Geometric Mean Intensity Research Articles

Bancroftian filariasis in Kwale District of Kenya. I. Clinical and parasitological survey in an endemic community.

A cross-sectional parasitological and clinical survey for Wuchereria bancrofti infection was carried out in an endemic community of south-eastern Kenya, to obtain background epidemiological information for detailed serological studies on bancroftian filariasis in the same community. Quantitative examination of night blood samples (100 microliters) from 1129 individuals (76% of the population), using the counting chamber technique, revealed circulating microfilariae in 13.7% of the study group. Prevalence increased with age, and was higher in males (15.9%) than in females (11.6%). The geometric mean intensity among infected individuals was 223 microfilariae/ml blood (336 microfilariae/ml for males and 212 microfilariae/ml for females). Approximately 16.5% of the males aged > or = 15 years had hydrocele and 2.4% of the population had elephantiasis. The prevalence of these conditions increased with age; in those aged > or = 50 years, 23.8% of males had hydrocele and 6.1% of the total population had elephantiasis. Acute cases of epididymo-orchitis, adenolymphangitis and funiculitis were also seen. The microfilarial prevalence in males with hydrocele was not significantly different from that in asymptomatic males, but none of the elephantiasis cases had microfilaraemia. The striking difference in microfilaraemia pattern between hydrocele and elephantiasis patients may reflect different mechanisms underlying the development of these two chronic manifestations. The overall prevalences of microfilaraemia and clinical manifestations in this community were moderate when compared with those found in other studies carried out along the coast of eastern Africa.

The Boqueron Project after 5 years: a prospective community-based study of infection with Schistosoma mansoni in Puerto Rico.

The Boqueron Schistosomiasis Project is a prospective community-based study of Schistosoma mansoni infection after the interruption of transmission by nonchemotherapeutic control measures. The study methods and the parasitologic results of the first five annual stool surveys are described in this report. In the first year, 1972, among 904 inhabitants (88% of the total population) the prevalence of infection was 40%, and the geometric mean intensity of infection among positives was 16.1 eggs per gram (epg). Snail control was begun in early 1973 with molluscicides and habitat modification. Intensive monitoring every 2 weeks revealed only 63 noninfected Biomphalaria glabrata in the community during the subsequent 4 years. The incidence of new infectins among people negative in all previous surveys dropped from 17% in 1972 to 1% in 1974 and has remained negligible since then. Among young children and newborn, only four new infections (all less than 5 epg) were found after the first control year. Despite this low rate of transmission, prevalence only decreased from 37% to 34%, and the population geometric mean fecal egg output has not substantially changed in a cohort of 528 individuals examined in each of the six annual surveys. Possible reasons for the minimal change in parasitologic status are discussed and include water contact behavior outside the community and changes in laboratory techniques. Data from the first 5 years of the study suggest that in a population where mean intensity of infection with S. mansoni is low, further decreases in prevalence and intensity of infection occur slowly. The implications for control programs based on nonchemotherapeutic measures are discussed.

Magnetic properties of oceanic pillow basalts: evidence from Macquarie Island

Summary A combined pdaeomagnetic, hysteresis, thermomagnetic, and electron microprobe study has been carried out on pillow basalts from Late Tertiary oceanic crust exposed on Macquarie Island. Oriented cores were collected from four sites in the North Head region where the basalts are unmetamorphosed but have suffered pervasive seafloor weathering and at four sites near Langdon Point where the pillow lavas have experienced metamorphism to the greenschist facies. Field geologic and petrologic evidence suggests that the North Head samples are representative of the weathered zone of Late Tertiary oceanic crust while the specimens from Langdon Point are representative of pillow basalts from > 500m into the oceanic lithosphere. Geometric mean intensities of natural remanence (NRM) were 0.7 × 10−3 G for North Head samples and 0.6 × 10−3 G for specimens from Langdon Point. The geometric mean values of low field susceptibility (x) and Koenigsberger ratio (Q) for North Head samples (0.1 × 10−3 G Oe−1 and 9.8) are similar to those of DSDP basalts. The mean value of X and Q for Langdon Point samples (2.8 × 10−3 G Oe−1 and 0.3) is similar to those observed in metabasalt dredged from oceanic escarpments. Hysteresis, thermomagnetic and electron microprobe data indicate that the titanomaghemites in the North Head samples have suffered a high degree of low temperature oxidation. Langdon Point samples show reversible thermomagnetic curves with a magnetic Curie temperature (580° C), indicating that metamorphism has altered the magnetic mineralogy. This metamorphism is thought to occur at or near the ridge during the seafloor spreading process. Thus, the NRM of the metamorphosed pillow basalts would still record geomagnetic reversals and contribute to magnetic lineations. The low NRM intensities are ascribed to prolonged seafloor weathering of North Head samples and to metamorphism of the basalts from Langdom Point. Since the 10−3 G NRM intensities of pillow basalts are too low to account for the observed amplitude of marine magnetic anomalies, the underlying basaltic and doleritic sheeted dyke complex must also contribute to the anomalies.

Geology of the oceanic crust: Magnetic properties of oceanic rocks

A diverse suite of rocks has been sampled from ocean basin escarpments in the North Atlantic and the Caribbean: fresh and weathered basalts, metabasalts (zeolite and greenschist facies), gabbros, metagabbros (greenschist and amphiolite facies), serpentinized peridotites, and actinolite rocks. One hundred and three representative specimens were chosen from this diverse suite of rocks, and the natural remanent magnetization (NRM) intensity and susceptibility for each specimen were measured in the laboratory. Unaltered and unweathered basalts had the highest measured NRM intensities (geometric mean, 10.69 × 10−3 emu/cm3). The weathered basalts, metamorphosed basalts, gabbros, metagabbros, and actinolite rocks all had NRM intensities much lower than those of the fresh basalts (geometric means, 0.11–1.60 × 10−3 emu/cm3). The serpentinized peridotites, however, are strongly magnetic, the geometric mean intensity being 7.86 × 10−3 emu/cm3. The serpentinized peridotites also had high susceptibilities (geometric mean, 3.298 × 10−3 emu/cm3), whereas all the other rock types had low susceptibilities (geometric means, 0.047–0.480 × 10−3 emu/cm3). The measured magnetic parameters for the several rock types are compared with published models of the oceanic crust based on measurements of the magnetic field over the oceans. These geophysical models suggest that the upper part of the oceanic crust is a strongly magnetized layer of basalt and the majority of the crust below is weakly magnetized. The fresh basalts measured in this study have intensities and susceptibilities compatible with those in the geophysical models. The metabasalts, gabbros, metagabbros, and actinolite rocks have weak intensities and therefore could be, on the basis solely of their magnetic character, volumetrically important components of the weakly magnetized part of the oceanic crust. Serpentinized peridotite has intensities and susceptibilities too high to be an important component of the oceanic crust.