parasite resistance trends and practical implementation

98 AABP PROCEEDINGS — VOL. 52 — NO. 2 — SEPTEMBER 2019

Endnotes

a https://www.openepi.comb https://www.openepi.com/PDFDocs/TwobyTwoDoc.pdf

Acknowledgement

I want to thank Grace Curry and Makenna Hughes for helping with data extraction. Ms. Hughes was funded by the ISU undergraduate research scholars’ program.

References

1. Arora AK, Killinger AH, Mansfield ME. Bacteriologic and vaccination stud-ies in a field epizootic of infectious bovine keratoconjunctivitis in calves. American journal of veterinary research 1976;37:803-805.2. Butler WF. Treatment of bovine cutaneous papillomata with autogenous vaccine. Veterinary Record 1960;72:1016-1017.3. CheolYong H, SonIl P, HongRyul H. Effects of autogenous toxoid-bacterin in lactating cows with Staphylococcus aureus subclinical mastitis. J Vet Med Sci 2000;62:875-880.4. Clark BL, Stewart DJ, Emery DL, et al. Immunisation of cattle against interdigital dermatitis (foot-rot) with an autogenous Bacteroides nodosus vaccine. Australian Vet J 1986;63:61-62.5. Connor AMO, Brace S, Gould S, et al. A randomized clinical trial evaluat-ing a farm-of-origin autogenous Moraxella bovis vaccine to control infec-tious bovine keratoconjunctivis (pinkeye) in beef cattle. J Vet Internal Med 2011;25:1447-1453.6. Daimi SGR, Waghmare SP, Bijwal DL, et al. Effect of autogenous killed vaccine against subclinical mastitis in cows. Indian Vet J 2006;83:15-17.7. Davidson HJ, Stokka GL. A field trial of autogenous Moraxella bovis bacterin administered through either subcutaneous or subconjunctival injection on the development of keratoconjunctivitis in a beef herd. Canadian Vet J 2003;44:577-580.

8. Dudek K, Bednarek D, Ayling RD, et al. An experimental vaccine composed of two adjuvants gives protection against Mycoplasma bovis in calves. Vaccine 2016;34:3051-3058.9. Funk L, Connor AMO, Maroney M, et al. A randomized and blinded field trial to assess the efficacy of an autogenous vaccine to prevent naturally occurring infectious bovine keratoconjunctivis (IBK) in beef calves. Vaccine 2009;27:4585-4590.10. Hoedemaker M, Korff B, Edler B, et al. Dynamics of Staphylococcus aureus infections during vaccination with an autogenous bacterin in dairy cattle. J Vet Med Series B 2001;48:373-383.11. House JK, Ontiveros MM, Blackmer NM, et al. Evaluation of an autogenous Salmonella bacterin and a modified live Salmonella serotype Choleraesuis vaccine on a commercial dairy farm. Am J Vet Res 2001;62:1897-1902.12. Hughes DE, Kohlmeier RH, Pugh GW, et al. Comparison of vaccination and treatment in controlling naturally occurring infectious bovine kerato-conjunctivitis. Am J Vet Res 1979;40:241-244.13. Hughes DE, Pugh GW, Kohlmeier RH, et al. Effects of vaccination with a Moraxella bovis bacterin on the subsequent development of signs of corneal disease and infection with M. bovis in calves under natural environmental conditions. Am J Vet Res 1976;37:1291-1295.14. Mills L. Cross-protection of feedlot calves against Pasteurella endotoxemia with an Re mutant Salmonella typhimurium bacterin-toxoid. Agri-Practice 1991;12:35-36, 38-39.15. Olson NE. Studies on the effects of vaccination of dairy cattle against staphylococcic mastitis. Dissertation Abstracts 1966;26:5377-5378.16. Pearson JKL. Autogenous toxoid vaccine in the prophylaxis of staphylo-coccal mastitis in cattle. J Dairy Res 1959;26:9-16.17. Pearson JKL, Kerr WR, McCartney WDJ, et al. Tissue vaccines in the treat-ment of bovine papillomas. Vet Rec 1958;70:971-973.18. Reeves HE, Lotz SB, Kennedy E, et al. Evaluation of an autogenous vac-cine in cattle against Escherichia coli bearing the CTX-M-14 plasmid. Res Vet Sci 2013;94:419-424.

