By Nicholas Brown, DVM
Professional Services Veterinarian
Huvepharma, Inc.
nicholas.brown@huvepharma.us
Synthetic anticoccidials, also referred to as “chemicals,” are often considered the most efficacious tools to mitigate coccidiosis in poultry, particularly in situations where other options such as vaccination and ionophores have failed.
Despite their effectiveness, however, veterinarians and producers sometimes hesitate to incorporate them into control programs due to the concern of chemical-resistant Eimeria strains developing.
Anticoccidials in what’s known as the “coccidiostatic” category, such as monensin, salinomycin, zoalene or amprolium, are thought to develop resistance more slowly, allowing a low level of cycling and lesion scores to persist. This concept of how much cycling each medication permits is also commonly referred to as “leakage.”
Because of their ability to prevent cycling and allow less leakage, chemicals are often thought to be more prone to resistance development due to the higher pressure for resistance selection. There is a lack of published data to support this belief, however. That leaves veterinarians and producers no choice but to base decisions on short-term factors and observations, as well as anecdotal and outdated information.
Assessing performance
Considering the economic costs of coccidiosis in broilers, it’s important to pay special attention to selecting and evaluating any anticoccidial program. Although there are methods that attempt to assess the effectiveness of anticoccidials to a given Eimeria population — anticoccidial sensitivity testing (AST), lesion scoring and oocysts per gram, to name a few — predicting the long-term performance of each anticoccidial can be much more difficult.
Historically, AST has been used to confirm or measure anticoccidial resistance in suspected field samples. When conducted in a controlled environment, AST can eliminate variability in management factors, drug-inclusion levels and presence of other confounding elements.
This method of confirming resistance in field samples has significant disadvantages, however. For example, AST might use elevated doses of Eimeria in testing versus challenge levels encountered by broilers in production.1,2 The AST model also relies on what oocysts — the eggs of parasitic protozoa such as Eimeria — can be recovered and isolated from field samples that may vary in quality and quantity.
Comparison study
To compare the performance of three synthetic anticoccidials and assess the development of anticoccidial resistance among Eimeria species commonly found in broilers, Huvepharma conducted a study with Poultry Research Partners, LLC, Athens, Georgia. It’s important to note while most of the medications tested in this experiment have been used for more than 50 years, little recent published research on resistance development is available.
Materials and methods
For the study, we obtained 2,200 1-day-old Cobb 500 by-product male chicks from a commercial hatchery. They had received no vaccinations prior to transport to the research facility.
We placed chicks at a 0.8 square foot (0.074 square meter) per bird density in each 4-foot-by-8-foot floor pen with re-used pine-shaving litter. Each treatment consisted of 10 pens with 40 birds, each with the following treatments:
- No anticoccidial in feed or vaccination (control)
- No anticoccidial in feed with one 0.25 mL dose oral gavage coccidiosis vaccination at day 1 (AdventTM, Huvepharma, Inc)
- Clopidol at 113 g/ton in all feeds
- Amprolium at 113 g/ton in all feeds
- Zoalene at 113 g/ton in all feeds.
At 21 and 28 days, we conducted lesion scoring on one randomly selected bird per pen consisting of microscopic Eimeria maxima scoring of the jejunum and gross Eimeria acervulina scoring of the duodenum, according to the Johnson and Reid system.3
The experimental design was carried out identically for seven trials with 21 days average downtime between cycles. Minimal litter de-caking and replacement was done between trials. Pens were maintained as the same treatment throughout the seven trials.
Figure 1. Floor pen setup (8 feet x 4 feet)
Study results
Final weights and feed-conversion ratios (FCRs) for each trial are presented in Figures 2 and 3.
In general, weights for the anticoccidial treatments were higher than the control, with the difference being statistically significant for clopidol in 5 of 7 trials, for amprolium in 5 of 7 trials and for zoalene in 3 of 7 trials.
Significant improvements in FCR over the control were seen in 6 of 7 groups treated with clopidol, 7 of 7 groups treated with amprolium and 4 of 7 groups treated with zoalene.
Mortalities, which were due to mixed bacterial causes (osteomyelitis, sepsis, etc.) and not related to coccidiosis or necrotic enteritis, ranged from 2% to 6% throughout all trials and showed very few significant differences between treatments.
Figure 2. Final 42-day weights for each treatment for trials 1-7
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Figure 3. Final 42-day mortality adjusted FCRs for each treatment for trials 1-7
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Figures 4 and 5 illustrate the weight and FCR improvements of each treatment over the control throughout the trial. Peak improvements occurred during trials 3 and 4, followed by a gradual decline to almost zero in all groups except for the amprolium treatment, which still showed significant benefits over the control in trials 6 and 7. Like mortality, both microscopic E. maxima and gross E. acervulina lesion scores were more variable and showed very few significant differences between treatments (Figures 6 and 7).
