Sheep and goats, collectively referred to as small ruminants, are an important source of meat, milk, and wool for people throughout the world. Gastrointestinal nematode parasites, commonly called roundworms, can cause serious illness in these animals. Unfortunately, roundworms in small ruminants are increasingly becoming resistant to antiparasitic drugs worldwide. The U.S. is no exception.
With the approval of ivermectin in 19841 and other macrocyclic lactones in the following years, U.S. veterinarians and small ruminant producers were able to easily, cheaply, and safely treat entire groups of sheep and goats for parasites with high initial effectiveness. Combining highly effective antiparasitic drugs with the practice of simultaneously treating all animals in a flock or herd—a practice still common today—can result, at first, in an almost 100 percent parasite kill rate. But recent scientific evidence shows that eliminating all parasites from a flock or herd is not sustainable due to the inevitable development of antiparasitic resistance.2
Antiparasitic resistance is the genetic ability of parasites to survive treatment with an antiparasitic drug that was generally effective against those parasites in the past. After an animal is treated with an antiparasitic drug, the susceptible parasites die and the resistant parasites survive to pass on resistance genes to their offspring. Widespread antiparasitic resistance threatens the health of small ruminants and can result in production losses for producers.
Small Market but Great Need
A drug company typically spends many years and millions of dollars to develop a new drug for animals, get it approved by FDA, and on the market. The limited number of FDA-approved antiparasitic drugs for small ruminants may be due to this high investment of time and money, combined with the relatively small U.S. market share of small ruminants compared to that of other food-producing animals, such as cattle. According to the 2017 Census of Agriculture (the most recent census results available), the combined U.S. sales of sheep, goats, and their products (milk, wool, and mohair) totaled a little over $1 billion, making up only 0.3 percent of all U.S. agriculture sales. In contrast, sales of cattle and calves totaled over $77 billion and sales of milk from cows brought in over $36 billion, accounting for over 19 percent and 9 percent of all U.S. agriculture sales, respectively.3
Sheep numbers in the U.S. peaked in 1942 at over 56 million head.4 As of the beginning of 2021, that number had dwindled to only a little over 5 million head. In January 2021, there were a little over 2.5 million goats in the U.S., compared to a little over 93.5 million head of cattle.5, 6
Despite their relatively low numbers in the U.S., small ruminants have a high need for effective antiparasitic drugs. Parasitic infections can harm animal health and lead to economic losses. For example, the roundworm Haemonchus contortus (commonly known as the barber pole worm) causes severe disease in sheep and goats, resulting in such signs as decreased activity level, decreased to no appetite, reduced growth, a very low red blood cell count (severe anemia), and death. When H. contortus become resistant to previously effective antiparasitic drugs, the flock or herd experiences increased health problems and the producer experiences greater economic losses.
Table 1 lists the antiparasitic drugs that are FDA-approved for use in sheep and goats in the U.S. However, not all these drugs are currently marketed and some are inconsistently available. Resistance has developed to each active ingredient listed in the table, including to moxidectin, which was approved in 2005 and is the newest antiparasitic drug on the U.S. market for sheep.7, 8, 9
Table 1: FDA-approved antiparasitic drugs for sheep and goats*,**
|Active Ingredient||Proprietary (Brand) Name||Species|
|Thibenzole®||Sheep and goats|
|Omnizole®||Sheep and goats|
|E-Z-EX Wormer Pellets®||Sheep and goats|
|Equizole®||Sheep and goats|
|TBZ 200 Medicated Feed Premix||Sheep and goats|
|Morantel tartate||Rumatel® 88||Goats|
|Albendazole||Valbazen®||Sheep and goats|
|Safe-Guard®||Goats and wild sheep (Rocky mountain bighorn sheep)
Not approved for use in domestic sheep.
|Moxidectin||Cydectin® Oral Drench||Sheep|
Extra-Label Drug Use
Because relatively few antiparasitic drugs are FDA-approved for small ruminants, veterinarians often use antiparasitic drugs approved for other species in sheep and goats in an extra-label manner. The Animal Medicinal Drug Use Clarification Act of 1994 amended the Federal Food, Drug, and Cosmetic (FD&C) Act to allow veterinarians to legally prescribe, under specified conditions, an approved human or animal drug for a use that isn’t listed on the drug’s labeling (this is called an extra- or off-label use). To prescribe a drug in an extra-label manner, the veterinarian must follow FDA’s requirements for extra-label drug use in animals, as stated in the FD&C Act and FDA regulations.
