Studies on the dynamics of transmission of onchocerciasis in a Sudan-savanna area of North Cameroon II. Seasonal and diurnal changes in the biting densities and in the age-composition of the vector population

Near the perennial breeding sites in the main water courses the highest monthly biting rates were recorded from June to August (early rainy season) and again from December to February (dry season), but near the rainy season breeding sites in the affluents, or at fly-catching sites, more than 2 km inland from the nearest main river breeding site, the highest rates were recorded between August and December (mid-rainy to early dry season). The variation of the biting rates over the year depended on the waterflow of the main rivers and on the differential dispersal of the flies from the breeding sites during the dry and rainy seasons. At the beginning of the rainy season, when the rivers started flowing, an immigration of non-local flies was observed at the river Mayo Rey near Tcholliré, presumably coming from Simulium breeding sites in the Adamaoua mountains to the South. During three years 77,374 flies were caught by daily catches between 06.00 and 18.30 hours. During the dry season the biting activity of nulliparous and parous flies was highest between 09.00 and 10.00 hours and 16.00 and 17.00 hours, 9% and 22% of the total day-catches respectively. In the rainy season the peak biting rates occurred between 06.00 and 07.00 hours (9%) and again from 16.00 to 17.00 hours (14%). The lowest biting rates were observed between 11.00 and 12.00 hours in both dry and rainy seasons (4% and 3% of the total catch). Nulliparous flies showed higher variations in biting activity than did parous flies, leading to the highest proporation of parous flies during midday.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lymphocyte proliferation and antibody responses to Plasmodium falciparum liver-stage antigen-1 in a highland area of Kenya with seasonal variation in malaria transmission

Lymphocyte proliferation and antibody responses to five peptides corresponding to the N- and C-terminal non-repeat and central repeat regions of Plasmodium falciparum liver-stage antigen-1 (LSA-1) were examined in residents of a highland area of Kenya where malaria transmission is episodic and varies with rainfall. The frequency of lymphocyte proliferation responses (stimulation index > 2) by children (persons > or = 6 years old) and adults (persons > or = 18 years old) was similar and did not differ significantly across seasons. In contrast, the proportion of individuals with IgG antibodies to LSA-1 peptides was higher in the rainy than dry season, and the frequency of these responses was greater for adults than children (39.4% versus 18.7% during the period of high transmission; P = 0.009). Antibodies to LSA-1 were primarily of the IgG1 and IgG3 subclasses, and these also varied with season (30.1% and 32.5% of individuals had IgG1 and IgG3 in the rainy season versus none and 10.9% in the dry season). There was no significant difference in the time to re-infection between groups of persons with or without IgG antibody or lymphocyte proliferation responses to LSA-1 peptides. These data indicate that age and transmission intensity independently affect IgG antibody responses to LSA-1 but do not influence lymphocyte proliferation in this highland area where malaria transmission is highly variable.     

A survey of the infective larvae of Gnathostoma spinigerum in swamp eels bought in a local market in Bangkok,Thailand

The livers were separated from the viscera of 2738 swamp eels (Monopterus alba) purchased from Klong Toey market, the largest market in Bangkok, between June 1999 and May 2000. When these livers were digested in artificial gastric juice, 524 (19.1%) were found to be infected with the human-infective, third-stage larvae (L3) of Gnathostoma spp. All the identified larvae were confirmed morphologically to be G. spinigerum. Prevalence of the infection varied with season, from a high of 38.3% in September to a low of 7.0% in April, being generally high during the rainy season and winter (June-February). The mean (S.E.) number of L3 recovered/infected liver, which was 3.99 (0.52) overall, also varied with the season, peaking at 5.38 (1.89) in January, but the month-on-month variation was not statistically significant. Although the results of an earlier study had indicated that the prevalence of eel infection decreased in November, after the rainy season, the most abrupt decrease observed in the present study occurred at the beginning of summer (March). However, the period covered by the present study was unusually wet, and the prevalence of eel infection may depend on rainfall more than season.     

