Experimentally induced infections of Parascaris equorum in worm-free pony foals required 14 to 17 days for migration of the larvae through the liver and lungs, and 79 to 110 days to become gametogenically functional. Treatment of experimentally infected or naturally exposed foals during the parenteral phase of development, using levamisole at 8 mg/kg, a mixture of levamisole at 8 mg/kg plus piperazine at 88 mg base equivalent/kg, or dl-tetramisole at 10 mg/kg, was quite efficacious in (1) reducing the number of P. equorum larvae recovered from the small intestines of the foals at necropsy, or (2)delaying the appearance of ascarid eggs in the feces of treated foals beyond the time period observed for the untreated foals. Three formulations of dichlorvos at doses of 10, 20, 30, or 40 mg/kg, and 2 formulations of trichlorfon at 20 or 40 mg/kg, were not effective when treatment was given on or before the 14th day after infective eggs were administered. Treatment with a gel formulation of dichlorvos at 20 mg/kg 17 to 28 days after experimental infection was 100% effective in removing ascarid larvae from the small intestines of poly foals.
Macrocyclic lactones (MLs) revolutionized parasite control in horses and other animals. They are unique in that they are effective against arthropods and nematodes. The first of the widely used avermectins was ivermectin. In 1983, it was marketed for use in horses as an injectable formulation but was withdrawn in 1984 after about a year and half on the market because of adverse problems. It was replaced by a paste formulation and an oral/stomach tube liquid formulation. Ivermectin is highly active on bots, ascarids, large and small strongyles, pinworms, strongyloides, stomach worms, and some other internal parasite species. Another ML, moxidectin, became available in 1997 as a gel formulation for oral administration. The parasiticidal activity of this compound is similar to ivermectin except efficacy is less on the common bot (Gastrophilus intestinalis) but high on encysted small strongyles. Recently however lower than initial activity on ascarids and small strongyles has been found for both ivermectin and moxidectin.
Twelve fur seal pups, which had not nursed their mothers, were used in an infectivity experiment. Pups were exposed to parasitic 3rd-stage larvae of Uncinaria lucasi from belly tissues of fur seal bulls, bachelors, and pregnant cows, to determine maturation capability of the larvae. Hookworms were not recovered from the intestines of 3 pups receiving larvae from belly blubber of bulls, 6 pups receiving larvae from belly blubber of bachelors, and 1 nonexposed pup. Maturation of hookworms did occur in 2 pups exposed to larvae from a mixture of belly blubber, mammary tissue, and milk of pregnant cows. Parasitic 3rd-stage hookworm larvae from belly tissues of pregnant and "non-pregnant" fur seal cows averaged 938.1 and 802.1 micron long, and 34.1 and 31.5 micron wide, respectively; however, larvae from belly tissues of a fur seal bull, bachelors, 2-year-old males, male and female yearlings and pups, and Steller Sea Lion subadults averaged 640.5-732.0 micron long and 20.9-24.9 micron wide.
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