A Field Trial Evaluation of the Effectiveness and Benefit of Cydectin Long-Acting Injectable and Ivomec Injectable as Used One Time in Grazing Stocker Cattle* T. A. Yazwinski, PhD C. A. Tucker, PhD Z. Johnson, PhD J. Powell, DVM Department of Animal Science University of Arkansas Fayetteville, AR 72701 CLINICAL RELEVANCE Use of moxidectin long-acting injectable and ivermectin injectable in female Bos taurus beef-type calves was evaluated in terms of efficacy (fecal egg counts) and performance parameters (weight gain). In this 150-day study, moxidectintreated calves gained 20% more weight than did ivermectin-treated and control calves. Mean fecal egg count reductions ranged from 76.7 to 99.0 for moxidectin and 0.8 to 83.4 for ivermectin. Moxidectin long-acting injection provided efficacious (immediate as well as long-term) egg count suppressions as well as enhanced animal productivity (weight gains). The study also showed that Cooperia spp appear poised to present the most immediate challenges once long-acting macrocyclic lactone treatments become available. INTRODUCTION Bovine gastrointestinal nematodiasis is a recurrent condition that requires intensive animal surveillance and periodic treatments for optimal animal performance and feed efficiency. 1 Since the early 1980s, the most routinely used parasiticides have been the macrocyclic lactones (MLs): avermectins (ivermectin, doramectin, and eprinomectin) and milbemycin (moxidectin). The performance of these endec- *This study was sponsored in part by Fort Dodge Animal Health, Overland Park, KS. tocides (effective against arthropod and helminth parasites) has been outstanding, but with extensive usage and parasite exposure, the inevitable appears to have occurred resistant populations of nematodes have arisen and the effectiveness of MLs has correspondingly been reduced. 2,3 Recently, eprinomectin and moxidectin have been formulated for long-acting injections, a development that may confer greater and more persistent efficacy for these compounds at a time when the nematocidal effectiveness of 43
Veterinary Therapeutics Vol. 7, No. 1, Spring 2006 their current formulations is waning. 4 Moxidectin is currently available for use in cattle as Cydectin Pour-On (Fort Dodge Animal Health). 5 Formulated as a long-acting injectable (Cydectin LA, Fort Dodge Animal Health), moxidectin has been reported to possess exceptional effectiveness against both internal 6 and external 7 parasites of cattle. In a recent field study, it was shown to reduce fecal nematode egg counts for more than 80 days after treatment and to confer significant improvements in weight gains relative to control cattle. 8 The current study provides a comparison of moxidectin as Cydectin LA with ivermectin as Ivomec (Merial), the latter being the most widely used endectocide in Arkansas over the past 24 years. 9 The data collected further elucidate the effectiveness currently conveyed by ivermectin in cattle and provide additional insight into the effectiveness and benefit of longacting, injectable moxidectin. MATERIALS AND METHODS This study was conducted in accordance with guidelines as supplied by the US FDA (Good Target Animal Study Practices: Clinical Investigators and Monitors), the European Union (Good Clinical Practice for the Conduct of Clinical Trials for Veterinary Medicinal Products), and the World Association for the Advancement of Veterinary Parasitology (WAAVP second edition of guidelines for evaluating the efficacy of anthelmintics in ruminants [bovine, ovine, caprine] and methods for the detection of anthelmintic resistance in nematodes of veterinary importance). Additionally, the animals in this study were maintained and handled as specified by both the sponsor (Fort Dodge Animal Health) and the University Animal Care and Use Committees. For the entire acclimation period and study, the animals were on one pasture and had ad libitum access to pasture herbage (predominantly fescue), water, and trace mineral supplement as a single group. The stocking rate was approximately two animals/acre. The study was conducted from August 2004 to March 2005; total rainfall and average temperature ranges during those months were 1.5 to 16.0 cm and 3.9 C to 22.2 C, respectively. Animals were provided grain supplement at the rates of 1.5 and 2.5 kg/head for the first 40 and last 110 days of the study, respectively. All animals were monitored for health and well-being on a daily basis. Animals and Allocations In total, 194 female Bos taurus beef-type