains of the virus, some into regions from which chikungunya has hitherto been absent. In parallel, over the past 30 years international trade has resulted in the spread of A. albopictus from its original range in Asia, to all continents but Antarctica, thereby extending the geographic area over which transmission can occur.

The effect of long-lasting insecticidal water container covers on field populations of Aedes aegypti (L.) mosquitoes in Cambodia.

Dengue in Cambodia is mainly transmitted by Aedes aegypti (L.) mosquitoes that primarily breed in large, concrete jars (> or =200 liters) used for the storage of water for domestic use. Following a preliminary risk assessment, long-lasting insecticidal netting (LN) treated with deltamethrin was incorporated into the design of the covers for these jars. Their effect on immature and adult female populations of Ae. aegypti in six villages in a peri-urban area of Cambodia were compared with populations in six nearby control villages before and for 22 weeks after distribution of the jar covers. There were significantly fewer pupae per house in intervention villages than in control villages (6.6 and 31.9, respectively, p<0.01). Fewer pupae were recovered from intervention houses than from control houses at every post-intervention assessment. Two weeks after the intervention, the average number of indoor resting female Ae. aegypti per house in the intervention villages had declined approximately three-fold, whereas in the controls there was only a slight reduction (16%). The magnitude of the difference between the two areas diminished over time, which contact bioassays confirmed was likely due to a gradual reduction of insecticidal effect of the jar covers. In the study area, insecticide-treated covers for large concrete water storage jars were efficacious for controlling Ae. aegypti in the protected water jars and with a demonstrable effect on adult densities and survival. Further studies of this targeted container strategy in Cambodia, and elsewhere, are recommended. However, improvements in technology that would extend the duration of insecticidal effectiveness of LN materials may be needed for the development of cost-effective public health applications.

Cost-effectiveness of annual targeted larviciding campaigns in Cambodia against the dengue vector Aedes aegypti.

OBJECTIVE: To assess the cost-effectiveness (CE) of annual targeted larviciding campaigns from 2001 to 2005 against the dengue vector Aedes aegypti in two urban areas of Cambodia with a population of 2.9 million people. METHODS: The intervention under analysis consisted of annual larviciding campaigns targeting medium to large water storage containers in households and other premises. The CE compared the intervention against the hypothetical alternative of no intervention. The CE was calculated as the ratio of disability adjusted life years (DALYs) saved to the net cost of the intervention (in 2005 US dollars) by year. A sensitivity analysis explored the range of study parameters. RESULTS: The intervention reduced the number of dengue cases and deaths by 53%. It averted an annual average of 2980 dengue hospitalizations, 11,921 dengue ambulatory cases and 23 dengue deaths, resulting in a saving of 997 DALYs per year. The gross cost of the intervention was US $567,800 per year, or US $0.20 per person covered. As the intervention averted considerable medical care, the annual net cost of the intervention was US $312,214 (US $0.11 per person covered) from a public sector perspective and US $37,137 (US $0.01 per person covered) from a societal perspective. The resulting CE ratios were: US $313/DALY gained from the public perspective and US $37/DALY gained from the societal perspective. Even under the most conservative assumption, the intervention remained cost effective from both perspectives. CONCLUSIONS: Annual, targeted larviciding campaigns appear to have been effective and cost-effective medium-term interventions to reduce the epidemiologic and economic burden of dengue in urban areas of Cambodia.

Inhibition of adult emergence of Aedes aegypti in simulated domestic water-storage containers by using a controlled-release formulation of pyriproxyfen.