Ixodes ricinus density and infection prevalence of Borrelia burgdorferi sensu lato along a North-facing altitudinal gradient in the Rhône Valley (Switzerland)

Questing Ixodes ricinus ticks were sampled monthly along a north-facing altitudinal gradient in the canton of Valais, Switzerland, from March 2004 to February 2005. Tick density and infection with Borrelia burgdorferi sensu lato were monitored. Ticks were collected by flagging vegetation at three different altitudes (750 m, 880 m, and 1020 m above sea level). Ticks were examined for Borrelia by polymerase chain reaction (PCR) followed by reverse line blot. At the three altitudes, questing tick activity was not observed under 10 degrees C and was reduced when saturation deficit was higher than 5 mm Hg, most questing tick activity was occurred between 2 mm Hg and 7 mm Hg. Tick density and peak tick density were highest at 1020 m. High saturation deficits at the lowest altitudes appear to impair the tick population. The prevalence of B. burgdorferi infection in nymphs and adults decreased with altitude. The prevalence of infection was higher in adult ticks (47%) than in nymphs (29%). Four B. burgdorferi sensu lato genospecies were detected: B. afzelii (40%), B. garinii (22%), B. valaisiana (12%) and B. burgdorferi sensu stricto (6%). Mixed infections were detected in 13% of infected ticks.     

Seasonal and social modulation of testosterone in Costa Rican rufous-collared sparrows (Zonotrichia capensis costaricensis)

Previous work shows that most birds breeding in northern temperate regions adjust production of testosterone in response to stage of the breeding cycle and in some cases following social interactions. In contrast, prior research suggests that tropical breeding birds are less likely to modulate testosterone in response to social interactions (the propensity to increase testosterone in response to social instability is known as the challenge hypothesis). To further test the challenge hypothesis in tropical birds, we investigated whether variation in season affects reproductive condition, aggressive behavior, and social modulation of testosterone in two populations of Costa Rican rufous-collared sparrow, Zonotrichia capensis costaricensis. We conducted our study at three distinct times of year: the dry season (March and May); the veranillo, a hiatus in the rainy season (July); and the late rainy season (November). Significantly more birds were in breeding condition in the dry season than in the rainy season or veranillo. In each time period, we collected baseline testosterone samples and conducted simulated territorial intrusions (STIs). Our study shows that testosterone is modulated with season independent of breeding condition, as testosterone levels were affected by season, breeding condition, and an interaction of the two factors. Males breeding in the dry season had higher plasma testosterone levels than non-breeding males in the dry season and both breeding and non-breeding males in the veranillo and rainy season. Males did not socially modulate testosterone in any season. Aggressive behaviors expressed during STIs did not differ among seasons with the exception that males sang fewer songs during the rainy season.     

Seasonal fluctuation of antibody levels to Plasmodium falciparum parasitized red blood cell-associated antigens in two Senegalese villages with different transmission conditions

The recombinant R23, PfEB200, and GST-5 antigens derive from conserved antigens associated with the Plasmodium falciparum-infected erythrocyte membrane. They were identified as targets of protective antibodies in the Saimiri sciureus model. We have assessed here the humoral response to these antigens in humans. Cross-sectional surveys were conducted in two Senegalese villages with different levels of endemicity. The prevalence of specific IgG and IgM was similar and influenced by age in both localities. The anti-R23 antibodies decreased after the rainy season, particularly in the children less than ten years old. The anti-PfEB200 response did not show significant seasonal variation. The anti-GST-5 response increased in both the less-than 10-year-old and the greater-than 10-year-old groups after the rainy season in Dielmo, but only in the Ndiop villagers who were more than 10-years-old. Thus, antigen-specific seasonal variations of antibody levels were influenced differently by age in both villages. The isotype distribution was antigen-specific and differed for both seasons.     

