Technically Speaking

Dr. H. David Chapman, University of Arkansas, discusses new rotation strategies

Dr. H. David Chapman
Department of Poultry Science
University of Arkansas

Outbreaks of coccidiosis "recognized by obvious intestinal lesions and excessive mortality "occur less frequently in modern poultry operations than they did in the past. Nevertheless, subtle effects of the disease, such as reduced growth rate and impaired feed conversion, continue to reduce the performance and profitability of commercial birds.

Over the years, sound management practices, sanitary procedures and a broad range of anticoccidial feed additives have all helped to control coccidiosis. Still, Eimeria parasites remain widespread in U.S. poultry flocks, mostly because the parasites have developed resistance to most in-feed coccidiostats after prolonged usage. It is therefore important to continually seek improved methods of coccidiosis control.

Managing Immunity

Traditional approaches to coccidiosis control aim to identify the most effective drug in order to achieve optimal control of the disease. In the past, a succession of new drugs was introduced enabling a producer to select the most effective agent. Few truly new products have appeared in the last decade, however, and it has become increasingly difficult to benefit from the "new drug effect" usually seen with novel anticoccidials.

In recent years, an alternative approach involving the use of drugs that allows immunity to develop has become more popular. Recent research has shown that although drugs such as salinomycin do not prevent the acquisition of immunity, this process takes time and solid immunity is not acquired until birds are 6 to 7 weeks of age.

The development of immunity under these circumstances depends upon the accidental exposure to infective oocysts in the environment.

Acquisition of immunity also depends on the frequency and duration of exposure to infection, the species of Eimeria present, and environmental and management factors.

Natural exposure to infection cannot be guaranteed, so controlled exposure by intentional immunization (using live coccidiosis vaccines) is more appropriate if immunity is desired. The recent introduction of a new method of coccidiosis vaccination "spray cabinet in the hatchery - has made this more practicable for broiler production.

Reliance upon immunity development in medicated birds has the disadvantage that reduced performance due to subclinical coccidiosis may occur if birds are exposed to high levels of infection early in life before immunity has developed.

New Tools, New Strategies

The U.S. poultry industry's newest coccidiostat, diclazuril (Clinacox, Schering- Plough Animal Health), has a unique mode of action and provides broiler operations with an alternative method of control. The synthetic anticoccidial is effective against all species of Eimeria as evidenced by greater suppression of lesions in the intestines compared with other drugs. Diclazuril is also able to completely suppress oocyst production (Fig. 1), thereby reducing the risk of subclinical infection. It is therefore ideal for reducing infection levels or "cleaning-up" commercial houses.

When used in rotation programs with other anticoccidials, diclazuril will not only improve efficacy, but its "clean-up" effect should help improve the efficacy of other drugs. This is important because other feed additives used for coccidiosis control - ionophorous antibiotics such as salinomycin, monensin, narasin and semduramicin "belong to the same family of drugs and have a similar mode of action. As a result, strains of coccidia resistant to one drug show reduced sensitivity to the others.

In order to suppress these resistant strains, it is necessary to use a drug with an unrelated mode of action. Diclazuril is effective against ionophore-resistant strains and strains resistant to various "chemical" drugs such as amprolium, clopidol, robenidine, halofuginone and zoalene (Fig. 2). Ideally, diclazuril should be used with the flock following litter removal, as this should help reduce the numbers of any drug-resistant organisms.

Judicious Use

The consequences of feeding the same anticoccidial for prolonged periods are well known and almost always result in a progressive decline in efficacy. This has been demonstrated with the ionophore monensin, where strains of E. acervulina isolated from breeder farms (where the drug had never been employed) were more sensitive to the drug than strains from broiler farms with a long history of ionophore usage.

Like ionophores and other established coccidiostats, diclazuril has the potential to become less effective when used continuously. It is therefore important to understand its limitations and use it judiciously.

Trials were conducted to compare E. tenella resistance buildup to diclazuril with a quinolone (to which resistance develops rapidly) and amprolium (to which resistance develops slowly). After 10 generations, a reduction in sensitivity to diclazuril was observed (Fig. 3). However, the rate of development of resistance was more comparable with amprolium than with the quinolone.

It is clear that resistance to diclazuril will develop with prolonged use, just as it has to other coccidiostats. However, using the new anticoccidial responsibly will help to preserve the product's "clean up" power.

Alternating coccidiostats (rotation programs) has been practiced for many years, but the approach has been more empirical than systematic. For best results, diclazuril should be used for no more than two successive flocks before rotating to an ionophore or a chemical/ionophore shuttle program; e.g., nicarbazin followed by salinomycin in starter and grower feeds (Fig. 4). Rotating diclazuril with a coccidiosis vaccine such as Coccivac-B (Schering-Plough Animal Health) will also help to "rest" feed additives and preserve their effectiveness.

Diclazuril: Where It Fits

Should diclazuril be used in the starter feed, the grower feed or in both? It is important to include diclazuril in the starter ration to reduce the build-up of oocysts in the litter. Although lesions due to Eimeria are not often seen before 3 weeks of age, this is the period when the initial infection and multiplication of the parasite occurs.

Diclazuril can also be included in the grower ration because this is the period when the pathogenic effects of coccidiosis are most likely. Another reason to include diclazuril in both feeds is to eliminate any resistant strains "a goal that cannot be readily achieved in shuttle programs where a chemical is used only in the starter feed. This is because any oocysts of a "resistant" strain can survive in the litter for the brief period that a chemical is present in the starter feed. Published data suggest that when diclazuril is included in the grower feed, performance is similar to or better than other ionophore programs.

Rotation With a Vaccine

Coccidiosis vaccines have been employed for many years for the immunization of replacement pullets and broiler breeders. Over the past two years, however, the option for spray cabinet administration for one live vaccine, Coccivac-B, has caused usage in broilers to increase significantly. Published data have shown performance of vaccinated birds to be statistically equivalent to conventional coccidiostats in flocks 35 days of age or older.

It should be noted that Coccivac-B contains strains of Eimeria that were isolated many years ago and are inherently sensitive to in-feed coccidiostats. The product can therefore be used to replace wild, resistant strains of coccidia with the vaccinal ones. It has been demonstrated that the use of Coccivac-B can restore the sensitivity to drugs in farms where extended usage of coccidiostats has selected for resistance.

For example, in a study of the efficacy of monensin against isolates of Eimeria obtained from broiler flocks after vaccination with Coccivac-B (Fig. 5), medicated birds showed improved weight gain and produced fewer parasite oocysts in their droppings. Alternating drugs and vaccines may therefore be advantageous since it could result in a restoration of sensitivity to anticoccidial drugs.

Getting More Creative

The growing interest in coccidiosis vaccination and the arrival of diclazuril present new options for managing the disease and reducing the potential for resistance.

For example, following use of an ionophore such as salinomycin, feed diclazuril for two cycles and follow with Coccivac-B for the next two cycles (Fig. 6). Diclazuril would "clean-up" the broiler house and subsequent use of the vaccine should then repopulate the house with drugsensitive parasites. It should then be possible to return to salinomycin or other ionophore. An additional benefit of this approach is that use of the vaccine should prevent the emergence of resistance to diclazuril.

So far evidence to prove that rotation programs will improve the efficacy of anticoccidial drugs in the longer term has not been obtained.

Nevertheless, adoption of novel strategies is important because few new effective drugs will be available in the future for the control of coccidiosis.

Source: CocciForum Issue No.1, Schering-Plough Animal Health.

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