Estimation of survival and gonotrophic cycle length of Simulium metallicum sensu lato (Diptera: Simuliidae) in southern Mexico.

This study compares the use of a mark-recapture analysis and a time-series analysis to estimate the gonotrophic cycle length and survivorship of Simulium metallicum s.l. in southern Mexico. Daily collections were performed with human- and horse-baited traps at 3 sites in a coffee plantation. The mark-recapture and time series experiments on these collections conclusively yielded a gonotrophic cycle length estimate of 3 days. Horizontal estimates of daily survivorship ranged from 0.75 to 0.69 and these values were similar to that estimated vertically of 0.77. The survival to infective stage (9 days) ranged from 0.012 to 0.043, taking into account at least 12 days for development of 3rd-stage larvae of Onchocerca volvulus. Mark-recapture and time-series methods were found to be suitable for estimating the gonotrophic cycle length and daily survivorship of S. metallicum s.l. under field conditions in southern Mexico.

Determination of sample sizes for the estimation of Onchocerca volvulus (Filarioidea: Onchocercidae) infection rates in biting populations of Simulium ochraceum s.l. (Diptera: Simuliidae) and its application to ivermectin control programs.

Monthly samples of biting Simulium ochraceum s.l. Walker were collected before and after ivermectin treatment in southern Mexico and analyzed for Onchocerca volvulus Leuckart infection rates, infection intensity, and the characteristics of larval distribution among parous flies. The variance over mean ratio (VMR) indicated that in all cases this distribution departed from Poisson and was strongly aggregated (VMR > 1). The negative binomial was found to be an adequate model with a small value of the aggregation parameter k, but the degree of larval overdispersion increased as the mean larval load decreased, invalidating the use of a common kc value. A linear relationship between k and the mean (mu) was established, k(mu) = k1 mu, which permitted exploration of the relationship between the observed proportion of infected flies, p, and the estimated mean larval burden per fly, m (all larval stages in parous flies). This would allow mean numbers of larvae per parous fly to be predicted from presence-absence data (e.g., from infection rates provided by polymerase chain reaction methods applied to pools of flies), assuming that k1 is a known parameter. Given that both p and m are naturally low in S. ochraceum, their relationship was practically linear within the range of observed values. Predictions were tested with the Mexican data from which the clumping parameter was estimated as well as for Guatemalan data for which this information was not available. Results showed a highly satisfactory degree of agreement between predictions and observations. The sample sizes required to estimate mean larval loads from prevalence data for fixed levels of precision (defined as the ratio between SE[m] and m) were calculated for realistic S. ochraceum infection rates (those found in published pre- and postcontrol field surveys as well as in this work). For the special case in which the relationship between k and the mean is linear and goes through the origin, k(mu) = k1 mu, the number of flies to be examined for O. volvulus infections does not explicitly depend on the aggregation parameter, but rather on the unknown proportion of infected flies. Practical recommendations for the calculation of sample sizes are discussed. For infection levels < 0.2%, a minimum number between 6,000 and 13,000 parous flies would have to be examined to estimate the mean larval load with a precision between 0.20 and 0.30. The linearity between onchocercal infection rate and infection intensity in the fly population indicates that relationships between the former and onchocerciasis patterns in the human population should be further explored for the purposes of monitoring the impact of ivermectin control programs through entomological evaluations.

Abundance and parity rate of Lutzomyia cruciata (Diptera:Psychodidae) in an endemic focus of localized cutaneous leishmaniasis in southern Mexico.

Human bait catches were carried out from 5 through 27 March 1994 in an endemic focus of cutaneous leishmaniasis in the state of Campeche, Mexico. Females of Lutzomyia cruciata (Coquillet) were dissected and 67% were parous. The number of total and parous females collected per day was analyzed by time series, but neither the gonotrophic cycle length nor survivorship could be estimated. However, a survival rate per oviposition cycle of 0.68 was estimated from the least square regression of parous on total females. Additional evidence for L. cruciata as a vector of human leishmaniasis is discussed.

Population dynamics and biting rhythm of the anthropophilic sandfly Lutzomyia cruciata (Diptera: Psychodidae) in Southeast, Mexico.

