Volvulus Microfilariae Research Articles

Seasonal variation in the microfilarial skin density of Onchocerca volvulus and in the biting activity of Simulium spp. in Guatemala

The density of Onchocerca volvulus microfilariae in the skin of 11 carriers was examined by carrying out a monthly examination in a single endemic area of Guatemalan onchocerciasis. The results obtained suggested the possibility of a seasonal variation in the microfilarial skin density. Almost all of the volunteers participating had shown a peak concentration of microfilariae in September, followed by August or October. The highest biting densities of Simulium ochraceum, the principal vector of onchocerciasis in Guatemala, occurred from September to the following January. It thus appears that the seasonal variation in the microfilarial density may correspond, to some extent, with the biting activity of the vector, S. ochraceum, and/or with climatic changes in the area.

The development ofOnchocerca volvulusin two temperate blackfly species,Simulium ornatumMeigen andSimulium lineatumMeigen

In view of the promise shown by temperate simuliids for laboratory colonization, the ability of Simulium ornatum and S. lineatum to support the development of Onchocerca volvulus was investigated. An injection technique is described for the introduction of O. volvulus microfilariae from a chimpanzee into female simuliids in the absence of a bloodmeal. RPMI 1640 medium, with penicillin, streptomycin and foetal calf serum, proved satisfactory as an inoculation medium. Third stage larvae were obtained in both S. ornatum and S. lineatum after seven days at 27 degrees C. The rate of development of the parasite in S. ornatum is recorded.

The use of an Onchocerca volvulus microfilarial antigen skin test in an epidemiological survey of onchocerciasis in Guatemala

Onchocerca volvulus microfilariae obtained from onchocercal nodules were used as antigenic material for a skin test. This antigen is as specific as those previously employed in skin tests, detecting 85·2% of the persons with positive diagnostic signs of onchocerciasis in endemic areas. A low rate of false positives was found in the control group in non-endemic areas. The positive rate of reactions in the persons without positive signs in the endemic areas might be due to the presences of cases not detected as a result of palpation for nodules and two skin biopsies. In the examination of the residents of several populations, a close correlation was found between the positive rates of skin tests and the presence of infection. From these findings, we conclude that the skin test using the microfilarial antigen is a suitable procedure for the diagnosis of onchocerciasis. The skin test could also be useful as an epidemiological tool in assessing the effectiveness of control programmes.

Ultrastructural Characterization of the Cuticle of Onchocerca volvulus Microfilaria

Previous ultrastructural studies on various microfilariae have suggested the presence of a trilaminar membrane at the parasite cuticle. In the present report, the formation of the cuticle of Onchocerca volvulus microfilariae was studied at different developmental stages by transmission electron microscopy of samples obtained from subcutaneous nodules of untreated patients. The cuticle is formed by the layering of surface coat fibrillar components of the hypodermal cells. No plasma membrane is found at the cuticle of the microfilaria. The use of freeze-fracture replication confirmed the absence of a classic plasma membrane at the parasite cuticle. Our results suggest that immunogenic determinants of the microfilaria cellular elements are hidden from the exterior by the acellular cuticle. This may explain the lack of cellular reaction usually found around living O. volvulus microfilariae in the dermis of onchocerciasis patients.

Studies on the flight range and dispersal ofSimulium damnosum(Diptera: Simuliidae) in the rain-forest of Cameroon

Five marking/recapture experiments were carried out on Simulium damnosum in the rain-forest of the United Republic of Cameroon, in order to determine the regression in biting density at increasing distances from the marking point. Flies were captured unfed, marked with fluorescent dust, and then released on the bank of a large river. Recaptures were made over the six-seven days after marking, at seven stations situated from 200 m to 79 km along a large river, from the marking site. Marked flies, which showed fluorescent pigment when examined under an ultraviolet lamp, were recovered at all seven sites. The ratios of the number of recaptures per man-hour at each distant site to the recaptures per man-hour at the marking point were calculated to obtain values for the proportional biting density (Z). The value of Z declined in a logarithmic manner with increasing distance from the marking site, such that it fell to 0-1 at distances of 34-1 km on a large river, 5-5 km on a small river, and 1-5 km on a road leading away from a large river. The peak number of flies recaptured at the farthest sites along a large river (56 km and 79 km) occurred one day later than at the station nearest the marking point. The dispersal of marked flies which had fed on volunteers carrying Onchocerca volvulus microfilariae, was studied along a large river. Of three flies recaptured 24 km upstream from the marking point, one contained 10 infective larvae of O. volvulus.