Table 2. Reporting of randomization and blinding in autogenous vaccines studies in cattleAuthors Approach to allocation to treatment group Discussion of that assessment of clinical disease was made without

knowledge of the vaccine group?1 Allocation not described Blinding not mentioned by authors3 Random allocation (supporting evidence) No- outcome assessment not blinded4 Random allocation (supporting evidence) Yes - outcome assessment blinded5 Random allocation (no evidence) Blinding not mentioned by authors6 Random allocation (supporting evidence) Blinding not mentioned by authors7 Random allocation (no evidence) Blinding not mentioned by authors8 Random allocation (no evidence) Blinding not mentioned by authors9 Random allocation (supporting evidence) No- outcome assessment not blinded

10 Random allocation (no evidence) Blinding not mentioned by authors11 Allocation not described Blinding not mentioned by authors12 Allocation not described Blinding not mentioned by authors13 Allocation not described Blinding not mentioned by authors14 Allocation not described Blinding not mentioned by authors15 Allocation not described Blinding not mentioned by authors16 Allocation not described Blinding not mentioned by authors17 Random allocation (no evidence) Blinding not mentioned by authors18 Allocation not described Blinding not mentioned by authors

SEPTEMBER 2019 — VOL. 52 — NO. 2 — AABP PROCEEDINGS 99

Parasite resistance trends and practical implementation of fecal egg countsJ.D. Edmonds, DVM, PhD; M.D. Edmonds, DVM, PhDJohnson Research, LLC, Parma, ID 83660Corresponding author: Dr. J.D. Edmonds; [email protected]; 208-722-5829

Abstract

Anthelmintic resistance in the United States has recent-ly gained the attention of the veterinary and livestock com-munities. The rapid rise in resistance to 2 new drug classes shortly after approval outside the United States calls for new strategies and recommendations for maintaining an effective anthelmintic program even as new drugs become available. The development of resistance is product-dependent, with observable differences between classes. Methods used to test anthelmintic efficacy allow the veterinary practitioner to make informed treatment decisions; however, the results should be interpreted carefully due to the limitations for the fecal egg count procedure.

Key words: anthelmintic resistance, bovine, fecal egg count reduction test, FECRT

Résumé

La résistance anthelminthique aux États-Unis a récem-ment retenu l’attention de la communauté vétérinaire et d’élevage du bétail. La croissance rapide de la résistance à deux nouvelles classes de drogues peu après leur homologa-tion à l’extérieur des États-Unis exige de nouvelles stratégies et recommandations pour maintenir l’efficacité du pro-gramme anthelminthique avec l’arrivée de nouvelles drogues. Le développement de la résistance varie d’un produit à l’autre avec des différences notables selon la classe. Les méthodes utilisées pour tester l’efficacité anthelminthique permettent au praticien vétérinaire de faire des choix éclairés. Toutefois, les résultats devraient être interprétés avec prudence en raison des limites de la procédure de comptage des œufs dans les fèces.

Introduction

The ability to achieve an effective anthelmintic control program is both an economic and health concern for United States cattle operators and veterinarians. Since 1964 when the first case of resistance to benzimidazoles was reported,4 scientists have been working to discern the causes of resis-tance and develop methods to diagnose and prevent new occurrences. Fifty years of research has led to many dis-coveries, but resistance remains the greatest challenge to parasite control.19

Resistance typically begins at the farm level and de-velops more rapidly when animals are regularly dewormed. Management practices such as underdosing and improper dosing are important drivers of resistance due to the inherent selection of resistant and/or tolerant worms. Similarly, the use of generic products, which in some cases do not perform as well as the pioneer products, are also implicated. Regard-less of the process, anthelmintic resistance is now common. Along with ongoing research to understand the mechanisms of resistance, there is a need for improved cost-effective diagnostic tools so that intervention strategies can be imple-mented to maintain the economic and health benefits gained from adequate parasite control.

