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Observations were made for the second grazing season (April through September of 1960) on the effects of management systems, including medication, on the development of internal parasitism in lambs. The systems included drylot confinement (Band 1), transfer of animals to "clean" pastures at biweekly intervals (Band 2), and rotation among separate, contaminated pastures at approximately biweekly intervals (Bands 3 and 4). Purified phenothiazine (average particle size 6.9 A) was used for all medication involving animals in Bands 1, 2, and 4. A National Formulary (N.F.) grade product (average particle size 7.0 A) was used for Band 3. All bands had free access to a 1:9 phenothiazinemineral mixture, and therapeutic doses for the most part were given in accordance with a predetermined plan. Antemortem data, including hematocrit determinations and parasite egg counts, were obtained from 20 lambs of each band; worm counts were obtained at necropsy from 10 lambs of each band. Bands 1 and 2 did not develop clinical parasitism. The average minimum hematocrit levels for the two bands were 33.0 and 33.1%, respectively. Bands 3 and 4 developed haemonchosis despite the treatment regimens employed. The packed red-cell volumes (PCV) remained below 30% in Band 3 during the entire posttreatment phase. The PCV in Band 4 dropped below those in Band 3 during July and early August but markedly improved following therapeutic treatment. The necropsied lambs of Band 3 (N.F. phenothiazine) had more worms, especially Haemonchus contortus, than those of Band 4 (purified phenothiazine), even though doses of N.F. phenothiazine were increased to 50 g, or double those of the purified product. Under the conditions of these experiments, purified phenothiazine was superior to an N.F. grade product of comparable particle size for the control of Haemonchus infections in lambs. A study of the effects of different management systems on the development of internal parasitism in lambs was initiated at Beltsville, Maryland, in 1959. In the first year (Lindahl et al., 1963) the average levels of parasitism acquired by the lambs were related directly to the degree of exposure to infective larvae. Also, clinical haemonchosis in lambs was not controlled satisfactorily by medication with N.F. phenothiazine. Limited evidence indicated, moreover, that the ineffectiveness of phenothiazine may have been due to a tolerance of the parasite for the drug. Phenothiazine resistance had been reported by Drudge Received for publication 20 September 1967. * Animal Disease and Parasite Research Division, ARS, Beltsville, Maryland. t Animal Husbandry Research Division, ARS, Beltsville, Maryland. tPresent Address: Division of Research Grants, NIH, Bethesda, Maryland. $ Present Address: Animal Disease and Parasite Research Division, ARS, University Park, New Mexico. ? Present Address: Food and Drug Administration, BSSE, DTE, Washington, D. C. II Biometrical Services, ARS, Beltsville, Maryland. et al. (1957a, b), Leland et al. (1957), and Levine and Garrigus (1961). A relationship between the purity of phenothi zine and its efficacy against gastrointestinal nematodes was noted by Douglas et al. (1959) and Baker et al. (1959). Therefore, it seemed desirable to compare the efficiency of N.F. and purified phenothiazine products of comparable particle size in controlling haemonchosis in lambs as a phase of long-term investigations at Beltsville, Maryland. Studies conducted in 1960, the 2nd year of the project, were designed to make this comparison. MATERIALS AND METHODS The general plan was similar to that followed during the 1st year, 1959, and involved three basic management systems, namely: drylot confinement (Band 1), transfer of animals to "clean" pastures at biweekly intervals (Band 2), and grazing of animals on contaminated pastures (Lindahl et al., 1963). However, in the present study two bands of lambs were included in the system involving contaminated pastures to permit comparative evaluations of N.F. (Band 3) and purified phenothiazine products (Band 4) under similar conditions. During the pretrial period, all lambs were creepfed with pellets containing 65% alfalfa meal, 30% ground barley, and 5% soybean oil meal. |