Season, fever prevalence and pyrogenic threshold for malaria disease definition in an endemic area of Mali

BACKGROUND: Modelling malaria parasitaemia as function of fever has been proposed as best alternative to estimate the attributable fraction of malaria fever and the sensitivity and specificity of different case definitions of malaria disease. OBJECTIVES: To determine the prevalence of fever and its relation to malaria parasitaemia and to establish a pyrogenic threshold for malaria disease in the area. METHODS: We conducted two cross-sectional surveys in children of 6 months to 9 years of age (2434 during the rainy season of 1993 and 2353 during the dry season of 1994) randomly selected from 21 areas of Bandiagara district, Mali. RESULTS: The relationship between fever and Plasmodium falciparum parasitaemia depends strongly on the season, thus affecting the malaria-attributable fraction of fever cases and the sensitivity and specificity of malaria case definitions. The overall proportion of fever attributable to malaria parasitaemia was 33.6% during the rainy season and 23.3% during the dry season, with the highest proportion occurring among the youngest children. The cut-off value, where the sensitivity curve crosses the specificity curve, was around 3200 pf/microl for all age categories during the rainy season and 200 pf/microl during the dry season. CONCLUSIONS: Malaria remains a main cause of fever in this area of Mali. The pyrogenic threshold of parasitaemia depends strongly on the season, and different cut-off levels of parasitaemia should be used during the two seasons to define malaria cases in this area.     

超声测量卵圆孔未闭直径与球囊伸展径的比较

目的 比较卵圆孔未闭（PFO）大小的判断方法。方法 47例PFO患者,年龄9~68（42±17）岁。经食管超声心动图（TEE）测量大小,经胸超声心动图声学造影（cTTE）测量右向左分流量（RLS）及球囊导管测量PFO伸展径（BSD）。结果 cTTE静息状态下PFO无RLS组TEE测量PFO大小为（3±1）mm,有RLS的大小为（5±1）mm,有RLS组PFO直径明显大于无RLS组（P<0.01）。TEE 测量PFO最大径为（4±1）mm,球囊测量伸展径为（9±4）mm,伸展径明显大于TEE最大径（P<0.01）,两者显著相关（r=0.98,P<0.01）。简单型PFO的TEE测量最大径,与球囊测量伸展径非常接近,相关程度更好,复杂型PFO的TEE测量值明显小于BSD,相关程度较低。结论 cTTE静息状态下RLS分流量越多,PFO的TEE裂隙越大;TEE测量PFO最大径明显小于其伸展径;TEE结合cTTE静息有无RLS有助于判断大型PFO。     

Seasonal variation in wing size and shape between geographic populations of the malaria vector, Anopheles coluzzii in Burkina Faso (West Africa)

The mosquito, Anopheles coluzzii is a major vector of human malaria in Africa with widespread distribution throughout the continent. The species hence populates a wide range of environments in contrasted ecological settings often exposed to strong seasonal fluctuations. In the dry savannahs of West Africa, this mosquito population dynamics closely follows the pace of surface water availability: the species pullulates during the rainy season and is able to reproduce throughout the dry season in areas where permanent water bodies are available for breeding. The impact of such environmental fluctuation on mosquito development and the phenotypic quality of emerging adults has however not been addressed in details. Here, we examined and compared phenotypic changes in the duration of pre-imaginal development, body dry mass at emergence and wing size, shape and surface area in young adult females An. coluzzii originated from five distinct geographic locations when they are reared in two contrasting conditions mimicking those experienced by mosquitoes during the rainy season (RS) and at the onset of the dry season (ODS) in Burkina Faso (West Africa). Our results demonstrated strong phenotypic plasticity in all traits, with differences in the magnitude and direction of changes between RS and ODS depending upon the geographic origin, hence the genetic background of the mosquito populations. Highest heterogeneity within population was observed in Bama, where large irrigation schemes allow year-round mosquito breeding. Further studies are needed to explore the adaptive value of such phenotypic plasticity and its relevance for local adaptation in An. coluzzii.     