Anthelmintic Resistance Trends

Negative effects of parasitism are often expressed subclinically and typically only appreciated in the cattle industry through decreased animal performance. Reduced animal performance is often multifactorial, and any reduction in anthelmintic efficacy can go unrecognized. Anthelmintic resistance occurs when a previously susceptible worm popu-lation survives treatment and passes its resistant genes to the next generation. A diminished fecal egg count reduction tests is the primary method for detecting reduced drug ef-ficacy. Research into benzimidazole resistance has shown reduced egg count reductions are only evident once 25% of the gastrointestinal nematodes are resistant.1

Gastrointestinal nematode (GIN) resistance to the avermectin/milbemycin (AM) drugs class is usually of most concern, since these drugs have been the cornerstone of parasite control since first gaining approval for use in cattle in 1984. Although avermectin resistance was first reported in small ruminants in South Africa,22 the first report in United States cattle was in 2004 when resistant Cooperia punctata and Haemonchus spp worms were confirmed at necropsy in a grazed stocker operation.7 Further evidence was confirmed in a controlled efficacy study where necropsy of treated animals demonstrated that avermectin was ineffective in reducing developing or arrested Ostertagia ostertagi and adult Cooperia onchophora.5

These findings along with the rapid rise in domestic and global publications on anthelmintic resistance in live-stock20 raised the importance of this issue for producers, the veterinary community, and regulatory agencies. As a result, the FDA Center for Veterinary Medicine (CVM) hosted a


public meeting in 2012 with recognized experts in veterinary parasitology. This meeting led to a CVM-directed education campaign titled Antiparasitic Resistant Management Strategy (ARMS) that called for selective use of antiparasitic drugs along with the promotion of management practices to help maintain effectiveness. These strategies were published,10 presented at educational meetings, and provided on the CVM website.21 In 2018, the CVM requested that animal drug com-panies voluntarily revise drug labels of livestock and equine antiparasitic drugs within 1 year to include information on antiparasitic resistance.

The shortage of new anthelmintic classes in the mar-ketplace and the need to preserve the effective anthelmintics currently available, has led some researchers to use computer simulation models to provide insight into parasite dynamics and identify potential management interventions.2,12,23 Re-sults from computer modeling have challenged previously held principles and advanced new recommendations. For example, it is no longer recommended to annually rotate anthelmintic classes and instead many now advocate for a single anthelmintic class be used until it is no longer effective. Although modeling is a useful tool, there is a large complexity of variables that must be considered when making anthel-mintic resistance predictions. Variables include, but are not limited to, parasite and host biology, genetics, host immunity, pharmacokinetic dynamics, individual management deci-sions, and environmental nuances. Computer simulations predicted that the development of anthelmintic resistance to a new drug class would be markedly delayed if treatment intervals were significantly restricted11 or if 2 drugs from different anthelmintic classes were used simultaneously.13 Haemonchus contortus resistance, however, was reported to a new class of anthelmintic, monepantel, in sheep after only 4 annual treatments. Monepantel, an amino-acetonitrile derivative (AAD), was the first drug from a new anthelmintic class released since ivermectin 30 years prior.18 Monepantel was released with instructions for dosing regimens to reduce selection pressure for anthelmintic resistance. However, re-ports of resistance arose after only 2 years of use in New Zea-land and then shortly after in the United Kingdom.9 In 2014 in Australia, a second new class of anthelmintic, derquantel (a spiroindole), was released in combination with abamectin, and within 2 years of the release reduced efficacy against H. contortus egg shedding was reported.18 Results from these studies demonstrate the complexities and pitfalls involved in predicting the development of anthelmintic resistance, and the current models may not be applicable to new classes of anthelmintics.

