Porcine necrotic enteritis
Porcine necrotic enteritis is a severe and usually lethal disease that affects the intestinal tract of piglets. It is an enterotoxaemia produced by Clostridium perfringens, characterized by a necro hemorrhagic intestinal mucosa.
Necrotic enteritis is a worldwide disease. Economic losses are high in unvaccinated batches, since mortality usually exceeds 50%. In vaccinated batches, the disease can still appear, although sporadically and milder, involving delayed pigletsâ€™ growth.
Etiology of porcine necrotic enteritis
The disease is caused by b toxin produced by Clostridium perfringens type C, a gram-positive and obligate anaerobic bacterium. It usually affects piglets from 1 to 5 days of age, but it can affect piglets throughout the lactation period.
Î˛ toxin is a very powerful exotoxin, which is sensitive to heat and trypsin. Bacteria enter the cells of the small intestine and produce the toxin, which causes necrosis of the intestinal villi.
Transmission of porcine necrotic enteritis
It is not common to find Clostridium perfringens type C in healthy animals; thus, it is considered a primary pathogen. Transmission occurs mostly through feco-oral route, and sows are the main source of infection for newborn piglets.
Risk factors of necrotic enteritis
The ability of the bacteria to cause the disease depends on various predisposing factors that create a favourable environment for its proliferation, adherence and production of toxins.
Clostridium perfringens type C infections are frequent in newborn animals (in the first 5 days of life) of any type of species because it is able to colonize rapidly in the absence of a balanced intestinal flora. In addition, the action of the toxin is favoured by trypsin deficiency of newborns and protease inhibitors present in colostrum.
Other identified predisposing factors include nutrition or the presence of other intestinal pathologies:
Intestinal microbiota in animals that have already started consuming solid feed is highly influenced by diet. Different components of the diet have been identified as predisposing to the growth of the bacteria, like the type of cereal and the quantity and quality of protein used.
Addition of high levels of wheat, barley, rye or oats, containing an important amount of non-digestibleÂ fibre, increase viscosity of the intake and decrease its digestibility. Inclusion of high levels of protein, especially from animal origin, such as fishmeal, the use of poorly digestible protein or the presence of antinutritional factors (ANF), create an ideal environment for bacterial proliferation. Undigested feed is used by intestinal microbiota, and may disturb its balance and the major types of microorganisms and, therefore, favour the growth of Clostridium over other species.
Any pathology that affects the integrity of the intestinal epithelium decreases the absorption of nutrients and increases the permeability of the intestinal barrier and, therefore, facilitates the development of necrotic enteritis. Non-absorbed feed and plasma proteins resulting from intestinal lesions are the ideal substrate for Clostridium proliferation.
Clostridium acts as secondary agent generally after infection by: transmissible gastroenteritis virus, rotavirus, porcine epidemic diarrhea virus (PEDV) or coccidia. Mycotoxicosis is also a disease that disturbs the intestinal epithelium and allows the growth of Clostridium perfringens.
Clinical signs and injuries of necrotic enteritis
Presentation of the disease varies between hyperacute, acute or chronic. Acute presentation in piglets of 1 to 3 days is seen as depression, severe abdominal pain, hemorrhagic diarrhea, collapse and death in less than 24 hours. More chronic cases show persistent diarrhea during 1 or 2 weeks, with pasty greyish stools without blood, dehydration and progressive weight loss.
The disease appears epizootically in batches without previous immunity (non-vaccinated sows). Morbidity is usually greater than 50% and lethality close to 100%. Intestinal lesions are usually severe and extensive, and death is often related to the toxemia produced by the bacteria. Hypoglycemia and bacteremia collaborate in the raise of mortality rate.
Disease tends to be enzootic in immunized batches, appearing milder cases sporadically. The outbreak of acute processes in this situation indicates poor group immunity, either by introduction of unvaccinated gilts in the batch or because piglets do not receive proper colostrum volumes.