calves of mixed breeding and approximately 4 to 7 months of age were delivered to the research unit for a 30-day acclimation period and health, disposition, and fecal egg count determinations. Based on the above criteria, the most suitable 162 animals were ranked and blocked by magnitude of strongyle eggs per gram of feces (EPG) counts (six blocks with 27 animals/ block) and then re-ranked within each block according to body weight (BW) as obtained on day 1 of the study. Starting at the top of each final ranking, the animals were bracketed into three-animal replicates and randomly assigned to treatment groups within each replicate. Treatment Groups The three treatment groups were moxidectin long-acting injectable (Cydectin LA), ivermectin injectable (Ivomec), and control, with 54 animals assigned to each group (see above). The moxidectin preparation (a 10% solution of moxidectin [w/v]) was delivered SC into the dorsal aspect of the proximal third of the ear with an 18-gauge, 0.75-inch needle at the rate of 1 ml/100 kg BW (1 mg moxidectin/kg BW). The ivermectin injectable was a commercially available solution of ivermectin (lot 44
[ ] Control Group Mean Treated Group Mean % Egg Count Reduction = 100 Control Group Mean number NBC0060; expiry 01-2008). It was delivered SC in the front of the left shoulder with a 16-gauge, 0.75-inch needle at the rate of 2 ml/100 kg BW (0.2 mg ivermectin/kg BW). Dosages of both experimental treatments were rounded to the next highest 0.5 ml. Control animals received no treatments or placebos. Data Collection BWs were obtained for each animal on study days 1, 0, 22, 50, 90, 106, 149, and 150. Fecal samples (approximately 200 g) were obtained rectally from each animal on delivery to the test facility (study day 41 or 35) and on study days 0, 22, 50, 106, and 149. All samples were refrigerated until egg count determinations and coproculturing were conducted, which occurred within 3 days of sample collection. Egg counts and coproculture to identify infective larvae were done on an individual sample basis. Egg counts were conducted on 0.5-g amounts of feces. Feces was homogenized in saturated magnesium sulfate and centrifuged once for 3 minutes; eggs were allowed to float to a coverslip for transfer to a slide and were then counted in total at 10 to 40 (sensitivity of 2 EPG). Coproculture was conducted if the calculated EPG count exceeded 9. For each coproculture, approximately 100 g of feces was homogenized with approximately 20 g of pulverized corn cob and formed into a mold of suitable consistency and dimension in a 10-oz plastic cup. The cup was covered and left at 22 C for 10 to 14 days, at which time the culture was flooded with water for 6 hours and all motile larvae collected, fixed with formaldehyde and heat, identified to genus, and counted (first 100 larvae/sample). Injection sites on all moxidectin treated animals were palpated on study days 22 and 50; thereafter, injection sites were palpated on weighing days but only on animals that had swelling on day 50. Statistical Analysis The experimental design was a randomized complete block with each animal being an experimental unit. For all analyses, the F-test for the main effect of treatment was tested for significance at the 5% level before treatment group least squares means were compared by repeated t-tests (P <.05) (SAS, 1999; SAS Institute, Cary, NC). Percent egg count reductions expressed in this paper are based on the least squares means egg counts according to standard equation (see box, above). Unscheduled Event A severe outbreak of infectious bovine keratoconjunctivitis (IBK) occurred in the experimental herd approximately 40 days after the animals were treated. In total, 43 moxidectintreated, 41 ivermectin-treated, and 36 control calves displayed visible signs of the infection with corneal opacity, excessive lacrimation, and conjunctivitis. These animals received individual injections of oxytetracycline (Liquamycin LA-200, Pfizer Animal Health, or Duramycin 72-200, Durvet) at the rate of 9 mg/0.45 kg BW on a weekly basis for up to 4 consecutive weeks. In addition, because of concern over potential coincident Pasteurella infections, all study animals were given chlortetracycline (Aureomycin 100 Granular, Alpharma) in the grain supplement for 5 continuous days. Each calf was given 2.5 kg/day of the 77% cracked corn/17% soybean oil meal medicated grain 45
