Control of Coccidiosis in Chicken

The use of prophylactic measures against coccidiosis is essential in modern poultry production, although the objective of the anticoccidial program may vary depending on the type of birds to be treated. For example, in broiler production prophylaxis should not only prevent disease and mortality but also subclinical infections. Even minor lesions of the intestinal wall may be detrimental to the growth and feed conversion of the birds, jeopardising the profitability of the operation.

In the breeders segment, unlike broilers, mild infections are desirable to induce a challenge immunity. If this immunity is not achieved, there is a danger of considerable damage being caused by coccidiosis in subsequent commercially important periods. This is the reason for the increase in the use of a coccidial vaccine in this segment. Nonetheless, it is important to balance the severity of the infection and the therapeutic treatment.

Some of the considerations for developing a coccidia control programme are:

Vaccines: Commercial vaccines in the form of standarised low doses of sporulated oocysts of the varous coccidial species are administered in drinking water during the first week of life. Because the vaccine serves only to introduce infection, chickens re-infected by the progeny of the vaccine strain.

Key points to consider to achieve a successful vaccination:

• Immunity to avian coccidia is strongly species-specific, so that all the species that may be encountered in commercial chickens of any particular class should be included in any vaccine intended for them.
• Following vaccination, immunity is stimulated initially by the developing life cycles of the vaccinal strains, and is boosted and maintained by successive re-infections initiated by the combination of the progeny of vaccinal oocysts and local wild-type strains.
• The infectiousness and fecundity of oocysts are crucial to the efficacy of a live anticoccidial vaccine, since it is the oocyst stage of Eimeria that initiates the vaccinal infection, hence stimulating immunity and during subsequent recycling infections, maintaining it.
• The efficacy of a vaccine is considerably influenced by the efficiency of the method of administration.

Anticoccidial drugs: Many drugs are available for the prevention and treatment of coccidiosis in chickens. Detailed instructions for use are provided by all manufacturers, and all warnings should be carefully observed.  Relatively low dosages may be used continuosly for prevention or if development of immunity is desired. 

Anticoccidials are generally given to poultry in feed to prevent acute disease and the economic loss often associated with subacute infection.  Prophylactic use is preferred because most damage occurs before signs become apparent, and delayed treatment may not benefit the entire flock. 

Producers may use one anticoccidial continuosly through successive flocks, rotate anticoccidials every 4 to 6 months, or change anticoccidials during a single grow-out (shuttle programmes). Continous use of anticoccidial drugs may result in selection for and survival of drug-resistant strains of coccidia.

There are two classes of anticoccidials:

• Coccidiostats which arrest or inhibit the growth of intracellular coccidia, giving rise to latent infection after drug withdrawal.  In this class we can find the ionophores that mainly affect both the extra and intracellular stages of the parasite (by forming complexes with various ions and transporting them into and through biological membranes, which accounts for their activity). Some ionophores depress weight gain and recent surveys suggest that drug tolerance is widespread).
• Coccidiocidals which destroy coccidia during their development. 

Some examples are:
 
Amprolium: Structurally similar to thiamine and a competitive antagonist that affects coccidia during its dividing phases, where it needs great amounts of thiamine. Due to the fact that it has poor activity against some Eimeria spp, its spectrum has been extended by using it in mixtures (sulfaquinoxaline, ethopabate or folic acid). Development of resistant strains is a problem.

Clopidol stops the development of sporozoites or trophozoites and acts against the electron transport system within the parasite mitochondrion. It has a broad species spectrum, but development of resistance is a problem.   

Folic acid antagonists include sulfonamides and ethopabate. These compounds are structural antagonists of PABA, which is incorporated into folic acid. They are efficacious because coccidia need nucleic acids, mainly during the second schizont stage. Resistance to these compounds is widespread.

Toltrazuril (Baycox®) damages all intracellular development stages of Eimeria. Toltrazuril affects schizonts and micro- and macrogametes, but not the tissue cells of the host animals, as was shown in light and electron microscopic studies.

These findings suggest that toltrazuril interferes with the division of the nucleus and with the activity of the mitochondria, which is responsible for the respiratory metabolism of coccidia.

In the macrogametes, toltrazuril damages the so-called wall-forming bodies. In all intracellular developmental stages, severe vacuolisation occurs due to inflation of the endoplasmatic reticulum.

The special mode of action results in the following advantages:

• toltrazuril acts on all intracellular developmental stages,
• toltrazuril does not interfere with the development of immunity,
• follow-up treatment usually is not necessary,
• even an advanced infection (after 3 – 5 days; gametogony) can still be treated successfully,
• the efficacy of toltrazuril is independent of the severity of the infection.

Programmes rotating in-feed coccidiostats, vaccines and toltrazuril stand-alone treatments are useful approaches to maintaining the longevity of coccidiostats, vaccines, and toltrazuril.