Concrete domestic water-storage jars are a common larval habitat of the dengue vector Aedes aegypti in countries of Southeast Asia. The efficacy of a novel controlled-release formulation of the insect growth regulator pyriproxyfen, designed to inhibit adult emergence for 6 months (the approximate duration of the main dengue transmission season in many endemic countries) was tested in Cambodia against a local strain of Ae. aegypti in 200-liter jars. The resin-based formulation contained 4.8% active ingredient (AI). At target dosages of 18, 27, and 36 ppb of AI, inhibition of adult emergence remained above 95% for at least 2 months. After 3 months at 18 ppb AI, the residual efficacy was significantly lower than for the higher dosages (P < 0.05). At the higher dosages, inhibition of adult emergence was > or = 87% for 6 months. At a dosage of 27 ppb AI, monthly removal and replacement of two thirds of the water did not reduce efficacy (P > 0.05). Potential operational advantages and challenges of using the formulation are discussed. Larger-scale efficacy studies are recommended in community settings.

Cost-effectiveness of a pediatric dengue vaccine.

To ascertain the economic feasibility of a pediatric tetravalent dengue vaccine, we developed and calibrated a cost-effectiveness model of vaccinating children at 15 months in Southeast (SE) Asia using a societal perspective. We assumed that full immunization would require two doses at prices of US$ 0.50 and US$ 10 per dose in the public and private sectors, respectively. The gross cost per 1000 population (of all ages) of the vaccination program would be US$ 154. Due to projected savings in dengue treatment, the net cost per capita would be only US$ 17 (89% below the gross cost). The cost per disability adjusted life year (DALY) saved by a pediatric vaccine would be US$ 50, making the potential vaccine highly cost-effective. Eventually, vaccination may be able to replace environmental control as a strategy for dengue prevention and be cost saving.

Control of Culex quinquefasciatus in pit latrines, using shredded, waste polystyrene

As an alternative to the use of commercially available, expanded-polystyrene beads, a study was made of the effectiveness of shredded, waste polystyrene (SWAP) for the control of Culex quinquefasciatus. The relevant physical properties of the SWAP were first investigated in the laboratory. Then, under field conditions in wet pit latrines, mosquito emergence rates were measured before and after application of the material. In the laboratory, when compared with the commercial product, the irregular shape of the SWAP particles greatly reduced their capacity to spread over the water surface and the interstitial air spaces also permitted respiration and development of mosquito larvae. Nevertheless, under field conditions, with careful application of the SWAP over the water surfaces in wet pit latrines, almost complete control of mosquito breeding was observed within a few days. This was sustained for 60 days, at which time observation were discontinued. SWAP appears to offer an effective, cheap and readily available alternative to the commercial product for the control of C. quinquefasciatus. The practicality of using it in community-based, mosquito-control programmes warrants further study.(AU)

Community participation in dengue prevention and control: a survey of knowledge, attitudes, and practice in Trinidad and Tobago

The priorities of public health planners are often at variance with the community's own environmental sanitation priorities and perspectives. Public opinion about individual, collective, and governmental responsibilities in addressing these issues and priorities is of particular importance when designing community-based programs. In a study conducted in Trinidad and Tobago on knowledge, attitudes, and practice regarding dengue, its prevention and control, a high level of awareness about dengue and its etiology was evident, but there was poor understanding of the symptoms and hence little concern about the health risks associated with it. The most important household pest problem identified by the respondents was related to mosquito nuisance, particularly from night-biting mosquitoes. Rodents were also a major concern perceived as being responsible for economic losses, ruined food, and a health hazard. Unreliable water supply, a factor associated with Aedes aegypti abundance, was an environmental sanitation issue of major importance to householders in rural areas. No correlation was found between knowledge of dengue and levels of Ae.aegypti abundance as measured by larval surveys of the respondents' premises. The study gave a clear indication of the need for broad-based environmental sanitation strategies when planning community-based vector control initiatives for the prevention and control of dengue in Trinidad and Tobago (AU)

Mass chemotherapy with diethylcarbamazine for the control of bancroftian filariasis: a twelve-year follow-up in Northern Trinidad, including observations on mansonella ozzardi

