Malaria infection increases attractiveness of humans to mosquitoes

Do malaria parasites enhance the attractiveness of humans to the parasite's vector? As such manipulation would have important implications for the epidemiology of the disease, the question has been debated for many years. To investigate the issue in a semi-natural situation, we assayed the attractiveness of 12 groups of three western Kenyan children to the main African malaria vector, the mosquito Anopheles gambiae. In each group, one child was uninfected, one was naturally infected with the asexual (non-infective) stage of Plasmodium falciparum, and one harboured the parasite's gametocytes (the stage transmissible to mosquitoes). The children harbouring gametocytes attracted about twice as many mosquitoes as the two other classes of children. In a second assay of the same children, when the parasites had been cleared with anti-malarial treatment, the attractiveness was similar between the three classes of children. In particular, the children who had previously harboured gametocytes, but had now cleared the parasite, were not more attractive than other children. This ruled out the possibility of a bias due to differential intrinsic attractiveness of the children to mosquitoes and strongly suggests that gametocytes increase the attractiveness of the children.     

A new Y-tube olfactometer for mosquitoes to measure the attractiveness of host odours

The behavioural responses of flying western flower thrips (Frankliniella occidentalis Pergande) (Thysanoptera: Thripidae) to the colour yellow and the odour anisaldehyde were examined. In a wind tunnel, upwind flight by female thrips was common in an airflow of 0.11 m s^–1 but was impeded at 0.22 m s^–1. In the absence of anisaldehyde, flying female thrips exhibited an oriented response towards a yellow cue in the wind tunnel at a wind speed of 0.11 m s^–1. The main response of females to anisaldehyde in the wind tunnel was flight inhibition. There was no evidence of an odour-induced visual response, an odour-induced anemotactic response or chemotaxis by female thrips to anisaldehyde in wind tunnel bioassays, but chemokinesis was implicated. With a matrix of yellow or black water traps with and without anisaldehyde in a greenhouse sweet pepper crop, yellow traps with anisaldehyde caught more thrips adults than yellow traps without anisaldehyde, black traps with anisaldehyde and black traps without anisaldehyde (1.3, 28 and 721 times for males respectively and 2.4, 9 and 117 times for females, respectively). Differences between respective traps were statistically significant in almost all cases. Trapping experiments using a centre-baited trap design to reduce the interaction of anisaldehyde between baited and unbaited traps were undertaken in tomato and sweet pepper greenhouse crops. When the spatial distribution of the thrips adult population within the greenhouse was taken into account, yellow water traps with anisaldehyde caught between 11 and 15 times more female and 3 and 20 times more male F. occidentalis adults than yellow traps without anisaldehyde.     

Composition of human skin microbiota affects attractiveness to malaria mosquitoes

The African malaria mosquito Anopheles gambiae sensu stricto continues to play an important role in malaria transmission, which is aggravated by its high degree of anthropophily, making it among the foremost vectors of this disease. In the current study we set out to unravel the strong association between this mosquito species and human beings, as it is determined by odorant cues derived from the human skin. Microbial communities on the skin play key roles in the production of human body odour. We demonstrate that the composition of the skin microbiota affects the degree of attractiveness of human beings to this mosquito species. Bacterial plate counts and 16S rRNA sequencing revealed that individuals that are highly attractive to An. gambiae s.s. have a significantly higher abundance, but lower diversity of bacteria on their skin than individuals that are poorly attractive. Bacterial genera that are correlated with the relative degree of attractiveness to mosquitoes were identified. The discovery of the connection between skin microbial populations and attractiveness to mosquitoes may lead to the development of new mosquito attractants and personalized methods for protection against vectors of malaria and other infectious diseases.     

Selection for virulent dengue viruses occurs in humans and mosquitoes

Dengue is the most common mosquito-borne viral disease in humans. The spread of both mosquito vectors and viruses has led to the resurgence of epidemic dengue fever (a self-limited flu-like syndrome) and the emergence of dengue hemorrhagic fever (severe dengue with bleeding abnormalities) in urban centers of the tropics. There are no animal or laboratory models of dengue disease; indirect evidence suggests that dengue viruses differ in virulence, including their pathogenicities for humans and epidemic potential. We developed two assay systems (using human dendritic cells and Aedes aegypti mosquitoes) for measuring differences in virus replication that correlate with the potential to cause hemorrhagic dengue and increased virus transmission. Infection and growth experiments showed that dengue serotype 2 viruses causing dengue hemorrhagic fever epidemics (Southeast Asian genotype) can outcompete viruses that cause dengue fever only (American genotype). This fact implies that Southeast Asian genotype viruses will continue to displace other viruses, causing more hemorrhagic dengue epidemics.     

