Neuromuscular effects of acute 2,4-dichlorophenoxyacetic acid (2,4-D) exposure in dogs

2,4-Dichlorophenoxyacetic acid (2,4-D) was administered once orally to adult female mongrel dogs at 0, 25, 50, 75, 100 or 125 mg/kg of body weight (n = 4 per group). Clinical neurological examinations, electromyography (EMG) and motor nerve conduction velocity measurements were carried out before exposure, and on days 1, 3, 7, 14, 21 and 28 post-exposure. On day 7, two dogs per group were killed. Samples of the medial and lateral plantar nerves were removed for teased fiber and semi-thin section analysis; samples of appendicular muscles were removed for histology and histochemistry. Necropsy included histopathological examination of the brain and spinal cord at multiple levels. Remaining dogs were killed on day 28 and their tissues were processed similarly. Transient generalized myotonic discharges were found in skeletal muscles of dogs receiving 50 mg/kg or more of 2,4-D. A few dogs in the higher dosage groups also had mild muscle stiffness, myotonic dimpling and/or lethargy, lasting up to 3 days post-exposure. Other parameters remained within normal limits.