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Technically Speaking

The Effects of Selection Pressure on Eimeria Populations

Rick Phillips, DVM Director, Worldwide Technical Service, Poultry Schering-Plough Animal Health Corporation

Coccidiosis of poultry is caused by intracellular protozoan parasites belonging to several species of the genus Eimeria. As poultry producers know, coccidiosis can significantly impair growth rate, feed utilization and flock uniformity.

Coccidia organisms have a complex life cycle. There is a portion inside the bird (endogenous) and a portion outside (exogenous) in the litter/environment.

Selection pressure

All living organisms, including protozoa, are influenced by their environment. This environmental influence is referred to in biology as selection pressure.

Selection pressure can be natural or man made. When applied to a given biological population, it can give certain species or strains in that population an advantage or disadvantage to their survival and reproductive capabilities.

All living creatures are under a constant natural selection process. In mammals, selection pressure occurs slowly over thousands of years due to the relatively slow generation time. In the rapidly proliferating world of microorganisms such as bacteria, viruses and protozoa, however, selection pressures can have a profound rapid effect on population dynamics.

To truly understand and predict the population’s behavior, analysis of these populations is a quantitative discipline and mathematical models are required. Unfortunately, no specific model has been developed to predict the behavior of the Eimeria population in today’s poultry production units.

On the other hand, there is a tremendous amount of scientific data that enables basic predictions to be made about how Eimeria behave in a controlled environment with defined conditions or pressures, such as a poultry house.

Two major selection pressures placed on the Eimeria population in today’s production facilities include host immunity and anticoccidial programs, such as live coccidial vaccines and drugs.

1. Host immunity

The immune system response of poultry to coccidial parasites is complex. Both cell-mediated and antibody immune responses are activated during a coccidial challenge; yet, the cell-mediated immune response plays a larger role in disease resistance.

The host immune system is a naturally occurring selection pressure that constantly influences the dynamics of the coccidia population in poultry houses. Less immunogenic coccidial species and strains that have the ability to evade the host immune system will have a higher survival percentage over the life of the flock. The immune response can vary greatly between different poultry breeds and/or strains. This is why the breed/strain, age and sex of the host (bird) can have a dramatic effect on the level of coccidia that is shed in a poultry house.

2. Anticoccidials

Synthetic chemicals: Synthetic chemicals, especially the highly effective ones, place strong selection pressure on coccidia populations in poultry houses.

Ironically, synthetic chemicals are too effective with respect to long-term control of coccidia populations in poultry facilities. Coccidial strains most sensitive to the drug are not allowed to replicate in the gut of the bird, allowing the resistant and/or less drug-tolerant species or strains to rapidly multiply without natural competition (Figure 1).

This rapid selection for natural resistant strains prevents the continuous use of synthetic chemicals in a poultry operation. When resistance is low to an effective synthetic chemical, maximum flock performance may be realized with respect to good coccidial control.

Polyether ionophores: In contrast, polyether ionophores are not as effective in coccidia control as an effective synthetic chemical. They place less selection pressure on the coccidial population, allowing for some coccidial replication or shedding to occur.

With the ionophores, there is enough replication of coccidia to allow the host immune system to respond effectively. The long-term success of the ionophores has been due to this simple yet complex scenario.

Live vaccines: Once a shift occurs in a coccidial population from drug-sensitive to drug-resistant/tolerant, it may take many years or growing cycles before the coccidial population has the opportunity to naturally shift back to its original state once the anticoccidial product is removed/rotated. However, the use of effective live, drug-sensitive coccidial vaccines shortens the time it takes for a population to shift back.

Live coccidial vaccines, by nature, place selection pressure on an existing coccidial population in the poultry house. When applied appropriately, live coccidial oocysts will replicate and replace drug resistant/tolerant field strains. Over a relatively short period of time, the coccidial population will shift to more sensitive strains.


In order for poultry integrators to maximize flock performance, a good understanding of the two key selection pressures mentioned above — host immunity and anticoccidials — is imperative. Adding drug-sensitive, live coccidial vaccines in rotation with conventional coccidiostats will help the industry maximize product efficacy and coccidial control.

Source: CocciForum Issue No.5, Schering-Plough Animal Health.

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