Effect of antiparasitic medication in ponies on pasture

Twenty Shetland ponies, 6 to 7 months old, were naturally infected with gastrointestinal nematodes and stomach bots. The ponies were allotted to 2 groups of 10 and were maintained on separate similar pastures that were free of infective larvae at the beginning of the study. The ponies in 1 group were treated monthly for 17 months with a therapeutic dose of a thiabendazole and piperazine mixture; those in the other group were not treated. During the 3rd and 5th months of the experiment, the ponies in the treated group were also given therapeutic doses of dichlorvos to remove bots. Various physical, hematologic, parasitologic, and blood chemical observations were made at weekly intervals. Each group of ponies was weighed at biweekly intervals. At the end of the experiment, the greatest differences between the treated and control ponies were in the mean number of worm eggs in fecal samples (0 vs 1,866 eggs/g), mean body weight (151.9 vs 117.0 kg), mean heart girth (126.5 vs 116.3 cm), mean packed cell volume (36.4 vs 30.8%), and mean serum protein content (8.47 vs 9.33 mg/100 ml), especially beta-globulin content (mean of 1.9 vs 3.4 mg/100 ml). The treated ponies remained clinically normal and were more spirited and more difficult to restrain for blood sampling and weighing than were the untreated controls. Parasitic nematode larvae were not recovered from grass samples from the pasture grazed by the treated ponies, but many such larvae (up to 500/300-g sample) were recovered from grass samples from the pasture grazed by the untreated ponies.