Abamectin toxicity in farm dogs

Authors: Parton KH
Publication: New Zealand Veterinary Journal, Volume 57, Issue 1, pp 69, Feb 2009
Publisher: Taylor and Francis

Animal type: Companion animal, Dog
Subject Terms: Animal remedies/veterinary medicines, Anthelmintics, Parasite control, Parasites - internal, Diagnostic procedures, Treatment/therapy, Toxicology
Article class: Abstract

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Abstract: Farm dogs in New Zealand exposed to macrocyclic lactone parasiticides (ML), including abamectin pour-on products, are at risk of lethal poisoning. Although sensitivity to ML of Collie and Collie crossbreds with the MDR1 gene mutation is well documented, toxicity may occur in breeds without this mutation. The MDR1 mutation causes a defect of p-glycoprotein, a large trans-membrane protein in the blood brain-barrier, which allows ML such as ivermectin and abamectin to enter the brain. Because ML act as γ-aminobutyric acid agonists, they increase the effects of inhibitory nerve pathways in the central nervous system, leading to depression and stupor.
In beagles, the LD50 (dose that kills 50% of the exposed population) is ≥40 mg/kg ivermectin and 8 mg/kg abamectin. In breeds with the MDR1, mutation the LD50 is only 0.2 mg/kg ivermectin. Clinical signs associated with ML toxicity such as abamectin include dilated pupils, blindness (reversible), muscle tremors, weakness, hypermetric gait, a progressive ataxia, vomiting, hyper-salivation, depression, coma and death. Seizures have been described in some cases.
No gross pathology or histological lesions are seen in adults, however pathological lesions have been reported in 11–12-week-old Beagle puppies given more than 9.4 mg/kg ivermectin. Lesions included haemorrhage in the thymus, congestion and oedema of the lungs, acute suppurative pneumonia, and oedema of the skin.
Analytical tests to measure abamectin in the plasma of dogs are available in New Zealand (Hill Laboratories, Hamilton). A DNA test is available through Gribbles Veterinary Pathology, to identify dogs with the MDR1 mutation.

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