Sandflies attracted by human bait were caught in an endemic focus of localized cutaneous leishmaniasis in the state of Campeche, Mexico. Catches were carried out monthly from February 1994 to January 1995 between 18:00 and 22:00 h. Lutzomyia cruciata was the only species caught. The highest population peak of Lu. cruciata was found in March with lesser peaks in February, December 1994, and January 1995. Maximum biting rate of Lu. cruciata was found between 18:00 and 19:00 h. The host-seeking females of Lu. cruciata were directly related to levels of humidity between 88 and 100%. Low and high temperature had a negative effect upon Lu. cruciata activity. The possible role of Lu. cruciata as vector of leishmaniasis in the state of Campeche, Mexico is discussed.

Estimating the gonotrophic cycle and survivorship of Simulium ochraceum (Diptera: Simuliidae) during routine vector surveillance in southern Mexico.

The gonotrophic cycle, survivorship, and daily parity rate of Simulium ochraceum were estimated from specimens collected during routine vector surveillance in southern Mexico, using a vertical (time-specific) method. Series of sequential data analysis on parity of the entomological data obtained in February 1994 showed the highest significant (P < 0.05 alpha) correlation indices (r = 0.63 and r = 0.67) for a 4-day time lag, indicating that the mean gonotrophic cycle length corresponded to 4 days. Daily survival rate was estimated to be 0.80, the survival to infective age (12 days) to be 6.9%, and the daily parity rate to be 0.41. These estimations are in accordance with those previously obtained by others from mark-release-recapture experiments (horizontal method) in Guatemala.

Trophic status, oogenic stages, and insemination of resting populations of three Simulium species (Diptera: Simuliidae) in Chiapas, Mexico.

Of 176 black fly females collected resting at the edge of El Rosario River, 77% were caught in rocky shelters, whereas the others were captured on damp stretches of river bank. Simulium ochraceum predominated in the catches (74%), followed by Simulium metallicum (21%), and Simulium callidum (5%). Percentages of females without blood in their gut were 70, 85, and 67%, respectively. Of 55 S. ochraceum females that had recently oviposited (sacculate), 29 had the remains of a blood meal in the gut. Of 64 nulliparous females, 48 exhibited early oogenesis (Christopher's stage I-II) indicating recent emergence. At Las Golondrinas village, 36% (71) of 199 specimens were S. ochraceum, of these 85% had no blood in the midgut. Five females of S. ochraceum at this locality had red blood and 6 had black blood in their gut. Of 43 parous and 28 nulliparous females, 36 and 25 had ovaries at stages I-II, respectively. Most females of S. metallicum had no blood in their gut (92%). Nullipars were abundant (91) and stage N (24 h old) was observed in 66 of them.

[Seasonal variations in Aedes Aegypti populations in Monterrey, Mexico]. / Variación estacional de las poblaciones de Aedes Aegypti en Monterrey, México.

Seasonal variation of the populations of the dengue vector mosquito Aedes aegypti, in the city of Monterrey, N.L., Mexico, showed a bimodal pattern. The first peak is lower and appeared at early June, while the second and higher one was observed in the second week of October. 1,419 female mosquitoes were caught in this study. Females with fresh red blood in the stomach were present in all the catches, gravid females (mature eggs in the ovaries) being more abundant than the non-gravid ones. The gravid females with red blood are the oldest of the vector population. Nevertheless, in October, 40 per cent of females had red blood and of these, 37 per cent were gravid. In relation to the biting rate, a 10 females/hour-man rate was recorded for the first peak and a 13.3 females/hour-man rate for the second one. There were 32 per cent of females involved in multiple feedings during the weekly catches. This incidence increases to 40 per cent in October. These rates were calculated on the basis of females with fresh blood in the stomach; from these the gravids and non-gravids have at least three and two multiple feedings, respectively. Finally the epidemiological importance of these multiple feedings on dengue transmission is discussed.

[The effect of ivermectin on the transmission of Onchocerca volvulus in southern Mexico]. / Efecto de la ivermectina sobre la transmisión de Onchocerca volvulus en el sur de México.