It was recently suggested that moxidectin may have a role in the control of macrocyclic lactone-resistant nema-todes. Although cross resistance between avermectins and moxidectin occurs, a greater resistance to avermectins is often reported. Moxidectin is a milbemycin and compared to avermectins has a different potency, pharmacokinetic pro-file, a higher safety profile, and perhaps a unique resistance

profile.16 In vitro studies suggest that the small structural differences between avermectins and milbemycin alter re-ceptor binding and physiological outcomes such as larval pharyngeal pumping, motility, and development.15 Also, since moxidectin is lipophilic, it has a wide volume of distribu-tion to the adipose tissue, longer half-life (14 days vs 7 days for avermectins) and tissue persistence.25 However, in the face of anthelmintic resistance, judicious use of moxidectin should always be implemented to mitigate greater selection pressure.

In recent years, feedlots and grazing operators have often given 2 or more anthelmintics concurrently to achieve desired production endpoints. Published US feedlot studies using combination anthelmintic therapies are limited and varied, but some have shown that there is a performance advantage (weight gains and carcass data) for animals treated with a combination macrocyclic lactone/benzimidazole compared to a macrocyclic lactone alone.17 A 118-day graz-ing study compared the efficacy of concurrent treatment at pasture turnout with a macrocyclic lactone/benzimidazole or an injectable macrocyclic lactone with extended activity. In the first 32 days, only the concurrent therapy provided nearly 100% efficacy based on fecal egg count reduction test (FECRT) and a weight gain advantage compared to controls. At the conclusion of the 118-day study, both the macrocyclic lactone/benzimidazole therapy and the extended release macrocyclic lactone had statistically similar weight gains, and the gains were statistically greater than controls. In this study, cattle were harboring macrocyclic lactone-resistant nematodes, and a single combination treatment at the be-ginning of the grazing season provided benefits throughout the entire period and may have helped to preserve refugia.6

Practical Implications of Fecal Egg Counts

Anthelmintic treatment selection is generally based on the product label indications, perceived efficacy, and cost. Producers and veterinarians are often unaware of the real-time anthelmintic efficacies of the products they are using. The 2 primary methods for testing product efficacy are the controlled efficacy test and the fecal egg count reduc-tion test (FECRT). The controlled efficacy test determines the actual number of worms present in animals before and after treatment by timed necropsy collection. Given the cost and expertise required for controlled studies, some version of the FECRT is the primary method used in the field to de-termine product efficacy.3 Results from the FECRT must be interpreted with the knowledge that reduced post-treatment strongyle egg counts do not always correlate with nematode killing. Also, a FECRT does not report the total number of worms, strongyle nematode genera present, immature stages or surviving female worms that temporarily stop shedding eggs after anthelmintic treatment.24

Regardless of the limitations, the FECRT is the best in-field test available for monitoring treatment response. Typi-


cally, the FECRT is conducted by performing fecal egg counts (FEC) on samples from approximately 15 to 20 animals before and after treatment. If the FEC is reduced by 95% or more after treatment, then the treatment is considered effective. Fecal egg counts can be conducted in the veterinary clinic or submitted to a veterinary diagnostic laboratory. Based on the 2007 to 2008 USDA National Animal Health Monitoring Service, only 5.7% of beef cattle operations surveyed conduct fecal egg counts. A recently completed 2017 survey will provide further insight when reported.

A comparison of different fecal egg counting methods revealed that the Mini-FLOTAC technique was the most accurate and precise FEC method in ruminants when com-pared to the standard Wisconsin or McMaster methods.13 However, the Mini-FLOTAC device is not sold commercially in the United States and the method is only available through veterinary research laboratories. When requesting or per-forming a FEC, understanding test sensitivity is critical for result interpretation since procedures differ by location. In most laboratories, the Modified Wisconsin or McMasters methods are available, but the sensitivities can vary widely. For example, the Modified Wisconsin typically ranges from 3 to 5 eggs per gram (epg) while the McMaster procedure can range for 8 to 50 epg.