Seasonal Transmission of Opisthorchis viverrini sensu lato and a Lecithodendriid Trematode Species in Bithynia siamensis goniomphalos Snails in Northeast Thailand

Seasonal changes play roles in the transmission success of fish-borne zoonotic trematodes (FZT). This study examined the seasonal transmission patterns of Opisthorchis viverrini sensu lato (s.l.) and a virgulate cercaria (family Lecithodendriidae) in the snail intermediate host, Bithynia siamensis goniomphalos in northeast Thailand. Snail samples were collected monthly during the rainy, cool, and hot seasons during 2012–2013 to determine the prevalence and intensity of larval trematode infections. The prevalence of O. viverrini s.l. varied significantly with season, being 0.31%, 1.05%, and 0.37% in the rainy, cool, and hot seasons, respectively (P < 0.05). Similarly, the prevalence of virgulate cercariae was 3.11%, 6.80%, and 1.64% in the rainy, cool, and hot seasons, respectively (P < 0.05). The intensity of larval trematode infections also varied between seasons and peaked in the hot season (P < 0.05) in both species. The snails infected with O. viverrini s.l. were significantly smaller (P < 0.05) and those infected with virgulate cercariae were significantly larger (P < 0.05) than uninfected snails. Seasonal variation and the different sizes of B. s. goniomphalos parasitized by O. viverrini s.l. and virgulate trematodes indicate complex host–parasite interactions with important implications for the epidemiology of O. viverrini s.l.     

Influence of container size, location, and time of day on oviposition patterns of the dengue vector, Aedes aegypti, in Thailand

We conducted a study to determine the effect of container size and location on oviposition site selection by Ae. aegypti in large outdoor field enclosures (10 x 10 x 4 m high). There was a strong positive relationship between increasing container diameter, container volume, and water surface area with egg numbers over both high (rainy, July) and low (cool-dry, January) dengue transmission seasons. Location of containers (indoors versus immediately outdoors and underneath houses) did not influence the number of eggs deposited for containers 5-32 cm in diameter in either season. No trends based on container color (black, brown, or grey) were observed. A slight trend with a greater numbers of eggs laid outdoors in the largest containers (42 cm diameter) during the dry season was observed. Three separate models were run using the mixed model procedure in SAS for each container attribute. Controlling for season, time, and date, the most important container attribute predicting total egg numbers was container volume (total capacity) explaining 88% of the variation, followed by water surface area (85%), and container diameter opening (83%). Oviposition peaked in the afternoon at 1600 hrs and 2000 hrs in the dry and rainy seasons, respectively. Few eggs were laid overnight (2000 hrs-0600 hrs). Our results indicate that physical attributes of oviposition sites, such as size, light-dark contrasts, and specular reflectance from water surfaces, play a significant role in oviposition site selection.     

Spatial distribution of mosquito larvae and the potential for targeted larval control in The Gambia

We examined the distribution of aquatic stages of malaria vectors in a 400-km(2) area in rural Gambia to assess the practicality of targeting larval control. During the rainy season, the peak period of malaria transmission, breeding sites were 70% more likely to have anopheline larvae in the floodplain of the Gambia River than upland sites (P < 0.001). However, mosquitoes were found in some examples of all habitats, apart from moving water. Habitats most often colonized by anopheline larvae were the largest water bodies, situated near the landward edge of the flood-plain, where culicine larvae were present. In the wet season, 49% of sites had anophelines versus 19% in the dry season (P < 0.001). Larval control targeted at specific habitats is unlikely to be successful in this setting. Nonetheless, larval control initiated at the end of the dry season and run throughout the rainy season could help reduce transmission.     

Characterising African tick communities at a wild–domestic interface using repeated sampling protocols and models

