El Niño Southern Oscillation and vegetation dynamics as predictors of dengue fever cases in Costa Rica.

Dengue fever (DF) and dengue hemorrhagic fever (DHF) are growing health concerns throughout Latin America and the Caribbean. This study focuses on Costa Rica, which experienced over 100 000 cases of DF/DHF from 2003 to 2007. We utilized data on sea-surface temperature anomalies related to the El Niño Southern Oscillation (ENSO) and two vegetation indices derived from the Moderate Resolution Imaging Spectrometer (MODIS) from the Terra satellite to model the influence of climate and vegetation dynamics on DF/DHF cases in Costa Rica. Cross-correlations were calculated to evaluate both positive and negative lag effects on the relationships between independent variables and DF/DHF cases. The model, which utilizes a sinusoid and non-linear least squares to fit case data, was able to explain 83% of the variance in weekly DF/DHF cases when independent variables were shifted backwards in time. When the independent variables were shifted forward in time, consistently with a forecasting approach, the model explained 64% of the variance. Importantly, when five ENSO and two vegetation indices were included, the model reproduced a major DF/DHF epidemic of 2005. The unexplained variance in the model may be due to herd immunity and vector control measures, although information regarding these aspects of the disease system are generally lacking. Our analysis suggests that the model may be used to predict DF/DHF outbreaks as early as 40 weeks in advance and may also provide valuable information on the magnitude of future epidemics. In its current form it may be used to inform national vector control programs and policies regarding control measures; it is the first climate-based dengue model developed for this country and is potentially scalable to the broader region of Latin America and the Caribbean where dramatic increases in DF/DHF incidence and spread have been observed.

Re-emergence of Plasmodium malariae In Trinidad, West Indies

A focus of Plasmodium malariae infection has recently occurred on the island of Trinidad, some 30 years after a successful eradiction programme. Examination of bloodsmears revealed 22 cases of P. malariae in the Nariva-Mayaro area of Trinidad between August 1994 and September 1995. Most (77 percent) of the cases were male and, as seven were aged >25 years of age, it appeared that transmission had been renewed, probably by the vector Anopheles bellator. However, none of the 3000 mosquitoes tested by ELISA for circumsporozoite protein of P. malariae proved positive. Use of IFAT to check blood samples for P. malariae appeared more sensitive than direct examination of bloodsmears, indicating that 42 (13 percent) of the 325 samples tested were seropositive (at titres of 1:256 or greater). The levels of transmission of the parasite may therefore be even higher than indicated by examination of blood smears. The surveillance measurers adopted to understand the epidemiology of this outbreak of P. malariae in Trinidad are described. The need to maintain malaria surveillance in all the countries where P. malariae parasites once existed (prior to eradication) is emphasised. (AU)

Re-emergence of Plasmodium malariae in Trinidad, West Indies.

A focus of Plasmodium malariae infection has recently occurred on the island of Trinidad, some 30 years after a successful eradication programme. Examination of bloodsmears revealed 22 cases of P. malariae in the Nariva-Mayaro area of Trinidad between August 1994 and September 1995. Most (77%) of the cases were male and, as seven were aged < 25 years of age, it appeared that transmission had been renewed, probably by the vector Anopheles bellator. However, none of the 3000 mosquitoes tested by ELISA for the circumsporozoite protein of P. malariae proved positive. Use of IFAT to check blood samples for P. malariae appeared more sensitive than direct examination of bloodsmears, indicating that 42 (13%) of the 325 samples tested were seropositive (at titres of 1:256 or greater). The levels of transmission of the parasite may therefore be even higher than indicated by examination of bloodsmears. The surveillance measures adopted to understand the epidemiology of this outbreak of P. malariae in Trinidad are described. The need to maintain malaria surveillance in all the countries where P. malariae parasites once existed (prior to eradication) is emphasised.