Often the main obstacle to conducting a FEC is the cost of the procedure. Veterinary diagnostic laboratories charge upwards of $15 per sample and only some laboratories pro-vide discounts for multiple-sample submissions. To reduce sample costs, individual fecal samples can be composited to reduce the number of fecal egg count procedures needed. Composited fecal samples should be prepared from animals that are physiologically similar (e.g. calf or cow) and include the same amount of feces from each animal so that there is equal animal representation within the composite. A recent article also detailed a method for utilizing composited fecal samples for detection of anthelmintic resistance in cattle.8 This article provided detailed instructions for preparing composite fecal samples, counting the eggs, and evaluating the results. Based on the results presented, a population of nematodes with greater than 95% FECR or less than 80% FECR could be reasonably classified as susceptible or re-sistant, respectively. However, results between 80 to 95% should be interpreted with caution.

Conclusions

Anthelmintic resistance to currently available drugs is a significant and complex issue for the livestock industry. In United States cattle, gastrointestinal nematode resistance to avermectins was first reported in 2004. Increased re-porting of reduced drug efficacies has resulted in greater awareness by the cattle industry, veterinary professionals, and regulatory agencies. There is a need for new classes of anthelmintics and new strategies to extend their efficacy in the field. Concurrent use of 2 products from different classes

is often practiced in the face of anthelmintic resistance. The rapid development of resistance outside the US to 2 new drug classes is alarming considering the effectiveness of these products was projected to be much longer. Resistant development is product-dependent, with potential differ-ences between the avermectin and milbemycins. Detection of anthelmintic resistance in the field is dependent on the fecal egg count reduction test. However, the veterinary prac-titioner must recognize and appreciate test limitations and laboratory variance when applying test results to treatment recommendations.

Acknowledgement

The authors declare no conflicts of interest.

References

1. Anderson TJC, Blouin MS, Beech RN. Population biology of parasitic nematodes: Applications of genetic markers. Adv Parasitol 1998; 41:219-283.2. Barnes EH, Dobson RJ, Barger IA. Worm control and anthelmintic resis-tance: Adventures with a model. Parasitol Today 1995; 11:56-63.3. Coles GC. Drug resistance and drug tolerance in parasites. Trends Parasitol 2006; 22:348.4. Drudge JH, Szanto J, Wyant ZN, Elam G. Field studies on parasite control in sheep: Comparison of thiabendazole, ruelene, and phenothiazine. Am J Vet Res 1964; 1512-1518.5. Edmonds MD, Johnson EG, Edmonds JD. Anthelmintic resistance of Os-tertagia ostertagi and Cooperia oncophora to macrocyclic lactones in cattle from the western United States. Vet Parasitol 2010:224-229.6. Edmonds MD, Vatta AF, Marchiondo AA, Vanimisetti HB, Edmonds JD. Con-current treatment with a macrocyclic lactone and benzimidazole provides season long performance advantages in grazing cattle harboring macrocyclic lactone resistant nematodes. Vet Parasitol 2018; 252:157-162.7. Gasbarre LC, Smith LL, Lichtenfels JR, Pilitt PA. The identification of cattle nematode parasites resistant to multiple classes of anthelmintics in a commercial cattle population in the US. Vet Parasitol 2009; 166:281-285.8. George MM, Paras KL, Howell SB, Kaplan RM. Utilization of composite fecal samples for detection of anthelmintic resistance in gastrointestinal nematodes of cattle. Vet Parasitol 2017; 240:24-29.9. Hamer K, Bartley D, Jennings A, Morrison A, Sargison N. Lack of efficacy of monepantel against trichostrongyle nematodes in a UK sheep flock. Vet Parasitol 2018; 257:48-53.10. Kornele ML, McLean MJ, O’Brien AE, Phillippi-Taylor AM. Antiparasitic resistance and grazing livestock in the United States. J Am Vet Med Assoc 2018; 1244:1020-1022.11. Learmounta J, Taylor MA, Bartram DJ. A computer simulation study to evaluate resistance development with aderquantel–abamectin combination on UK sheep farms. Vet Parasitol 2012; 187:244-253.12. Leathwick DM, Hosking BC. Managing anthelmintic resistance: Model-ling strategic use of a new anthelmintic class to slow the development of resistance to existing classes. N Z Vet J 2009; 57:203-207.13. Leathwick DM. Modelling the benefits of a new class of anthelmintic in combination. Vet Parasitol 2012; 186:93-100.14. Paras KM, George MM, Vidyashankar AN, Kaplan RM. Comparison of fecal egg counting methods in four livestock species. Vet Parasitol 2018; 257:21-27.15. Prichard R, Ménez C, Lespine A. Moxidectin and the avermectins: Con-sanguinity but not identity. Int J Parasitol Drugs Drug Resist 2012; 2:134-153.16. Prichard RK, Geary TG. Perspectives on the utility of moxidectin for the control of parasitic nematodes in the face of developing anthelmintic resistance. Int J Parasitol Drugs Drug Resist 2019; 10:69-83.


