CPRC Update: March 2010

Necrotic enteritis (NE) is a common disease of broiler chickens caused
by the bacterium Clostridium perfringens, which produces toxins that
damage gut tissues.

What is necrotic enteritis?
Necrotic enteritis (NE) is a common disease of broiler chickens caused by the bacterium Clostridium perfringens, which produces toxins that damage gut tissues. Left unchecked, this damage can impair nutrient absorption and in some cases open the door to other gut infections such as coccidiosis. An NE-affected flock may suffer from increased mortality, but often “sub-clinical” cases don’t present any signs other than reduced flock performance. Without obvious signs, these cases can go unnoticed and therefore untreated. Reduced performance, increased mortality and correlation with other diseases add up to significant losses for the industry, with some estimates as high as $2 billion a year globally.

Necrotic enteritis is currently controlled with antibiotics; however, increasing concern over antibiotic resistance is putting pressure on the industry to find alternative methods of disease control.  Dr. John Prescott and his team at the University of Guelph are looking at the possibility of using vaccination to protect chickens against NE.

Immunity to NE
Despite its prevalence, there is relatively little known about immunity to NE. Dr. Prescott’s team has made significant advances in understanding of the subject. They first showed that it is possible to confer resistance to NE by oral infection with virulent strains of C. perfringens. “Virulent” strains are those capable of producing disease. A number of proteins secreted by these strains were shown to elicit immune reactions. Among these proteins were alpha-toxin (AT) and others, which are given the short names FBA, GPD, HP and PFOR. Intramuscular immunization with these proteins revealed all could protect birds from mild challenge. AT, HP and PFOR could protect against more severe challenge.  Alpha toxin has historically been considered in the literature as the most important factor causing NE. However, more recent work from around the world is casting some doubt on the importance of its role in causing disease. Dr. Prescott’s work further suggests that certain secreted proteins, in addition to AT, are involved in immunity to NE.

Towards an NE vaccine
With this information in hand, Dr. Prescott and his team set their sights on developing an NE vaccine that could deliver one or more of these proteins, or at least fragments of the proteins, that elicit an immune response. They reasoned that an effective NE vaccine would deliver these protein fragments or “antigens” directly into the intestine. There are existing Salmonella vaccines that do just that. Dr. Prescott decided to start with one of these vaccines and modify it so it could confer resistance to Clostridia in addition to Salmonella. A number of such vaccine vectors were constructed and tested.

Salmonella vaccines expressing FBA and HP were able to protect chickens from NE challenge. These chickens produced antibodies to antigens from both Salmonella and Clostridia. Dr. Prescott’s team wanted to increase the level of protection and also wanted to test a vaccine that expressed AT. The researchers made a number of modifications to the vaccine system. They fine-tuned the protein fragments expressed by the vaccine, increased the production levels of the antigens and changed the backbone of the vaccine vector. Disappointingly, the modified AT vaccine only protected birds from moderate challenge. The HP-based vaccine protected them from more severe challenge, but protection levels were not as good as in previous experiments. Neither vaccine protected birds from Salmonella infection. Further studies showed the genetic modifications weakened the vaccine strain, rendering it less effective.

What’s next?
Although disappointing, these results revealed important information on vaccine design.  Dr. Prescott’s team is now using this information to better define the parameters for development of a successful NE vaccine based on improved Salmonella vaccine carriers that can also deliver other antigens. A successful multivalent vaccine approach such as this will have significant impact on both food safety and poultry health.

Funding for this work was provided by CPRC in partnership with Agriculture and Agri-Food Canada and the Natural Sciences and Engineering Research Council. Sponsors for previous work also include the Ontario Ministry of Agriculture, Food and Rural Affairs, the Poultry Industry Council, and the Saskatchewan Chicken Industry Development Fund.

For more details on any CPRC activities, please contact Gord Speksnijder at The Canadian Poultry Research Council, 483 Arkell Road, R.R. #2, Guelph, Ontario, N1H 6H8, phone: 289-251-2990, fax: 519-837-3584, e-mail: info@cp-rc.ca, or visit us at www.cp-rc.ca.