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Attempts to achieve apparent (in the field) nematocidal efficacy while using popular anthelmintics with varying degrees of anthelmintic effectiveness
Yazwinski, T.; Tucker, C.; Wray, E.
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Objectives: The studies reported herein were conducted to further assess the effectiveness of routinely-applied anthelmintic treatment and then attempt to achieve efficacious treatment via means not normally used but which should be put in place if the debilitating effects of nematode parasitism are to be addressed. For more than a decade, it has been well recognized that there has been the lack of a commercially-available, single active, broad spectrum, efficacious anthelmintic for use in ruminants. Producers and veterinarians who have recognized this shortfall, have attempted to achieve effective nematode control in a variety of husbandry (pasture management, dietary inclusions, grazing management, etc.) and treatment (frequency, dose level, targeting, combinations, etc.) schemes. Anthelmintic combinations, formulated as such, represent a logical means of biding our time until a new, efficacious compound becomes available. Said combinations have not become available in the USA, where it is now common that combinations be actuated “animal side”; with the effectiveness of some of these treatment practices evaluated in the studies reported here.
Materials and Methods: In three recent field studies, we looked at apparent anthelmintic efficacies via the fecal egg count reduction test (FECRT) in naturally-infected replacement and stocker cattle; assessing various single and combined anthelmintics. In all instances, at least ten animals were placed in each treatment group, with fecal egg counts recorded for all animals on the days of treatment and again 14 days later. Coprocultures were conducted on all individual fecal samples with egg counts > 20 eggs per gram (EPG).
Results: In the first field study (utilizing beef steers), injectable ivermectin and oral fenbendazole were evaluated at 1X and 2X prescribed dose rates, and as a combination of both at 1X. Neither treatment with ivermectin as the sole anthelmintic resulted in a FECR rate of > 47%. All treatment regimens wherein fenbendazole was used resulted in FECR rates > 99%. In the second field study (utilizing replacement heifers),1X dose rates of moxidectin injectable, moxidectin topical, ivermectin injectable, doramectin injectable and eprinomectin long-range were evaluated. During the 14-day post-treatment period, fecal egg counts actually rose for ivermectin treated cattle (42%) and for doramectin treated cattle (9%). Treatment with moxidectin topical, moxidectin injectable and eprinomectin long range decreased egg counts by 6, 26 and 56%, respectively. As a follow-up to the second field study and utilizing cattle from the same herd, oxfendazole and levamisole drenches, given separately at labeled dose rate, were evaluated. Both treatments resulted in FECR rates > 98%. For the cattle used in the 3 field studies, Cooperia punctata and C. oncophora accounted for > 90% of the coproculture larvae harvested on a treatment group basis at both pre- and post-treatment.
Conclusions: It is generally accepted that FECR rates > 90% are necessary in order for an anthelmintic to be assumed to be effective. Given the results obtained herein, it is apparent that the avermectins (ivermectin, doramectin, eprinomectin) are not effective at normal or double dose rates, and that moxidectin is not efficacious at 1X. Fenbendazole, oxfenda- zole (benzimidazoles) and levamisole (an imidazothiazole) administrations are still proving to be efficacious (> 90% FECR); a guarded assessment of true nematocidal effectiveness giv- en the potential disparity between egg count and worm count reductions. Combinations of a macrocyclic lactone (ML) with a benzimidazole or imidazothiazole also result in acceptable FECR percentages, most likely due to the non-ML in the combination. The implications of these findings, coupled with the findings of many others, are that macrocyclic lactone use in cattle should be augmented with combination. Clearly, gastro-intestinal nematodiasis is not being effectively abated with ML use; use that results in: (1), prolonging the deleterious effects of subclinical infections, (2) perpetuation of the selection and propagation of ML-resistant parasites, and (3), continued economic losses (wasted monies spent on non-efficacious products and continued poor animal health and productivity). At the present time, we are clearly past the time of routine “ML remedy” for nematodiasis in herbivores. The looming query at hand is what we do presently to preserve animal health.
Keywords: Cattle Nematodes, Effective Anthelmintics.
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Affiliation of the authors at the time of publication
Animal Science Department, University of Arkansas, Fayetteville Arkansas, United States
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