Selective inhibition of butyrylcholinesterase in vivo in horses by the feed-through larvacide Equitrol.

This research studied the effects of an organophosphate insecticide, tetrachlorvinphos (TCVP), on the cholinesterase activities in horses. It found that the TCVP-containing larvicide Equitrol predominantly inhibited butyrylcholinesterase activity in horse plasma, with no notable impact on acetylcholinesterase activity in the muscle or erythrocyte cholinesterase, thus posing minimal risk to normal cholinergic neurotransmission in horses.

Investigation of the Impact of Tetrachlorvinphos on Horses

• The active ingredient in larvacides like Equitrol is an organophosphate insecticide known as tetrachlorvinphos (TCVP). These larvacides are used in controlling flies in environments raising horses.
• The researchers were interested in studying whether thus insecticide affected the enzyme acetylcholinesterase in horses, as it is known to cause toxicity by inhibiting this enzyme, leading to an accumulation of the neurotransmitter acetylcholine, which can cause signs of cholinergic toxicity.
• Before this study, little was known about how TCVP affected acetylcholinesterase or other cholinesterase enzymes in horses.

Components of the Study

• The study was carried out both in vitro (outside a living organism) and in vivo (within a living organism). The in vitro studies showed that all cholinesterase activity in horse plasma was butyrylcholinesterase, while muscle contained over 90% acetylcholinesterase activity.
• In the in vivo section of the study, adult male horses were given feed supplemented with Equitrol daily for three weeks and their health was closely monitored. Blood samples were taken regularly, and muscle biopsies were also done before and after the study period.
• Noticeably, no signs of overt toxicity were noted at any time during the study from the ingestion of the TCVP-containing larvicide.

Findings of the Study

• Results showed that plasma cholinesterase activity was significantly inhibited in larvicide-treated horses one day after treatment started, with peak inhibition observed at around one week and three weeks into the study. However, cholinesterase activity gradually improved after cessation of the treatment.
• Noteworthy was that neither erythrocyte cholinesterase activity nor plasma carboxylesterase activity was affected by the larvicide treatment during the study. Also, there was no suggestion of a treatment-related reduction in muscle cholinesterase activity.
• The in vivo findings from this study acted as a confirmation for the previous in vitro studies, which indicated a marked difference in the sensitivity of horse plasma and erythrocytes cholinesterase to inhibition by TCVP.
• The research concluded that recommended dosing levels of the TCVP-containing larvicide in horses are unlikely to affect acetylcholinesterase activities or disrupt cholinergic neurotransmission in target tissues.