New toxoid vaccine can enhance NE prevention strategy

By Diana Delmar

“Necrotic Enteritis (NE) has become a top concern in the poultry industry, particularly among producers growing broilers without the use of in-feed antibiotic growth promoters,” says Dr. Aris Malo, global technical manager, Intervet/Schering-Plough Animal Health. The disease also costs the global poultry industry millions of dollars annually, he notes. Clinical NE results in severe intestinal necrosis and increased morbidity and mortality. “In a gut ravaged by NE, the villi actually appear to be melted together. You can imagine the big reduction in the intestinal absorptive surface,” the veterinarian adds.

Subclinical NE is more common, though not always readily apparent. It causes poor weight gain and growth, impaired feed conversion, predisposition to secondary infections, and increased time to market, he says.

NE is a primary reason that therapeutic antibiotics are administered to broilers, according to a nationwide survey conducted by researchers at the University of Liverpool in the UK.

Alpha-toxin is a leading cause

A leading cause of NE is believed to be an alpha-toxin protein secreted by Clostridium perfringens, an opportunistic bacterium that flourishes in the gut if it has a favourable environment. NetB, another toxin reported by Australian researchers to be crucial in the development of NE, was found in only a minority of American chickens with the disease, and was also found in chickens without NE, investigators at the University of Connecticut reported.

When Canadian investigators evaluated several proteins secreted by C. perfringens for their ability to protect against a virulent strain of the pathogen, the greatest protection occurred when broilers were primed with an alphatoxoid and boosted with the active toxin. They concluded that while other proteins may be involved, alpha-toxin plays a key role in the development of NE. In India, researchers who used poly-merase chain reaction to test clinical isolates from broilers with NE concluded that the alpha-toxin gene may play a significant role in the pathogenesis of NE in broilers.

Coccidiosis ‘primary trigger’

An important predisposing event for the development of NE is clinical or subclinical coccidiosis. Dr. Charles Hofacre, poultry veterinarian and professor at the University of Georgia, US, calls coccidiosis a “primary trigger” for NE, which he says is a risk with any anticoccidial program that allows coccidial oocyst cycling. Coccidial cycling causes damage to the intestinal cells, leading to an imbalance in gut bacteria and mucus production. Mucus is a food source for C. perfringens, Hofacre says, according to a recent article in CocciForum’s Intestinal Health.

Other NE triggers, says Malo, include reduced antibiotic sensitivity in countries where in-feed antibiotics are still used, and high protein diets, which supply amino acids that benefit clostridia. High levels of animal byproducts, particularly fish meal, can be heavily contaminated with clostridia spores.

In addition, high levels of wheat, rye and barley have been associated with the development of NE since they can irritate the gut and be difficult to digest, resulting in more mucus production, and more nutrients for clostridia. Edible litter, such as rice or oat hulls, is consumed by birds, increasing ingestion of sporulated coccidial oocysts. Even a rapid change in feed can irritate the gut, thereby making it a favourable environment for

C. perfringens, Malo says.

Controlling NE

One way to control NE, says Hofacre, is to maintain a healthy balance of intestinal bacteria to prevent clostridia from gaining a foothold. This can be accomplished by using coccidiosis vaccines, competitive exclusion products and a C. perfringens toxoid. Toxoids for control of clostridia have been used successfully in other food animals for years, but until recently there were none available for poultry.

Coccidiosis vaccines administered at day one of age enable birds to develop immunity to coccidiosis early in life, when the disease has less impact than it does later in the production cycle. Once vaccinated birds develop immunity to coccidia, they’ll be less susceptible to NE, he says.

Competitive exclusion products help maintain a healthy balance of gut bacteria, Hofacre continues, noting that other products under investigation for NE control include organic and inorganic acids in water and feed, digestive enzymes, essential oils, and herbal extracts. A toxoid vaccine for poultry, called C. perfringens type A toxoid and marketed outside the US as Netvax, has been developed by Intervet/Schering-Plough Animal Health and was recently approved for use in the EU. When two doses are administered to hens, they pass immunity against NE to their off spring. To date, the vaccine has been used primarily by producers in the US and Canada growing antibiotic-free birds.

Vaccination against coccidiosis

In Canadian field trails, broilers from hens that received the toxoid were also vaccinated with the coccidiosis vaccine Coccivac-B. The birds were less prone to NE outbreaks, and if NE outbreaks occurred, they were less frequent and severe. In addition, their weight surpassed the breed standard (Figure 1), says Dr. Linnea Newman, a consultant to Intervet/Schering-Plough Animal Health who presented the results at the May 2008 Ontario Association of Poultry Practitioners.

“In antibiotic-free bird flocks, the toxoid may make the difference between success and failure, but it must be used in combination with proper management and a good ration formula,” Newman says. In another Canadian trial, broilers from hens that received the toxoid received ionophores for coccidiosis control.

“The toxoid had a measurable, positive effect on broiler performance,” says Newman, who notes that use of the toxoid is cost-effective for NE control since only hens and not their progeny have to be vaccinated.

Controlled trials were also conducted in two European countries. Five batches of broilers from over 11,000 hens that received the toxoid had no NE lesions, compared to 5.7% in controls at one site and over 17% at the other; mortality was also lower in the treatment versus control group. Vaccinated breeder hens and their progeny had significantly higher levels of C. perfringens alpha-toxin antibodies (Figure 2), Dr. Luciano Gobbi, a technical manager with Intervet/ Schering-Plough Animal Health reported at the 2008 World’s Poultry Congress in Brisbane.

Besides the toxoid, Malo says other measures that may reduce the risk of NE include feeding a lower protein diet, vegetable diets, and more synthetic amino acids.

“Avoid by-products of questionable quality as well as grains such as wheat and edible types of litter. Using enzymes and microflora treatments that improve digestibility might also help producers prevent NE in the flocks and the resulting economic damage,” Malo concludes.