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Biomedical subjects

W Takken

Publications and source records attributed to W Takken.

At least 37 records · Page 2Linked to original sources
PubMed2000

Sampling gravid Culex quinquefasciatus (Diptera: Culicidae) in Tanzania with traps baited with synthetic oviposition pheromone and grass infusions.

The effectiveness of traps baited with (5R,6S)-6-acetoxy-5-hexadecanolide (the synthetic oviposition pheromone) and grass infusions in sampling a population of gravid Culex quinquefasciatus Say was conducted in Muheza, Northeast Tanzania. A counterflow geometry (CFG) trap baited with pheromone and set outdoors, adjacent to a pit latrine building, collected more gravid Cx. quinquefasciatus than a CDC trap baited with pheromone and operated without light. Inside pit latrine buildings, significantly more gravid Cx. quinquefasciatus were collected in a CFG trap-baited with pheromone or grass infusion than in traps baited with tap water. CFG traps baited with either grass infusion or pheromone and set outdoors, away from known breeding sites, caught significantly more gravid Cx. quinquefasciatus than traps baited with tap water. CFG traps baited with pheromone + grass infusion caught significantly more gravid Cx. quinquefasciatus than CFG traps baited with either grass infusion or pheromone. In both cases, the proportion of gravid mosquitoes increased as traps were moved away from a natural emergence site. More gravid Cx. quinquefasciatus were collected in a pheromone-baited CFG trap than were egg rafts deposited in a jar with pheromone-treated water. It is concluded that CFG traps baited with oviposition attractants can be used effectively to sample gravid Cx. quinquefasciatus.

Animals↗
PubMed1999

[A return of endemic malaria to the Netherlands is highly unlikely].

The Netherlands has been free from malaria since the early 1960, due to a combination of factors: active search and treatment of patients and parasite carriers, targeted use of insecticides, changes in farming and in housing of man and cattle, pollution of surface water with phosphates and the fact that surface waters became fresher. These factors reduced the mosquito population that is dependent on brackish water. The Dutch malaria mosquito cannot transmit the parasite of tropical malaria. The mosquito population could possibly increase due to measures to 'develop nature' but the number of parasite carriers, the acute disease manifestations, the quality and organization of the health care system make it extremely unlikely that local transmission will occur. Fears that malaria may become endemic and that the population in the western parts of the country will have to apply malaria chemoprophylaxis in the near future, are unfounded.

Animals↗
PubMed1999

Odor-mediated behavior of Afrotropical malaria mosquitoes.

The African mosquito species Anopheles gambiae sensu lato s.l. and Anopheles funestus rank among the world's most efficient vectors of human malaria. Their unique bionomics, particularly their anthropophilic, endophagic and endophilic characters, guarantee a strong mosquito-host interaction, favorable to malaria transmission. Olfactory cues govern the various behaviors of female mosquitoes and here we review the role of semiochemicals in the life history of African malaria vectors. Recent evidence points towards the existence of human-specific kairomones affecting host-seeking A. gambiae s.l., and efforts are under way to identify the volatiles mediating this behavior. Based on examples from other Culicidae spp., it is argued that there is good reason to assume that mating, sugar feeding, and oviposition behavior in Afrotropical malaria vectors may also be mediated by semiochemicals. It is foreseen that increased knowledge of odor-mediated behaviors will be applied in the development of novel sampling techniques and possibly alternative methods of intervention to control malaria.

Africa↗
PubMed1999

Biological problems with the replacement of a vector population by Plasmodium-refractory mosquitoes.

Attempts are being made to backcross into Anopheles gambiae s.s. the gene(s) which cause zoophily in Anopheles quadriannulatus. Such a backcrossed strain might be preferable to a Plasmodium-refractory strain as a basis for genetic control because a refractory strain could select for evasion of refractoriness in the wild Plasmodium population. The species composition of the malaria vector population in several Tanzanian villages was overwhelmingly An. gambiae s.s. in a normal rainy season, but consisted of four species, all proved by ELISA and/or PCR to carry P. falciparum sporozoites, at the time of the heavy rains associated with El Niño. Thus any scheme, for malaria transmission control by replacement of vectors by genetically-manipulated non-vectors, would have to be able to replace more than one species.

Animals↗
PubMed1998

Differential responses of mosquito sibling species Anopheles arabiensis and An. quadriannulatus to carbon dioxide, a man or a calf.

