Field Evaluation of Indoor Thermal Fog and Ultra-Low Volume Applications For Control of Aedes aegypti in Thailand.

Efficacies of a handheld thermal fogger (Patriot™) and a backpack ultra-low volume (ULV) sprayer (Twister™) with combinations of 2 different adulticides (pyrethrin, deltamethrin) and an insect growth regulator (pyriproxyfen) were field-tested and compared for their impact on reducing indoor Aedes aegypti populations in Thailand. The effectiveness of the indoor space sprays was evaluated by sampling the natural Ae. aegypti population in houses and determining their physiological status, by monitoring mortality of sentinel caged mosquitoes (AFRIMS strain) and by assessing larval mortality in laboratory bioassays using water exposed to the spray. A total of 14,742 Ae. aegypti were collected from Biogents Sentinel traps in this study. The combination of ULD® BP-300 (3% pyrethrin) and NyGuard® (10% pyriproxyfen) sprayed either by the Patriot or Twister significantly reduced some Ae. aegypti populations up to 20 days postspray relative to the control clusters. The addition of pyriproxyfen to the adulticide extended how long household mosquito populations were suppressed. In 2 of the 4 products being compared, the Twister resulted in higher mortality of caged mosquitoes compared with the Patriot. However, neither machine was able to achieve high mortality among Ae. aegypti placed in hidden (protected) cages. The larval bioassay results demonstrated that the Twister ULV provided better adult emergence inhibition than the Patriot (thermal fogger), likely due to larger droplet size.

Sample-size requirements for developing strategies, based on the pupal/demographic survey, for the targeted control of dengue.

Several methods to determine the sample size required for a reliable and practical assessment of the number of Aedes aegypti pupae in a community in Puerto Rico have been explored. Because the pupae were highly aggregated, the data were fitted to a negative binomial distribution. Classical statistical-inference methods for sample-size determination demanded the sampling of >3,000 premises for a reliable estimation of the mean number of pupae/person (with a 15% error). This number was reduced to 1,000-1,200 premises after applying a finite-population correction. Database sub-sampling simulations, with increasing sample sizes, showed that the variability in the mean relative abundance of container types and in the mean number of pupae/container substantially decreased after sampling 186 and 310 premises, respectively. Sequential sampling was applied to test the hypotheses that the number of female pupae/person was at least 0.19 (considered the dengue epidemic threshold) or no greater than 0.10 (arbitrarily set as the safe level). After sampling only 25 premises in the first survey and 125 in the second, it was determined that the densities of female pupae were above the epidemic threshold. Thus, sequential sampling provided substantial reductions in the sample size required to determine if vector control was needed. Validation of the Ae. aegypti thresholds required for dengue transmission could confer viability and efficiency to dengue-vector surveillance and control programmes.

Medical examiner samples: a source for dengue surveillance

Post-mortem medical examiner samples may be useful for sentinel surveillance of disorders usually detected by antibody determinations on specimens from ill patients or from surveys. We found anti-dengue IgM positivity in 3(23/780) and anti-dengue IgG positivity in 77(597/777) of sera obtained at the Puerto Rico medical examiner (Institute of Forensic Sciences) in December 2000, April 2001, and October 2001. This approach may be a useful alternative for estimating the population prevalence of serologic markers for dengue and other infectious diseases.

Analysis of survival of young and old Aedes aegypti (Diptera: Culicidac) from Puerto Rico and Thailand.