Veterinary Therapeutics Vol. 7, No. 1, Spring 2006 TABLE 1. Mean (SEM) Animal Body Weight (kg) by Treatment Group and Study Day Day of Study Treatment 149/150 Group 1/0 (mean) 22 50 90 106 (mean) Cydectin LA 221.4 (2.8) 230.1 (2.9) 229.0 (2.9) 260.7 (3.3) a 262.6 (3.3) a 300.2 (3.9) a Ivomec 223.4 (2.6) 232.4 (2.6) 227.2 (2.6) 250.7 (3.0) b 252.1 (3.0) b 287.9 (3.5) b Control 220.1 (2.5) 227.7 (2.5) 223.4 (2.6) 247.3 (2.9) b 249.0 (2.9) b 285.6 (3.4) b a,b Means in the same column with unlike superscripts are significantly different (P <.05). supplement, which delivered 5,000 mg chlortetracycline/animal/day for the 5 days. RESULTS Performance Parameters Mean animal weights by treatment group are presented in Table 1. Treatment group mean BWs were comparable until day 90, at which time, and for the remainder of the study, moxidectin-treated cattle were significantly (P <.05) heavier than calves in the other two groups. No significant differences in mean animal BWs were seen between ivermectin-treated and control animals for the entire study. Animal weights were also analyzed with the effects of treatment, IBK status, and interaction in the model. No significant effects of IBK status or interaction were found (P <.05; data not presented). Average daily gains by treatment group and study interval are given in Table 2. No significant differences were seen between groups for the first and final study intervals. From study day 22 to 50, a period when weight loss occurred across all groups, moxidectin-treated calves lost significantly less weight than did calves in the other two groups (P <.05). From day 50 to 106, moxidectin-treated calves outgained both ivermectin-treated and control calves (P <.05). From day 106 until the end of the study, moxidectin-treated calves outgained calves from the other two treatment groups, albeit without significant differences. Over the entire 150-day study, moxidectin-treated calves significantly (P <.05) outgained calves in the other two treatment groups, with average daily gains by treatment group being 0.54, 0.44, and 0.44 kg for moxidectin, ivermectin, and control calves, respectively. Animal weight gains were also analyzed with the effects of treatment, IBK status, and interaction in the model. No significant effects of IBK status or interaction were found (P <.05; data not presented). Parasitologic Parameters Strongyle EPG counts by treatment group and study day are presented in Table 3. Counts were comparable between groups on day 0. On all subsequent sample dates, mean EPG counts in the moxidectin group were lower than those in the control group (P <.05). Animals treated with ivermectin had egg counts that were significantly lower than controls on study day 22 only; whereas at the trial s end (day 149), they had counts significantly higher than did the controls. Relative to their respective day 0 levels, EPG counts for moxidectin-treated calves were reduced by 99.9%, 99.1%, 76.7%, and 90.4% on trial days 22, 50, 106, and 149, respectively. In contrast, calves treated with ivermectin experienced peak egg count reductions (day 22) of only 83.4%. 46
TABLE 2. Average (SEM) Daily Gains (kg) by Treatment Group and Study Day Interval Study Day Interval Treatment 106 to 1/0 to Group 1/0 to 22 22 to 50 50 to 106 149/150 149/150 Cydectin LA 0.39 (.05) 0.04 (.04) a 0.60 (.02) a 0.87 (.03) 0.53 (.02) a Ivomec 0.41 (.05) 0.19 (.03) b 0.44 (.02) b 0.83 (.03) 0.44 (.02) b Control 0.34 (.04) 0.15 (.03) b 0.46 (.02) b 0.85 (.03) 0.44 (.02) b a,b Means in the same column with unlike superscripts are significantly different (P <.05). Mean percentages of coproculture larvae by genus, study day, and treatment group are presented in Table 4. Larvae of Cooperia, Ostertagia, and Haemonchus genera were the most abundant; Trichostrongylus and Oesophagostomum larvae were present in low numbers throughout the study. On day 0, all coproculture larval populations were comparable between treatment groups. Relative to control animal larval population percentages, treatment with either ML significantly depressed the values for Ostertagia and Haemonchus and significantly elevated those of Cooperia (P <.05), a change in coproculture larvae percentages from control calf values through days 50 and 149 posttreatment for ivermectin and moxidectin, respectively. Injection-Site Observations Only two of the 54 animals injected with moxidectin developed injection-site swellings; neither swelling was larger than 1 tablespoon in volume. One animal had swelling on day 50 after treatment only; swelling was noted in the other on posttreatment days 22 to 106. No other adverse