Field studies on responses of two mosquito sibling species, Anopheles arabiensis Patton and An. quadriannulatus Theobald, to a man, a calf and different release rates of carbon dioxide (man, calf and cow equivalents) were conducted in north-eastern South Africa. Various combinations of baits were compared in two-choice tests, using two mosquito nets, placed 2.5 m apart and 10 cm off the ground. Mosquitoes attracted to the baits were able to enter the nets from below and were collected by means of a suction tube. In a two-choice test between a man and CO2 (human equivalent, 250 ml/min), 81% of the An. quadriannulatus were caught with CO2. The reverse was seen for An. arabiensis, where only 20% of the total catch was caught with CO2 compared to man. High release rates of CO2 (cow equivalent, 800 ml/min) attracted significantly more An. quadriannulatus than the low release rate (250 ml/min), whereas no significant effect of the release rate of CO2 on the total catch of An. arabiensis was seen. In the latter species, up to 33% of the attraction of human emanation is attributable to carbon dioxide. Anopheles quadriannulatus was equally attracted to a calf and CO2 (calf equivalent, 180 ml/min). Catches of other mosquito species showed consistent differences between all treatments which appear to be associated with differences in host-preference, suggesting that the importance of CO2 in host-seeking behaviour of mosquitoes increases with the degree of zoophily.

Animals↗
PubMed1998

Effect of body size on host seeking and blood meal utilization in Anopheles gambiae sensu stricto (Diptera: Culicidae): the disadvantage of being small.

The survival, metabolic reserves, and host-seeking response of unfed Anopheles gambiae Giles sensu stricto females of different size classes were investigated from 1 to 6 d after emergence. In addition, the effect of blood meal size and frequency of feeding on the accumulation of metabolic reserves and reproductive development were investigated during the 1st gonotrophic cycle. Large females survived longer and contained significantly more protein, glycogen, and lipid at emergence than did small females. Over 6 d, the host-seeking response of large mosquitoes was greater than that of small ones. There was no gonotrophic development in unfed small females after emergence, whereas < or = 52% of large females developed oocytes to Christophers stage II. One blood meal, given by enema, resulted in gonotrophic development to stage II in small females, whereas only large females matured oocytes to Christophers stage V. In both large and small females, egg development was related to the size of the blood meal, but large females were better able to utilize the meals. Two successive blood meals enhanced the ability of small females to develop eggs. The pregravid condition commonly observed in An. gambiae appears to be determined by insufficient metabolic reserves at emergence, and the probability for small individuals to be pregravid is greater than for large mosquitoes because of the need to build up sufficient energy reserves before gonotrophic development can occur. The consequence of this behavior in relation to malaria transmission is discussed.

Animals↗
PubMed1998

Olfactory responses of female Culex quinquefasciatus Say (Diptera: Culicidae) in a dual-choice olfactometer.

Olfactory responses of individual female (n = 1010) Culex quinquefasciatus Say (Diptera: Culicidae) to various odor stimuli were studied in a dual-choice olfactometer. Responses (i.e., the number of mosquitoes entering either of both olfactometer ports) were studied towards clean conditioned air (control), human foot skin emanations (collected on worn stockings), carbon dioxide (4.5% in clean air), moistened air, and various combinations thereof. Skin emanations were significantly more attractive (chi 2 = 23.0, p < 0.001) than clean stockings (control). The mosquito was also significantly more attracted (chi 2 = 7.7, p < 0.01) to skin emanations than to a clean stocking to which water (an equivalent of that absorbed by a worn stocking) was added. A moistened (1 g H2O) clean stocking, however, was slightly more attractive than a dry stocking (chi 2 = 6, p < 0.025). Carbon dioxide (4.5%) did not elicit higher responses than clean air, and no synergistic effect was observed in combination with skin emanations. With the aim of developing an odor-baited trap, our results indicate that Cx. quinquefasciatus responds well to human body odors which can be collected on polyamide materials.

Animals↗
PubMed1996

Synthesis and future challenges: the response of mosquitoes to host odours.

