Find Out If Your Dewormers Are Actually Working
Anthelmintic (dewormer) resistance is a growing problem in cattle operations across the United States. When parasites develop resistance, products that once worked become partially or completely ineffective. The hard truth is that many producers don't know their dewormers aren't working until significant production losses have already piled up.
Resistance testing tells you which products still work in your herd. That information helps you make smarter treatment decisions, stop wasting money on products that don't do the job, and slow down the spread of resistance.
Why Test for Resistance?
The Hidden Problem
Subclinical infections reduce performance without causing obvious disease. You assume treatment worked because there's no acute illness, but production losses accumulate silently in the background.
Resistance is more common than most producers realize, varies by region, operation, and product, and once it's present, it doesn't reverse. The parasites that survive treatment pass those resistant genes to the next generation.
Economic Reality
| Wasted Resource | Cost |
|---|---|
| Product wasted | $3-10 per animal |
| Labor wasted | Time for handling |
| Continued losses | Weight, condition, health |
| Accelerated resistance | In surviving parasites |
The Fecal Egg Count Reduction Test (FECRT)
Gold Standard for Resistance Detection
The FECRT is the primary method for detecting dewormer resistance in the field. It compares parasite egg output before and after treatment. Effective treatment produces a dramatic drop in egg counts. Ineffective treatment shows minimal change. Partial resistance falls somewhere in between.
How FECRT Works
The process starts with a pre-treatment sample: identify and tag animals for tracking, then submit fecal samples for egg counts. On treatment day, dose correctly for each product and record all treatment details. Ten to fourteen days later, collect post-treatment samples and submit for repeat fecal egg counts. Then calculate the percent reduction.
FECRT Interpretation
``` FECRT% = [(Pre-treatment FEC - Post-treatment FEC) / Pre-treatment FEC] × 100 ```
For example, with a pre-treatment count of 500 EPG and a post-treatment count of 50 EPG, the reduction is (500-50)/500 x 100 = 90%.
| Result | Interpretation | Recommendation |
|---|---|---|
| >95% reduction | Effective | Continue using this class |
| 90-95% | Suspicious | May have early resistance |
| <90% | Resistance present | Consider alternative class |
| <50% | Severe resistance | Don't use this class |
Conducting a FECRT
Sample Selection
Aim for a minimum of 10 animals per treatment group, though 20 or more gives you better statistical confidence. Include high, medium, and low shedders from a randomly selected or representative sample, and exclude any animals that were recently treated.
Sample Collection
Collect pre-treatment samples on day zero and post-treatment samples on days 10 to 14. Collect fresh fecal samples directly from the rectum or from freshly deposited, identified manure. Use clean gloves and a clean container, label clearly with animal ID and date, and keep samples cool (not frozen) until they're processed.
Aim for at least 5 to 10 grams, avoid contamination with soil or bedding, and refrigerate if there will be any delay before processing.
Treatment Administration
| Factor | Why It Matters |
|---|---|
| Accurate weight | Under-dosing causes treatment failure |
| Correct dose | Follow label exactly |
| Proper route | Injectable vs pour-on vs oral matters |
| Product handling | Proper storage, not expired |
Laboratory Analysis
Several labs can run fecal egg counts, including university extension labs, private parasitology labs, and some veterinary clinics.
| Method | Best For | Notes |
|---|---|---|
| McMaster | Most FECRT work | Standard, 25-50 EPG detection |
| Wisconsin | Research, low counts | Higher sensitivity |
| FLOTAC | Research | High sensitivity |
| Mini-FLOTAC | Field-friendly | Good sensitivity |
Interpreting Results
Reading the Numbers
| Animal | Pre-treatment FEC | Post-treatment FEC | Reduction |
|---|---|---|---|
| Cow 1 | 450 | 25 | 94.4% |
| Cow 2 | 800 | 100 | 87.5% |
| Cow 3 | 300 | 50 | 83.3% |
| Cow 4 | 600 | 0 | 100% |
| Cow 5 | 500 | 75 | 85.0% |
| Average | 530 | 50 | 90.6% |
What Different Results Mean
>95% (Effective): Continue your current approach and retest in 2 to 3 years to monitor for emerging resistance.
90-95% (Suspicious): The product still provides some benefit, but plan to rotate to a different class and retest sooner (1 to 2 years).
<90% (Resistant): The product isn't providing adequate control. Switch to a different class and reserve this one for combination use or emergencies only.
<50% (Severe resistance): Stop using this class as a primary treatment. Preserve it for possible future combination use and focus your program on effective alternatives.
