Effects of plumbagin on development of the parasitic nematodes Haemonchus contortus and Ascaris suum.

1. Plumbagin (5-hydroxy,2-methyl-1,4-napthoquinone) inhibited the motility and survival of Haemonchus contortus first-stage larvae (L1) with an ED50 of 1 microgram/ml, but was less effective in preventing the development of H. contortus to infective third-stage larvae in a faecal slurry assay. 2. Of the structural analogs tested, plumbagin was the most potent in preventing development of L1 followed in decreasing order of potency by 1,4-naphthoquinone, 5-hydroxy-1,4-napthoquinone (juglone) and 1,2-napthoquinone. 3. Plumbagin had a biphasic effect on development of the fourth-stage Ascaris suum larvae that caused an increase in growth at low concentrations but was lethal at higher doses. 4. Plumbagin and 1,2-napthoquinone partially inhibited embryonation of A. suum eggs.