There is ample evidence that host seeking in mosquitoes is mediated by semiochemicals emanating from the host. Olfactory cues (kairomones) are detected through an intricate pathway, beginning with sensilla located on the antennae (odour) and palpi (CO2). Age and physiological state of the mosquito determine whether detection of kairomones results in a behavioural response. Only a few kairomones have been described so far. CO2 is a kairomone for most mosquito species and signifies the presence of a potential host because of its occurrence in the volatile emissions of all vertebrates. Other chemicals are likely to play a role in mosquito-host interaction as well, notably L-lactic acid, fatty acids and 1-octen-3-ol. Species-specific host preference is thought to be olfactory based and related to the presence of specialized sensilla or perception at the central olfactory pathway. Host specificity is genetically determined, as demonstrated by inherited differences within members of the Anopheles gambiae complex. In view of the available evidence, challenging research areas are: (1) the function of olfactory receptors and the level of specialization; (2) the identification of general and host-specific kairomones; (3) demonstration of behavioural responses to laboratory-identified olfactory cues in the field; (4) studying whether mosquito host-locating behaviour can be manipulated by kairomones; (5) the genetics of the regulation of olfactory behaviour; and (6) determining whether there is an olfactory basis for the evolution of mosquito-host interactions.

Animals↗
PubMed1996

Model stimulations to estimate malaria risk under climate change.

The current geographic range of malaria is much smaller than its potential range. In many regions there exists a phenomena characterized as "Anophelism without malaria." The vectors are present but malaria transmission does not occur. Vectorial capacity often has been used as a parameter to estimate the susceptibility of an area to malaria. Model computations with global climatological data show that a dynamic concept of vectorial capacity can be used as a comparative risk indicator to predict the current extent and distribution of malarious regions in the world. A sensitivity analysis done in 3 distinct geographic areas shows that the areas of largest change of epidemic potential caused by a temperature increase are those where mosquitoes already occur but where development of the parasite is limited by temperature. Computations with the model presented here predict, with different climate scenarios, an increased malaria risk in areas bordering malaria endemic regions and at higher altitudes within malarious regions under a temperature increase of 2-4 degrees C.

Animals↗
PubMed1995

Mapping the densities of malaria vectors within a single village.

Small scale spatial variation and temporal heterogeneity in mosquito densities can have important consequences for disease transmission, but the extreme variation which is observed in populations of malaria vectors makes it difficult to obtain good predictions of densities for short time periods over limited areas. We have applied Bayesian techniques derived for use in cancer epidemiology in order to map densities of Anopheles gambiae s.l. and A. funestus in a Tanzanian village where there is intense transmission of Plasmodium falciparum malaria. Estimates derived in this way should prove useful in vector population biology and in improving estimates of exposure-response relationships of the human host to malaria. The same methods can be applied in other fields of animal ecology.

Animals↗
PubMed1994

Odor-induced host location in tsetse flies (Diptera: Glossinidae)

Four aspects of olfaction in host location by tsetse flies, Glossina spp., are discussed as follows: (1) host location and its mechanism, (2) factors affecting host location, (3) kairomones and host location, and (4) kairomones and host selection. Flight behavior in the various phases of host location (i.e., ranging, activation, orientation, and landing) in the absence and presence of olfactory cues is summarized. Movement toward an odor source is effected inter alia through optomotor-steered, upwind anemotaxis. It is still unclear how tsetse employ upwind anemotaxis to realize host location, considering the often highly variable wind direction. Olfactorily induced activation is governed by the olfactory cue perceived and threshold levels set by the internal state of the fly. The former depends on the odor source and distance from it; the latter is determined by species, sex, and physiological state. Wind direction and speed, as well as vegetation and the mobility of the host, interfere with successful completion of odor-induced host location. Close-range olfactory cues (including composition and concentration gradients), visual cues, and nutritional state determine whether a fly, having reached the host animal, will land on it. Carbon dioxide is important in host location because it induces landing and long-range attraction. The role of the other kairomones (acetone, 1-octen-3-ol, 4-methyl-phenol, and 3-n-propyl-phenol) is less clear. Apart from the complacency of various host species under tsetse attack, host choice by tsetse is predominantly opportunistic and primarily the result of the frequency of successful tsetse-host encounters. Nevertheless, host selection based on olfactory cues cannot be ruled out.

Animals↗
PubMed1994

Influence of human breath on selection of biting sites by Anopheles albimanus.

The selection of biting sites by Anopheles albimanus on a human host was studied in the laboratory. The perception of exhaled breath guided the mosquitoes towards the head region, and was associated with a preference (P < 0.001) for biting this part of the body. Subsequent removal of breath resulted in a strong reduction of the number of bites on the head (from 49 to 20%). The significance of these findings in relation to host location is discussed.