It generally is assumed that the daily probability of survival of wild adult mosquitoes is independent of age. To test this assumption we conducted mark-release-recapture studies in Puerto Rico and Thailand to determine if estimated daily survival rates between two different age cohorts of the dengue vector Aedes aegypti (L.) were the same. Survivorship was estimated with nonlinear regression analysis using bootstrapping to obtain estimates of errors. Initial recapture success of the younger cohort was greater than the older cohort at both locations. Our analysis revealed a significantly greater survival rate for the younger cohort of females in Puerto Rico, and no significant differences between age cohorts in Thailand. For comparison, a traditional approach for analyzing these type of data, linear regression of log-transformed captures over time (exponential model), was used to calculate the probability of daily survival based on slopes of linear regression lines for recaptured mosquitoes. With this method, the estimated daily survival rate of older females (13-23 d old) was significantly greater than survival of younger ones (3-13 d old) in Puerto Rico and Thailand. In addition, short-range movement of mosquitoes was observed in Puerto Rico; maximum dispersal distance detected was 79 m. Survival rates of adult Ae. aegypti may be age-dependent and nonlinear regression analysis is a sensitive approach for comparing patterns of mosquito survival based on mark, single release, multiple recapture data.

The dengue and dengue hemorrhagic fever epidemic in Puerto Rico, 1994-1995.

From June 1, 1994 to May 31, 1995 a total of 24,700 cases of dengue (7.01/1,000 population) were reported to the laboratory-based surveillance system in Puerto Rico (1991-1994, annual average: 2.55/1,000). Dengue virus 2 predominated. The earliest indicator of epidemic activity was the virus isolation rate in May 1994 (14.0% versus 5.7% average). The male-to-female ratio among cases was 1:1.1; 65.4% were younger than 30 years (the 10 to 19 year age group had the highest incidence, 11.8/1,000). At least 5,687 cases (23.0%) showed a hemorrhagic manifestation; 4,662 (18.9%) were hospitalized, and 40 died (0.2%; 10 laboratory-positive). Two cases documented by laboratory were transmitted by unusual routes--intrapartum and through a bone marrow transplant. Among 2,004 hospitalized cases reported by infection control nurses, 139 (6.9%) fulfilled the criteria for dengue hemorrhagic fever (DHF) and another 13 cases (0.6%) had dengue shock syndrome. This epidemic produced the largest number of hospitalizations, DHF cases, and deaths from any dengue epidemic in Puerto Rico. Severity did not change throughout the year. Surveillance capabilities were maintained by temporary, simplified reporting methods, none of which could be recommended as the single method of choice for surveillance; each must be used (on site, or as a service available from a reference laboratory) at the right time in the epidemic cycle. The utility of comparisons of current and previous data underscores the value of long-term surveillance. Our analysis was unable to document whether significantly increased transmission occurred more often in cities where the water supply was rationed or where the local landfill was closed.

Dengue activity in Puerto Rico during an interepidemic period (1995-1997).

From 1995 to 1997 dengue was reported in Puerto Rico at an average annual rate of 1.75/1,000 population, compared to 6.73 in 1994, an epidemic year. Dengue virus serotypes 1 (DEN-1), -2, and -4 were isolated each year, with DEN-2 predominating in 1995 and 1996, and DEN-4 in 1997. From 1995 through 1997 incidence was highest (0.61-0.77/1,000) in persons under 30 years of age; males and females were equally affected. Among positive cases, 28.3% to 37.9% were hospitalized; 28.9% to 35.2% had hemorrhagic manifestations; at least 1.1% to 1.6% fulfilled the criteria for dengue hemorrhagic fever/dengue shock syndrome; and 0.2% to 0.3% died. Neither hurricane preparations (1995) nor widespread floods (1996) seem to have affected dengue incidence. Most municipalities with the highest laboratory-diagnosed dengue rates in 1995 were in the eastern foothills of the central mountains, an area relatively spared by the 1994 epidemic. In the next two years, at least half of the municipalities with the highest laboratory-diagnosed dengue rates were in the west. The most intense municipal outbreak of this period (DEN-2, Villalba, 1995, rate of 11.67/1,000) is described to highlight the importance of local conditions and epidemiologic history in determining the risk of dengue.

Mosquito vector control and biology in Latin America--an eleventh symposium.