Figure 4. Weight improvements over control for all trials
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Figure 5. FCR improvements over control for all trials
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Figure 6. Mean microscopic E. maxima scores throughout all trials (21 and 28 days)
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Figure 7. Mean gross E. acervulina lesion scores throughout all trials (21 and 28 days)
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Figure 8. Average weight improvements over control treatment throughout all trials by day of measurement
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Figure 9. Average FCR improvements over control treatment throughout all trials by day of measurement
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Conclusions
FCRs and weight gain
We observed remarkable consistency throughout the trials, with the control performing the most poorly, the vaccine group only intermediate and groups administered synthetic anticoccidials (chemicals) performing the best.
The variation in the baseline performance numbers, especially for FCRs in trial 2 and weight gain in trial 7, can most likely be attributed to seasonal differences over 1.25 calendar years and/or differences in quality of the by-product chicks used in the study.
Throughout all trials, most differences in weights over the control occurred in the final feeding phase (28 to 42 days, Figure 8). However, for FCR, most differences over the control seemed to appear in the grower phase (14 to 28 days, Figure 9). These observations make it unclear exactly which point in the trial may have correlated with the peak lesion scores and coccidiosis challenge.
Lesion scores
The variability and lack of statistical differences in lesion scores was also somewhat unexpected. Possible reasons for the inconsistency include:
- Sample size
Due to the desire for performance parameters to be the primary dependent variables, only two birds per pen were sacrificed for lesion scoring, which was done at 21 and 28 days. Coccidiosis-cycling patterns differ between anticoccidial treatments, especially between chemical treatments and vaccine. The limited sample window may have missed peak lesion-score appearance for some treatments while capturing high lesion-score windows for other treatments.
- E. acervulina and E. maxima scoring
As shown in Figures 6 and 7, average lesion scores for both E. acervulina and E. maxima were relatively low throughout the experiment, ranging from 0 to 0.7 out of 4. This may indicate a relatively low natural challenge level in the litter or sampling outside the window for peak lesion scoring.
Development of resistance
The prevailing view in the poultry industry is that the so-called strong chemicals — those that provide little if any leakage — develop resistance more rapidly than those that allow some leakage.
Going into this study, we therefore expected zoalene and amprolium to be less prone to resistance than clopidol. That was not the case, however.
In our study, performance declined at a similar rate across all chemicals throughout this trial, suggesting that the anticoccidials used in this study may have similar patterns of resistance development.
Closing remarks
Biological systems are complex and nearly impossible to duplicate in controlled trials, inevitably leading to flaws in experiments attempting to replicate them. Furthermore, the cost and time associated with evaluations like these can seem daunting or difficult to justify.
To understand the long-term performance of synthetic anticoccidials, it is essential to simulate field conditions as much as possible using several types of studies:
- Floor pens — Allow for more open floor space and increased airflow when compared to the typical broiler house.
- Sample size — Average of 20,000 to 60,000 birds can live in one house on a commercial poultry farm, whereas each treatment group in this study was comprised of 440 birds.
- Litter-moisture uniformity — In this trial, there were no large areas of wet/dry litter hosting coccidiosis “hot spots” or “cold spots,” which may occur in live production.
- Pen construction — The material (mesh wire) used to construct the floor pens in this study may allow oocyst passage between groups.
- Downtime — Unlike commercial operations, downtime between flocks in this experiment was not standardized due to logistical factors.
- Initial oocyst inoculum — For this study, the inoculum was not fully characterized for pre-existing resistance.
Despite these variations, the model used in our floor pen trial may still be useful for predicting anticoccidial performance in broiler production systems. The consistency of treatment performance and lesion scoring when compared to control groups is a demonstration of its integrity and ability to generate a coccidiosis challenge using a realistic dose.
There is an ongoing need for further characterization of resistance development to other anticoccidials such as ionophores or chemicals like robenidine, decoquinate and nicarbazine. The lack of published, current and detailed information on this class of anticoccidials creates space for antiquated narratives to thrive. Having a thorough understanding of anticoccidial chemicals allows for optimized use and improved decision making.
References
1 Jeffers TK. Eimeria Tenella: Incidence, Distribution, and Anticoccidial Drug Resistance of Isolants in Major Broiler-Producing Areas. Avian Dis. 1974;18(1):74-84.
2 Jeffers TK. Eimeria Acervulina and E. Maxima: Incidence and Anticoccidial Drug Resistance of Isolants in Major Broiler-Producing Areas. Avian Dis. 1974;18(3):331-42.
3 Johnson J, Reid WM. Anticoccidial drugs: Lesion scoring techniques in battery and floor-pen experiments with chickens. Exp Parasitol. 1970;28(1):30-36.
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