Behaviour of Anopheles albimanus in relation to pyrethroid-treated bednets.

Responses of the malaria vector Anopheles albimanus to pyrethroid impregnated bednets made of cotton or nylon, compared with untreated nets, were investigated in houses occupied by two people inside and/or outside two bednets, in coastal Chiapas, México. The pyrethroid used was lambdacyhalothrin 30 mg a.i./m2. Bioassay mortality rates of An.albimanus exposed to treated nets for 3 or 15 min, rose from 40-55% to 90-100% for nylon nets 3-19 weeks post-treatment, but were consistently lower for treated cotton nets. An.albimanus females (collected unfed on human bait) were released in houses surrounded by curtains for trapping mosquitoes that exited from the house. Floor sheets were used in and around each experimental house for retrieving any mosquitoes knocked-down and/or killed. During post-treatment assessment for 17 weeks, An.albimanus blood-feeding success rates were 23-24% with untreated nets, 14-18% with treated cotton nets and 8-15% with treated nylon nets, significantly reduced when both human baits were inside the treated bednets, but not when one or more baits were outside the treated bednet(s) within the house. Proportions of mosquitoes leaving houses < 3 h post-release were 53-59% from houses with untreated bednets versus 65-78% with treated bednets. Except in one case (when both humans were outside treated cotton nets), these increased early exit rates were significant, whether or not the human baits were inside the treated bednets indoors. Mortality rates of An.albimanus females exiting overnight (22.00-06.00 hours) averaged 15-39% from houses with treated cotton and 16-46% with treated nylon nets, very significantly greater than the control mean rates of 6-8% mortality with untreated nets. Observations on wild-caught An.albimanus females marked with fluorescent powder and released indoors revealed that few mosquitoes (3-11%) actually contacted the bednets unless both human baits remained under them - when contact rates were 22% on treated nylon, 23% on treated cotton and 42% on untreated nets (P approximately 0.05). The mean resting time was significantly longer on untreated (14.4 min) than on treated nylon (5.8 min) or cotton (9.5 min) bednets, whereas mean resting times on other surfaces indoors were 16.5-19.8 min. Proportions exiting within 2h of release were significantly more from houses with treated houses (33-35%) than with untreated nets (8%). However, mortality rates of mosquitoes that landed on treated nets were very significantly greater (90-100%) than after landing on untreated nets (10%). Thus, despite some excito-repellency, lambdacyhalothrin-impregnated bednets (especially made of nylon) proved to be effective as an alternative to house-spraying against An.albimanus.

Control of Anopheles albimanus mosquitos in southern Mexico by spraying their preferred indoor resting sites.

The traditional indoor spraying technique to control Anopheles albimanus mosquitos was compared with a selective method that targets their preferred resting sites in coastal villages of southern Mexico. We also determined whether mosquitos changed their preferred resting sites following insecticide applications. In the selective spraying approach, two 1-m wide horizontal swaths of bendiocarb insecticide were applied 0.75-1.75 m from the base of the walls and at 1-m upwards from the base of the inner roof at a target dose of 0.4 g/m2. A 2-year follow-up (1988-90), which included four insecticide treatments, showed that the selective spraying was as effective as full spraying for controlling A. albimanus. Also the mosquitos preferred resting sites that were not modified by the insecticide applications. Finally, the use of this techniques required 46% less time and cost 67% less than conventional spraying.

Intradomiciliary behavior of Anopheles albimanus on the coastal plain of southern Mexico: implications for malaria control.

The postfeeding indoor resting behavior of Anopheles albimanus in experimental houses in southern México was investigated by using a mark-recapture procedure. The majority of mosquitoes rested inside houses after taking a blood meal indoors. There was a higher landing frequency on interior surfaces other than walls and roofs; however, mosquitoes rested for longer periods on these 2 surfaces. Successive landings on walls after short flights showed that mosquitoes gradually increased their mean landing height from 1.0 to 1.4 m. Similarly, mosquitoes resting at the base of inner roofs had a successive landing height range of about 0.5 m. Based on these observations and the potential for reduction of nearly 50% in the quantity of insecticide used and the time needed to apply it, village-scale studies involving the selective spraying of a 1-m-wide swath of insecticide on walls and on roofs are recommended in this area.

Indoor low-volume insecticide spray for the control of Anopheles albimanus in southern Mexico. Village-scale trials of bendiocarb, deltamethrin and cyfluthrin.

A comparative insecticide village-scale trial was carried out to determine the efficacy of low-volume (LV) indoor spray of bendiocarb, deltamethrin and cyfluthrin for the control of Anopheles albimanus in a coastal plain of southern Mexico. Low-volume spray was conducted with knapsack mist-blowers, giving an average discharge rate of 215 ml/min, which deposited droplets of 50-100 microns. Using this technique, 25 houses were treated/sprayman/day as compared with 8 houses/sprayman/day using conventional hand compression pumps. Indicators of LV mortality showed a residual activity of 10 wk as compared with 13 wk of activity for wettable powder (WP) (bendiocarb). When comparing expenditures, LV spray costs were 43% less when using bendiocarb, 83% less with deltamethrin and 74% less with cyfluthrin with respect to the conventional WP spray of the same insecticides. Low-volume spray time was reduced by one-third, with respect to WP. Malaria incidence was reduced by 53 and 56%, respectively, in the bendiocarb LV and deltamethrin LV treated villages as compared with the untreated village.

