Ileitis, also known as porcine proliferative enteropathy, is an important intestinal disease that affects the performance of nursery and finishing pigs. Its causative agent is Lawsonia intracellularis. Intestinal diseases not only affect pig welfare but also increase antibiotic use and cause significant economic losses in intensive pig production systems. Among the most common infectious diseases in pigs, ileitis is estimated to cause economic losses of 0.50 to 1.00 euros per infected pig. This figure may seem small, but because infection is widespread, subclinical cases are numerous, and reduced growth rate and feed efficiency can go unnoticed, losses at the herd level are often underestimated. Ileitis is “underestimated” mainly because affected pigs lack specific clinical signs; common signs are limited to diarrhea, slower growth, and reduced feed efficiency. In cases of subclinical ileitis, clinical signs may be entirely absent. Detecting the disease and making a rapid, accurate diagnosis are prerequisites for confirming subclinical or clinical ileitis, and are also the basis for implementing effective treatment and preventive measures to minimize losses. This article summarizes and analyzes the literature in “Ileitis in nursery and finishing pigs—an underestimated disease_Wang Jingjing.pdf.”
I. Clinical Signs and Pathological Diagnosis
In addition to nonspecific clinical signs, affected pigs may show typical macroscopic and/or microscopic histological lesions. In acute ileitis, catarrhal inflammation or hemorrhage may be visible in the ileal mucosa and intestinal wall, with enlarged mucosal folds. Chronic enteritis is characterized by immature proliferative epithelial cells replacing the epithelial cells of the intestinal villi and crypts, resulting in marked thickening of the ileal and jejunal walls. These lesions indicate that ileitis is not simply a matter of “diarrhea”; it impairs intestinal epithelial renewal and barrier function, thereby affecting nutrient absorption and growth.
Lawsonia intracellularis is a bacterium with complex nutritional requirements that can only be cultured and maintained in cell culture medium; it causes ileitis in infected pigs. Therefore, Lawsonia intracellularis is not usually isolated for testing. Instead, tissue samples are directly stained using nonspecific staining methods, or polymerase chain reaction (PCR) is used to test infected tissue, digesta, and fecal samples. Compared with other diagnostic methods, PCR can produce more specific and sensitive results by amplifying a specific nucleotide sequence. Real-time quantitative PCR has shown a positive correlation between the severity of lesions in the ileal tissue of affected pigs and the concentration of Lawsonia intracellularis genome equivalents in feces from the same pig. The HE score reflects histological damage visible after HE staining and ranges from 0 to 28; the amount of Lawsonia intracellularis in feces is positively correlated with the HE score. In addition, clinical impact, measured by the average daily gain of unvaccinated or vaccinated pigs, correlates with the concentration of Lawsonia intracellularis detected in feces by PCR. This makes qPCR the preferred method for diagnosing porcine ileitis. At the herd level, detecting specific antibodies in serum by enzyme-linked immunosorbent assay (ELISA) helps determine when infection occurred, but this method is not suitable for confirming the disease. In other words, qPCR is more focused on confirming the pathogen and assessing its load, while ELISA is more focused on determining the timing of infection in the herd; the two methods cannot replace each other.
Two. Control and prevention
If acute ileitis occurs, the farm needs to treat affected pigs with antibiotics, such as macrolides and tetracyclines. However, given the need for prudent antibiotic use and the threat of bacteria developing antibiotic resistance, the long-term strategy for controlling and preventing swine ileitis should be based on a vaccination program tailored to the farm, biosecurity, and measures to reduce risk factors. This approach is important: antibiotics can address acute clinical problems, but cannot replace pathogen control and risk management at the herd level. The subclinical nature of ileitis means that treating pigs with diarrhea alone is far from sufficient. Farms need to combine monitoring, vaccination, biosecurity, and husbandry management to reduce pathogen transmission and factors that trigger disease.
