Ivermectin and Moxidectin Toxicosis
Basic Information 
Epidemiology
Risk Factors
• Drug administered: Toxicity is more frequent from the administration of moxidectin than ivermectin.
• Owner administration: Owners frequently do not follow the label directions, and the equine paste and gel formulations are easily overdosed in foals, miniature donkeys, miniature horses, and debilitated adult horses.
Clinical Presentation
History, Chief Complaint
• Oral overdose of ivermectin at 10 times the label dose or at three times the label dose for moxidectin
• IV administration of bovine formulations
• Onset of CNS signs is immediate after IV injection and within 8 to 12 hours of PO administration.
• Presence of clinical signs as described below if exposure is unknown
Physical Exam Findings
• Depression, ataxia, stupor, coma, mydriasis, blindness, tremors, hypersalivation, decreased respiratory rate, and drooping lower lip. In foals, the clinical signs are similar, but in addition, include a protruding tongue, vacant stare, and recumbency.
• Because ivermectin and moxidectin are γ-aminobutyric acid (GABA) agonists, seizures are not seen with toxicity.
Etiology and Pathophysiology
• Equine formulations of moxidectin and ivermectin are dosed at 0.4 mg/kg body weight and 0.2 mg/kg body weight, respectively. Moxidectin is not labeled for use in foals younger than 4 months.
Over-the-counter pastes and gels contain enough drug to treat 520 to 600 kg of body weight, and owners frequently administer the entire syringe regardless of the horse’s actual weight.
• Moxidectin is 100 times more lipid soluble than ivermectin, so it distributes to a greater degree in fat and has a much longer plasma elimination half-life (23 days for moxidectin; 4 days for ivermectin).
In animals with low fat stores, such as debilitated adults and neonates, plasma concentrations of moxidectin are higher, so the neurotoxic dose is lower.< div class='tao-gold-member'>
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