Antiparasitic Resistance in Cattle and Small Ruminants in the United States: How to Detect It and What to Do About It
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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 Introduction • Parasites in untreated animals, called host- based refugia. Internal parasite infections and external parasite infestations harm animal health and can result in • Eggs and larvae already on the pasture when significant production losses in food-producing the animals are treated, called environmental species, such as cattle, sheep, and goats. refugia. Antiparasitic animal drugs are used to treat and • Life stages of the parasite that are unaffected control parasitic infections and infestations in animals. by drug treatment, such as some larval stages. The parasites that a given drug is effective against are listed in the indication on the drug’s label. Why is preserving refugia important? Antiparasitic resistance is the genetic ability of Preserving refugia maintains drug-sensitive parasites to survive the effects of an antiparasitic drug (susceptible) parasites. The presence of some drug- to which they were previously susceptible. Antiparasitic sensitive parasites decreases (dilutes) the proportion resistance becomes a problem when an increasing of resistant parasites within the parasite population on percentage of a parasite population carries resistance a farm. genes, allowing the parasites to survive treatment with an antiparasitic drug that has been effective in the past. The Importance of Preserving Refugia Australia, New Zealand, South Africa, and South America have struggled with antiparasitic resistance in Parasite population within Parasite population within livestock species for the past few decades. Recent the herd: the herd: scientific data indicate antiparasitic resistance is now emerging in livestock species in the United States. Refugia After an animal is treated with an antiparasitic drug, the susceptible parasites die and the resistant Treat entire herd, so no Treat only 50% herd, so parasites survive to pass on resistance genes to their refugia is preserved. some refugia is preserved. offspring. If not enough susceptible parasites remain in the environment and in the animal, they cannot dilute the increase in resistant parasites that occurs after treatment. This scenario occurs when there is a lack of refugia. What is refugia? All susceptible parasites Some susceptible parasites die. Only resistant parasites remain to dilute the Refugia is the proportion of the total parasite remain to breed and pass resistant parasites, slowing population that is not selected for antiparasitic drug on resistance genes to their the development of a fully offspring. resistant parasite treatment—essentially, those parasites that are in population. “refuge” from the drug. Therefore, there's no selection pressure on these parasites to develop resistance. Key: Refugia maintains a proportion of susceptible Resistant parasite Susceptible parasite parasites on the farm and includes: 1 presence of antiparasitic resistance may Factors Contributing to Antiparasitic worsen the resistance. Resistance Relying solely on antiparasitic drugs to Parasite biology, genetics, and control parasites, rather than changing pathogenicity. management practices: using only drugs instead of incorporating methods to Immune status of the host animal. preserve refugia and better manage the pasture can speed up antiparasitic Treatment factors, such as dose and resistance. frequency of dosing. Using antiparasitic drugs for unapproved Drug factors, such as mechanism of uses, such as to increase weight gain: in action and half-life. the short-term, this may improve profitability; however, using antiparasitic Certain livestock drugs for unapproved uses increases the opportunities to eliminate management practices. susceptible parasites, leaving Newly hatched strongyle larvae. Photo courtesy of only resistant parasites Livestock Management Dr. Ray Kaplan, College of Veterinary Medicine, behind. In the long-term, this Practices Contributing University of Georgia. may lead to significant to Antiparasitic economic losses due to the Resistance negative effects of antiparasitic resistance on Treating every animal herd health. in the herd: treating the entire herd Detecting eliminates susceptible Antiparasitic parasites from all Resistance animals at once, increasing the Currently, the most common proportion of resistant on-the-farm test to detect parasites in the antiparasitic resistance is the population. Fecal Egg Count Reduction Test (FECRT). The FECRT is the only practical, widely Frequent routine deworming without available test to evaluate the effectiveness of performing diagnostic tests or antiparasitic drugs in the field. determining