Blindness in lambs due to inadvertent closantel overdose

CLOSANTEL is a salicylanilide drug used for the treatment and control of fasciolosis in sheep and cattle. It is also active against Haemonchus contortus and the nasal bot, Oestrus ovis. In combination with benzimidazole derivatives, it is a broad-spectrum anthelmintic for the treatment and control of both nematodes and liver fluke infestations in sheep and cattle. Blindness in sheep has been associated with overdoses of closantel in Brazil (Borges and others 1999) and Australia (Gill and others 1999). Blindness due to a closantel/albendazole mixture was also recorded in Zimbabwe (Obwolo and others 1989). An episode of possible closantel toxicity occurred in the UK in 1998 (J. Wilmington, unpublished observations). This short communication describes the pathological findings of a confirmed incident in the UK. Retinopathy, optic neuropathy and myelin vacuolation in the brain stem, as previously described in association with closantel toxicity in sheep (Gill and others 1999, Obwolo and others 1989, Jubb and Huxtable 1993), were identified. The affected animals were in a group of 80 threeto sixmonth-old lambs, running with a further 100 older lambs and 80 ewes. They were grazing permanent pasture which had been used over a number of years. As some animals had diarrhoea, the owner decided to treat all of them with a combined fluke and worm drench containing 50 mg/ml closantel and 57 mg/ml mebendazole (Mebadown Super; Downland Marketing). Lambs in both groups were supposedly dosed with 5 ml and the ewes with 8 ml of anthelmintic. Two weeks later, the sheep were moved and eight of the 80 lambs were noticed to be blind. Three affected animals were submitted to the Veterinary Laboratories Agency (VLA) Langford for anteand postmortem examination. The animals weighed 12, 17 and 19 kg. The smallest had mild hindlimb incoordination. All three were blind with no menace reflex and had dilated pupils. Euthanasia was carried out using intravenous barbiturate. The eyes, with optic nerves attached, and brain were dissected out in each case. The eyes and brain appeared grossly normal. However, there was some narrowing of the optic nerves rostral to the optic chiasma at the site of the intracanalicular segments. No other abnormalities were detected in the rest of the carcases. The eyes, optic nerves and brain from all three animals were fixed in 10 per cent neutral buffered formol saline, processed routinely, sectioned at 5 Mtm and stained with haematoxylin and eosin. In all three animals, severe bilateral degenerative changes were observed in the retina. These included bullous detachment from the retinal pigmented epithelium, a generalised loss of the photoreceptor layer and depletion of both outer and inner nuclear layers (Fig 1). The depletion of the nuclear layers was more severe in the outer nuclear layer, which was reduced to a row of cells and was effaced in places. In the intraorbital optic nerves of two lambs, severe bilateral vacuolar degeneration, especially towards the periphery of the nerve, with infiltration by gitter cells, was observed (Fig 2). The predominant feature in the intracanalicular portion of the optic nerves was gitter cell accumulation. Perivascular cuffing by mononuclear cells was present around occasional vessels. In the brainstem of the most severely clinically affected animal, the cerebellar pedunFIG 1: Depletion of the outer (arrow) and inner (arrowhead) nuclear layers of the retina (R) in a lamb. x 120. C Choroid, S Sclera