Observations on the value of small daily doses of phenothiazine for the control of trichostrongylosis in sheep

Abstract

Summary Three preliminary experiments each involving two, pairs of twin lambs, and a confinnatory experiment in each of the two parts of which six pairs of twin lambs were used, were carried out to ascertain the effect of daily one gramme doses of phenothiazine on the rate of acquisition of infestation and on the worm burden of lambs kept at a constant rate of artificial infestation with Trichostrongylus axei . When a daily dose of one gramme of phenothiazine is given to lambs within a few minutes of a dose of infective trichostrongylid larvae the treatment does appear to, have some effect in reducing the number of inf ective larvae which develop to maturity. When there is an interval of several hours between the dose of phenothiazine and the dose of larvae each day, the treatment has no effect on the establishment of the larvae in the host. Daily doses of one gramme of phenothiazine have no effect in reducing the number of mature worms present in sheep. Daily doses of one gramme of phenothiazine do not prevent the development of severe parasitic gastritis in sheep continually exposed to infestation when there is an interval of several hours between the administration of the phenothiazine and the consumption of the larvae. Daily doses of one gramme of phenothiazine prevent the development of infective trichostrongylid larvae from eggs passed in the faeces of treated sheep. The effect of daily doses of one gramme of phenothiazine on the egg count in treated sheep was not constant. In some experiments no effect was observed, but in others the treated sheep had a lower egg count than the control group. It is suggested that the good results reported from the field from the use of phenothiazine-salt mixture in the control of parasitic gastritis must be referable to a reduction in the pasture larval count due to the property of small daily doses of phenothiazine of preventing the development of infective larvae from eggs passed in the fazces and not to an actual reduction in worm burden. It is probable, therefore, that good effects will be produced in lambs running with ewes which are kept on phenothiazine-salt treatment, but that adult sheep already infected are unlikely to benefit from such treatment. The efficacy of the treatment may be vitiated if a substantial number of the sheep refuse to take the medicament and the flock should be watched carefully to ascertain whether certain sheep habitually refuse the lick.