Limited fly load and development of Onchocerca volvulus microfilariae in Guatemalan Simulium ochraceum.

Fly load of Onchocerca volvulus microfilariae and their early development in Simulium ochraceum were examined at intervals of 10, 30 and 60 min after ingestion and then hourly up to 72 hours. Many microfilariae ingested were damaged by the cibarial armature of the vector, limiting the fly load, but some of these damaged microfilariae were able to penetrate the gut before they died. Thus, the mean number of the living parasites per fly in the head and thorax decreased gradually with elapse of time after ingestion. The parasites ingested by S. ochraceum did not develop beyond the microfilarial stages during the first 24 hours; thereafter only thoracic forms (1st-stage larvae) were found to be alive. The high mortality of the infected fly might be caused by the high intakes of microfilariae and also by normal and/or partly wounded microfilariae invading various organs. The number of microfilaria ingested from different biting sites were usually high in comparison with the densities observed in each site; this indicates that the microfilariae might be attracted by the biting females.

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.

Life span and distribution of Onchocerca volvulus microfilariae in mice.

Microfilariae of Onchocerca volvulus obtained from onchocercomas were inoculated subcutaneously into the inguinal region of mice. Microfilariae disseminated rapidly, invading the tail and lungs within 1 hr and the ears within 3 hr postinoculation. During the early period of infection microfilariae appeared in the eyes, ears, viscera, pelt, carcass, and tail; later, the microfilariae tended to accumulate in the tail. Liver microfilariae were recovered up to 12 wk postinoculation. Intraperitoneal inoculation and subcutaneous inoculation into the inguinal region resulted in the concentration of microfilariae in the tail, whereas inoculation into the scalp caused microfilarial accumulation in the ears.

The distribution of microfilariae in the skin of Guatemalan onchocerciasis patients: an evaluation of diagnostic potentials.

Quantitative examinations for the distribution and density of microfilariae were carried out on male Guatemalan onchocerciasis patients by taking 6 or 13 skin biopsies each from 74 subjects. We found that microfilariae were distributed most frequently on the iliac crest and secondarily in the scapular region; higher detection rates and greater microfilarial densities were seen in these areas. From these results we recommend the taking of biopsies from both of these anatomical regions so as to avoid false negative cases. The infected patients were classified into 3 groups (light, moderate and heavy infections) on the basis of the numbers of negative skin snips out of the 13 biopsies taken. In heavy infections, a higher densities of microfilariae were detected in the head and neck regions, as compared with the densities in the lower extremities (calf). The high concentration of microfilariae in these regions is of importance in relation to the development of eye lesions and/or transmission. Of patients who had microfilariae in the anterior segment of the eye some had negative skin biopsies frm the outer canthus, retroauricular region and neck.

A sero-epidemiological study of onchocerciasis with the indirect hemagglutination test.

A sero-epidemiological study of onchocerciasis was carried out with the IHA test in Guatemala. In the endemic area, 94.8% of 191 subjects with microfilariae, 82.1% of 172 cases with onchocercal nodules, and 22.5% of 236 cases without either microfilariae or nodules gave positive reactions in the IHA test. On the other hand, the positive rate was 3.4% and 2.0% in each of 2 nonendemic populations. A follow-up study of reactors in the IHA with neither microfilariae nor nodules showed that 11 (42%) of 26 cases were microfilaria positive 6--7 mo after the first examination. A clear correlation was found between the results of the IHA test and skin biopsy in surveyed communities. The age distribution curve of the IHA test was closely associated with skin biopsy although the former was higher. IHA titers rose in proportion to microfilarial density. A sex-related difference was evident in the IHA positive rate in the subjects from medium and low endemic areas, but little difference was shown in individuals from a higher endemic area.

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 nonendemic 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.