if treatment is necessary: treating animals too often increases the How the test works – opportunities to eliminate susceptible parasites, leaving only resistant parasites Fecal samples are collected from a proportion of behind. animals in a herd just before treatment with an antiparasitic drug and again after treatment Deworming when environmental refugia is (usually 14 days, but varies depending on the low: treating animals when there are few drug). Parasite eggs are counted in both the pre- eggs on the pasture, such as after a and post-treatment fecal samples. If the egg harsh winter or hot, dry summer, numbers in the post-treatment sample are not increases the proportion of resistant eggs reduced by at least 90 percent, resistance may in the environment. be present in the herd. Ninety percent has been the historical numerical value used to detect Giving an antiparasitic drug without resistance based on FECRT results. knowing if it will be effective on the farm: treating with less effective drugs in the Because the level of antiparasitic resistance varies from farm to farm, not all farms will have 2 the same percentage reduction in post-treatment Anemia in a goat due to egg counts after treatment with the same Haemonchus contortus antiparasitic drug. One farm may show a 98 (barber pole worm), as percent reduction while a neighboring farm may detected by assessing the color of the inner have a 91 percent reduction. For this reason, lower eyelid. Photo percentage reductions in post-treatment egg courtesy of Dr. Ray counts should be compared over time on the Kaplan, College of same farm. Veterinary Medicine, University of Georgia. If repeated FECRTs on the same farm show a consistent decrease in percentage reduction (for example, 98 percent reduction→96 percent Currently, these tests have some limitations, reduction→91 percent reduction), resistance mainly high cost. Also, the Egg Hatch Test and may be developing, even if the values are over LDA are only available for specific antiparasitic 90 percent. drugs. The FECRT is most accurate in sheep and goats. Other ancillary tests used to Several biological factors in cattle affect the determine if treatment with an sensitivity of the FECRT, including: antiparasitic drug is necessary and to Cattle shed fewer parasite eggs as they identify which parasites are present: age. FAMACHA© Eye Color Chart Scores Cattle-specific parasites generally On-the-farm test only for sheep and reproduce less frequently and produce goats that evaluates an animal’s load less offspring than small ruminant- of Haemonchus contortus (barber pole specific parasites. worm). Worm load is associated with the color of the inner lower eyelid Cattle manure has a higher water content which correlates to the degree of than small ruminant manure. The water anemia. dilutes the concentration of parasite eggs, resulting in fewer eggs per fecal Coproculture sample. Identifies the parasites that are Other tests to detect antiparasitic resistance are present in a herd. available only through select laboratories: How the test works – Polymerase Chain Reaction (PCR) Test Genetically identifies resistant Fecal samples are collected and parasites. cultured from select animals in a herd. The parasite eggs hatch and the larvae Egg Hatch Test are identified to the species level. Evaluates resistance to Managing Antiparasitic Resistance benzimidazoles. To slow the development of antiparasitic resistance on a farm, encourage producers to Larval Development Assay (LDA) use antiparasitic drugs as just one part of an overall parasite control program that fits the Evaluates in vitro resistance to needs of their herd. benzimidazoles, levamisole, and some macrocyclic lactones. 3 Tips for managing antiparasitic resistance: Use management practices along with antiparasitic drugs to treat and control Use clinical signs and diagnostic test parasites. Some good management results to determine which parasites are practices include: present on the farm, as well as to determine the level of infection and level Preserving refugia within the herd. of resistance. Base management and treatment decisions on this information. Quarantining new livestock. Use only antiparasitic drugs that are Rotating pastures with other livestock effective based on recent diagnostic test results and approved for the particular species or horses. parasites present on the farm. Always follow the directions on the drug’s label. Dragging or harrowing pastures to break up manure piles. Don’t use antiparasitic drugs for unapproved uses. Keeping pasture grass sufficiently tall. Because most parasite larvae stay within 1 inch from the ground, grass Ideally, identify and cull the animals that that is too short exposes animals to are the highest fecal egg shedders. more