Fatal attraction phenomenon in humans: cat odour attractiveness increased for toxoplasma-infected men while decreased for infected women

Background

Latent toxoplasmosis, a lifelong infection with the protozoan Toxoplasma gondii, has cumulative effects on the behaviour of hosts, including humans. The most impressive effect of toxoplasmosis is the “fatal attraction phenomenon,” the conversion of innate fear of cat odour into attraction to cat odour in infected rodents. While most behavioural effects of toxoplasmosis were confirmed also in humans, neither the fatal attraction phenomenon nor any toxoplasmosis-associated changes in olfactory functions have been searched for in them.

Principal Findings

Thirty-four Toxoplasma-infected and 134 noninfected students rated the odour of urine samples from cat, horse, tiger, brown hyena and dog for intensity and pleasantness. The raters were blind to their infection status and identity of the samples. No signs of changed sensitivity of olfaction were observed. However, we found a strong, gender dependent effect of toxoplasmosis on the pleasantness attributed to cat urine odour (p = 0.0025). Infected men rated this odour as more pleasant than did the noninfected men, while infected women rated the same odour as less pleasant than did noninfected women. Toxoplasmosis did not affect how subjects rated the pleasantness of any other animal species'' urine odour; however, a non-significant trend in the same directions was observed for hyena urine.

Conclusions

The absence of the effects of toxoplasmosis on the odour pleasantness score attributed to large cats would suggest that the amino acid felinine could be responsible for the fatal attraction phenomenon. Our results also raise the possibility that the odour-specific threshold deficits observed in schizophrenia patients could be caused by increased prevalence of Toxoplasma-infected subjects in this population rather than by schizophrenia itself. The trend observed with the hyena urine sample suggests that this carnivore, and other representatives of the Feliformia suborder, should be studied for their possible role as definitive hosts in the life cycle of Toxoplasma.     

Electrical activity of the brain and oxygen supply for cognitive-mnestic activity in humans under different degrees of hypoxia

Comparative analysis of the indices of oxygen supply for the body and the brain and the functional state of the brain was carried out in three experimental situations: performance of cognitive-mnestic tests under normoxic (TN) and hypoxic conditions (TH) and exposure to hypoxia in the absence of cognitive tasks (H). Each subject participated in all of the experiments. Hypoxic conditions were created by breathing of a hypoxic gas mixture (HGM) consisting of oxygen and nitrogen for 25 min. The first group (eight subjects) was exposed to moderate hypoxia, with the gas mixture containing 12% O₂ (HGM-12); the second group (eight subjects) was exposed to severe hypoxia, with the gas mixture containing 8% O₂ (HGM-8). The cognitive-mnestic activity (CMA) under both normoxia and hypoxia was continuous and included the following tests: “Arithmetic calculations,” “Memory for numerals,” “Colored figures”, and “Sensorimotor response time.” The CMA efficiency was significantly impaired only under severe hypoxia (HGM-8). The CMA efficiency was higher in some subjects of the first group (HGM-12) in the TH series as compared to that in the TN series. The EEG spectral power (SP) during CMA was decreased as compared to the background in all subjects in the TN series and in most subjects of the first group, exposed to HGM-12, whereas it was increased in all subjects of the second group, exposed to HGM-8. The EEG SP was lower in most subjects of both groups studied in the TH series as compared to that found in the H series. The rheographic index of cerebral blood flow rate was not changed compared to the background in the TN series and was increased in the H and TH series during HGM-8 treatment. The increase in cerebral blood flow was less pronounced in the TH series as compared to the H series in most subjects of the second group (HGM-8). Oxygen consumption by the body was elevated by 10–20% in the TN series. A significant increase in oxygen consumption was found in the subjects of both groups studied during hypoxia treatment (H), and it was greater in HGM-12. The following differences were found between the subjects of the two experimental groups: the increase in oxygen consumption in most subjects of the first group (HGM-12) was higher in the TH series as compared to the H series, whereas, in most subjects of the second group (HGM-8), the increase in oxygen consumption was higher in the H series. The data are discussed from the point of view of synergic and concurrent relationships between different forms of energy expenditure on structural and functional reorganization and organ-specific functions.     

Physical attractiveness and reproductive success in humans: evidence from the late 20th century United States

Physical attractiveness has been associated with mating behavior, but its role in reproductive success of contemporary humans has received surprisingly little attention. In the Wisconsin Longitudinal Study (1244 women, 997 men born between 1937 and 1940), we examined whether attractiveness assessed from photographs taken at age 18 years predicted the number of biological children at age 53–56 years. In women, attractiveness predicted higher reproductive success in a nonlinear fashion, so that attractive (second highest quartile) women had 16% and very attractive (highest quartile) women 6% more children than their less attractive counterparts. In men, there was a threshold effect so that men in the lowest attractiveness quartile had 13% fewer children than others who did not differ from each other in the average number of children. These associations were partly but not completely accounted for by attractive participants' increased marriage probability. A linear regression analysis indicated relatively weak directional selection gradient for attractiveness (β=0.06 in women, β=0.07 in men). These findings indicate that physical attractiveness may be associated with reproductive success in humans living in industrialized settings.     