Factors Affecting Results
| Management Factor | Explanation |
|---|---|
| Under-dosing | Didn't give enough product |
| Expired product | Reduced potency |
| Poor storage | Degraded product |
| Wrong timing | Post samples too early or late |
| Re-infection | New parasites acquired during test |
| Statistical Factor | Explanation |
|---|---|
| Low starting FEC | Hard to show reduction from low numbers |
| Sample variation | Natural day-to-day fluctuation |
| Lab error | Counting mistakes |
Testing Schedule
When to Test
Baseline testing establishes where your herd stands and identifies any existing resistance. Routine monitoring every 2 to 3 years tracks whether resistance is developing. Triggered testing is warranted after any suspected treatment failure, when changing products or protocols, when animals aren't responding as expected, or when purchased animals come from an unknown resistance status.
Which Products to Test
Test all three major dewormer classes: benzimidazoles (fenbendazole, albendazole), macrocyclic lactones (ivermectin, moxidectin), and imidazothiazoles (levamisole). If you're considering switching classes, test the one you're considering before committing.
Other Resistance Tests
Egg Hatch Assay (EHA)
This lab test exposes eggs to specific drug concentrations and is the research standard for benzimidazole resistance. It requires a specialized lab and is more complex than FECRT, but it's precise.
Larval Development Assay (LDA)
The LDA detects resistance mechanisms by exposing larvae to drugs at various concentrations. It's not commonly available and has significant technical requirements.
Molecular Tests
PCR-based tests can detect specific resistance genes, and some are available now for benzimidazoles. Research is ongoing for other classes, and these tests may become more widely available in the future. Their biggest advantage is the ability to detect resistance before treatment failure shows up in the field.
Working with Your Veterinarian
Veterinary Role
Your veterinarian can design an appropriate FECRT protocol, collect and handle samples properly, interpret results in the context of your operation, recommend management changes, and access additional testing resources.
What to Discuss
Before testing, cover your current parasite control program, products being used, any suspected treatment failures, and herd health history. After testing, talk through what results mean for your specific operation, what changes to make in your parasite control program, and what the monitoring plan looks like going forward.
Using Test Results
If Products Are Effective
Preserve that effectiveness. Don't use products unnecessarily, consider targeted selective treatment (TST) approaches that leave some animals untreated (maintaining a susceptible parasite population), and monitor periodically to catch any changes early.
If Resistance Is Detected
Stop using the ineffective product as a primary treatment. Identify effective alternatives through testing, consider combination treatments (using two effective classes together), enhance non-chemical control measures like pasture rotation and grazing management, and document everything going forward.
Combination Treatments
When resistance is present to one class but not others, combining two effective classes can improve overall efficacy. For example, pairing a benzimidazole with a macrocyclic lactone may give better results than either alone. This approach doesn't help if resistance exists to both classes, but it may slow resistance development when used strategically.
Frequently Asked Questions
Bottom Line
Testing beats guessing every time. The only way to know which dewormer products actually work in your herd is to run a fecal egg count reduction test. The standard is straightforward: anything below 90% reduction means resistance is present, and that product isn't pulling its weight.
Test all three major dewormer classes so you know your full range of options before you need them in an emergency. Let the results guide your parasite control decisions rather than sticking with a product out of habit. Document everything so you can track resistance status over time and spot trends early.
Work with your veterinarian throughout the process. Professional guidance on protocol design, sample handling, and result interpretation improves outcomes and helps you build a sustainable parasite management program.
Related Articles
- Internal Parasite Control Strategies
- Fecal Egg Count Monitoring
- Dewormer Selection and Rotation
- Pasture Management for Parasite Control
References
- Kaplan, R.M. (2020). Biology, epidemiology, diagnosis, and management of anthelmintic resistance in gastrointestinal nematodes of livestock. Veterinary Clinics of North America: Food Animal Practice, 36(1), 17-30.
- Coles, G.C., et al. (1992). World Association for the Advancement of Veterinary Parasitology (WAAVP) methods for the detection of anthelmintic resistance in nematodes of veterinary importance. Veterinary Parasitology, 44(1-2), 35-44.
- Sutherland, I.A. & Leathwick, D.M. (2011). Anthelmintic resistance in nematode parasites of cattle: A global issue? Trends in Parasitology, 27(4), 176-181.
- Texas A&M AgriLife Extension. (2024). Detecting Dewormer Resistance in Cattle. Publication ASWeb-167.
- American Association of Bovine Practitioners. (2024). Parasite Control Guidelines: Resistance Testing.
- Gasbarre, L.C., et al. (2009). The identification of cattle nematode parasites resistant to multiple classes of anthelmintics in a commercial cattle population in the US. Veterinary Parasitology, 166(3-4), 281-285.
