How the AST improves management of coccidiosis and flock performance

By
Vasil Stanev, DVM
Formerly technical manager, intestinal health, poultry
Zoetis

It’s been over a century since coccidial oocysts were first observed in the ceca of a chicken. Since the 1940s, many anticoccidials have been developed to combat these protozoan parasites. Nevertheless, coccidiosis in poultry remains one of the greatest threats to flock health and the economic viability of the poultry industry.

In-feed anticoccidials — both ionophores and synthetics — have proved to be valuable tools in preventing coccidiosis. However, as all of us in the poultry industry know, their long-term use has naturally led to some coccidial resistance to these compounds.

Developing a long-term, strategic rotation program employing all types of anticoccidial tools — ionophores, synthetics and vaccines — is therefore necessary to maintain and even restore the efficacy of these products. But even then, poultry managers need to know which in-feed anticoccidial will be most effective against the species of Eimeria causing problems on a particular farm. This information not only builds the foundation for better disease management, it also helps to ensure money isn’t wasted on products that won’t work.

There is only one diagnostic tool that allows us to predict the sensitivity of a specific Eimeria isolate to different anticoccidial drugs: the anticoccidial sensitivity test (AST). I’ve heard some researchers and veterinarians question the value of the test. They argue the results cannot accurately reflect field conditions, but I beg to differ. In my experience, the AST has proved to be a valuable, cost-saving tool for most poultry operations.

How the AST is performed

To understand the AST’s value, let’s look first at how the test is performed with Eimeria. i

It’s important to understand that the AST can’t be performed in vitro — outside the parasite’s natural environment — because Eimeria parasites won’t complete their life-cycle stages unless they are in the animal. In addition, different drugs affect different stages of that life cycle. Consequently, the AST has to be performed  in vivo — in the animal — which makes it more laborious, expensive and time-consuming compared to in vitro testing.

Birds used for the AST are 2 to 3 weeks of age. They are free of Eimeria, and so is the environment where the AST is performed. For each anticoccidial to be tested, there is an untreated, uninfected group and an untreated, infected group; these two groups serve as controls. There are also a number of groups that are treated with different anticoccidials and infected.

Birds are individually infected with coccida by oral gavage. The coccidia are field isolates obtained from a specific farm that we reproduce for the test. The dose should be high enough to cause coccidiosis in the untreated, infection group evidenced by a lesion score of three.

Several parameters, such as weight gain, feed consumption, lesion scores and oocyst shedding are evaluated; we also statistically analyze the results and then compare them to results in the control groups. The test itself takes 7 days, but preparation for the test can take 3 to 4 months because repetitive inoculations and re-isolations of Eimeria are needed to determine the proper inoculation dose, as well as to obtain the needed amount of occysts.

Simply sensitivity

Those critical of the AST contend the test doesn’t reflect real-world experience because the infection is so much more severe than field pressure. They further argue the test doesn’t take into account bird immunity development nor the recovery growth of birds. All this is true, but it doesn’t minimize the value of the test, which is simply to demonstrate the sensitivity of Eimeria isolates to different anticoccidials.

Consider that the pattern of coccidial sensitivity or resistance depends largely on the previous use of anticoccidials. Each Eimeria population consists of individuals; some are more sensitive and some are more resistant to any anticoccidial. Whatever anticoccidial is used will negatively affect the sensitive segment of the population and the resistant segment will flourish.

For this reason, resistance will prevail over time, and the AST captures this phenomenon very well. We have carried out the AST numerous times and have found that when certain integrators have a history of overusing any one anticoccidial, the isolates from those farms are resistant to that anticoccidial upon AST testing.

We also know from pharmacology that when a pathogen is resistant to a certain drug, it will also be resistant to other drugs that have a similar molecular structure and mode of action. Eimeria and anticoccidials are no exception. This is confirmed quite well by the AST. A good example is the family of monovalent ionophores; when we see resistance to one of them, such as salinomycin, we see the same degree of resistance to other monovalents, such as monensin or narasin.

Real-world example

I’d like to share a case history involving a large integrator that demonstrates the value of the AST. Each flock was treated with a nicarbazin/monovalent ionophore combination in the first phase of production, followed by a monovalent ionophore in the second phase. The integrator continued with this program for a substantial period of time and thought flock performance was satisfactory.

When monthly coccidiosis-lesion scoring started to indicate the efficacy of the program was on the decline, an AST performed at the French National Institute for Agricultural Research demonstrated the development of resistance to the anticoccidials in use.

The isolate exhibited the highest sensitivity to the synthetic anticoccicial decoquinate, which this integrator had never used before, and to the divalent ionophore lasalocid. The integrator switched to these two anticoccicials — and the results were impressive. Monthly lesion scores improved dramatically and so did flock performance. The average daily weight gain increased by 1.5 g (0.003 lb) and, at different company production sites, the feed conversion ratio dropped by 3 to as many as 8 points.

The integrator was astounded — not only to learn it was possible to achieve such performance, but also that it was possible to have flocks with a zero total mean coccidial-lesion score. The integrator realized the economic impact of subclinical coccidiosis had been seriously underestimated.

In conclusion, we can predict the sensitivity of a specific Eimeria strain with the AST. The test may be laborious, time-consuming and costly, but it can really pay off in the long run by improving coccidiosis management and flock performance. It’s also the only diagnostic tool with predictive value. The only other option for an integrator would be “trial and error,” but losses could markedly escalate during the process.