The intervals between the blood-meals of man-biting Simulium damnosum (Diptera: Simuliidae).

Six marking-recapture experiments were carried out with Simulium damnosum in the Cameroon Republic, five in the rain-forest and one in the Sudan-savanna zone. Adult flies were marked by applying a small spot of oil paint to the mesonotum while they were engorging with blood on the legs of volunteers infected with Onchocerca volvulus microfilariae. Catches of wild flies were carried out for 12-13 days after the first day of marking, and all marked flies recaptured were examined. Some of the flies recaptured on days 0, 1 and 2 were nulliparous and had probably been disturbed by the application of the paint before they had completed their blood-meal. The frequency distribution of recaptured marked parous flies returning for their "second" blood-meal rose to a peak early on day 4, but more flies returned earlier,three days after taking a blood-meal, than later on day 5. After day 5, the numbers of recaptured flies were too low to demonstrate any peaks corresponding to "third" and later blood-meals. The longest surviving fly was recaptured 10 days after marking. Twenty-six percent of the flies recaptured on days 3 and 4 contained developing O. volvulus larvae three-five days old, which had presumably been ingested as microfilariae during the blood-meal taken on day 0. Infective larvae first appeared in flies returning late on day 6, and the highest percentage of infective flies occurred on day 7. Infective larvae were found in recaptured flies until day 10, the last day on which marked flies were recovered. Of 929 flies marked on the Sudan-savanna experiment, three (0-32%) flies were recaptured. Two returned on day 4 and one on day 6.

QUANTITATIVE STUDIES ON THE EMERGENCE OF ONCHOCERCA VOLVULUS MICROFILARIAE FROM SKIN SNIPS

The authors made studies on the emergence of O. volvulus microfilariae from skin snips in order to assess accurate MFD in onchocercal infections from quantitative view point. The results obtained are as follows : Skin snips should not be teased into small pieces, but the intact snips should be incubated for a longer period than the teased ones. The distribution of microfilariae in a minute skin area is quantitatively even in most cases. This finding suggests the usefulness of this method to compare MFD of the adjacent skin regions to each other. However, the comparison of MFD with extremely different-sized snips should be avoided. There were no significant changes in MFD by warming skin surface.

Experimental studies on the transmission of Guatemalan and West African strains of Onchocerca volvulus by Simulium ochraceum, S. metallicum and S. callidum

The distribution of O. volvulus microfilariae in the skin of Guatemalan patients is described, and compared with the distribution of the bites of S. ochraceum, S. metallicum and S. callidum. Full development of the microfilariae of the Guatemalan strain of O. volvulus was recorded on a quantitative basis in large numbers of S. ochraceum, S. metallicum and S. callidum from the Finca El Amparo in an endemic onchocerciasis zone; and similar development in the same species was observed at the Finca El Faro in a zone where no endemic onchocerciasis has been reported. S. ochraceum, S. metallicum and S. callidum are compared in regard to (a) the ingestion of microfilariae, (b) survival after infected blood meals, (c) development of infective larvae, and (d) probable potentialities as vectors in the field. A 10–25-fold concentration of microfilariae of the Guatemalan strain of O. volvulus was observed in feeding S. ochraceum, S. metallicum and S. callidum, and it is suggested that this phenomenon might make possible the use of a xenodiagnostic test. Quantitative observations were made on the intake and development of West African forest and sudan savanna strains of O. volvulus in Guatemalan simuliids, and comparison is made with those seen in S. damnosum. Central American onchocerciasis is considered to be an indigenous infection differing in many respects from the African disease.