Broiler intestines with moderate necrotic enteritis lesions.

Necrotic enteritis is a worldwide problem in the broiler industry. The subclinical form of the disease is considered to be more of an economic burden than the clinical form, because of its insidious nature. It has been estimated that annual economic loss due to necrotic enteritis outbreaks amounts to some US$2 billion. Although the disease is primarily caused by bacteria, dietary factors have been identified as one of the most important predisposing factors for necrotic enteritis.

Necrotic enteritis causes 

Enteric diseases are of paramount concern to the poultry industry because of production loss, increased mortality, reduced welfare of birds and increased risk of contamination of poultry products for human consumption. Necrotic enteritis  is one such enteric infectious disease, caused by Clostrodium perfringens, a ubiquitous, gram-positive, spore-forming, extremely prolific, and toxigenic bacteria that can be found in soil, dust, feces, feed, and used poultry litter.

There are five types of C perfringens (A, B, C, D and E) and necrotic enteritis is caused by toxins produced by C perfringens type A or C. Recently, it was established that alpha toxin is not an essential virulence factor in necrotic enteritis in broiler chickens and only certain strains of C perfringens are capable of producing necrotic enteritis under specific conditions

The disease usually occurs in broiler chickens at 2-6 weeks of age. Necrotic enteritis  has been reported in many bird species. Among chickens, the disease is more common in broilers; however outbreaks in layer pullets and adult layer chickens have also been reported. The disease may occur as either an acute or subclinical condition. The subclinical form of the disease has become an economically significant problem for the broiler industry worldwide, as the routine use of antibiotic growth promoters have been banned from poultry feed due to concern over antimicrobial resistance and this has contributed to the higher prevalence of the disease.

Predisposing factors for necrotic enteritis 

The key factor for the development of necrotic enteritis is an intestinal environment that favours growth of C perfringens. Feed composition and feeding practices are assumed to affect disease incidence of necrotic enteritis in broilers.

Dietary factors 

The nature of the diet is an important factor that influences the incidence of necrotic enteritis.

• Diets with high levels of indigestible, water-soluble, non-starch polysaccharides predispose to necrotic enteritis. Thus the feeding of high levels of wheat or barley can predispose to outbreak of necrotic enteritis due to higher level of indigestible fibre, which increases digesta viscosity, decreases digesta passage rate and reduces nutrient digestibility. A highly viscous intestinal environment increases the proliferation of facultative anaerobes such as gram-positive bacteria and enterobacteria
• The use of higher levels of protein meals, such as fish meal or meat and bone meal is often associated with an increased risk of necrotic enteritis. In general, protein-rich diets containing high levels of poorly digestible protein lead to higher protein concentration in the gastrointestinal tract which act as a substrate for bacteria.
• The dietary fat source also influences the incidence of necrotic enteritis. It has been observed that animal fat increase the counts of C perfringens compared to vegetable oil.
• The physical form of the feed may also influence the occurrence of necrotic enteritis. Feed containing large-sized and many small-sized particles predispose more to necrotic enteritis than feed containing uniform particle size.
• Frequent alteration in the feeding regime increases the incidence of necrotic enteritis.
• Poultry feed that has encouraged mold growth and mycotoxin production increases the incidence of necrotic enteritis.

Other factors 

Broiler intestine exhibiting severe necrotic enteritis lesions.

• The occurrence of necrotic enteritis is often associated with an outbreak of coccidial infection. The coccidial infection causes severe damage to cells that line the small intestine making the gut susceptible to infection by C perfringens. Actually, the sloughed cells from the damaged villi provide nitrogen to the caeca in addition to the undigested feed, which leads to necrotic enteritis. However, it is interesting to note that intestinal coccidiosis (but not caecal coccidiosis) invites necrotic enteritis.
• Improper cleaning of houses, utensils and equipment between batches of chickens increases the incidence of necrotic enteritis on the farm.
• Intestinal parasites (especially round worms), immune suppression due to mycotoxins, stress and viral disease such as chicken anemia virus, Gumboro disease and Marek’s disease are also the causes for necrotic enteritis in broilers.

Clinical signs and lesions 

The clinical appearance of necrotic enteritis may vary significantly from nothing more than impaired performance to sudden and high mortality. The disease can affects birds of almost any age, but most commonly occurs in broilers of two to six weeks of age. Early sign of necrotic enteritis are often wet litter and diarrhea. Usually there will be depression, ruffled feathers, closed eyes and dark coloured diarrhea in the infected birds. There will be depression in growth rate and feed efficiency, which is generally noticed at a later stage due to damage to the intestine and subsequent reduction in digestion and absorption of food.

The disease pattern may vary, depending on the factors such as immunity and management including types and programme of in-feed additives. Mortality rates may sometimes rise to 50%. Necrotic enteritis can be diagnosed from the typical gross intestinal lesions from post-mortem findings and a gram-stained smear of a mucosal scraping that exhibits large, gram-positive rods.

Preventive measures  

Necrotic enteritis is a bacterial disease and routine use of antibacterial compounds such as Bacitracin (50 g/ton feed), Lincomycin (2g/ton feed), Virginamycin (20 g/ton feed) in the feed of broilers can prevent the occurrence of necrotic enteritis in broilers. However, legislation prohibits the use of antibiotics as growth promoters in animal feed in key broiler production areas of worldwide.

This has necessitated the industry to evaluate suitable alternate control measures. Diet is an important contributor to necrotic enteritis in broilers, and feeding practices can reduce the disease’s incidence. Among dietary practices that can be followed to reduce the incidence of necrotic enteritis are the following:

• The formulation of diets should limit the inclusion of viscous grains (wheat, barley, rye, oat, etc.) and animal protein supplements (fish meal, meat and bone meal).
• Poor quality protein sources should be excluded. It has been observed that dietary inclusion of potato protein increases the incidence of subclinical necrotic enteritis compared to soybean meal protein. The potato protein-based diet had higher trypsin inhibitor activity and a lower lipid content that could have contributed to the increased incidence of subclinical necrotic enteritis.
• Ensure proper storage of ingredients as well as prepared feed. The correct handling of feed and storage equipment is equally important to reduce necrotic enteritis incidence.
• Addition of organic acids such as butyric acid, medium chain fatty acids (C₆ to C₁₂ but mainly lauric acid) and/or essential oils (thymol, cinnamaldehyd, eucalyptol) may help prevent necrotic enteritis in broilers. These essential oils also control coccidial infections and consequently may reduce necrotic enteritis. All of these compounds have antimicrobial effects.
• Dietary addition of lactose (2.5%) could be used as a potential alternative to growth-promoting antibiotics to control necrotic enteritis. Lactose promotes lactobacilli species to develop throughout the intestinal tract. It is hypothesized that the increase in lactobacilli could promote the production of lactate, and the pH of the intestinal tract would decrease, negatively affecting certain bacteria, including C perfringens.

Necrotic enteritis  is a widespread disease in the broiler industry of significant economic importance. The disease is primarily caused by a bacteria (C perfringens), and can be prevented effectively by antibiotics addition thorough feed.

Feed composition and feeding practices are one of the important predisposing factors for necrotic enteritis in broilers. If proper feeding practices and good management are followed, the incidence of necrotic enteritis can be effectively minimized in broilers in the era of antibiotic-free poultry production.