Several antiparasitic drugs are approved for only sheep, so veterinarians often prescribe these drugs for goats in an extra-label manner. Due to differences in drug absorption and metabolism between sheep and goats, veterinarians commonly use a dose in goats that is 1.5 to 2 times the approved sheep dose. Because the effective dose in goats is currently unknown, veterinarians may be under- or over-dosing animals, which may contribute to the development of antiparasitic resistance.
When using a drug in an extra-label manner in food-producing animals, such as small ruminants, the prescribing veterinarian is responsible for establishing a substantially extended withdrawal period supported by appropriate scientific information. This information can be obtained from such sources as scientific literature, academia, or the Food Animal Residue Avoidance Databank (FARAD).
Good Management Practices Help
Internationally, veterinarians and veterinary parasitologists are investigating various management practices that may help delay the development of antiparasitic resistance in grazing livestock. Major sheep-producing countries, including Australia and New Zealand, already follow these recommendations:10, 11
- Identify and then avoid management practices that contribute to antiparasitic resistance, such as treating every animal in the flock or herd and frequent routine deworming without performing diagnostic tests or determining if treatment is necessary;
- Preserve refugia by not treating all animals at the same time. Refugia are the proportion of the total parasite population that is not selected for antiparasitic drug treatment—essentially, those parasites that are in “refuge” from the drug. Therefore, there’s no selection pressure on these parasites to develop resistance. Preserving refugia maintains a proportion of drug-sensitive (susceptible) parasites on the farm. The presence of some drug-sensitive parasites decreases (dilutes) the proportion of resistant parasites within the parasite population on a farm.
- Choose antiparasitic drugs wisely. Use only those that are effective based on recent diagnostic test results and approved for the particular parasites present on the farm. Always follow the directions on the drug’s label; and
- Quarantine new livestock. Before introducing new animals into a flock or herd, examine them for roundworm infections and treat accordingly.
The ARMS of FDA
In the U.S., FDA’s Center for Veterinary Medicine created the Antiparasitic Resistance Management Strategy (ARMS) to promote sustainable use of approved antiparasitic drugs in cattle, small ruminants, and horses. The ARMS group is collaborating with veterinary, parasitology, and agricultural organizations on an outreach and educational effort regarding antiparasitic resistance in grazing livestock, including sheep and goats. One goal of this effort is to help producers and veterinarians adopt strategies that maintain the effectiveness of antiparasitic drugs.
Animals, including small ruminants, will always have a need for new antiparasitic drugs. No animal parasite species is close to becoming wiped out, and parasites will continue to evolve drug resistance. However, antiparasitic drug discovery is declining for various reasons, namely economic changes in the animal drug industry. Because of mergers and acquisitions among the traditional animal health companies, fewer groups are working on antiparasitic drug discovery. Furthermore, the companies that do presently exist have little incentive to invest in researching and developing new antiparasitic drugs because resistance is not yet recognized as a widespread problem and the currently available drugs still seem effective.12
Unfortunately, with this trend toward less investment in researching and developing antiparasitic drugs, especially ones for small ruminants,13 the likelihood that new antiparasitic drugs for sheep and goats will be available in the U.S. in the near future is low. Despite this challenge, the ARMS group thinks that today is the best time for drug companies to pursue approval of new antiparasitic drugs for sheep and goats. Companies should start the drug development process now to ensure that new antiparasitic drugs will be available in the next decade. FDA is interested in working with companies on innovative ways to meet the drug approval requirements. For example, companies may be able to use published literature and foreign data to satisfy some of these requirements.
A best-case scenario is for a drug company to develop a new antiparasitic drug and get it FDA-approved, while simultaneously, producers and veterinarians modify their current management practices and drug use strategies. Based on evidence from New Zealand, these modified practices and strategies not only help delay resistance to the newly-approved antiparasitic drug, they also slow resistance to currently-marketed antiparasitic drugs.14 The U.S. small ruminant industry needs new antiparasitic drugs, and the sustainable use of these drugs—as promoted by FDA’s ARMS—will help maintain their effectiveness for as long as possible.
