1. Background
Ileitis or porcine proliferative enteropathy (PPE) is a wasting disease of pigs. PPE includes all forms of disease characterized by thickening of the small intestine, including the ileum, due to the proliferation of cells, even in the colonic mucosa. Clinical symptoms are diarrhea, wasting and reduced performance (decreased average daily weight gain, ADG) in growing pigs. In fattening and breeding animals, proliferative hemorrhagic enteropathy (PHE) may result in the discharge of blood in the feces. The disease can also occur in a chronic form in fattening herds, accompanied by mild diarrhea and poor weight gain (porcine intestinal adenomatosis, PIA) [
1,
2].
PPE is ubiquitous in pig producing countries all around the world. Holtkamp et al. estimated the economic losses caused by the disease at USD 4.65 per fattening pig, with American pig farmers’ losses at USD 56.1 million annually [
3].
Diagnosis of the disease is possible based on pathological and histopathological lesions. Immunohistochemistry or PCR may be required for causal diagnosis.
Jensen et al. (1997) reported on the monitoring of porcine enteric disease in a slaughterhouse scoring system similar to that regarding mycoplasmosis [
4]. In their studies, the ileac sections of 39 fattening pigs slaughtered were examined and graded by visual and palpation monitoring. According to their results, it is possible to monitor and classify the intestines after slaughter visually and by palpation based on the rigidity and thickness of the ileum. In their view, it is essential to demonstrate typical proliferation or to detect
Lawsonia intracellularis bacteria.
In Hungary, a slaughterhouse belonging to a significant pig integrator realized that the number of small intestines suitable for further processing decreased significantly (by almost 60–70%) during the production process due to inflammation of the small intestine, rupture, and significant narrowing of the intestinal lumen. Histological examinations confirmed the intracellular presence of
Lawsonia intracellularis in samples showing macroscopic lesions [
5].
Based on the above, it has become necessary to develop a slaughterhouse ileitis monitoring method that is simple, feasible to perform at modern slaughter lines, leads to immediate results and is cost effective. It should be doable during slaughterhouse processing without disrupting the slaughter processes themselves. The results of the method should correlate with the results of other laboratory diagnostic tests for ileitis (histology, immunohistochemistry, herd serology, fecal PCR). It is important that the results of the method can be interpreted immediately in terms of the performance of the herd from which the animals arrived for slaughtering. The practical experiences of applying the method are presented below.
4. Discussion
Among non-notifiable infectious diseases that threaten the profitability of modern pig farming, those causing respiratory symptoms (PRDC) and gastrointestinal damage play a prominent role. In the latter disease group, in the recent period, the incidence of swine dysentery damage has been reduced through the production of free herds and compliance with internal disease control rules.
However, another group of gastrointestinal diseases increased: proliferative enteropathy (PPE) in pigs in the form of chronic disease (porcine intestinal adenomatosis (PIA) in pigs, in the form of acute, severe clinical symptoms (porcine hemorrhagic enteropathy in pigs, PHE)). Porcine proliferative enteropathy (PPE) is a ubiquitous disease in pig-keeping countries around the world, with producers and veterinarians surveyed in the UK and Australia estimating that clinical symptoms have spread to 30–56% of herds [
7,
8]. In the same place, serological surveys of fattening pigs show that 80–100% of the herds are infected with
Lawsonia intracellularis [
9,
10,
11]. In Germany, Denmark, Spain, the Netherlands, and the United Kingdom, at least one positive sample was found in 90.3% (79.2–100%) of 144 herds investigated by quantitative PCR [
12]. Máté et al. [
13] also found 100% infection in 27 pig farms in Hungary. The pathogen of the disease is
Lawsonia intracellularis, an obligate intracellular bacterium. The disease occurs not only in pigs but also in other species (e.g., horses) [
14].
Clinically and sub-clinically ill animals excrete the pathogen in the feces to different extents. The infection occurs orally. The first histological changes in the small intestine are detected 8–10 days after infection. The pathological symptoms depend on which of the degenerative and reparative processes following the infection predominates.
The most common age group to be infected ranges from 6 weeks of age (young nursery pigs) to the end of fattening (20 weeks). The clinical symptoms are mild; it is also common to have detectable lesions only after slaughter. Average daily weight gain and feed conversion ratio decreases among diseased pigs. Sick animals are anorexic and occasionally have diarrhea. Mortality can be high.
Diagnosis of the disease is possible based on pathological and histopathological lesions. Immunohistochemistry or PCR may be required for causal diagnosis. Confirmation of PPE diagnosis is typically based on the observation of characteristic microscopic lesions. Difficulties in routine cultivation of
Lawsonia intracellularis have led to the use of several alternative methods to detect infection, such as immunohistochemistry (IHC) or, less commonly, in situ hybridization. Immunohistochemical staining was the gold standard, with special use of the
Lawsonia intracellularis specific antibody to identify the bacterium in PE lesions [
15]. The presence of
Lawsonia intracellularis can be confirmed by electron microscopy as a typical curved intracellular organism.
CR assays have been widely used to confirm
Lawsonia intracellularis in feces or intestinal mucosal samples [
16]. The susceptibility of PCR techniques to
Lawsonia intracellularis has increased significantly, especially in the last decade. Many PCR assays can detect up to 100 organisms per gram of feces.