The 11th annual Latin American symposium presented by the American Mosquito Control Association (AMCA) was held as part of the 67th Annual Meeting in Dallas, TX, in February 2001. The principal objective, as for the previous 10 symposia, was to promote participation in the AMCA by vector control specialists, public health workers, and academicians from Latin America. This publication includes summaries of 45 presentations that were given orally in Spanish or presented as posters by participants from 8 countries in Latin America. Topics addressed in the symposium included results from chemical and biological control programs and studies; studies of insecticide resistance; and population genetics, molecular, ecological, and behavioral studies of vectors of dengue (Aedes aegypti), malaria (Anopheles albimanus and Anopheles aquasalis), leishmaniasis (Lutzomyia), murine typhus, and Chagas' disease (Triatoma). Related topics included biology and control of Rhodnius, scorpions, Loxosceles spp., Chironomus plumosus, and Musca domestica.

Dengue surveillance in Florida, 1997-98.

Recent dengue outbreaks in the Caribbean and Central and South America and the presence of competent mosquito vectors increase the likelihood of future autochthonous transmission in Florida. During April 1997 to March 1998, a laboratory-based active surveillance program detected 18 cases of dengue involving all four dengue serotypes. All patients reported recent travel to countries with indigenous dengue transmission. These results demonstrate that dengue infections are imported into Florida at a much higher rate than reflected by previous passive surveillance; therefore, the risk for local dengue transmission may be increasing.

Longitudinal studies of Aedes aegypti (Diptera: Culicidae) in Thailand and Puerto Rico: blood feeding frequency.

We used a histologic technique to study multiple blood feeding in a single gonotrophic cycle by engorged Aedes aegypti (L.) that were collected weekly for 2 yr from houses in a rural village in Thailand (n = 1,891) and a residential section of San Juan, Puerto Rico (n = 1,675). Overall, mosquitoes from Thailand contained significantly more multiple meals (n = 1,300, 42% double meals, 5% triple meals) than mosquitoes collected in Puerto Rico (n = 1,156, 32% double meals, 2% triple meals). The portion of specimens for which frequency of feeding could not be determined was 31% at both sites. We estimated that on average Ae. aegypti take 0.76 and 0.63 human blood meals per day in Thailand and Puerto Rico, respectively. However, frequency of multiple feeding varied among houses and, in Puerto Rico, the neighborhoods from which mosquitoes were collected. In Thailand 65% of the mosquitoes fed twice on the same day, whereas in Puerto Rico 57% took multiple meals separated by > or = 1 d. At both sites, the majority of engorged specimens were collected inside houses (Thailand 86%, Puerto Rico 95%). The number of blood meals detected was independent of where mosquitoes were collected (inside versus outside of the house) at both sites and the time of day collections were made in Puerto Rico. Feeding rates were slightly higher for mosquitoes collected in the afternoon in Thailand. Temperatures were significantly higher and mosquitoes significantly smaller in Thailand than in Puerto Rico. At both sites female size was negatively associated with temperature. Rates of multiple feeding were associated positively with temperature and negatively with mosquito size in Thailand, but not in Puerto Rico. Multiple feeding during a single gonotrophic cycle is a regular part of Ae. aegypti biology, can vary geographically and under different climate conditions, and may be associated with variation in patterns of dengue virus transmission.

Longitudinal studies of Aedes aegypti (Diptera: Culicidae) in Thailand and Puerto Rico: population dynamics.

Aspiration collections of adult Aedes aegypti (L.) were made weekly from inside and outside of houses for 3 yr in a rural Thai village (n = 9,637 females and n = 11,988 males) and for 2 yr in a residential section of San Juan, Puerto Rico (n = 5,941 females and n = 6,739 males). In Thailand, temperature and rainfall fell into distinct seasonal categories, but only temperature was correlated with fluctuations in female abundance. Average weekly temperature 6 wk before mosquitoes were collected and minimum weekly temperature during the week of collection provided the highest correlations with female abundance. Accounting for annual variation significantly improved Thai models of temperature and mosquito abundance. In Puerto Rico, temperature, but not rainfall, could be categorized into seasonal patterns. Neither was correlated with changes in female abundance. At both sites the vast majority of females were collected inside houses and most contained a blood meal. Most teneral females were collected outside. Wing length--an indicator of female size--and parity, egg development or engorgement status were not correlated, indicating that feeding success and survival were not influenced by female size. At both sites, females fed almost exclusively on human hosts (> or = 96%), a pattern that did not change seasonally. In Puerto Rico more nonhuman blood meals were detected in mosquitoes collected outside than inside houses; no such difference was detected in Thailand. Gut contents of dissected females indicated that females in the Thai population had a younger age distribution and fed more frequently on blood than did Ae. aegypti in Puerto Rico. Our results indicated that aspects of this species' biology can vary significantly from one location to another and 1 yr to the next.

