Antibiotic Sensitivity Testing Helps Compare Anticoccidial Programs for Poultry Operation

Anticoccidial medications are widely used throughout the commercial broiler production industry.¹ They are cost-effective, reduce the burden of coccidiosis in flocks and improve profitability for poultry producers.²

As a result of their ongoing ubiquitous use, there have been reports of reduced efficacy.^{3, 4, 5, 6} Producers have implemented the use of shuttle and rotation programs to help limit the development of antibiotic resistance.⁷ But reports of lost efficacy have continued for all commercially available products currently in use.^7,8

So the question becomes: How can poultry producers test the efficacy of anticoccidials against each other to ensure they are using the one that will have the greatest impact on their flock?

Antibiotic sensitivity testing (AST), has long been used to assess the efficacy of anticoccidials against field strains of coccidiosis. AST can provide useful data to help develop a cocci control program, but it is not without limitations. 

In this article, I will discuss what AST is, how it is performed, and the strengths and weaknesses of this form of testing on different coccidiosis treatment options, ionophores and weak and strong chemicals

How To Perform Antibiotic Sensitivity Testing

At a high level, AST uses coccidia from the field to challenge birds that are consuming an anticoccidial of interest. Performance and lesion scores of these birds are evaluated and compared to birds that did not receive the same anticoccidial product. The results can be used to assess the efficacy of the product.

To perform AST, a fecal sample or moist litter is collected from six farms in a complex with birds two to five weeks old. At the lab, the coccidia will be isolated from the samples and a standardized dose developed. The lab will provide an unmedicated feed to day-of-age birds for approximately two weeks.

At 14 days of age, the test birds are transitioned to feed with the selected anticoccidial(s) to be tested, with one group kept as a positive control. The birds are separated into four groups:

Birds are allowed to consume the medicated feed for a few days before all groups, except the negative control, are gavaged with a standardized dose of the test coccidia. The birds are tracked for one week before they are euthanized and the cocci lesions are scored. Feed consumption, feed conversion, weight gain and mortality are also calculated for each group.

By comparing results between the groups, an assessment of each anticoccidial is made.

Strengths and Weaknesses of AST on Different Classes of Anticoccidials

While AST is often a good option for testing multiple anticoccidials against each other, there may be limitations depending on the class of anticoccidials being tested. In some cases, AST may not be worth it.

To understand the strengths and limitations of AST, it is necessary to have a high-level understanding of cocci infections and the mode of action of different anticoccidial classes.

Once coccidia enters the bird, it takes approximately seven days before new coccidia are excreted in the bird’s feces. In untreated birds that are naturally exposed, or with the use of a cocci vaccine, multiple cycles of coccidia need to occur for the bird’s immune system to react to the infection and reduce cocci levels in the bird. 

To address AST use, anticoccidials can be put into three categories:

Ionophores

Ionophores eliminate most, but not all, coccidia in the bird. This reduces the challenge but allows enough to remain for the bird’s immune system to build immunity.

This cocci leakage allows ionophores to be used for prolonged periods without a decrease in performance. Based on this mode of action, combined with the short-challenge duration, a test may show poor results even when field performance is excellent. At Elanco, we do not recommend the use of AST for ionophores as there is little to no value in predicting success in the field.

Weak chemicals

The term “weak” describes the chemical's effect against coccidia, not the bird. While having a different mode of action, weak chemicals have a similar outcome to ionophores and allow some cocci leakage with subsequent immunity, allowing more flexibility in duration of use. Similar to ionophores, testing weak chemicals could create more questions than answers. While some producers value weak chemical AST, Elanco finds limited value in testing them. 

Strong chemicals

When used against sensitive coccidia, strong chemicals are very effective in eliminating coccidia, often with no coccidia seen during posting sessions. However, once resistance starts, it rapidly progresses and performance is severely impacted. AST testing is a great fit for strong chemicals due to these factors, with AST being able to replicate field use even in a short duration test. 

Discuss AST Results With Your Technical Consultant

AST can provide valuable information and is most applicable with the use of strong chemicals. However, even when AST shows good efficacy, it doesn’t mean that sensitivity will last through the duration of the anticoccidial program. 

AST results, in combination with intended duration of use, time since the chemical was last used and historical performance, should be discussed with your technical consultant before implementing an anticoccidial program. 

Poultry producers can contact their Elanco technical consultant to discuss selecting the right anticoccidial program for their flock or learn about other Elanco Intestinal Integrity products.