INHIBITION OF CHOLINESTERASE IN THE CATTLE GRUB HYPODERMA BOVIS (DIPTERA: OESTRIDAE) AFTER TREATMENT OF THE HOST CATTLE WITH SYSTEMIC INSECTICIDES

Abstract

Cattle grubs, first-instar larvae of Hypoderma bovis (L.), were removed from nine slaughtered heifers. Two of these heifers had previously been treated with coumaphos, three with Ruelene, and two with trichlorfon. The remaining two heifers were not treated with insecticides and served as sources of control grubs. The grubs, after removal from the hosts and fixation in formalin, were incubated with one of the two cholinesterase substrates, acetylthiocholine iodide or 5-bromoindoxyl acetate. Cholinesterase was completely inhibited in the central nervous systems of the grubs whose hosts were treated with Ruelene. This inhibition was also virtually complete in grubs from heifers treated with trichlorfon and was usually complete in those from the heifers treated with coumaphos. This is presented as histochemical evidence that the three systemic insecticides inhibit the cholinergic enzymes in the grubs, thus causing them to die while still in the body of the host.

Some Factors Involved in Systemic Insecticide Toxicosis: Esophageal Lesions in Heifers Treated With Coumaphos, Crufomate and Trichlorfon

“…In the treated larvae, the color faded in proportion to the depletion of the ChEs and ranged from buff to dark brown. The variation in the staining characteristics of the brains and ganglia of the treated larvae could be used to make reliable, albeit arbitrary, estimates of the concentration of ChEs in the larval tissues (Steward, 1969).…”

“…Systemics inhibit cholinesterases (ChEs) (Khan, 1969a ChEs in the grubs were determined by a histochemical technique in which acetylthiocholine iodide was used as a substrate (Steward, 1969 ChE inhibition in cattle grubs collected fr:om the treated heifers ChEs were depleted in the brain and ganglia of the larvae of both species of Hypo' derma. The depletion was faster and more pronounced and uniform in the larvae exposed to trichiorfon or crufomate than in those exposed to coumaphos (Table 1).…”