Adult↗
PubMed1993

Absence of seasonal variation in malaria parasitaemia in an area of intense seasonal transmission.

Parasitological surveys carried out in two villages of the Kilombero district of Tanzania indicated a very high prevalence of Plasmodium falciparum parasitaemia throughout the year (all ages mean prevalence = 69.2%) and a low, unstable prevalence of P. malariae (all ages mean prevalence = 4.5%). Fevers (temperature > or = 37.5 degrees C) in both children and adults showed irregular changes in prevalence over time, but there was no seasonal pattern. Neither was there seasonal variation in either P. falciparum parasite prevalence or parasite densities. This was despite marked seasonality in vectors caught in CDC light-traps and in estimated sporozoite inoculations determined by ELISA. The estimated mean annual inoculation rate was extremely high, over 300 infectious bites per person per year, the main vectors being members of the A. gambiae complex and Anopheles funestus. There was considerable variation between houses but even in houses with relatively low mosquito numbers the inoculation rate was sufficient to maintain a maximal P. falciparum prevalence. Heterogeneities in exposure cannot explain why the parasite prevalence is not always 100%. In areas of such high transmission, parasitaemias are likely to be determined mainly by the interaction of schizogony and anti-blood stage immunity, since parasites arising from new inoculations generally comprise only a small proportion of the total in the circulation. In any one individual, this will lead to periodic fluctuations in levels of parasitaemia. These are unlikely to show a close relationship to either seasonal variation in inoculations or to differences between households in the local inoculation rate.

Adolescent↗
PubMed1993

Effects of adult body size on fecundity and the pre-gravid rate of Anopheles gambiae females in Tanzania.

The influence of adult body size on the pre-gravid state and fecundity was studied in Anopheles gambiae Giles females hand-caught inside houses and virgin females collected as pupae in Tanzania. Blood-fed mosquitoes were kept for 2-3 days before dissection and examination for insemination and ovarian condition. Those females which did not develop eggs were classified as pre-gravid. The number of mature eggs in those mosquitoes which became gravid was counted. Virgin females were fed and kept for egg maturation in the laboratory. Wing-length of females was measured as an index of mosquito size. The overall pre-gravid rate in the resting An.gambiae population was found to be 21% and, of these, 66% had been inseminated. In the virgin females the pre-gravid rate was 92.6%. The mean wing-length of wild females which became gravid was significantly larger than those which remained pre-gravid. There was a positive correlation between fecundity and wing-length. Smaller females tended to require two or three bloodmeals to facilitate completion of the first gonotrophic cycle. The critical size permitting oviposition from the first blood-meal was a wing-length of 3 mm.

Animals↗
PubMed1990

Field studies on the potential of butanone, carbon dioxide, honey extract, 1-octen-3-ol, L-lactic acid and phenols as attractants for mosquitoes.

Various combinations of six candidate attractants--butanone, carbon dioxide (CO2), honey, octenol, lactic acid and mixed phenols--were tested against natural populations of mosquitoes in Everglades National Park, Florida, U.S.A., using unlighted CDC-baited traps. With few exceptions, the attractancy of these candidate compounds to mosquitoes, when used alone, was less than that of CO2 alone. The exceptions were that octenol and honey extract alone attracted larger numbers of Coquillettidia perturbans (Walker). Addition of lactic acid and/or octenol to CO2 increased trap collections of Aedes taeniorhynchus (Wiedemann), Anopheles atropos D. & K., and An. crucians Wiedemann by 1.4-13.8 times. Culex nigripalpus Theobald collections were increased 2.7 times by the addition of lactic acid, while the addition of octenol produced mixed results. Whereas the addition of lactic acid reduced collections of Cx (Melanoconion) spp., the addition of octenol generally increased collections. The opposite happened for Wyeomyia mitchellii (Theobald). For the biting midge, Culicoides furens (poey), octenol (1.6-23.4 x ) and phenol (2.7 x ) alone attracted larger numbers, and lactic acid alone attracted approximately the same numbers as CO2 alone. The combinations octenol + phenol and octenol + 200 ml/min CO2 increased C. furens collections c. 100 times over CO2 alone. The combination of octenol + CO2 increased (1.6 x ) collections of the tabanid Diachlorus ferrugatus (Fabricius). Butanone appeared to decrease the trap collections of all species when combined with CO2 or octenol + CO2.

Animals↗