Due to this high sensitivity, the presence of the pathogen in the feces does not necessarily mean clinical importance in a given herd. When evaluating a negative IHC result, the distribution of PPE lesions in the intestinal tract should also be considered and it may be necessary to examine several segments to confirm the presence of the pathogen.
Sampling protocols should consider the occurrence of clinical signs (diarrhea, poorer weight gain, feed intake) in different age groups.
Antibiotics (e.g., tetracyclines, tiamulin, tylosine) are commonly used to treat or pre-vent PPE, and it is common practice to treat groups of animals continuously for extended periods. There are no data available to suggest that
Lawsonia intracellularis develops resistance to commonly used antibiotics [
17].
A few years ago, a vaccine appeared on the market that had to be administered orally. The results obtained with this vaccine were reliable with accurate dosing, but the method of application made it difficult, in many cases impossible, to apply it to large-scale pig farms. There has been a definite need to develop and market a precisely designed parenteral vaccine that provides safe administration. The recognition of the significant economic damage of the disease, its worldwide spread, the urgent need for prudent antibiotic use, and the possibility of a new vaccination strategy have created the need to develop an immediate and inexpensive manner of evaluating the impact of Lawsonia intracellularis, e.g., by monitoring pigs entering the slaughterhouse.
Jensen et al. (1997) first reported the monitoring of porcine enteric disease [
4] in a slaughterhouse similar to mycoplasmosis. Their studies examined the ileal sections of 39 fattening pigs slaughtered at the slaughterhouse. Their visual and tactile monitor method suggested the following classifications of the examined ileum:
0 = normal proportions, rigidity, and thickness of the intestinal wall;
1 = rigidity and thickness of the intestinal wall increased slightly;
2 = intestinal wall thickened and rigidity up to 4 mm, lymphoid tissue hyperplasia,
hyperemia, and mild edema in the mucosa;
3 = thickening greater than 4 mm in the intestinal wall with partial stenosis.
The results of their studies show that it is possible to monitor the intestines by visual and palpation aspects after slaughter, and that the intestines can be classified into groups that can be checked by histopathological examinations based on the rigidity and thickness of the ileum. The intestines at the abattoir were macroscopically characterized for PPE-associated regional ileitis: hyperemia, sub serosal edema, and increased thickness of the rigid intestinal wall. However, other intestinal disorders may result in macroscopically similar results, so it is essential to demonstrate typical proliferation or to detect Lawsonia intracellularis bacteria. The increased wall thickness of the rigid intestines did not suggest any active infection, as the lesions were characterized by chronic, nonspecific, or late reactions to enteritis: hypertrophy of the tunica muscularis, Peyer’s patches, goblet cell hyperplasia, infiltration with mononuclear cells. Lawsonia intracellularis was detected by PCR in eight guts with increased thickness and one with a grade 0 test result. Of the nine PCR-positive intestines, three were also immunohistochemically positive and their feces were also found to be PCR positive. In two of the immunohistochemically positive intestines, Lawsonia intracellularis was detected only in macrophages.
In Hungary, in a slaughterhouse owned by a major pig integrator, it was found that the number of small intestines suitable for further processing decreased significantly (by almost 60–70%) during the production process due to inflammation, rupture and significant narrowing of the small intestine. Histological examinations confirmed the intracellular presence of
Lawsonia intracellularis in several cases of samples showing macroscopic lesions. Vaccination against ileitis in farms producing slaughter pigs has significantly reduced the amount of small intestine that cannot be processed, while at the same time leading to a significant improvement in the use of antibiotics in that farm [
5].
The essence of the method we have developed is that, similar to the scoring of the lungs, it can be performed during slaughterhouse processing so that it does not interfere with or make it impossible to carry out normal technological processes. The method can be applied without reducing the speed of the slaughtering line. The results of the procedure are correlated with the results of other laboratory diagnostic tests for ileitis (histology, immunohistochemistry, herd serology, fecal PCR). It is noteworthy that the results of the method can be used to immediately estimate the impact of Lawsonia intracellularis infection on the performance of the herd from which the slaughter animals come. Based on studies performed on a total of 27 swine units with nearly 850 fattening pigs, the method is suitable for achieving all the set objectives. Its introduction opens a new opportunity to estimate the losses caused by the disease, but also to regularly check and continuously monitor the practical effectiveness of therapies and preventive vaccination-based methods. The studies will allow the development of site-specific strategies (at what age to be vaccinated) adapted to the ileitis status of the herds. Regular slaughterhouse monitoring may also provide a basis for a total or partial reduction in antibiotic use (e.g., discontinuation of antibiotic use in feed).
The developed method uses the experience gained from previous studies with low animal numbers (Jensen et al. (1997) [
4]) as well as the results of estimating the losses caused by ileitis (Holtkamp 2019 [
6]). We are convinced that, with the introduction of the method, the same decisive tool will be put into practice in the hands of veterinarians and animal keepers as the examination of lung scoring.
As we had even less practice in the initial phase of applying the method, the difference in the use of the visual and tactile methods was mostly due to this. Overall, we believe that the visual and tactile method, with some practice, leads to the same result under practical conditions as the results of mucosal and intestinal examination after intestinal dissection. Therefore, in the following, we consider it sufficient to use the visual and tactile method. This is also particularly important in fulfilling the requirement that the test does not cause extra contamination during the normal cutting process.