Epidemiological approach to the control of horse strongyles

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Abstract
An investigation of the spring rise in strongyle egg output of grazing horses on 2 commercial horse farms in northern USA in 1982 and 1982 revealed 2 distinct spring and summer rises in fecal egg counts, with peaks in May and Aug./Sept. There was a marked rise in the concentration of infective larvae on pasture 2-4 wk after the peaks in egg output, so that grazing horses were at serious risk from June onwards and pasture larval counts on 1 farm did not fall to low levels until June of the following year. The spring and summer rises in fecal egg counts appeared to be seasonal in nature, to be derived largely from worms developing from previously ingested larvae, rather than from newly ingested larvae, and to be unrelated to the date of foaling. An epidemiological approach to strongyle control based on prophylactic treatments in the spring successfully eliminated the spring rise in egg output, but was inadequate to control the summer rise or subsequent escalation of pasture infectivity in Septemer. It was, nevertheless, superior to a conventional treatment program at 8 wk intervals, using the same drug, pyrantel pamoate. Prophylactic spring/summer treatments proved to be much more effective. Both pyrantel pamoate at 4 wk intervals and ivermectin at 8 wk intervals kept fecal egg counts at low levels during spring and summer. As few as 2 ivermectin treatments (May 11, July 6) resulted in a 6-fold reduction in pasture larval counts on November 9 and January 3 for the treated group (8872, 8416 stage 3 larvae [L3]/kg) compared to the control group (52,824, 50,984 L3/kg). This strategy had far reaching prophylactic effects in preventing the late season rise in pasture infectivity and ensuring a low level of residual pasture infectivity at the start of the next grazing season.