We assessed the transmission potential and the mean infected and infective biting density of S. ochraceum in an onchocerciasis endemic community in the southern Mexican state of Chiapas. In addition, the impact of first treatment of ivermectin during the peak of natural infection of host-seeking S. ochraceum populations was also evaluated. Monthly entomological sampling was carried out during the year before treatment to assess the seasonal biting behavior of parous host-seeking S. ochraceum females, and either infected (with whatever stage of O. volvulus) or infective females (with only third larval stage of O. volvulus). The mean of infective biting density was unimodal with the peak in February. Higher densities of infective females were detected from January to March. Annual infective biting density estimated was 95.2 and the annual transmission potential of 152.3. After treatment with ivermectin, entomological collections were carried out during the December-April period. In these months, the first ivermectin mass treatment produced a reduction of natural infection rates of 55 per cent in the mean infected females density (p < 0.001) and of 30 per cent in the transmission potential. However, significative differences were found in parity rates of biting females during the evaluation periods. Decrease of infection rates in vector populations was caused by immediate effect of ivermectin over the skin microfilariae. Microfilariae community load available for transmission declined significantly from 14 Dmg/mg to 5.9 Dmf/mg. These means were statistically different (p < 0.001) and were reduced by 58 per cent from the baseline value.

[Effect of sublethal concentrations of abate on biological parameters of Aedes aegypti]. / Efecto de concentraciones subletales de abate sobre algunos parámetros biológicos de Aedes aegypti.

The effect of abate (Temephos) applied at the rate of 0.016, 0.020 and 0.025 mg/l on fourth stadium larvae and its effect upon pupae weight, adult weight, ingested blood weight and longevity of Aedes aegypti were investigated. The insecticide was applied on F2 generation larvae that were fed an artificial diet under laboratory conditions, from larvae collected at urban level in Monterrey, N.L., Mexico. In relation to the adult, females were fed with a 10 per cent honey solution and rabbit blood for each gonotrophic cycle, while males were fed with only a honey solution. The effect of abate showed a trend to reduce the pupae weight, adult, weight and ingested blood, and to increase longevity too. Although there was not a difference for these variables in the females emerged from larvae exposed to the insecticide, the effects were significant in the males. In this sex, the weight of pupae and adult decreased by 31 and 33 per cent respectively, and longevity increased from 26 to 31 days. Possible advantages and disadvantages of these effects are discussed from a control viewpoint.

Observations on the flight and mating behavior of Anopheles pseudopunctipennis under insectary conditions.

Anopheles pseudopunctipennis showed a bimodal daily pattern in flight activity with the largest peak at 2000 h and the smaller one at 0400 h. In the first 10 days as adults, the maximum swarming activity was observed between 2-6 days of age. Also, sexual encounters were registered at the same time interval.

Effects of sublethal dosages of Abate upon adult fecundity and longevity of Aedes aegypti.

Sublethal concentrations of Abate (temephos) were applied to F2 generation Aedes aegypti larvae, and fecundity and longevity were recorded in the emerged adults. Females exposed to Abate oviposited only in the first 2 gonotrophic cycles, meanwhile control females laid a few eggs after taking the third blood meal. Dosages of 0.009, 0.013 and 0.015 mg/liter of Abate decreased the mean egg production per gonotrophic cycle 37, 47 and 69%, respectively, in relation to the control. Females that were exposed as larvae to Abate lived longer than the control females.

Diurnal pattern and behavior of oviposition of Toxorhynchites theobaldi in the field.

The diurnal pattern and oviposition behavior of Toxorhynchites theobaldi natural populations were studied in 25 artificial containers in the field. The mosquito exhibited a bimodal oviposition pattern with the lower peak at 1100 hr and a mean of 15.7 eggs per container. The higher peak was observed at 1900 hr with a mean of 80.9 eggs per container. Each female flew from 21 to 58 elliptic vertical circles before ejecting one egg upon the surface. In 270 oviposition events, the average was 31.4 ellipses, and the frequency distribution of flights number with different ellipse numbers was fitted to a Poisson distribution. There was a significant linear correlation (r = 0.70) between the oviposition rate and the container surface area.