The sharing of habitat by wild and domestic animals may result in pathogen transmission, notably via ectoparasite vectors such as ticks. Interfaces between protected and communal lands constitute sharp transitions between areas occupied by host communities that are extremely contrasted in terms of composition, diversity and density. Empirical characterizations of tick communities and of their vertebrate hosts are strongly relevant for understanding the mechanisms leading to disease transmission between wild and domestic animals. In the present study we aimed at depicting the pattern of spatial variation in the density of immature ticks at such an interface located in Zimbabwe. At the end of the 2011 rainy season, we applied a hierarchical repeated protocol to collect ticks. We used the drag-sampling method in the vegetation surrounding water pans used by ungulates in 3 distinct landscape compartments (i.e. national park, mixed compartment and communal lands) characterized by a differential use by wild and domestic hosts. We combined generalized linear mixed models with site occupancy models to (1) assess tick aggregation levels at different spatial scales, (2) identify and disentangle factors which influence the density and probability of tick detection, and (3) compare robust estimations of tick densities among the landscape compartments. Ticks belonging to the Amblyomma and Riphicephalus genuses were found to be the most abundant. At small scale, ticks were more often detected in the afternoon and were more abundant close to water pans for Amblyomma and Riphicephalus genuses. Riphicephalus spp. density was also higher in grassland and bushland vegetation types as compared to woodland vegetation type. At large scale, for the three detected genuses, density was much higher near water pans located in the communal lands as compared to the national park and mixed compartment. Given that host community's diversity is much lower in the communal areas than in the two other landscape compartments, these results are compatible with a dilution effect but not sufficient to demonstrate this effect without additional studies. Up to date, it is the first utilization of these rigorous sampling and statistical modelling methodologies to estimate tick density in African ecosystem simultaneously at large and small scales.     

Ability of Anopheles gambiae mosquitoes to transmit malaria during the dry and wet seasons in an area of irrigated rice cultivation in The Gambia

The seasonality of malaria transmission was studied in a Gambian village situated in an area where rice was cultivated. Observations were made during two dry seasons, when pump-fed irrigation was used to grow rice, and in the intervening rainy season, when rice was cultivated using a combination of irrigated and rain-fed paddies. Clinical episodes of malaria were mainly confined to the months during and soon after the rainy season. In the wet season the prevalence of parasitaemia was higher in febrile subjects than in afebrile controls but the reverse applied during the dry seasons. However, the biting rates of Anopheles gambiae complex mosquitoes in the two dry seasons (2.5 and 0.8 bites/child/night respectively) were greater than or similar to that in the rainy season (0.6 bites/child/night). The proportion of human bloodmeals (0.53 vs 0.75) and the survival of mosquitoes (parity rates of 0.41 vs 0.58) were both lower in the dry seasons than in the rains. The low prevalence of morbidity due to malaria in the dry season and the observed fall in the sporozoite rate may therefore have been due to a reduction in the vectorial capacity of the An. gambiae population. However, reduced transmission in the dry season may also have been due to the direct effect of high temperatures on the parasite in the vector.     

Variation in the relationship between anti-MSP-1(19) antibody response and age in children infected with Plasmodium falciparum during the dry and rainy seasons

Malaria remains a major parasitic disease in Africa, with 300-500 million new infections each year. There is therefore an urgent need for the development of new effective measures, including vaccines. Plasmodium falciparum merozoite surface protein-1(19) (MSP-1(19)) is a prime candidate for a blood-stage malaria vaccine. Blood samples were collected from children aged 10 days to 15 years in the months of January-March (N = 351) and October-November (N = 369) corresponding to the dry and rainy seasons, respectively. P. falciparum infection was determined by microscopy and enzyme linked immunosorbent assay (ELISA) was used to determine the total IgG and IgG subclasses. There was a significant increase in the mean anti-MSP-1(19) antibody titre in the dry season (p < 0.05), compared to the rainy season. A significantly positive correlation between the anti-MSP-1(19) antibody titre and parasite density (p < 0.01, r = 0.138) was observed. In the rainy season, unlike in the dry season, P. falciparum positive children had higher anti-MSP-1(19) antibody titres than P. falciparum negative children and this difference was significant (p < 0.05). When all individuals were grouped together, the anti-MSP-1(19) antibody titre increased with age in both seasons (r = 0.186 and 0.002), this increase was more apparent in the dry season. However, when the study population was divided into P. falciparum positive and negative groups, it was observed that in the rainy season, there was a negative correlation between anti-MSP-1(19) titre and age in P. falciparum positive individuals, while those who were P. falciparum negative had a positive correlation between anti-MSP-1(19) titre and age. Analysis of anti-MSP-1(19) IgG subclass showed that IgG1 and IgG3 mean titres were highest in both the dry and rainy seasons with an increase in the mean antibody titres for IgG1, IgG2 and IgG3 in the rainy season. In the dry season there was a positive correlation between IgG1, IgG2, and IgG3 titres with age, while IgG4 was negative, whereas in the rainy season there was a positive correlation between IgG2 and IgG4 (non-cytophilic antibodies) with age and a negative correlation for IgG1 and IgG3 (cytophilic antibodies) with age. Seasonal differences in the level of MSP-1(19) IgG subclass titres were observed for P. falciparum negative and positive individuals. Only samples, which were positive for IgG2 and IgG4, showed positive correlation between parasitemia and total IgG. The incidence of P. falciparum infection, which increases during the rainy season, might be an important determinant of anti-MSP-1(19) antibody levels in children living in Igbo-Ora and the results point to the fact that non-cytophilic antibodies to MSP-1(19) in children might be associated with an increase in total IgG and parasitemia.     

