Visual deficits and retinotoxicity caused by the naturally occurring anthelmintics, Embelia ribes and Hagenia abyssinica.
The naturally occurring anthelmintics, Embelia ribes (Enkoko) and Hagenia abyssinica (Kosso), have been reported to possibly cause optic atrophy among the Ethiopian population. In this study we found retinal pathology and defects in visual behavior in chicks treated with Enkoko, Kosso, or embelin, a crystalline extract of E. ribes. The chicks were fed orally with a high dose of 0.25 g (5 g/kg) or a low dose of 0.025 g (0.5 g/kg) per day of Enkoko or Kosso, beginning on Day 2 of life. The high dose for Enkoko was administered for 1 or 5 days, while that for Kosso was administered for 1 or 9 days. For the low dose of both Enkoko and Kosso, the dosing regime was for a period of 1, 4, or 9 days. Embelin was administered at a dose of 0.001 g (0.02 g/kg) per day for 9 days. Control chicks were force fed an equivalent amount of chick feed. Treatment with Enkoko or Kosso significantly reduced the ability of chicks to detect a moving bead introduced into the peripheral field of vision. The degree of constriction of the visual field for detection was dependent upon the total amount of drug administered. Performance on a visual discrimination task, which required discrimination of feed grains from pebbles, was also impaired in chicks treated with total doses of 0.200 and 0.250 g of Enkoko or Kosso. Thus, the extent of deficit in visually guided tasks was found to be dose dependent. The visual deficits observed in Enkoko-treated chicks were mimicked by embelin, which suggests that embelin may be responsible for the visual defects. Anatomical evidence of degeneration of ganglion cells was found in retinae exposed to high doses of Enkoko (1.25 g) and Kosso (2.25 g). However, no retinal lesions were detected in chicks following treatment with cumulative doses of less than 0.25 g of Enkoko or Kosso. Similarly, retinal pathology was not observed in embelin-exposed retinae.