17. Reinhardt CD, Hutcheson JP, Nichols WT. A fenbendazole oral drench in addition to an ivermectin pour-on reduces parasite burden and improves feedlot and carcass performance of finishing heifers compared with endec-tocides alone. J Anim Sci 2006; 84:2243-2250.18. Sales N, Love S. Resistance of Haemonchus sp to monepantel and reduced efficacy of a derquantel/abamectin combination confirmed in NSW, Australia. Vet Parasitol 2016; 228:193-196.19. Sangster NC, Cowling A, Woodgate RG. Ten events that defined anthel-mintic resistance research. Trend Parasitol 2018; 34:553-563.20. Sutherland IA, Leathwick DM. Anthelmintic resistance in nematode parasites of cattle: A global issue? Trend Parasitol 2011; 27:176-181.21. van Wyk JA, Malan FS. Resistance of field strains of Haemonchus contortus to ivermectin, closantel, rafoxanide and the benzimidazoles in South Africa. Vet Rec 1988; 123:226-228.

22. U.S. Food and Drug Administration. Antiparasitic resistance. 2019. Available at: www.fda.gov/animal-veterinary/safety-health/antiparasitic-resistance. Assessed Aug 29, 2019.23. van Wyk JA. Refugia-overlooked as perhaps the most potent factor concerning the development of anthelmintic resistance. Onderstepoort J Vet Res 2001; 68:55-67.24. Yazwinski TA, Tucker CA, Wray E, Jones L, Reynolds J, Hornsby P, Powell J. Control trial and fecal egg count reduction test determinations of nema-tocidal efficacies of moxidectin and generic ivermectin in recently weaned, naturally infected calves. Vet Parasitol 2013; 195:95-101.25. Zulalian J, Stout SJ, daCunha AR, Garces T, Miller P. Absorption, tissue distribution, metabolism, and excretion of moxidectin in cattle. J Agric Food Chem 1994; 42:381-387.


New era of parasite control—BMPs for beef cattleChristine B. Navarre, DVM, MS, DACVIM-LASchool of Animal Sciences, Louisiana State University, Baton Rouge, LA 70803

Abstract

Control of gastrointestinal nematodes (GIN) may have economic and health impacts in beef cattle operations. In the past several decades, GIN control has relied almost exclusively on the use of anthelmintics. With the increase in anthelmintic resistance (AR) new strategies must be developed. Knowl-edge of GIN biology and epidemiology in the region based on climate and weather, and specific information from the ranch, such as quantitative fecal egg counts, estimates of AR through fecal egg count reduction tests, ages of the cattle and pasture management are necessary to develop GIN control programs. Control programs should integrate grazing man-agement, management of the immune system so cattle can resist infection, and anthelmintic use.

