Introduction
Adequate supply of finely balanced dietary nutrients is essential for optimal growth of modern fast growing meat birds owing to their high genetic potential. All the critical nutrients must be supplied in feed to maintain this genetic potential. As per the recommendations of National Research Council, Poultry diets are mostly a mixture of ingredients like cereal grains, soybean meal, animal by-product meal, fats, vitamins and mineral premixes. Together with water, these compounds deliver proteins, amino acids, carbohydrates, fats, minerals, vitamins and energy that the birds need to grow, reproduce and stay healthy. However complete utilization of these nutrients requires a fully capable and functional intestinal system or gut. Alteration in the functional capabilities of the gut in poultry is associated with number of infectious and parasitic diseases ofwhich coccidiosis are probably one of the most intensively studied.
Coccidiosis is a parasitic disease affecting mainly the intestinal tract and caused by several species of a protozoa Eimeria. They invade and destroy the intestinal epithelium of chickens, and as a consequence, infected birds display reduced feed intake, have bloody diarrhea, and have reduced weight gains and mortality. Worldwide the annual costs inflicted by coccidiosis to commercial poultry have been estimated at € 2 billion, stressing the urgent need for more efficient strategies to control the parasite. Various feed components and physical form of the feed seem to influence Eimeria parasites thereby coccidial infection. Essential nutrients including all macronutrients and micronutrients have been examined for their potential use as dietary supplements for coccidiosis control. Some of these nutrients have a direct beneficial effect on the development of parasite within the host and are in fact coccidiosis promoting compounds. Alternatively some nutrients actively enhance the host’s resistance against Eimeria by stimulating the immune system, thus decreasing the pathological effects of a coccidiosis infection.
1. Feed Texture
The size of various ingredients comprising feed largely determine the feed structure or texture which in turn primarily depend upon the presence of cereals and the extent to which they have been ground during the milling procedure. Processing of feed into pellet or crumb at the factory may further decrease the average particle size of poultry feedstuffs. The physical texture of feed influences the development of intestinal tract, gizzard, secretion of enzymes and development of gut flora. Presence of fine particles in feed has a strong inhibiting effect on the contraction and reflux activity of the intestine and its development in comparison to coarse feed with large particle size. Some workers concluded that birds housed in cages supplied with whole cereals up to 60-70% and high protein concentrations (40%) in their diet, showed reduced oocyst shedding and mortality most prominently for E. tenellabut also E. acervulinaand E. maxima in a mixed coccidiosis infection model. Heavier gizzards as well as lower pH in the intestines were found in the study. It was hypothesized that the mechanical function of the better developed gizzards will have destroyed more oocysts, while the lower pH results in a more hostile environment for excysted sporozoites, thus mitigating the coccidiosis infection. However, there are studies which have shown contrasting results. So the effect of the texture on the course of coccidiosis infectionneeds to be explored further.
2. Feed Management
Feed restriction is frequently applied to commercial poultry, especially breeding broilers in order to control growth. Feed control has been shown to have an inhibiting effect on E. tenellainfections. This process possibly improves the immune response, after the initial stress induced by restricted feeding has disappeared. Another explanation for the greater resistance of feed restricted birds is the lower production of trypsin, which contributes to excystation of sporozoites in this chicken.
3. Feed Composition
In addition to feed texture, particular nutrients or feed ingredients may have a direct effect on the Eimeria parasite by influencing certain stages of its life cycle, while other components may modulate indirectly a coccidiosis infection by enhancing the immune response or improving the recovery after infection.
Feed Grains
Today’s poultry diet consists of corn or wheat as main energy source. Beneficial effect of a corn based diet on a coccidiosis infection is observed especiallyin E. tenellaand E. acervulinainfected broilers fed corn when compared to wheat. This different effect on a coccidiosis infection is possibly connected to variances in their micro nutritive constituents, while corn is rich in vitamin A and E that may boost the immune response, wheat contains about twice as much niacin and riboflavin, which are beneficial to the parasite. Moreover, a wheat-based diet may also alter the intestinal flora favoring the development of parasite.
Carbohydrates
The primary source of metabolizable energy of the birds is carbohydrates which can influence the intestinal microbial activity depending upon the type of carbohydrate. Carbohydrates have been investigated frequently for their influence on a coccidiosis infection; most attention has been given to the non-digestible carbohydrates (i.e. NSP). Increased saccharide content of the diet, has negative effect on growth of birds while the oocyst excretion is enhanced. These effects are attributed to modification of the intestinal flora by high saccharide content of the diet. NSP’s are polysaccharides that cannot be degraded by endogenous enzymes and therefore reach the colon almost indigested where they are fermented by microbes. The increased intestinal viscosity produced by NSP could favor the development of coccidiosis. The coccidiosis promoting effect may be lowered by enzymes that reduce viscosity like pentosanases.