Potential for aging female Aedes aegypti (Diptera: Culicidae) by gas chromatographic analysis of cuticular hydrocarbons, including a field evaluation.

Gas chromatography with flame-ionization detection was used to measure the time-associated, quantitative changes in the cuticular hydrocarbons of female Aedes aegypti (L.). Cohorts of unstressed Ae. aegypti, Rockefeller strain, were reared and held at 3 constant temperatures (24, 28, and 30 degrees C). Five females from each cohort were taken at 33 degree-day (DD) intervals from 0 to 231 DD (using 17 degrees C as the threshold temperature). Quantitative changes over time of cuticular hydrocarbons associated with gas chromatographic peaks 1 and 5 were identified as having promise for age grading. The relative abundance of peak 1 (pentacosane) decreased linearly from 0 to 132 DD, whereas peak 5 (nonacosane) increased linearly over the same period. Suboptimal larval conditions (crowded and starved), which resulted in physiological stress (decreased size), had negligible effect on the relative abundance of pentacosane and nonacosane. Additionally, the rate of change in the relative abundance of pentacosane and nonacosane were the same for both a recently colonized Chachoengsao (Thailand) strain of Ae. aegypti compared with the long-colonized Rockefeller (Caribbean) strain over a 0-99 DD interval. Two linear regression models, one based on the relative abundance of pentacosane and the other on the logit transformation of these values, were developed for aging female Ae. aegypti. A blind study using laboratory-reared mosquitoes and a mark-release-recapture experiment using field mosquitoes validated these age-grading models and produced promising results for aging females up to 132 DD (19, 12, and 10 calendar days at 24, 28 and 30 degrees C, respectively). Therefore the regression models, based on the relative abundance of these 2 cuticular hydrocarbons, appeared to be a useful approach for age-grading Ae. aegypti up to at least 12 d of age regardless of environmental conditions (temperature and stress) and population history (origin and colonization time).

Dengue in Guyana.

There have been dramatic increases in dengue fever (DF) and dengue haemorrhagic fever in South America. Guyana has reported less than five cases per year for most of the past decade. We evaluated patients in a clinic in Georgetown, Guyana, over 2 days and found evidence of 50 cases of dengue infection.

PIP: This research letter evaluates the incidence of dengue fever and dengue hemorrhagic fever in Guyana, South America, in a study conducted at the Vector Control Medical Center on July 20-21, 1998. Venous blood samples were collected from 112 patients (99 men and 13 women ranging in age from 9 to 60 years) who were inflicted with fever, chills, malaise, and/or headache. 50 samples were detected to be dengue virus- positive, illustrating a 45% prevalence of recent dengue virus infection among the tested patients. This study revealed that the incidence of dengue fever in Guyana is under-reported. Thus, improvements should be made in the surveillance measures for dengue infection in Guyana and its surrounding countries.

Increased fecundity of Aedes aegypti fed human blood before release in a mark-recapture study in Puerto Rico.