Anopheles albimanus (Diptera: Culicidae) host selection patterns in three ecological areas of the coastal plains of Chiapas, southern Mexico.

The host-feeding patterns of Anopheles albimanus Wiedemann were described and the effect of host availability on these patterns was assessed in three different ecological areas of coastal Chiapas, Mexico. Resting mosquitoes were collected indoors and outdoors during rainy seasons. A 20% sample of blood-fed mosquitoes was tested to determine the source of the blood meal using an ELISA. The unweighted human blood index (HBI) of An. albimanus in the three areas ranged from 0.11 to 0.21. This mosquito species fed more frequently on bovines than on any other host, but the forage ratio indicated that there was also a high preference for equines. Some females tended to rest or complete their gonotrophic cycle indoors after feeding on animals, but females also fed on man and rested outdoors. Host availability and ecological conditions appeared to be responsible for differences observed in the HBI among areas.

[Effectiveness of a carbamate insecticide as a household low-volume spray for malaria control]. / Efectividad de un insecticida carbamato en rociado intradomiciliar a bajo volumen para el control del paludismo.

A comparative regional-scale evaluation of the epidemiological impact of low volume (LV) spray of bendiocarb and the conventional spray of DDT against malaria in an endemic area of northern Chiapas was carried out. Three Anopheline species were found: Anopheles pseudopunctipennis, An. albinanus and An. argyritarsis. The most prevalent was An. pseudopunctipennis, a species we suspect may be involved in the transmission of most malaria cases. This species showed high levels of resistance to DDT. However, this insecticide had a long residual effect in wall bioassays, with mortalities > or = 95 per cent for up to 21 weeks in wood, sticks and plaster. Susceptibility to bendiocarb was total, and mortality to LV bendiocarb was > or = 75 per cent for up to 16 weeks in wood, sticks and straw. Very low numbers of mosquitoes were found throughout the evaluation, although malaria transmission continued in control villages (sprayed with DDT), as well as treated villages (sprayed with LV bendiocarb). No plasmodium infected mosquitoes were found, perhaps due to a very small sample size. The LV spray methodology was found to be 1.7 times more effective than conventional spraying in reducing malaria incidence. A net reduction of 1.6 times in insecticide application time was also found, which would allow spraying at the right time, especially when urgent control measures have to be applied, such as in malaria outbreaks. Finally, the new methodology costs 2.2 times more than the conventional ddt spraying, but if the potential of using spray workers in other activities is considered, costs would be comparable to those of DDT spraying.

Tests for the existence of genetic determination or conditioning in host selection by Anopheles albimanus (Diptera: Culicidae).

The existence of sympatric populations within Anopheles albimanus Wiedemann with specific host preferences and the question of whether host selection has a genetic basis or is a learned characteristic were investigated. Progeny of wild females collected from corrals or human bait were reared in an insectary. F1 females were fed on a cow or on a human host, allowed to oviposit, and then were released in an experimental hut divided into three compartments. The side rooms occupied by two human or one calf baits were connected by a "funnel-like" cone with the central room in which mosquitoes were released. Females were released at 2000 hours and recaptured the next morning at 0600 hours. A fixed proportion of mosquitoes selected either host (approximately 65% cow and 35% human), irrespective of their parental origin or source of the first blood meal. The data indicate that the existence of cryptic populations with anthropophilic habits is unlikely and that a "learned" host selection also may be ruled out.

Gonotrophic cycle and survivorship of Anopheles albimanus (Diptera: Culicidae) in southern Mexico.

The length of the gonotrophic cycle of Anopheles albimanus was estimated by 12 mark-release-recapture studies conducted in corrals in southern Mexico from 1987 to 1990. The initial set of three mark-release studies indicated that the gonotrophic cycle takes at least 4 d based on the day when gravid mosquitoes were first recaptured. However, in later experiments, mosquitoes recaptured seeking hosts at 48 h after release were in Sella's and Christophers' stages I and II, but parity rates had nearly doubled, indicating that eggs may have developed in less than 48 h and that mosquitoes returned to refeed immediately following oviposition. Two gonotrophic cycles probably exist, one of 48 h dominated by parous mosquitoes and one of 4 d comprised of nulliparous, pregravid mosquitoes. Daily survivorship was estimated by regression from the decrease in the daily recapture rate (0.46-0.68) was less than that estimated by the parity rates (0.67-0.69), with no important differences found between wet and dry seasons. Estimates of the probability of a mosquito living long enough to transmit malaria were 2% (range, 1.8-2.5%).

Scanning electron microscopic observations of Anopheles albimanus (Diptera: Culicidae) eggs.