A focus of plasmodium malariae in Trinidad, West Indies

This report describes a focus of Plasmodium malariae on the island of Trinidad, some 30 years after a successful eradication programme. A total of 22 cases of P. malariae was detected using blood smears in the Nariva-Mayaro area of Trinidad from August 1994 to September 1995, with 77.3 percent of the cases being males and 22.7 percent being females. Seven of the 22 cases were individuals < 25 years of age suggesting that malaria transmission was renewed by the anopheline vectors Anopheles bellator and Anopheles homunculus. None of the three thousand mosquitoes tested by ELISA for cirumsporozoite protein of P. malariae proved positive. Blood samples tested using polymerase chain reaction (PCR) proved negative for P. malariae, while indirect fluorescent antibody (IFA) serology proved more sensitive. For example, from 325 IFA samples tested, 42 (12.9 percent) had titres of 1:256 for higher. These results suggest that malaria transmission levels were higher than those reported by blood smears and PCR. The surveillance measures adopted to understand the epidemiology of this outbreak of P. malariae from Trinidad are described. The need of maintain malaria surveillance in all countries where P. malariae parasites existed prior to eradication is emphasized.(AU)

Factors influencing the duration of blood-feeding by laboratory-reared and wild Aedes aegypti (Diptera: Culicidae) from Trinidad, West Indies.

The time taken by Aedes aegypti to take a bloodmeal was determined under laboratory and field conditions in Trinidad. Eggs from field-collected females were reared under nutrient-limited (low diet) and nutrient-adequate (high diet) conditions. Blood-feeding times indicated that 71% of the females from the low-diet group were fast feeders (< 2 min) compared with 40% of the female from the high-diet group. Subsequent tests of the F1 and F2 progeny of the fast-feeding females indicated that the fast-feeding characteristic was not maintained. Similarly, testing of colonized Ae. aegypti (ST JOSEPH) indicated that, although 48% of the first, adult females were fast feeders (< 2 min), there was no apparent selection for the fast-feeding characteristic in mass-rearing of the F1 and F2 progeny from these females. Mean wing length (range = 1.66-2.94 cm) of the colonized females (2.67 cm) was more similar to that of females from the high-diet group (2.62 cm) than to that of females from the low-diet group (1.85 cm). Although wing lengths of females from landing collections in St Joseph, Trinidad, varied from 2.28-2.76 cm [mean (S.D.) = 2.46 (0.11) cm], there was no significant correlation between wing length and blood-feeding time on humans. The mean (S.D.) duration of blood-feeding for the wild Ae. aegypti, 179 (32) s, was comparable with that for females reared in the laboratory under nutrient-adequate conditions. Although the blood-feeding times for the wild females ranged from 107-282 s, only 2% (1 of 40) exhibited fast feeding times (< 2 min). Even though it is well known that mosquito size determines the probability of 'feeding success,' the duration of blood-feeding by wild Ae. aegypti on humans appeared independent of mosquito size (although the sample size was small). There may therefore be a genetic component to mosquito feeding speed, this trait being quickly lost during colonization and modulated by environmental factors.

Diel oviposition and fecundity of Anopheles oswaldoi in Trinidad, West Indies.

The diel oviposition periodicity of wild-caught Anopheles oswaldoi collected from the forest-ecotone habitat in Valencia, Trinidad, was studied in the laboratory by recording the performance of egg-laying individuals and a colony at 2-h intervals. Oviposition was almost exclusively nocturnal, with 95.3% of eggs and 91.3% of oviposition occurrences being observed during the scotophase. During the rest of the day, only 4.7% of eggs (8.7% occurrences) were recorded after dawn (0600-0800 h). Wild-caught females engorged on human blood under laboratory conditions matured, on average, 61.1 +/- 32.3 follicles (range 56-135). Ranges of 50-69, 70-89, 90-109, and > 110 follicles were matured by 12, 4, 4, and 3 gravid females, respectively. These findings provide vector operators with a window of time to maximize the impact of insecticides on An oswaldoi populations.

Diel oviposition and fecundity of Anopheles oswaldoi in Trinidad, West Indies

The diel oviposition periodicity of wild-caught Anopheles oswaldoi collected from the forest-ecotone habitat in Valencia, Trinidad, was studied in the laboratory by recording the performance of egg-laying individuals and a colony at 2-h intervals. Oviposition was almost exclusively nocturnal, with 95.3 percent of eggs and 91.3 percent of oviposition occurrences being observed during the scotophase. During the rest of the day, only 4.7 percent of eggs (8. 7 percent ocurrences) were recorded after dawn (0600-0800 h). Wild caught females engorged on human blood under laboratory conditions matured, on average, 61.1 +/- 32.3 follicles 9range 56-135). Ranges of 50-69, 70-89, 90-109, and > 110 follicles were matured by 12, 4, 4, and 3 gravid females, respectively. These findings provide vector operators with a window of time to maximize the impact of insecticides on An oswaldoi populations. (Au)

Country-wide prevalence of Ascogregarina culicis (apicomplexa: lecudinidae), a protozoan parasite of Aedes aegypti in Trinidad, West Indies.