Cerebral malaria in Malawian children hospitalized with Plasmodium falciparum infection

A hospital-based, prospective study was undertaken at Mangochi District Hospital (MDH) and Kamuzu Central Hospital (KCH) in Malawi. The malaria-transmission patterns in the catchment areas of these two hospitals are very different, transmission being continuous around MDH and seasonal, occurring mostly during the rainy season, around KCH. The main purpose of the study was to determine and compare the prevalences of cerebral malaria (CM) among young, hospitalized children (aged < 5 years) at both sites. Among 8600 of such children admitted to the two hospitals, the overall prevalence of CM was 2.3% (2.2% at KCH and 2.5% at MDH). The prevalences of CM on admission were similar at the two sites during the rainy season (at 3.2%), but the prevalence at MDH during the dry season was statistically higher than that at KCH over the same period (2.1% v. 1.0%; P = 0.0078). A nearly significant difference was noted between the two sites in the prevalences of parasitaemia on admission (11.9% at KCH v. 9.2% at MDH; P = 0.07), and of severe malarial anaemia (SMA) on admission (5.4% at KCH v. 4.2% at MDH; P = 0.06). No inter-site differences were noted in the prevalences of CM or SMA when analysed by mean age, weight, haemoglobin, body temperature, weight-for-age Z-scores, duration of hospitalization, or proportion with high parasite score on admission. These findings differ from those by researchers in other parts of sub-Saharan Africa, where the prevalence of CM has been found to be higher in areas with seasonal transmission patterns. It appears that the epidemiology of CM can differ within the same country, with location and season. Whenever possible, therefore, plans to control CM in any sub-Saharan country should be based on locally generated data.     

Etiology of diarrhea in children less than five years of age in Ifakara, Tanzania

A total of 451 stool specimens were collected from children less than five years of age with acute diarrhea from Ifakara, Tanzania and processed to detect bacterial enteropathogens, parasites, and rotaviruses. These specimens were divided into 348 from the dry season and 103 from the rainy season. Overall, diarrheogenic Escherichia coli (35.7%) were the predominant enteropathogens, with enterotoxigenic E. coli, enteroaggregative E. coli, and enteropathogenic E. coli being the most prevalent. Moreover, enteroaggregative E. coli (63% versus 35.5%; P < 0.05), Shigella spp. (24% versus 12%; P < 0.05), and rotavirus (23% versus 4%; P < 0.05) were more prevalent in the dry season than in the rainy season and enterotoxigenic E. coli (51.6% versus 20%; P < 0.05) and Giardia lamblia (14% versus 1%; P < 0.05) were more prevalent in the rainy season.     

