Significance of ‘Spinal-stage’ Hypoderma Larvae in Systemic Insecticide Toxicity

Abstract

SUMMARY Heifers naturally injected with first-instar larvae of Hypoderma bovis (L.) were treated with systemic insecticides at the time the larvae were lodged in the spinal canal. Ataxia or other signs of injury to the spinal cord or spinal nerves, or to both, were not seen in the treated heifers although, at necropsy, congestion and other signs of injury were seen in the epidural tissue around or near the larvae that were affected by the insecticides. The larval injury to the spinal tissues was almost identical to that seen in untreated heifers. It is concluded that larval injury to the epidural tissues is not severe enough to cause ataxia and particularly paralysis of the hind limbs in cattle treated with systemic insecticides. Further, these signs have been observed in some cattle treated with these compounds which were either not infected with H. bovis larvae or did not harbour them in the spinal canal. These signs have been seen also in sheep treated with these compounds and are probably of neurotoxic origin in both cattle and sheep.