The prevalence of the gregarine parasite Ascogregarina culicis was determined for Aedes aegypti populations in Trinidad, West Indies. Over 5 months, a total of 5,651 Ae. aegypti larvae were collected and identified from 419 sites throughout Trinidad as part of a national dengue surveillance program. Ascogregarina culicis infections were detected in 13.4% of 1,986 Ae. aegypti larvae dissected. In 7 of 8 counties or major subdivisions where this parasite was detected in Trinidad, the prevalence per locality ranged from 9.6 to 17.1% with an associated parasite density of 19.1-51.9 trophozoites/larva. Confirming that Ae. aegypti populations in Trinidad are highly susceptible to infection with A. culicis parasites, experimental studies showed that a strain of A. culicis from Florida readily infected 3 strains of Ae. aegypti from Trinidad with essentially no associated mortality. This report extends the known geographic range of A. culicis in Ae. aegypti populations and represents one of the first country-wide surveys for this gregarine parasite.

Country-wide prevalence of Ascogregarina culicis ( apicomplexa: lecudinidae), a protozoan parasite of Aedes aegypti in Trinidad, West Indies

The prevalence of the gregarine parasite Ascogregarina culicis was determined for Aedes aegypti populations in Trinidad, West Indies. Over 5 months, a total of 5,651 Ae. aegypti larvae were collected and identified from 419 sites throughout Trinidad as part of a national dengue surveillance program. Ascogregarina culicis infections were detected in 13.4 percent of 1,986 Ae. aegypti larvae dissected. In 7 of 8 counties of major subdivisions where the parasite was detected in Trinidad, the prevalence per locality ranged from 9.6 to 17. 1 percent with an associated parasite density of 19.1-51.9 trophozoites/larva. Confirming that Ae. aegypti populations in Trinidad are highly susceptible to infection with A. culicis parasites, experimental studies showed that a strain A. culicis from Florida readily infected 3 strains of Ae. aegypti from Trinidad with essentially no associated mortality. This report extends the known geographic range of A. culicis in Ae. aegypti populations and represents one of the first country-wide surveys for this gregarine parasite. (Au)

Blood-digestion kinetics of four Anopheles species from Trinidad, West Indies.

The blood-digestion kinetics of Anopheles albitarsis, An. aquasalis, An. bellator and An. homunculus were determined in the laboratory using females collected from two field sites in Trinidad. Anopheles aquasalis displayed the highest rate of haemolysis (giving an absorbance of 0.36 at 410 nm), followed by An. albitarsis (0.16), An. bellator (0.07) and An. homunculus (0.05). Trypsin activity peaked 12-24 h after blood feeding and then declined to zero at 60 h in all four species. Anopheles albitarsis had significantly higher maximum trypsin activity (0.69 units) than An. aquasalis (0.28), An. bellator (0.18) or An. homunculus (0.12) (P < 0.01). Aminopeptidase activity patterns were similar for An. aquasalis, An. bellator and An. homunculus, with peak activity at 18 h. Among the An. albitarsis mosquitoes, peak aminopeptidase activity occurred at 24 h. The peritrophic membrane developed 18, 30, 30 and 36 h post-feeding in An. aquasalis, An. albitarsis, An. bellator and An. homunculus, respectively. Stage V ovarian follicles began to mature 36 h after An. albitarsis and An. bellator fed to repletion and after 48 h in An. aquasalis and An. homunculus. Ovarian development in the four species was not affected by patterns of erythrocyte haemolysis, proteolytic enzyme activity or peritrophic-membrane development. The inter- and intra-specific variations observed in the blood-processing physiology of the four species of Anopheles are briefly discussed in terms of phylogeny.

Blood-engorgement kinetics of four anopheline mosquitoes from Trinidad, West Indies.

