The aim of this study was to screen crude extracts of M. discoidea
and H. africanum
plants that are popularly used in the traditional treatment of onchocerciasis in North West Cameroon for their activities against O
and as potential sources of novel O. volvulus
filaricidal lead compounds. The lack of a suitable macrofilaricide for the treatment of human onchocerciasis and the emergence of ivermectin resistant nematodes of veterinary importance underscores the urgency for novel drugs/phytomedicines or lead compounds. Previous assays for drug screens in onchocerciasis have been based on the use of O. lienalis
or O. gutturosa
] which are not as closely related to O. volvulus
as is O. ochengi
, or on use of O. volvulus
itself which is expensive and difficult to obtain from infected humans through surgery. The O. ochengi
model is now considered the most suitable for screening for anti-O. volvulus
drugs because of the similarities between both parasites which share the same Simulium
vector, and are equally susceptible to ivermectin and suramin [16
], in addition to the availability and relatively low cost of obtaining O. ochengi
Sixteen (16) crude extracts of different polarities from the 2 plant species were tested. Four (4) of the extracts showed activity on microfilariae only (Table ), while none showed activity on adult worms in the primary screen. The remaining 12 extracts were inactive on adult worms and mfs. All 4 microfilaricidal extracts were non-polar. It is thus likely that the active compounds in M. discoidea
and H. africanum
are predominantly non-polar. This corroborates previous studies which showed that non-polar compounds such as essential oils are nematocidal [18
]. The root extracts of M. discoidea
exhibited a higher activity against mfs than the leaves as seen from the IC50
values of 45 μg/mL and 62.5 μg/mL versus 62.5 μg/mL and 125 μg/mL for the roots and leaves respectively. The roots extract also had a higher SI value compared to the leaves. On the other hand, only the leaves of H. africanum
were used in this study as is the practice in traditional medicine of the people concerned. The methylene chloride extract of H. africanum
(HLC) recorded the overall best activity with the least IC50
. Interestingly, this extract recorded an SI of 1.5 (Table ). An SI value of greater than 1 for a crude extract increases the likelihood that its toxic and filaricidal components are different. Thus, elimination of these toxic components may yield more suitable filaricidal drug leads or phytomedicines. Also, the higher the SI value, the greater the probability of isolating safe leads from an extract. Where the anti-parasitic IC50
is low, and the SI value is high, the formulation of a phytomedicine for local use is encouraged.
The acute toxicity studies in mice revealed that the two active extracts, MRH and HLH with SI values greater than 2 (Table ) are non-toxic at 5 × IC50
despite their relatively low SI values. This may imply a detoxification mechanism in the liver of mice in vivo
. One other study [19
] also did not find any acute toxicity and no adverse change in behavior in mice following oral administration of an aqueous extract (3200 mg/kg body weight) of M. discoidea
. This finding in mice lends credence to the ethnopharmacologically observed lack of toxicity or adverse effects in humans.
In addition to the compound groups detected in the promising extracts (MRH and HLH) (Table ), other workers have reported the presence of alkaloids in these plants [10
]. Alkaloids may be found in other parts of the plant (not tested by us) as indicated in the literature. Some of these compounds may be responsible for the filaricidal activity of the plant extracts. However the purification and further antifilarial screening of the pure compounds from the extracts will be required to determine their full potential.
Overall, the effect of the active extracts on mfs increased with incubation time (Figure ), and there was a general decrease in % viability of mfs with increasing extract concentration, suggesting a dose dependent action. However, the observation that the extracts MRH, MLC, HLH and HLC were only microfilaricidal and not macrofilaricidal could probably be due to lack of the target for the active components in the adult worms.