Three. European field prevalence data
The prevalence and impact of Lawsonia intracellularis on European pig farms are currently unclear. No data are available on the incidence of enteric disease caused by Lawsonia intracellularis infection. Therefore, surveys in EU member states are needed to determine how widespread this pathogen is on pig farms. The study tested pig herds in Denmark, France, Germany, Spain, the Netherlands and the United Kingdom. These countries were selected because their combined pig population accounts for more than two-thirds of Europe’s total pig population, with a combined total of 1.005 hundred million pigs, compared with 1.5 hundred million pigs across Europe.
At the end of the study, 144 pig herds from 6 European countries were tested for Lawsonia intracellularis infection. Faecal samples were collected from 15 weaned pigs (10–25 kg), 15 growing pigs (25–40 kg) and 15 finishing pigs (40–100 kg) selected from each herd tested. The sample size, calculated using qPCR, was sufficient to detect Lawsonia intracellularis in approximately 300 pigs of the same age, with a confidence level of 95% and an estimated prevalence of 20%. Based on the history of enteric disease in pigs, 24 herds were selected for the trial in each participating country. As season affects the incidence of porcine ileitis, faecal samples were collected over a one-year period starting in 2017 10. Trial pigs were selected at random, and whether a pig had diarrhoea at the time of sampling did not affect its eligibility. Faecal samples were collected manually from the rectal ampulla of each pig, using a new disposable glove for each pig to prevent cross-contamination. All faecal samples were tested by qPCR for the concentration of a Lawsonia intracellularis-specific genomic fragment. The first available data came from 12 herds in Denmark, 11 herds in Germany, 11 herds in Spain, 11 herds in the Netherlands, 8 herds in France and 5 herds in the United Kingdom. The data were test results from samples collected from 2017 10 2 to 2018 3 30.
100% of the faecal samples from participating herds were tested for Lawsonia intracellularis. The proportion of positive faecal samples varied significantly among herds and countries. Approximately 48% of faecal samples from Danish herds tested positive for Lawsonia intracellularis genomic fragments; positivity rates were lower in Spain and France, at 20% and 18%, respectively. Overall, 34% of all faecal samples tested were positive. Positivity rates varied particularly widely among herds in the United Kingdom and the Netherlands. The positivity rates in faecal samples from different age groups also varied considerably between herds within each country and between countries. On average, Lawsonia intracellularis genomic fragments were detected in the faeces of 30% of weaned pigs, 42% of growing pigs and 33% of finishing pigs. Among the age groups, the highest positivity rate in weaned pigs was in Denmark, at 83%; in growing pigs, the highest rates were in the United Kingdom, Germany and Spain, at 60%, 57% and 33%, respectively; and in finishing pigs, the highest rates were in the Netherlands and France, at 54% and 29%, respectively. These interim results will be updated as more data become available.
IV. Conclusions and production implications
Since Lawsonia intracellularis genomic fragments were detected in all sampled pig herds, Lawsonia intracellularis can be considered a ubiquitous pathogen, at least in herds with diarrhea. Detecting antibodies in serum is essential to confirm Lawsonia intracellularis infection, but a final conclusion about the pathogen’s prevalence can only be made after data collection is complete. Lawsonia intracellularis infection can be confirmed by qPCR testing of feces from pigs with diarrhea for Lawsonia intracellularis, or by pathological lesions identified during post-mortem examination. Initial results indicate differences in Lawsonia intracellularis prevalence between countries, herds, and age groups. Possible causes and influencing factors are recorded in the questionnaire and will be analyzed and published after data collection is complete.
Overall, controlling ileitis should focus on three key points:
First, recognize that subclinical infection exists and do not focus only on pigs with diarrhea;
Second, use qPCR testing of feces and histopathological lesions as the basis for diagnosis, and use ELISA antibody testing to determine the timing of infection in the herd;
Third, use macrolide and tetracycline antibiotics appropriately in acute cases. In the long term, rely on a farm-specific immunization program, biosecurity, and measures to reduce risk factors.
European data further suggest that the prevalence of Lawsonia intracellularis varies considerably across countries, herds, and age groups. Infections occur in nursery, grower, and finisher pigs, so control efforts should not focus on just one stage. Only by integrating diagnosis, monitoring, immunization, and biosecurity into a closed loop can the hidden losses in production performance caused by ileitis in nursery and finisher pigs be reduced.