Proteins
Effect of variable concentration of feed protein content on coccidiosis has been evaluated by various authors. Low levels of dietary protein (≤ 13%), which are unfavorable for growth performance, seem to diminish coccidiosis mortality, oocyst shedding and coccidiosis lesions. This is attributed to a reduction of trypsin activity in the small intestine, thereby limiting sporozoite excystation and subsequent parasite invasion. High dietary protein may also favor bacterial growth in the intestines and enhances the development of coccidiosis lesions due to E. tenella. Raw soybeans as protein source have a protective effect against coccidiosis-induced growth retardation and lesions scores by several Eimeriaspp., attributed to protease inhibitors limiting excystation.
Lipids
Lipids are important in poultry feed as concentrated source of energy, which are essential components of cellular and subcellular membranes. The effect of lipid on coccidiosis infection is dependent upon the composition of fatty acids. Unsaturated fatty acids seem to promote clinical and pathological signs of coccidiosis when compared to saturated fatty acids. Chickens fed diets enriched with coconut oil, composed of medium chain saturated fatty acids, showed better performance after an E. acervulina infection than those birds given soy oil (unsaturated fatty acids).
Essential fatty acids
Essential fatty acids (EFAs) must be obtained from diet as they cannot be synthesized from other components by almost all land animals. There are two families of EFAs: ω-3 (omega-3 or n-3 or α-linolenic acid) and ω-6 (omega-6, n-6 or linoleic acid). EFA ω-3 belonging to the family of polyunsaturated fatty acids can be found abundantly in fish oil, flaxseed oil and whole flaxseed which can be supplemented to the feed. In E. tenellainfection experiments, lesion scores and growth retardation were significantly reduced in birds supplemented with 2.5, 5 and 10% fish oil and 10% flaxseed oil compared to unsupplemented diets. The beneficial effect of ω-3 fatty acids was explained by the retarded development of E. tenella and ultrastructural changes of both asexual and sexual stages induced by these compounds. These results are consistent with reports on the influence of ω-3 fatty acids on other parasites and suggest that a state of oxidative stress is created which is harmful to the parasite.
Minerals
Dietary minerals form the inorganic part of poultry feed and are needed to maintain the osmotic balance, as co-factors for enzymes, for cellular activity and skeleton physiology. High dietary calcium concentrations (approximately 2% or higher) exceeding the optimum required in a poultry diet, consistently had a negative influence on the outcome of coccidiosis infections with both E. tenellaand E. acervulina. The stimulation of trypsin (important in excystation) by calcium, may explain the coccidiosis enhancing effect of high dietary calcium. In terms of micro minerals, zinc and copper have a positive effect on the growth and weight gain of chickens infected with E. acervulina and E. tenella. Selenium also improves the immunity against coccidiosis caused by E.tenella; moreover it reduces mortality and increases body weight in challenged birds.
Vitamins
Vitamins are bio-molecules, which are involved in chemical reactions as catalysts or as substrates. A number of vitamins like, biotin, thiamine, nicotinic acid, folic acid and riboflavin are known to be necessary for the complete development of parasite within the host. Vitamin A has a positive effect on the growth performance, reduction in mortality and oocyst excretion in E. acervulinaor E. tenellainfected chickens. High doses of vitamin D enhance E. tenellacoccidiosis due to its immune suppressive properties. Adding γ-tocopherol to the diet at a concentration of 8 ppm to chickens infected with E. maxima improved the weight gain and reduced the amount of lesions, and oocyst excretion. Vitamin K is characterized by its coagulation promoting properties and its supplementation reduced mortality induced by an E. necatrix and E. tenellainfection, which are associated with blood loss, but no effect was found on growth, blood losses in faeces and hematocrit. The B-vitamins are essential for the development of Eimeria parasites and explains the coccidiosis enhancing effect of some of them. Hence, many anticoccidial drugs are antagonists of analogues of one or more representatives of the vitamin B complex. Vitamin C is an antioxidant, which stabilizes membranes and could therefore be beneficial in coccidiosis.
Although there are many studies concerning the effect of diet form and composition on Eimeria parasites, more research is needed in this sphere and any control of coccidiosis in future is unlikely to be achieved solely through feed composition or management and further to that usage of anti-coccidial drugs in feed will be a prime factor in devising any absolute control strategy.
References upon request