Studies on the dynamics of transmission of onchocerciasis in a Sudan-savanna area of North Cameroon III. Infection rates of the Simulium vectors and Onchocerca volvulus transmission potentials

Populations of Simulium damnosum s.str. and S. sirbanum were examined for infections with filarial parasites during three years in the areas of Tcholliré and Touboro, at sites at different distances from Simulium breeding rivers, and in relation to villages with different endemicities of onchocerciasis. A total of 60,353 flies from 23 fly-catching sites were dissected. The overall infection rate was low, 11.8% of 35,357 parous flies dissected. 1681 flies (4.8% of the total parous) contained 3557 infective larvae, 68.8% of which were morphologically indistinguishable from Onchocerca volvulus and 31.2% were infective 'Type D' larvae of non-human origin, indicating a high degree of zoophily of the fly populations. It was estimated that only 20-40% of all bloodmeals were taken from man. The majority (54%) of all infective O. volvulus larvae were found in the heads of the flies, the remainder being in the thorax (34%) and abdomen (12%). Only 54% of the O. volvulus infective larvae left their vectors during a bloodmeal which, however, was not completed in most cases. During the rainy season infection rates with O. volvulus infective larvae were 3.5% of the total parous flies, as compared with 1.8% during the dry season. The average number of infective larvae of O. volvulus per infective fly was 2.6 and 2.2 during the rainy and dry seasons respectively. These variations in the vectorial efficiency of the fly populations, as well as variations from one site to another, could be explained by different survival rates and man-biting habits of the various vector populations during the dry and rainy seasons and in different regions, rather than by different endemic profiles of onchocerciasis in the human population. The intensity of transmission varied seasonally and was highest (1609 to 3076 infective larvae/man/year) near the main breeding sites, where transmission was almost perennial. At distances of more than 3 km from the river transmission was mainly restricted to the rainy season and the Annual Transmission Potential was below 200, whereas low to zero levels of transmission were measured inside villages more than 3 km distant from the river. The coefficient of variation of the Annual Transmission Potentials over the three years of studies was from 31% to 192% of the mean, being higher that the variations in the corresponding levels of the biting rates, due to the low numbers of infective flies dissected at sites with low transmission.(ABSTRACT TRUNCATED AT 400 WORDS)     

Extensive variation and heteroplasmy in size of mitochondrial DNA among geographic populations of Drosophila melanogaster

Size variation and heteroplasmy in mitochondrial DNA (mtDNA) are relatively common in natural populations of Drosophila melanogaster. Of 92 isofemale lines of flies obtained from various geographic regions throughout the world, 75 lines were homoplasmic and showed a total of 12 different mtDNA size classes. The remaining 17 lines were heteroplasmic, each line carrying two different mtDNAs, and, in all but one case, the mtDNAs in these heteroplasmic lines differed in size; a total of nine size classes was represented among them. In cases where one type was predominant within an individual, it was usually the smaller mtDNA. This finding parallels what was observed in homoplasmic lines, in that the smaller mtDNAs were much more common than the larger variants in most populations. The data suggest a high rate of mutational occurrence of mtDNA size variants and some natural selection against them.     

Presence of multiple variants of Borrelia burgdorferi in the natural reservoir Peromyscus leucopus throughout a transmission season

White-footed mice (Peromyscus leucopus) serve as the principal reservoir for Borrelia burgdorferi and have been shown to remain infected for life. Complex infections with multiple genetic variants of B. burgdorferi occur in mice through multiple exposures to infected ticks or through exposure to ticks infected with multiple variants of B. burgdorferi. Using a combination of cloning and single strand conformation polymorphism (SSCP), B. burgdorferi ospC variation was assessed in serial samples collected from individual P. leucopus during a single transmission season. In individuals with ospC variation, at least seven ospC variants were recognized at each time point. One to four of these variants predominated at each time point; however, the predominant variants seldom remained consistent in an individual mouse throughout the entire sampling period. These results confirmed that mice in southern Maryland were persistently infected with multiple variants of B. burgdorferi throughout the transmission season. However, the presence of multiple ospC variants and the fluctuations in the frequency of these variants indicates that either new ospC variants are regularly introduced to this mouse population and predominate while the existing infections are cleared, or that the variation detected in the genetic profile at different time points reflects a complex mixture of B. burgdorferi populations whose relative frequencies may continually change.