Key words: beef cattle, gastrointestinal, parasites, nema-todes, anthelmintic resistance

Résumé

Le contrôle des nématodes gastro-intestinaux (NGI) peut avoir un impact économique et sanitaire sur les en-treprises de bovins de boucherie. Au cours des dernières décennies, le contrôle des NGI se faisait presqu’exclusivement à l’aide d’anthelminthiques. En raison de l’augmentation de la résistance anthelminthique (RA), de nouvelles stratégies doi-vent être développées. Afin de développer des programmes de contrôle des NGI, il est nécessaire d’avoir une connaissance de la biologie et de l’épidémiologie des NGI dans la région basée sur le climat et la météo et de l’information spécifique au ranch comme le compte d’œufs dans les fèces, l’estimé de la RA à l’aide de tests de réduction de l’excrétion fécal d’œufs, l’âge des bovins et la régie des pâturages. Les programmes de contrôle doivent intégrer la régie des pâturages, la régie du système immunitaire (pour faire en sorte que les bovins résistent à l’infection) et l’utilisation d’anthelminthiques.

Introduction

Control of gastrointestinal nematodes (GIN) may have positive economic and health impacts in beef cattle opera-tions in the United States (US). In the past several decades, GIN control has been based almost exclusively on the use of anthelmintics. The increase in anthelmintic resistance (AR) dictates that new strategies be developed. This paper will focus on the information needed to develop control strategies and give some basic recommendations that can be tailored to individual cow-calf and stocker operations.

Major GIN of Concern

The GIN of cattle that have significant prevalence and veterinary importance in the US are Cooperia spp (oncophora, punctate and pectinata), Haemonchus placei and contortus, and Ostertagia ostertagi.23,30 National Animal Health Monitor-ing System (NAHMS) data from 2007-2008 showed that over 80% of operations submitting fecal samples were positive for GIN.22

Ostertagia ostertagi is most the most important GIN of cattle because of its high pathogenicity and its impact on a wider age range of cattle.21 Haemonchus placei and contortus are also very pathogenic but usually only impact weanlings and yearlings. Cooperia spp are less pathogenic than either Haemonchus or Ostertagia, but in warm wet conditions may be present in very large numbers and become economically and clinically significant.

Anthelmintic resistance is covered in more depth in the previous paper, but warrants mention here. Level of AR is highly variable depending on location, but there is no ques-tion that AR is reported across the US for all of the major GIN. Of particular concern is the emergence of AR to Ostertagia.10,31 Deaths due to Ostertagia in adult cattle have been reported in Louisiana (C. Navarre, personal communication).

Goals of Control Programs

There are 2 goals in controlling GIN: control economic losses and control clinical disease. The level of infection de-termines both health and economic impacts of GIN. At low levels, there may be no economic impact and development of immunity can be protective to health. At moderate levels, production losses occur without evident clinical disease. The most studied impact from GIN is on weight gain in young growing animals, but impacts on reproductive efficiency and milk production in adult cows is also reported.21,29,30 A study from 2013 looking at efficacy and production benefits follow-ing use of extended-release injectable eprinomectin showed significant differences in weight gain of 66.9 lb, 42 lb, and 18.9 lb (30.4 kg, 19.1 kg, and 8.6 kg) in calves from Louisiana, Arkansas, and Missouri, respectively.13 In this study, efficacy was high with fecal egg count reductions > 95% at all sites, and these gains might not be obtained in the face of lower efficacy from AR. Clinical disease from high GIN burdens can occur, even in adults, especially when other stressors occur, particularly nutritional stress.

There is no question that there are economic and sometimes health benefits from controlling GIN in cattle, but predicting the outcomes of deworming on an individual

parasite resistance trends and practical implementation

Documents