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Amazonian onchocerciasis: parasitological profiles by host-age, sex, and endemicity in southern Venezuela
- S. VIVAS-MARTÍNEZ, M.-G. BASÁÑEZ, C. BOTTO, S. ROJAS, M. GARCÍA, M. PACHECO, C. F. CURTIS
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- Journal:
- Parasitology / Volume 121 / Issue 5 / November 2000
- Published online by Cambridge University Press:
- 02 January 2001, pp. 513-525
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- Article
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This paper describes, for the human onchocerciasis focus of southern Venezuela, the age profiles of Onchocerca volvulus microfilarial (mf) and nodule prevalence, mf intensity, and mf aggregation for the whole examined population (836 Yanomami people) living in 20 villages, and for these communities classified according to endemicity levels (hypoendemic: [les ] 20%; mesoendemic: 21–59%; hyperendemic: [ges ] 60% infected). Mf prevalence and intensity increased with age, particularly in the hyperendemic areas, and there were no marked differences between the sexes. The prevalence of nodules followed the same age pattern. Fifty percent mf prevalence was reached in the 15–19 year age-class when the population was taken as a whole; nearly in the 10 to 14-year-olds for the hyperendemic level, in those aged 20–29 years in mesoendemic areas, and not reached at all in hypoendemic villages. The degree of mf aggregation was measured by the k value of the negative binomial distribution and by the variance to mean ratio (VMR). The relationship between the standard deviation (S.D.) of mf counts and the mean mf density was also explored. These 3 indices (k, VMR, and S.D.) showed a tendency to increase with both mean mf load and host age. Since infection intensity and host age were themselves positively related, it was not possible to draw definite conclusions about age-specific changes of parasite aggregation. There was not a significant decrease of mf intensity after an earlier peak neither was there a shift towards younger ages of the maximum no. of mf/mg reached as the endemicity level increased. These results are discussed in relation to detection of density dependence in the human host, selection of an indicator age-group for rapid epidemiological assessment (REA) methods, and strategies of ivermectin distribution in the Amazonian focus. It is recommended that, for the Amazonian onchocerciasis focus, the indicator group for REA consists of all those aged 15 years and over.
Parasitological indicators of onchocerciasis relevant to ivermectin control programmes in the Amazonian focus of southern Venezuela
- S. VIVAS-MARTÍNEZ, M.-G. BASÁÑEZ, C. BOTTO, L. VILLEGAS, M. GARCÍA, C. F. CURTIS
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- Journal:
- Parasitology / Volume 121 / Issue 5 / November 2000
- Published online by Cambridge University Press:
- 02 January 2001, pp. 527-534
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- Article
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In the previous paper it was concluded that those aged [ges ] 15 years of both sexes could comprise the indicator group for rapid epidemiological assessment (REA) of onchocerciasis in the Amazonian focus. This paper explores relationships between community microfilarial (mf) prevalence, intensity, and nodule prevalence in 20 Yanomami communities, that would allow identification of REA methods in the region. The mean nodule ratio (prevalence of nodules/prevalence of mf) was 0·54 when onchocercomata in the indicator group were considered. The Spearman correlation coefficient between mf and nodule prevalence was 0·686 (P = 0·001). Palpation of nodules had 92% specificity and 32% sensitivity when compared to skin-snipping for the diagnosis of onchocerciasis. The predictive value positive increased from 75% to 81% when the indicator group was used. A microfilarial prevalence > 75% in this group would be indicative of hyperendemic status in the village, between 30 and 75% of mesoendemicity, and < 30% of hypoendemicity. For the assessment of infection intensity, biopsies may be taken from the iliac crest for all endemicity levels. Five of the hyperendemic villages surveyed in this work had a community microfilarial load (CMFL) greater than 10 mf/skin snip; the remaining 5 had a CMFL between 5 and 9. These levels of infection merit high priority ivermectin treatment. In Latin America, communities at both moderate and severe risk are included in mass chemotherapy programmes (i.e. when mf prevalence is over 20%). Roughly, a nodule prevalence in the indicator group > 10% would suggest a community mf prevalence > 20% with a sensitivity of 85% and a specificity of 71%. A multiple linear regression model of the arc-sine transformed mf prevalence in the village (all ages) on nodule prevalence in those aged [ges ] 15 years and altitude of the village explained 72% of the variance. The model combining nodule and altitudinal information had a sensitivity of 92% and a specificity of 71% in comparison to an estimated mf prevalence of 21% or more. It is suggested that the usefulness of the REA methods proposed be assessed in other areas of the Amazonian onchocerciasis focus.