To investigate the existence of subspecies of Anopheles albimanus Wiedeman in southern Mexico, the egg morphology of specimens obtained from several field populations and from insectary-adapted colonies of uniform pupal phenotype was examined. Scanning electron microscopic observations have shown that the eggs of An. albimanus are polymorphic in respect to the size and shape of their floats, but not in their ornamentation. Four types of eggs were found. Differences in the proportion of the various morphological types were statistically significant, although proportions of egg types were variable among individuals within the same population. These observations are suggestive of distinctive populations and warrant further studies using more sensitive methods to investigate sibling species in An. albimanus sensu lato.

Age structure and abundance levels in the entomological evaluation of an insecticide used in the control of Anopheles albimanus in southern Mexico.

Applications of bendiocarb produced a high insecticidal residual effect lasting up to 3 months on the most common indoor house surfaces. No significant decreases in mosquito man-biting rate levels were observed between treated and untreated villages. It was shown that almost equal proportions of intra- and peridomicillary mosquitoes came into contact with the insecticide, indicating that mosquitoes commonly enter houses, rest on treated surfaces and return to bite both indoors and outdoors. Although the insecticide was found to have a significant effect on the percentage parity (interpreted as abundance of older individuals) of intra- and peridomicillary Anopheles albimanus mosquitoes, parity recovered and continued a normal cyclic pattern that appeared to be dependent on relative abundance. The proximity of a treated village to an untreated village (1.2 km) can affect the age structure of mosquito populations through shared common resting and breeding sites.

Comparative use of bendiocarb and DDT to control Anopheles pseudopunctipennis in a malarious area of Mexico.

The state of Sinaloa has one of the highest and most persistent malaria transmission levels in Mexico. Due to this situation, with resistance of the vector Anopheles pseudopunctipennis Theobald to DDT, the carbamate insecticide bendiocarb was evaluated as an alternative to DDT for residual house-spraying in village-scale trials during 1985-87. Application rates of the active ingredient per square metre of sprayable surface (ai/m2) were 0.4 g bendiocarb 80% wettable powder (80WP) and 2 g DDT 75% WP. Both insecticides failed to control mosquito populations. Human-bait mosquito densities were not altered as a result of insecticide spraying and human-bait collected mosquito mortality rates were low, suggesting little pre-biting insecticide contact due to avoidance or insufficient resting time indoors. Lower densities of indoor-resting mosquitoes were observed with DDT as opposed to bendiocarb treated houses. Anopheline mortality was higher (98-100%) when exposed for 1 h to 1% bendiocarb in standard WHO susceptibility tests and wall bioassays. Mortality-rates of 15-48% due to 1 h exposure to 4% DDT indicated that this insecticide may continue to be partially effective. House curtain and mark-recapture mosquito studies indicated that DDT produced higher excito-repellency than bendiocarb, as reflected by more mosquito landings but lower feeding rates, shorter resting period and earlier exit time from DDT sprayed houses. In the absence of insecticide, more than 50% of blood-fed An.pseudopunctipennis females exited from houses within 2-4 h of release, showing exophilic behaviour. The outdoor/indoor density ratio indicated that the majority were exophagic. These behavioural characteristics limit the usefulness of any residual insecticide against An.pseudopunctipennis.

Anopheles vestitipennis, the probable vector of Plasmodium vivax in the Lacandon forest of Chiapas, México.

Baseline studies to evaluate the role of different anopheline mosquitoes as probable vectors of malaria in the Lacandon rainforest region in Chiapas (México) were carried out in 3 rural villages between June and November 1988. Anopheles vestitipennis was the most abundant species in all the villages, followed by An. albimanus, An. punctimacula and An. darlingi. Human bait collections showed that An. vestitipennis and An. albimanus tended to bite man either indoors or outdoors, while An. punctimacula preferred to bite outdoors. Monthly man-biting rates of An. vestitipennis tended to follow the rainfall pattern. Plasmodium vivax was detected in An. vestitipennis only, with an estimated overall infection rate of 4.67 per 1000 mosquitoes (range 0-54.42, of 3500 mosquitoes assayed). This is the first incrimination of An. vestitipennis as a probable vector of P. vivax in México.

Effect of indoor residual spraying of DDT and bendiocarb on the feeding patterns of Anopheles pseudopunctipennis in Mexico.

Intense and persistent use of DDT for malaria control has increased resistance and induced exophilic behavior of Anopheles pseudopunctipennis. An evaluation of bendiocarb and DDT to control this species in Sinaloa, Mexico, showed that, in spite of DDT-resistance, both insecticides produced similar effects. Feeding patterns were analyzed to explain these results. Resting mosquitoes were collected over the dry and wet seasons. Anophelines were tested in an ELISA to determine the source of the meals. The human blood index (HBI) ranged from 3.3 to 6.8% in DDT- and from 12.7 to 26.9% in bendiocarb-sprayed houses. Irritability and repellency in DDT-sprayed houses could explain the reduced HBI. In contrast, bendiocarb produced higher mortality. These effects could have affected different components of the vectorial capacity and similarly reduced malaria.

Epidemiology of a natural focus of Leishmania braziliensis in the Pacific lowlands of Colombia. III. Natural infections in wild mammals.

A natural infection of Leishmania braziliensis panamensis in the edentate, Choloepus hoffmanni is recorded from the Pacific Coast of Columbia.