Laboratory experiments suggest that utilization of blood rather than natural sugar sources for energetic needs affords female Aedes aegypti a reproductive advantage over conspecifics that use sugar. To test this hypothesis under field conditions, we carried out a mark-release-recapture study in Florida, PR. Adult females (F1) reared from field-collected eggs were provided with a diet of human blood alone or human blood plus a 20% honey solution before their release. Backpack aspirators were used to collect mosquitoes from release houses for 5 consecutive days beginning the 2nd day after release. Survival was estimated from the slope of the regression line of the log-transformed daily number of recaptures for each treatment group. To compare fecundity of the treatment groups, each recaptured female was dissected, ovaries were removed, oocytes counted, and Christophers' stages of oocyte development scored. Recapture rates were 30% for the blood-only group and 23% for blood plus honey group. The daily survival rate of the blood-only group (55%) was not statistically different from that of the blood plus honey group (69%) (t = 0.32, P > 0.05). By analysis of variance, fecundity (average number of stage III-V oocytes) was significantly higher in the females fed human blood alone (n = 103, 109 oocytes/female) than in the group fed on blood and honey (n = 50, 95 oocytes/female) (P = 0.0007). The observed gonotrophic cycle length of the recaptured females ranged from 3 to 7 days. Results from our field study are consistent with laboratory life-table experiments that suggest feeding exclusively on human blood provides a reproductive advantage for female A. aegypti.

Survival of starved Aedes aegypti (Diptera: Culicidae) in Puerto Rico and Thailand.

Survival of adult Aedes aegypti (L.) was studied in Thailand (1995) and Puerto Rico (1996) during periods of high and low dengue virus transmission. Resting males and females were collected inside houses by aspiration. Females were separated into different cages by their degree of engorgement and ovarian development. Teneral adults were obtained from pupae collected from natural breeding sites. All mosquitoes were given access to water, held at ambient temperature in the shade, and their survival monitored daily. We calculated median survival for each stage to estimate when mosquitoes had to feed again or die. No differences in survival between seasons were observed in Thailand. In Puerto Rico, except for wild males, survival was longer in the cool/dry season than in the hot/rainy season, indicating that mosquitoes may need to feed more frequently during the high than low dengue transmission season. During both study periods and at both sites, blood-engorged females survived as long or longer than mosquitoes in other gonotrophic or developmental stages. Except in Puerto Rico during the cool season, when females had a relatively high probability of surviving 3-4 d without feeding, females needed to feed approximately every other day to avoid death caused by starvation. Our results indicate that in some regions, there are seasonal differences in the length of time female Ae. aegypti can survive without feeding, females with a blood meal can survive for a longer time than those without blood, and teneral males can live longer without food than teneral females.

Life table study of Aedes aegypti (Diptera: Culicidae) in Puerto Rico fed only human blood versus blood plus sugar.

Life table studies were performed in 1996 with Aedes aegypti (L.) during the low (cool/dry) and high (hot/rainy) dengue virus transmission seasons in Puerto Rico. Mated adult females from field-collected pupae were placed individually in cages and divided into 2 treatment groups: one was fed only human blood and the other human blood plus a 10% sucrose solution. Survival and number of eggs laid were recorded daily for each female. During both seasons, age specific survivorship was higher for the blood plus sugar group, groups fed only human blood had higher reproductive outputs (mx), and net replacement rates (Ro) for blood only groups were higher than for those fed blood plus sugar. Intrinsic rates of growth (r) were the same for both treatments during the low (cool/dry) transmission season, but higher for the blood-only treatment during the high (hot/rainy) transmission season. Our results indicate that feeding on only human blood provides an evolutionary advantage to Ae. aegypti females in Puerto Rico. These results are similar to those from an earlier study carried out with Ae. aegypti in Thailand; the advantage of feeding on human blood does not seem to be restricted to a particular geographic region. We also found that the benefits associated with human feeding persist through epidemiologically different times of the year. We conclude that feeding on human blood is reproductively beneficial for Ae. aegypti, which may increase their contact with human hosts, and therefore may influence their vectorial capacity for dengue viruses through frequent feeding on blood.

Dengue and dengue haemorrhagic fever.

