Effects of oral tetrachlorvinphos fly control (Equitrol®) administration in horses: Physiological and behavioural findings

Abstract

Highly reactive horses may pose risks to humans involved in equestrian activities. Among the factors that may affect horses' reactivity to external stimuli are pesticides used for fly control in equine facilities. The organophosphorus (OP) insecticide tetrachlorvinphos (TCVP) is used as a feed-through larvicide to prevent completion of the fly larval life cycle in horse manure. TCVP exerts its effect by inhibiting the enzyme cholinesterase (ChE) leading to the accumulation of the neurotransmitter acetylcholine (AChE) in synapses of the central and peripheral nervous systems. The aim of the present study was to investigate alterations of whole-blood ChE levels associated with feeding a commercially available product (Equitrol, Farnam Companies, Inc.) to horses for fly control. A second aim was to report neurological, physiological and behavioural findings in addition to profiles of selected immune markers (IFN-gamma, IL-12p40 and COX-2) and serum thyroid hormones during and after a 30-day treatment period of TCVP feeding. The results indicated significant decreases in whole-blood ChE activity and concomitant behavioural alterations, manifested as increased reactivity and decreased controllability in treated horses. No changes were detected in physiological or neurological parameters, immune markers or thyroid hormones in treated (n=6) or control (n=4) horses during the course of the study.

Assessing intermittent pesticide exposure from flea control collars containing the organophosphorus insecticide tetrachlorvinphos

“…Consequently, it is now mostly applied dermally in poultry for the control of mites and flies and as an oral feed-through larvicide for fly control in horses, cattle, goats, and swine (USEPA, 1995;Berger et al, 2007). It is also used for the control of nuisance and public health pests (flies), as well as in the form of powder, aerosol, spray, or shampoo for the dermal treatment of pets (USEPA, 1995).…”

A review on the treatment and control of ectoparasite infestations in equids

“…The use of the organophosphate (OP) tetrachlorvinphos in horses leads to marked behavioural alterations in horses with riders having less control (Berger et al, 2008). The product was used as a feed-through larvicide and resulted in a significant decrease of whole blood ChE activity after a 30-day period of feeding (Berger et al, 2008). Efficacy for this product was not reported.…”

“…Pupal parasitoids from different species can contribute to controlling fly populations in horse farms by predating on certain species of Diptera (Machtinger, Geden, Kaufman, & House, 2015). Timing and location of the release of these predators is critical for efficacy (Machtinger et al, 2015 Marked behavioral changes noted in horses treated (Berger et al, 2008) Pyrethrin Topical Concentration to 0.5% (French et al, 2016) Pyrethroids Cypermethrin Topical…”

Control of ticks on horses using abamectin‐impregnated ear tags. A pharmacokinetic and pharmacodynamic study

“…Efficacy of topically and orally administered organophosphates and lindane against this tick species has been established in the 1960's; however, the safety of these treatments was not assessed (Drummond & Graham, 1964; Drummond & Ossorio, 1966). These compounds have been deemed to toxic for use on horses (Berger et al, 2008; Bradley et al, 2016; Bretveld et al, 2006). Neither for organophosphates nor for lindane have maximum residue limits been determined.…”