reactions to injections with moxidectin were observed. DISCUSSION This study began in September 2004 and ended in February 2005. All study animals were managed and maintained according to typical stocker cattle backgrounding practices, thereby yielding observations that apply to fall-time stocker calf placement, production, and parasitism in Arkansas. In addition, the widespread onset of IBK coincident with animal weight loss across all treatment groups provided insight into the compounding effects of maladies common to beef production and how effective remedy of one disease or stressor impacts greatly on the severity of a coincident disease or stressor. The weight loss that occurred across all treatment groups from day 22 to day 50 was seen to result from the IBK condition, a subacute Pasteurella infection, a negative nutritional status, or some combination thereof. Aggressive antibiotic therapy and the increase in grain supplement starting at day 40 are thought to account for the improved animal performance in all treatment groups after day 50. The parameters measured in this study were drug safety, animal weight gains, fecal strongyle egg counts, and the indirect identity and quantification of the strongyle eggs to genus through coproculture. Both parasiticides were entirely safe with no untoward effects. Only two of the 54 animals injected with moxidectin developed injection-site swellings, which were minor and spontaneously resolved before the end of the trial. Significant differences did develop between 47
Veterinary Therapeutics Vol. 7, No. 1, Spring 2006 TABLE 3. Strongyle EPG Counts (Least Squares Means [SEM]) by Treatment Group and Study Day Treatment Study Day Group 0 22 50 106 149 Cydectin LA 280.3 (33.4) 0.2 (.06) b 2.6 (1.5) b 65.4 (13.3) b 27.0 (5.6) c Ivomec 296.5 (40.7) 49.1 (9.3) b 136.0 (29.8) a 299.2 (36.7) a 195.3 (34.1) a Control 334.2 (47.3) 370.4 (49.9) a 183.3 (69.6) a 216.2 (40.8) a 119.7 (21.1) b a,b,c Means in the same column with unlike superscripts are significantly different (P <.05). treatment groups with regard to weight gains, with moxidectin-treated calves gaining 20.8% more weight than either the control or ivermectin-treated animals. On a treatment group basis, animal performance was negatively correlated with fecal strongyle egg counts and, presumably, parasite burden. For the first 22 days after treatment, weight gains were similar for animals in all three treatment groups. For study days 22 to 50, moxidectin-treated calves lost significantly less weight than did calves in the other two groups. Apparently, the effects of gastrointestinal nematodiasis were most abated for calves treated with moxidectin, rendering them most able to respond to the adverse effects of IBK, the subclinical Pasteurella infections, and/or a negative nutritional status. From day 50 to day 106, the significantly increased performance of moxidectin-treated calves continued, with moxidectin-treated calves outgaining calves in the other two groups by approximately 33%. From day 106 to the end of the study, moxidectin-treated calves outgained calves in the other groups by approximately 6%, a nonsignificant difference. The narrowing of the weight gain advantage by moxidectin-treated calves over calves in the other two groups at the end of the study was most likely the result of an accelerating acquisition of nematodes (notably Cooperia spp) by the moxidectin-treated calves and a corresponding lessening of susceptibility to Cooperia spp by calves in the other groups. Mean fecal egg count reduction percentages ranged from 76.7 to 99.0 for moxidectin and from 0.9 to 83.4 for ivermectin. The accepted minimum mean fecal egg count reduction percentage for a nematocide to be deemed effective is 90%, 10 a level of performance that ivermectin failed to display in this study. Published reports of avermectins failing to effectively reduce cattle EPG counts have come from New Zealand, 11 Argentina 12 and Great Britain. 13 In the United States, reports on avermectin resistance in cattle nematodes have been confined to papers at scientific meetings. 14 17 For the most part, the above cited papers 11 17 document the lack of avermectin effectiveness for Cooperia and Haemonchus spp in cattle. In the current study, coproculture larvae quantifications indicate Cooperia to be the nematode genus least adequately controlled by MLs. Given the posttreatment preponderance of Cooperia patencies after ML use and the apparent significance of this particular parasite genus in terms of animal production and vitality, 18 appropriate control measures for cooperiasis should be debated, prescribed, and researched relative to ML treatment. Since Cooperia spp are restricted to younger (<3 years) cattle, 19 ML use in this age group might be best potentiated by combination (coinci- 48