The incidence and geographical distribution of dengue have greatly increased in recent years. Dengue is an acute mosquito-transmitted viral disease characterised by fever, headache, muscle and joint pains, rash, nausea, and vomiting. Some infections result in dengue haemorrhagic fever (DHF), a syndrome that in its most severe form can threaten the patient's life, primarily through increased vascular permeability and shock. The case fatality rate in patients with dengue shock syndrome can be as high as 44%. For decades, two distinct hypotheses to explain the mechanism of DHF have been debated-secondary infection or viral virulence. However, a combination of both now seems to be the plausible explanation. The geographical expansion of DHF presents the need for well-documented clinical, epidemiological, and virological descriptions of the syndrome in the Americas. Biological and social research are essential to develop effective mosquito control, medications to reduce capillary leakage, and a safe tetravalent vaccine.

Aedes aegypti (Diptera: Culicidae) movement influenced by availability of oviposition sites.

Marked Aedes aegypti (L.) (5-6 d old) were released inside 2 groups of 5 houses (100 females per house) in a residential community in Florida, PR, to compare behavior of gravid females at sites where oviposition containers were absent to sites where containers were abundant (i.e., 2 tires and 10 ovipots were added to each yard). Two sequential releases were made so that both groups of houses were evaluated with oviposition containers removed and added. Mosquitoes resting inside the 10 release houses plus 20 additional neighboring houses were collected with backpack aspirators for 4 consecutive days, beginning 2 d after release. Because 172 of the 185 recaptured females (93%) were collected in the same houses in which they had been released, dispersal patterns were not directly comparable. However, the recapture rate in houses with containers added (13%) was significantly higher (P < 0.0001) than in houses with containers removed (6%). No difference was observed in the mean number of potential oviposition containers among the nonrelease houses at the 2 sites (3.9 versus 3.8 aquatic containers per house in the prerelease survey). Our results are consistent with the hypothesis that availability of oviposition sites is inversely correlated with the potential for female Ae. aegypti to disperse. These results have important implications because campaigns to reduce Ae. aegypti larval sites during dengue epidemics could have the undesirable effect of inducing the dispersal of infected adult female mosquitoes.

Using disability-adjusted life years to assess the economic impact of dengue in Puerto Rico: 1984-1994.

This study presents the disability-adjusted life years (DALYs), a non-monetary economic measure of impact, lost to dengue in Puerto Rico for the period 1984-1994. Data on the number of reported cases, cases with hemorrhagic manifestations, hospitalizations, and deaths were obtained from a surveillance system maintained at the Dengue Branch, Division of Vector-Borne Infectious Diseases, Centers for Disease Control and Prevention (San Juan, PR). The reported cases were divided into two age groups (0-15 years old and >15 years old), and then multiplied by predetermined factors (10 for 0-15 years; 27 for >15 years) to allow for age-related under-reporting of cases. Severity of dengue was modeled by classifying cases into three groups: dengue fever, dengue with severe manifestations, and hospitalized cases. Each group was assigned a different number of days lost because of dengue-related disability. Dengue caused an average of 658 DALYs per year per million population (SE = 114, range = 145-1,519). A multivariate sensitivity analysis, which simultaneously altered the values of six input variables, produced a mean of 580 DALYs/year/million population, with a maximum average of 1,021 DALYs/year/million population, and a maximum, single-year estimate for 1994 of 2,153 DALYs/million population. The most important input was the number of days lost to classic dengue. The DALYs/year/million population lost to dengue in Puerto Rico are much greater than previous estimates concerning the impact of dengue hemorrhagic fever alone. The loss to dengue is similar to the losses per million population in the Latin American and Caribbean region attributed to any of the following diseases or disease clusters; the childhood cluster (polio, measles, pertussis, diphtheria, tetanus), meningitis, hepatitis, or malaria. The loss is also of the same order of magnitude as any one of the following: tuberculosis, sexually transmitted diseases (excluding human immunodeficiency virus), tropical cluster (e.g., Chagas' disease, leishmaniasis), or intestinal helminths. The results objectively suggest that when governments and international funding agencies allocate resources for research and control, dengue should be given a priority equal to many other infectious diseases that are generally considered more important.