Assessment of Molluscicidal, Cercaricidal and Miracicidal Activities of Crude Extracts of Ocimum americanum, Bridelia micrantha and Chenopodium ambrosoides

Abstract

Schistosomiasis is a major health problem in both the tropics and subtropics. Niclosamide, the molluscicide in use is expensive, has poor water solubility and is harmful to non-target organisms such as fish. There is need of a molluscicide which is safe to a non-target host and which is capable of eliminating snail intermediate host and Schistosoma mansoni infective stages. This would be a better strategy of control as it would prevent infection of the definitive host and also interfere with transmission of the disease. Plants extracts have been found to be relatively safe to the environment and humans. The study was done to determine effect of selected plant extracts on Biomphalaria pfeifferi snails and microscopic stages of Schistosoma mansoni, miracidia and cercariae. Three plants: Ocimum americanum (whole plant); Bridelia micrantha (leaves and bark) and Chenopodium ambrosoides (leaves) were collected, dried and ground into powder. Extraction was done using methanol, hexane, and distilled water for B. micrantha, O. americanum and C. ambrosoides respectively. Plant extracts concentrations of 50µg/ml, 150µg/ml and 300µg/ml were prepared in the case of molluscicidal assay. Plant extract concentrations of 5µg/ml, 15µg/ml and 30µg/ml were prepared in the case of cercaricidal and miracicidal assays. Data analysis was done using SPSS version 16 to calculate mortality of snails and compare effect of the plant extracts on the snails. Finney Probit analysis was used to estimate LD₅₀ values of the extracts on the snails and LT₅₀ values of the extracts on cercariae and miracidia. B. micrantha had the highest molluscicidal activity (LD₅₀, 29.775µg/ml) followed by O. americanum (LD₅₀, 37.5920 µg/ml); C. ambrosoides (LD₅₀, 1909.13µg/ml) had least molluscicidal activity. O. americanum had highest cercaricidal activity followed by B. micrantha and lastly C. ambrosoides. At 30 µg/ml concentration, cercaricidal LT₅₀ values for O. americanum, B. micrantha and C. ambrosoides extracts were 53.85 minutes, 55.21 minutes and 79.14 minutes respectively. O. americanum had highest miracicidal activity followed by B. micrantha and lastly C. ambrosoides_. At 15 µg/ml concentration, miracicidial LT₅₀ values for O. americanum, B. micrantha and C. ambrosoides were 63.01 minutes, 69.86 minutes and 90.05 minutes respectively. In conclusion, B. micrantha (leaves/bark) methanol extract and whole plant hexane extract of O. americanum had high molluscicidal activity against B. pfeifferi snails which is not significantly different from the activity of Niclosamide (p≥0.05). Similarly whole plant hexane extract of O. americanum, methanol (leaves/bark) extract of B. micrantha and aqueous leaves extract of C. ambrosoides were shown to possess cercaricidal and micaricidal activities. The plants with best activity against B. pfeifferi snails, S. mansoni cercariae and miracidia were B. micrantha and O. americanum.

Keywords: Schistosomiasis, Schistosoma mansoni, Biomphalaria pfeifferi, Ocimum americanum, Bridelia micrantha, Chenopodium ambrosoides, Molluscicidal activity, Miracicidal activity, Cercaricidal activity, Lethal dose 50 (LD₅₀), Lethal time 50 (LT₅₀)