TABLE 4. Percentages (SEM) of Coproculture Larvae by Strongyle Genus, Study Day, and Treatment Group Treatment Study Day Strongyle Genus Group 0 22 50 106 149 Cooperia Cydectin LA 41.1 (4.3) 97.0 (5.3) a 81.5 (7.3) a Ivomec 48.3 (4.7) 93.8 (4.0) a 82.9 (4.6) a 51.1 (4.6) b 32.3 (5.4) b Control 42.3 (4.8) 57.3 (3.7) b 24.9 (4.3) b 42.6 (4.3) b 41.0 (5.5) b Ostertagia Cydectin LA 16.0 (2.2) 1.3 (4.7) b 10.1 (5.2) b Ivomec 13.2 (2.4) 0.3 (2.5) b 8.0 (3.6) b 36.6 (4.1) a 29.7 (3.8) a Control 17.2 (2.5) 20.5 (2.3) a 33.4 (3.4) a 38.0 (3.8) a 26.0 (3.9) a Haemonchus Cydectin LA 25.2 (3.6) 1.3 (2.1) b 6.0 (5.0) b Ivomec 28.5 (3.9) 5.7 (2.5) b 9.0 (4.2) b 8.4 (1.8) a 30.3 (3.7) a Control 27.1 (4.0) 16.0 (2.3) a 30.6 (3.9) a 12.1 (1.7) a 24.2 (3.8) a Trichostrongylus Cydectin LA 4.4 (1.4) 0.1 (0.8) 1.8 (1.6) Ivomec 2.4 (1.5) 0.1 (0.4) 0.1 (0.7) 0.9 (0.7) 4.9 (1.2) Control 3.2 (1.5) 1.3 (0.4) 4.1 (0.7) 3.5 (0.7) 5.9 (1.2) Oesophagostomum Cydectin LA 12.0 (2.2) 0.4 (1.1) 0.7 (1.1) Ivomec 7.4 (2.3) 0.2 (1.1) 0.1 (1.7) 3.0 (0.9) 2.8 (0.8) Control 10.3 (2.4) 5.0 (1.0) 7.0 (1.6) 3.9 (0.9) 3.0 (0.8) a,b Means of like genus and in the same column with different superscripts are significantly different (P <.05). = Insufficient nematode egg/g feces concentration to allow for accurate coproculture procedure larval development, harvest, and quantification. dent or alternated) with non-ml nematocides (e.g., benzimidazole, imidazothiazole) for optimized animal performance and effective treatment of parasitism. When using long-acting injections of ML parasiticides, treatment with an unrelated compound at posttreatment day 100 (approximate) appears to be indicated given the almost undiluted fecundity by Cooperia spp at that time. ML endectocides have been the mainstay of parasite control for most cattle producers since the early 1980s. Since then, the efficacy of these products has predictably diminished, most demonstrably in regard to Cooperia spp. Given the extensive exposure of bovine nematodes to MLs over the past two decades, long-acting injections of ML formulations that may become available for commercial use should be used with acute attention to retention of drug efficacy and the provision of enhanced animal welfare and performance. Regrettably, no new parasiticide compound of a class unrelated to available products is being evaluated for eventual commercial use in cattle, a situation that ensures that the availability of a new product is at least several years away. It is imperative, therefore, that our current array of products as used in cattle production be utilized in such fashion that their efficacies are not further diminished. CONCLUSION In this study, the long-term effectiveness and benefit of ivermectin (Ivomec) and moxidectin long-acting (Cydectin LA) injections as used in 49
Veterinary Therapeutics Vol. 7, No. 1, Spring 2006 a typical stocker calf production system were compared. As formulated, ivermectin was intended (label claim) to provide 14 to 28 days of efficacious levels of protection against infective larvae of the gastrointestinal nematodes detected by coproculture in this study. In contrast, moxidectin long-acting injection has been demonstrated to possess persistent efficacies (>90%) ranging from 90 days (Trichostrongylus spp) to 150 days (Haemonchus spp). 20 Additionally, because of the innate properties of the molecules themselves, moxidectin is approximately 100-times more lipophilic than ivermectin, a major element in the remnant persistence and efficacies of these two MLs. 21 As is evident from the above parasiticidal properties, moxidectin long-acting injection was expected to have a much greater impact than ivermectin on the nematode parasitism of the cattle and, correspondingly, posttreatment weight gains; such results were indeed evident in this study. Three overriding observations are apparent from this study. First, ivermectin injectable provided only transient egg count reductions that were not of efficacious (>90%) proportion. Combined with this lack of effectiveness was the lack of improved animal performance, a set of results that may be common following the use of any ML now commercially available. Second, moxidectin long-acting injection