Selective attractiveness of rodent-baited traps for female blowflies

Five species of Calliphoridae, mostly females, were collected using fly-traps baited with freshly killed rodent carcasses. Female blowflies were examined to determine the stages of ovarian development. The traps were selective for certain ovarian stages which varied according to the species. Phaenicia eximia (Wiedemann) was attracted to the traps mainly to oviposit, whereas females of Chrysomya albiceps (Wiedemann), C. putoria (Wiedemann) and C. megacephala (Fabricius) were captured in intermediate stages of oogenesis but seldom with eggs mature for oviposition. It is concluded that this kind of baited trap is unsuitable for the collection of male blowflies and that samples of female blowflies are specifically biased in favour of certain ovarian conditions.     

Ecological prerequisites for spread of bloodsucking mosquitoes

The spread of mosquitoes beyond the initial ranges and the ecological prerequisites for colonization of new territories are considered for several mosquito species which are active bloodsuckers and vectors of many disease agents. The invasion of homodynamic Aedes aegypti from Africa into the tropical zone of several continents has probably occurred during the last 400 years by means of transportation of the eggs and larvae mainly by sea ships. In the new areas mosquitoes occupied breeding places similar to those in their native ranges. On the contrary, the urban mosquito Culex pipiens f. molestus, introduced from the subtropical areas of Africa into Europe and other continents in the XX century, found a new type of habitat: underground constructions with polluted water where the mosquito develops the year round. This was preconditioned by such specific biological features as autogeny, stenogamy, and homodynamy (seasonal development without diapause). Aedes albopictus originated from Southeast Asia and invaded four continents (Europe, North and South America, and Africa) in 1980–2000 owing to its high capacity of adaptation to various climatic conditions (high viability of eggs, variation in the ability for diapause and mechanisms of diapause control between the strains, etc.). The eggs and larvae of this mosquito were transported by sea in used car tires. The models predicting the distribution of invasive species and the ways of their spread (by sea, air, motor transport) are discussed.     

Genetics of mosquitoes

Beginning in the mid-1950s, much progress has been made in studying various aspects of the genetics of mosquitoes, particularly involving several species of three principal genera,Aedes, Culex andAnopheles, that transmit important human diseases. Here I discuss selected areas of research involving formal genetics; genome structure, organization and evolution at the interspecific and intraspecific level; and evolutionary genetics of theAedes scutellaris group. Information and insights gained from in-depth analyses of these areas, particularly transmission genetics, cytogenetics and genetics of chromosomal rearrangements, and of mutagen-induced sexual sterility, have proved invaluable for the development of the theory and evaluation of feasibility of genetic control of natural populations. As a result, mosquitoes represent some of the best studied taxa at various levels of genetic organization. Recent developments in molecular genetics offer exciting possibilities for extension of these concepts.     

Control mechanisms for vitellogenin synthesis in mosquitoes

Earlier investigations of the hormonal control of vitellogenin (egg-yolk protein) synthesis in mosquitoes relied on microsurgical manipulations to identify organs that secrete the various hormones that stimulate or inhibit vitellogenesis. Advances in the last 10 years, using radioimmunoassays, HPLC, mass spectrometry and tissue-culture incubations, have shown that the control mechanism involves a complex interaction between ecdysteroids, juvenile hormones and peptide hormones.     

Testing a postulated case of intersexual selection in humans: The role of foot size in judgments of physical attractiveness and age

The constituents of attractiveness differ across the sexes. Many relevant traits are dimorphic, suggesting that they are the product of intersexual selection. However, direction of causality is generally difficult to determine, as aesthetic criteria can as readily result from, as cause, dimorphism. Women have proportionately smaller feet than men. Prior work on the role of foot size in attractiveness suggests an asymmetry across the sexes, as small feet enhance female appearance, yet average, rather than large, feet are preferred on men. Previous investigations employed crude stimuli and limited samples. Here, we report on multiple cross-cultural studies designed to overcome these limitations. With the exception of one rural society, we find that small foot size is preferred when judging women, yet no equivalent preference applies to men. Similarly, consonant with the thesis that a preference for youth underlies intersexual selection acting on women, we document an inverse relationship between foot size and perceived age. Examination of preferences regarding, and inferences from, feet viewed in isolation suggests different roles for proportionality and absolute size in judgments of female and male bodies. Although the majority of these results bolster the conclusion that pedal dimorphism is the product of intersexual selection, the picture is complicated by the reversal of the usual preference for small female feet found in one rural society. While possibly explicable in terms of greater emphasis on female economic productivity relative to beauty, the latter finding underscores the importance of employing diverse samples when exploring postulated evolved aesthetic preferences.