Antiparasitic Resistance in Cattle and Small Ruminants
by Jenny Alonge
Parasites are increasingly resisting current antiparasitic drugs. Countries such as Australia, New Zealand, South Africa, and South America have contended with antiparasitic-resistant livestock species for decades, and recent scientific evidence demonstrates that the United States is facing a similar emerging threat. The following article explains this problem, and offers advice on antiparasitic resistance detection and management.
Preserving refugia
Refugia is defined as the proportion of a farm’s parasite population that is not exposed to antiparasitic drug treatment—meaning that following treatment, some parasites remain susceptible to the antiparasitic medication. Refugia includes parasites in untreated animals, eggs and larvae in the environment when the animals are treated, and life stages unaffected by drug treatment, such as some larval stages. Preserving refugia is important to dilute the proportion of drug-resistant parasites in a farm’s overall parasite population.
Factors contributing to antiparasitic resistance
- Antiparasitic resistance is a complex phenomenon influenced by numerous factors, such as:
- Genetic factors — Some parasites possess genetic traits that confer resistance to certain antiparasitics whose prevalence in the parasite population increases over time.
- Overreliance on a single antiparasitic class — Frequent and exclusive use of a single antiparasitic class contributes to the selection pressure on parasites, favoring the survival of individuals with inherent resistance.
- Underdosing and improper administration — Administering incorrect dosages or using antiparasitics improperly (e.g., to improve weight gain) may result in parasite survival and passing on resistance traits.
- Incomplete treatment — Inconsistent or incomplete antiparasitic programs can leave parasites untreated, potentially leading to the survival of resistant individuals.
- Treating the entire herd — Treating every herd animal at once eliminates susceptible parasites from all animals and increases the proportion of resistant parasites.
- Treating at inappropriate times — Treating the herd when conditions for eggs aren’t favorable, such as after a harsh winter or hot, dry summer, increases the proportion of resistant eggs in the environment.
- Lack of fecal testing — Failure to conduct regular fecal testing to identify the specific parasites and their resistance status can result in the use of ineffective antiparasitics.
- High stocking densities — Overcrowding increases the likelihood of parasite transmission between animals, creating an environment where parasite resistance proliferates.
- Environmental persistence — Antiparasitics excreted by treated animals can persist in the environment and expose parasites to sub-lethal doses, contributing to resistant strain selection.
- Animal movement — Moving animals, especially those carrying resistant parasites, can introduce resistant strains to new areas or herds.
- Inadequate pasture management — Continuous grazing without proper pasture rotation allows parasites to complete their life cycle, increasing exposure risk and resistance development.
Fecal egg count reduction test
Currently, the fecal egg count reduction test (FECRT) is the most practical way to evaluate an anti-parasitic’s effectiveness in the field. A FECRT involves collecting fecal samples from a proportion of animals in the herd before and after antiparasitic treatment. The time between sampling is typically 14 days, but varies according to the antiparasitic. Parasite eggs are counted in both samples—if the egg numbers in the post-treatment sample aren’t reduced by 90%, resistance is suspected.
FECRT is more accurate in sheep and goats than in cattle, for the following reasons:
- Cattle tend to shed fewer parasite eggs as they age.
- Parasites that target cattle generally produce less frequently than small-ruminant-specific parasites.
- Cattle manure has a high water content that dilutes the parasite egg concentration, resulting in fewer eggs per fecal sample.
Other antiparasitic resistance detection methods
Other antiparasitic resistance detection methods include:
- Polymerase chain reaction (PCR) test that genetically identifies resistant parasites
- Egg hatch test that evaluates benzimidazole resistance
- Larval developmental assay that evaluates in vitro resistance to benzimidazoles, levamisole, and some macrocyclic lactones
- FAMACHA eye color chart scores for sheep and goats to evaluate their barber pole worm load by assessing the inner lower eyelid color
Antiparasitic resistance management
The goal of a well-managed and sustainable antiparasitic strategy for cattle and small ruminants should be allowing a low parasite burden in the herd to help preserve refugia. Management strategies include:
- Basing management and treatment decisions on clinical signs and diagnostic tests that determine the parasites present on the farm
- Maintaining good diagnostic and treatment records
- Following the antiparasitic drug’s label directions and using only drugs approved for the parasites present on the farm
- Identifying and culling the highest fecal egg shedding animals
- Treating animals at strategic times, such as when environmental temperature and humidity are high and herd females are periparturient
- Weighing animals when possible to avoid underdosing
- Quarantining new livestock
- Rotating pastures with other livestock or horses
- Dragging fields to break up manure piles
- Preventing overcrowding
- Keeping pasture grass above one inch
Antiparasitic resistance seriously threatens the health and productivity of cattle and small ruminants. However, veterinarians and producers who adopt a multifaceted approach that includes diagnostics, strategic deworming, and pasture management can combat resistance and ensure livestock health sustainability.
Comments
Related Articles
More news
Copyright © 2025 - All Rights Reserved
ISSN 2768-198X
List
Add
Please enter a comment