provided efficacious (immediate as well as longterm) egg count suppressions as well as enhanced animal productivity (weight gains). Third, Cooperia spp appear poised to present the most immediate challenges once longacting ML treatments become available (Haemonchus spp infections will assuredly be ones to consider as well). Given these findings (and interpretations), attention should be directed to developing strategies for a more effective use of current MLs (treatment repetitions and combinations, alternating with unrelated products, integration with nonchemical control measures, and the like) as well as establishing methods to help preserve the effectiveness of long-acting ML formulations once they do become available. ACKNOWLEDGMENTS The authors and the University of Arkansas thank Fort Dodge Animal Health, a division of Wyeth Pharmaceuticals, for the funding of this project. In addition, gratitude is expressed to P. Hornsby, A. Carte, and S. Krumplemann for their work in maintaining the animal herd. REFERENCES 1. Flack DE, Frank BN, Easterbrooks LH, et al: Thiabendazole treatment Effect upon weight gains, feed efficiency and cost of gain in commercial feedlot cattle. Vet Med Small Anim Clin 62:565 568, 1967. 2. Greary TG: Ivermectin 20 years on: Maturation of a wonder drug. Trends Parasitol 21(11):530 532, 2005. 3. Kaplan R: Drug resistance in nematodes of veterinary importance: A status report. Trends Parasitol 20(10): 477 481, 2004. 4. Anziani OS, Suarez V, Guglielmone AA, et al: Resistance to benzimidazole and macrocyclic lactone anthelmintics in cattle nematodes in Argentina. Vet Parasitol 122:303 306, 2004. 5. Yazwinski TA, Tucker CA, Copeland S, et al: Dose confirmation of moxidectin pour-on against natural nematode infections of lactating dairy cows. Vet Parasitol 86:223 228, 1999. 6. Ranjan S, Szwczyk E, Search R, et al: Evaluation of the period of protection of 10% moxidectin long-acting against D. viviparus, H. placei, T. axei and O. radiatum infections in cattle. Proc XIX Int Conf WAAVP:167, 2003. 7. Cleale RM, Lloyd JE, Smith LL, et al: Persistent activity of moxidectin long-acting injectable formulations against natural and experimentally enhanced populations of lice infesting cattle. Vet Parasitol 120:215 227, 2004. 8. Cleale RM, Hart KB, Hutchens DE, et al: Effects of subcutaneous injections of a long acting moxidectin formulation in grazing beef cattle on parasite fecal egg reduction and animal weight gain. Vet Parasitol 126:325 338, 2004. 9. Yazwinski TA, Tucker CA: Unpublished data, University of Arkansas Agricultural Experiment Station, Fayetteville, AR, 2001. 10. Coles GC, Bauer C, Borgsteede FH, et al: WAAVP 50
methods for the detection of anthelmintic resistance in nematodes of veterinary importance. Vet Parasitol 44:35 44, 1992. 11. McKenna PB: Anthelmintic resistance in cattle nematodes in New Zealand: Is it increasing? N Z Vet J 44:76, 1996. 12. Fiel CA, Saumell CA, Steffan PE, et al: Resistance of Cooperia to ivermectin treatments in grazing cattle of the Humid Pampa, Argentina. Vet Parasitol 97:211 217, 2001. 13. Coles GC: Sustainable use of anthelmintics in grazing animals. Vet Rec 151:165 169, 2002. 14. Yazwinski TA, Tucker CA, Powell J: A field trial on the effectiveness and benefit of Cydectin LA and Ivomec as used one time in grazing, stocker calves. Proc AAVP:36, 2005. 15. Smith LL, Gasbarre LC: Evaluation of individual or a combination of anthelmintics in a commercial cattle population where anthelmintic resistant parasites had been observed the previous year. Proc AAVP:63, 2005. 16. Gasbarre LC, Smith LL: Anthelmintic resistance in cattle nematodes What do we need to know? Proc AAVP:51, 2005. 17. Gasbarre LC, Smith LL, Pillit PA: Identification of cattle nematode parasites resistant to multiple classes of anthelmintics in a commercial cattle operation in the US. Proc AAVP:57, 2005. 18. Armour J, Bairden K, Holmes PH: Pathophysiological and parasitological studies on Cooperia oncophora infections in calves. Res Vet Sci 42:373 381, 1987. 19. Couvillion CE, Siefker C, Evans RR: Epidemiological study of nematodes in a grazing, beef cow-calf herd in Mississippi. Vet Parisitol 64:207 218, 1996. 20. Yazwinski TA, Tucker CA, Johnson Z, et al: The effectiveness of moxidectin long-acting injectable formulations when delivered to cattle in Arkansas at the levels of 0.75, 1.0 and 1.5 MPK of bodyweight. Proc AAVP:54, 2005. 21. McKellar QA, Jackson F: Veterinary anthelmintics: Old and new. Trends Parasitol 20(10):456 461, 2004. 51