A microfilaria survey was conducted in Trinidad in 1992, 12 years after mass treatment with spaced doses of diethylcarbamazine citrate (DEC-C) for the control of Bancroftian filariasis; 348 persons were examined using thick blood smears and a membrane filtration technique. They included 104 who had participated in the mass chemotherapy campaign in 1980. No Wuchereria bancrofti microfilariae were detected among 66 percent of the population examined. In 1980, 86 of 592 persons examined were found to be infected with W. bancrofti, 140 with Mansonella ozzardi and 44 with mixed infections, while in 1992, only M. ozzardi infections persisted despite treatment with DEC-C. Of the 104 persons reexamined 12 years later, 46 had M. ozzardi, of which five were new cases, but none had W. bancrofti. During both the 1980 and 1992 surveys, low microfilariae rates for M. ozzardi were observed among those 19 years of age or younger. Of the 302 persons newly examined in 1992, 29 were infected with significantly (P<0.001) more males (79.3 percent) than females (20.9 percent) being microfilaremic. The combined results showed similar prevalence rates for M. ozzardi from 23.3 percent to 21.6 percent in 1980 and 1992. Nuclepore membrane filtration and thick blood films were very efficient in demonstrating the presence of microfilariae. The usefulness of these methods and spaced treatment using DEC-C are also discussed (AU)

Water storage and dengue in Trinidad and Tobago: community perception and reality - abstract

A knowledge, attitude and practices (KAP) survey of 591 householders and inspection of 753 households for production of the dengue vector Aedes aegypti revealed that as many as 78 percent of households in Trinidad and Tobago stored water. Tanks (63 percent), drums/barrels (29 percent) and buckets (26 percent) were the commonest potential vector production sites, while drums >cans>buckets>tyres, proved to be the major actual producers of Ae aegypti. However, most of these were perceived as "useful by 100 percent of the population; 60 percent thought that old tyres were "useful" and only containers such as coconut shells and small bottles were perceived by 40 percent of the population as "garbage". Special messages of mosquito source reduction will need to be designed. Dengue was recognized as a "serious" illness but only 54 percent were aware of its manifestations. The urban population of Woodbrook where the experience of dengue (16.3 percent) was highest could better tell of the symptoms of dengue (56.8 percent). Conversely, in Tobago with no history of the disease, only 18.3 percent knew of any symptom of the illness. The community that had experienced the highest dengue prevalence as a whole showed least mosquito production - based on the house, container and Breteau indices for the whole country. The authors conclude that health education messages which emphasize the symptoms of Dengue and especially Dengue Haemorrhagic Fever (DHF) and link the transmission of these diseases to the nuisance mosquito, Ae, aegypti, may be one of the most important tools encouraging behaviour modification. This could lead to reduced mosquito production and a mitigation of dengue transmission (AU)

Mass chemotherapy with diethylcarbamazine for the control of bancroftian filariasis: a 12-year follow-up in North Trinidad including observations on mansonella ozzardi microfilaraemia - abstract

A microfilaria survey was conducted in Blanchisseuse, North Trinidad in 1992 twelve years after mass treatment with spaced doses of diethylcarbamazine citrate (DEC-C) for the control of Bancroftian filariasis; 348 persons were examined, including 104 who had participated in the mass-chemotherapy campaign in 1980. No W. bancrofti microfilarie were detected among 66 percent of the population examined. In 1980, 86 persons were found with W. bancrofti, 140 with M. ozzardi and 44 with mixed infections while in 1992 only Mansonella ozzardi infections persisted despite DEC-C treatment. From the 104 persons re-examined, 46 persons had M.ozzardi, of which 5 were new cases but none had W. bancrofti infections in 1992. During both the 1980 and 1992 surveys, low microfilariae rates of M.ozzardi were observed among the 1-4, 5-9 and 10-19-year age groups. From the 302 persons examined in 1992, 29 were infected, with significantly (p<0.001) more males (79.3 percent) than females (20.7 percent) being microfilaraemic. The combined results showed similar prevalence rates of M.ozzardi from 23.3 percent to 21.6 percent in 1980 and 1992, respectively. A good correlation was found between the results observed from Nucleopore membrane filtration and thick blood films. The usefulness of thes methods and spaced treatment using DEC-C is also discussed (AU)