Let’s Slow It Down
Antiparasitic resistance in small ruminants is a global and growing problem that can only be slowed, not stopped. Parasites naturally develop resistance to an antiparasitic drug through random genetic mutations. Resistant parasites survive treatment and continue to multiply, passing on their resistance genes to the next generation, resulting in even more resistant parasites. Poor management practices hasten this process. But by working together and focusing on the sustainable use of antiparasitic drugs, FDA, drug companies, veterinarians, and producers can help significantly slow the development of antiparasitic resistance in small ruminants.
FDA’s Center for Veterinary Medicine thanks Eve Waters for her contributions in researching and drafting this article. Dr. Waters is a 2015 graduate of the Virginia-Maryland Regional College of Veterinary Medicine.
1Freedom of Information Summary for Ivomec (ivermectin). New Animal Drug Application 128-409. New York: Merck & Co Inc., 1984.
2Kornele, ML, McLean MJ, O’Brien AE, et al. Antiparasitic resistance and grazing livestock in the United States. J Am Vet Med Assoc 2014;244:1020–1022.
3USDA National Agricultural Statistics Service. 2017 Census of Agriculture. Table 2. Market Value of Agricultural Products Sold Including Landlord’s Share, Food Marketing Practices, and Value-Added Products: 2017 and 2012. Available at: https://www.nass.usda.gov/Publications/AgCensus/2017/Full_Report/Volume_1,_Chapter_1_US/st99_1_0002_0002.pdf. Accessed Apr 16, 2021.
4USDA National Agricultural Statistics Service. Overview of the United States Sheep and Goat Industry. Available at: https://downloads.usda.library.cornell.edu/usda-esmis/files/sx61dm30x/st74ct34d/xs55mg16m/ShpGtInd-08-09-2011.pdf. Accessed Apr 15, 2021.
5USDA National Agricultural Statistics Service. Sheep and Goats. Available at: https://downloads.usda.library.cornell.edu/usda-esmis/files/000000018/41688945h/f47539332/shep0121.pdf. Accessed Apr 15, 2021.
6USDA National Agricultural Statistics Service. Cattle: Available at: https://downloads.usda.library.cornell.edu/usda-esmis/files/h702q636h/n009ww19g/9880wj45t/catl0121.pdf. Accessed Apr 15, 2021.
7Howell, SB, Burke, JM, Miller, JE, et al. Prevalence of anthelmintic resistance on sheep and goat farms in the southeastern United States. J Am Vet Med Assoc 2008;233:1913–1919.
8Terrill, TH, Kaplan, RM, Larsen, M, et al. Anthelmintic resistance on goat farms in Georgia: efficacy of anthelmintics against gastrointestinal nematodes in two selected goat herds. Vet Parasitol 2001;97:261–268.
9Kaplan, RM, Vidyashankar, AN. An inconvenient truth: global worming and anthelmintic resistance. Vet Parasitol 2012;186:70-78.
10Leathwick, DM, Besier RB. The management of anthelmintic resistance in grazing ruminants in Australasia—strategies and experiences. Vet Parasitol 2014;204:44-54.
11FDA Center for Veterinary Medicine. Helpful Information for Veterinarians—Antiparasitic Resistance in Cattle and Small Ruminants in the United States: How to Detect It and What to Do About It. Available at: https://www.fda.gov/media/85805/download. Accessed Apr 16, 2021.
12Geary, TG, Conder, GA, Bishop, B. The changing landscape of antiparasitic drug discovery for veterinary medicine. Trends Parasitol 2004;20:449–455.
14Leathwick, DM, Hosking, BC. Managing anthelmintic resistance: modelling strategic use of a new anthelmintic class to slow the development of resistance to existing classes. N Z Vet J 2009;57:203–207.
Resources for You
- Antiparasitic Resistance
- Helpful Information for Veterinarians – Antiparasitic Resistance in Cattle and Small Ruminants in the United States: How to Detect it and What to Do About It (PDF - 763KB)
- Antiparasitic resistance and grazing livestock in the United States (JAVMA, Vol 244, No. 9, May 1, 2014)
- Antiparasitic Resistance in Cattle, Small Ruminants, and Horses in the U.S. (video)
- Using Refugia to Manage Parasites in Cattle, Sheep, Goats, and Horses and Reduce Resistance to Dewormers (video)