Lesions usually appear in the small intestine (jejunum and ileus), but rarely in the colon. Necrosis of the intestinal mucosa is macroscopically seen as dark red and hemorrhagic intestines, with possible emphysema, presence of blood in the lumen and intestinal wall, and hemorrhagic mesenteric nodules.
Diagnosis of necrotic enteritis
Diagnosis is oriented by an evaluation of the history of the farm and the current clinical picture. Presumptive diagnosis is based on clinical signs, mortality pattern, macroscopic lesions and gram staining of an intestinal scraping where gram-positive bacilli are visible.
Confirmation of the disease needs intestinal histology of sick animals, seen as necrosis of the intestinal villi and colonization of the intestinal mucosa by gram-positive bacilli. Microbiological analysis also works as a confirmatory diagnostic technique. For C. perfringens isolation, both specific differential medium for the bacteria or a blood agar culture can be used.
PCR analysis would differentiate the different types of C. perfringens by identifying genes coding for toxins that each type of bacteria can produce and, thereby, distinguish between Clostridium perfringens type A and C.
Subacute and chronic forms in piglets from 6 to 14 days can be confused with an enteritis produced by Isospora suis due to the type of diarrhea. Histological examination allows to differentiate both diseases, since pathogens observed in the lesions are bacteria (necrotic enteritis) or protozoa (coccidiosis).
Treatment and prevention of porcine necrotic enteritis
Therapeutic treatment of acute cases, once symptoms appear, has low effectiveness since lesions are usually irreversible.
Prophylaxis is the best method to reduce mortality and prevalence of the disease, as it tends to recur in infected farms.
Vaccination with bacterial toxoid in pregnant sows, at 6 and 3 weeks before delivery, is recommended to confer passive immunity to piglets, combined with the administration of antimicrobials, such as bacitracin or lincomycin, from 2 weeks prepartum until the end of lactation. Vaccination of pregnant sows combined with the use of antimicrobials in piglets, such as penicillin, have also reported good results regarding the reduction of piglet mortality.
Vaccination must be added to good management practices to ensure a good quality and quantity of colostrum, as well as revaccination of sows 3 weeks prepartum in the following pregnancies to maintain the transfer of passive immunity to the coming litters.
In addition, to reduce the incidence of the disease, predisposing factors should be also controlled:
- Inclusion of ingredients such as low digestible protein or cereals with a high proportion of fibre, which enhance the growth of perfringens, should be reduced. In case it is not possible to diminish these ingredientsâ€™ proportion, non-starch polysaccharide enzymes and proteases must be included in formulation to reduce the risk of disease onset.
- Inclusion of mycotoxin binders in order to prevent the action of mycotoxins present in feed.
- Preventive measures for digestive parasitosis, such as coccidiosis, by the use of chemical or natural products.
Furthermore, suitable measures to maintain a healthy and balanced intestinal flora should be performed. Antibiotic growth promoters (AGPs) have usually been used to reduce effects of the change in the intestinal environment. Actual legal restrictions and the growing demand of consumers for alternatives have stimulated the study of different products, including probiotics and prebiotics, to favour competitive exclusion of C. perfringens, and the use of additives derived from plants.
Compounds of botanical origin based on cimenol ring have antimicrobial effect against intestinal pathogens, since these products are able to disintegrate the bacterial cell membrane and penetrate inside, release the cell content to the environment and therefore, cause the death of bacteria.
Necrotic enteritis is an important disease in pig farming due to economic losses caused in lactating piglets.
Basic clues for prevention and control of the disease in farms include: control of nutritional and infectious predisposing factors, immunization of pregnant sows and support of the intestinal health of the piglet.
Songer, J.G & Glock, R.D. (1998). Enteric infection of swine with Clostridium perfringens types A and C. Swine Health and Production 6(5):223-225.
Songer, J.G. & Uzal, F.A. (2005). Clostridial enteric infections. Journal of Veterinary Diagnostic Investigation 17:528-536.
Merck Veterinary Manual. 11th edition.