The blood-feeding kinetics of Anopheles albitarsis, An. aquasalis, An. bellator and An. homunculus were determined under laboratory conditions using females collected from two sites in Trinidad. Many An. aquasalis, An. albitarsis and An. bellator completed probing within 30 s but An. homunculus averaged > 50 s. Anopheles albitarsis (68.3 s), An. aquasalis (67.0 s) and An. homunculus (77.7 s) took similar times to fill their guts but An. bellator took longer (106.5 s). More than 75% of the An. albitarsis, An. aquasalis and An. homunculus but only 51% of the An. bellator showed apparent prediuresis. Anopheles aquasalis exhibited the shortest mean duration of prediuresis (77.3 s) and An. homunculus the longest (152.9 s). Most individual mosquitoes spent less time filling their guts than in prediuresis. Overall, An. aquasalis exhibited the shortest total feeding time (135.7 s) and An. homunculus the longest (197.0 s). There was a considerable degree of variation among individuals of each species in their ability to agglutinate human erythrocytes immediately upon blood engorgement. For example, within the same feeding experiments, some individuals agglutinated < 25% of the ingested erythrocytes whereas others agglutinated > or = 90%. The parity of the field-collected anophelines had no significant effect on probing, duration of blood-feeding, duration of prediuresis, or erythrocyte agglutination. The inter- and intra-specific variations in blood-feeding and erythrocyte processing observed for the four species of field-collected anophelines from Trinidad are discussed in terms of vector competence and the evolution of these behaviours.

Diel oviposition periodicity of anopheline mosquitoes (Diptera: Culicidae) from the Americas: Anopheles albimanus Wieldemann and Anopheles freeborni Aitken.

The diel oviposition patterns of Anopheles albimanus and An. freeborni derived from Panama and California, U.S.A., respectively, were studied in the laboratory by recording the egg-laying of individuals and colonies at 2-h intervals. Anopheles freeborni oviposition patterns were almost exclusively nocturnal, with 80% of eggs being laid during the scotophase. A large, well-defined peak of oviposition, comprising about 70% of eggs laid, occurred 4-6 h before sunrise. Anopheles albimanus oviposition patterns were bimodal with crepuscular and nocturnal components, 68% of eggs being laid shortly before lights went 'off' or 'on' and 32% during the scotophase. A small, well-defined peak of oviposition, comprising about 42% of eggs laid, occurred immediately following lights off (i.e. 18.00-20.00 hours). Nulliparous females allowed to engorge on human blood matured, on average, 104 follicles (range 80-142) for An. albimanus and 114 follicles (range 31-201) for An. freeborni.

Blood engorgement kinetics of four anopheline mosquitoes from Trinidad, West Indies

The blood-feeding kinetics of Anopheles albitarsis, An.aquasalis, An. bellator and An. homunculus were determined under laboratory conditions using females collected from two sites in Trinidad. Many An. aquasalis, An. albitarsis and An. homuculus averaged > 50 s. Anopheles albitarsis (68.3 s), An. aquasalis (67.0 s) and An. homunculus (77.7 s) took similar times to fill their guts but An. bellator took longer (106.5 s). More than 75 percent of the An. albitarisis, an. aquasalis and An. homunculus but only 51 percent of the An. bellator showed apparent prediuresis. Anopheles aquasalis exhibited the shortest mean duration of prediuresis (77.3 s) and An. homunculus the longest (152.0 s). Most individual mosquitoes spent less time filling their guts than in prediuresis. Overall, An aquasalis exhibited the shortest total feeding time (135.7 s) and An. homunculus the longest (197.0 s). There was a considerable degree of variation among individuals of each species in their ability to agglutinate human erythrocytes immediately upon blood engorgement. For example, within the same feeding experiments, some individuals agglutinated < 25 percent of the ingested erythrocytes whereas others agglutinated > or = 90 percent. The parity of the field-collected anophelines had no significant effect on probing, duration of blood-feeding, duration of prediuresis, or erythrocyte agglutination. The inter- and intra-specific variations in blood-feeding and erythrocyte processing observed for the four species of field-collected anophelines from Trinidad are discussed in terms of vector competence and the evolution of these behaviours (AU)