Proliferative enteropathies of pigs

Previous authors: G H K LAWSON AND M C WILLIAMS

Current authors:
M, JACOBSON, Professor, DVM, PhD, Dipl. ECPHM, Department of Clinical Sciences, SLU, Swedish University of Agricultural Sciences, Sweden

GENERAL INTRODUCTION: AEROBIC/MICRO-AEROPHILIC, MOTILE, HELICAL/VIBROID GRAM-NEGATIVE BACTERIA

Introduction

Proliferative enteropathy (PE) is a pathological description of the underlying intestinal lesion that occurs in a number of clinical conditions in pigs. The clinical diseases are known by their pathological description. The term describes the lesion, which is a persistent proliferation of enterocytes, and which in its early stages is often not accompanied by marked evidence of intestinal inflammation. All cases of PE in pigs show the presence of curved intracellular bacteria located in the apical cytoplasm of proliferating cells. Some other porcine enteric diseases may show some short-lived, minor epithelial proliferation but an absence of intracellular bacteria.² Other animal species may be affected by proliferative enteropathy with intracellular bacteria,^{22, 24, 27, 107, 149} while others may show proliferative enteropathy in the absence of intracellular bacteria.¹⁹ In all cases where enterocyte proliferation has been associated with intracellular bacteria, the evidence available to date suggests that the organism is Lawsonia intracellularis, or a closely related species.²

The disease in pigs differs clinically depending on the age of the affected pigs. Post-weaning animals fail to grow normally and show intermittent diarrhoea, and their condition is often described as porcine intestinal adenomatosis (PIA). Subclinical cases, manifesting as poor growth only, are commonly seen. Older animals are affected by a condition of intestinal blood loss referred to as proliferative haemorrhagic enteropathy (PHE). Changes superimposed on PIA can result in necrotic enteritis (NE) and  subsequently in regional ileitis (RI).^{46, 129}

Proliferative enteropathy was first described in North America in 1931,⁶ and now probably occurs wherever domestic pigs are reared.. In South Africa and probably elsewhere in Africa, the disease is still most commonly recognized as PHE, and occurs in large, intensive commercial units.^{29, 155, 158}

Aetiology

The demonstration of bacteria within the cells of the lesions of PE¹³¹ initiated a search for this organism that proved confusing to all involved.⁷⁴ The curved morphology of the intracellular bacteria indicated a possible relation with the genus Campylobacter and such bacteria could be recovered from the lesions.^{25, 33, 77, 143, 144}  A variety of these organisms came to be associated with the disease, but none proved capable of reproducing the condition experimentally.^{9, 87, 122, 123} It was only when the intracellular bacteria proved antigenically dissimilar to the porcine intestinal campylobacters that evidence for the presence of another distinct agent began to emerge.^{78, 89, 90} The new organism, an obligate intracellular bacterium that could be co-cultivated with rat enterocytes,⁷⁵ belonged to a new genus and, after a tortuous process, was named Lawsonia intracellularis.⁹¹ The intracellular organism had been described as Campylobacter-like organisms, intracellular organisms, or ileal symbiont intracellularis before the present name was approved.³⁰

The bacteria are variably curved or sigmoid Gram-negative rods, 1.25 to 1.75 μm long and 0.25 to 0.43 μm wide. The bacterial envelope consists of a wavy trilaminar outer layer separated from the cytoplasmic membrane by an electron-lucent zone.³⁰ Extracellularly, there is a single, unipolar flagellum.⁷² In the natural disease, bacteria are present as groups in the apical cytoplasm between the cell nucleus and the luminal border. They lie free in the cytoplasm and are not surrounded by a host cell membrane.¹²⁸ The bacteria adopt a similar location in cultured cells.⁹³

Bacteria have commonly been classified based on the sequences of the genes coding for the 16S rDNA.¹⁵⁴ On the evidence of the base sequences of bacterial 16S rDNA and the GroEL amino acid sequences, L. intracellularis is grouped into the Desulfovibrionaceae family in the delta subdivision of the Proteobacteria, and is taxonomically isolated from other characterized bacteria.^{20, 30, 91} The closest relatives are Bilophila wadsworthia and Desulfovibrio desulfuricans.^{30, 137} To date, six whole-genome sequences of L. intracellularis have been published, namely the porcine isolate PHE/MN1-00, originating from a gilt with PHE (ATCC PTA-3457),³⁵ the isolate N343 that came  from a sow with PHE from Minnesota,¹³⁵ three draft genome sequences of isolates from Japanese pigs with PIA,¹⁰⁹ and one isolate from a horse with a 99.63 per cent similarity to the porcine isolates.¹⁰⁴ Porcine strains from Europe, the USA⁶⁸ and Australia¹⁶  appear similar in characteristics in culture and identification by PCR or immunohistochemistry. An 18 kb prophage-associated genomic island may be a feature for adaptation to the porcine host.¹⁴⁷

The bacterium can be grown in a number of cell lines, notably the fast dividing Mc Coy cells (ATCC^® CRL-1696 ™), rat small intestinal cell line IEC-18 (ATCC CRL 1589) and INT-407 Henle intestinal cells (ATCC CCL 6).^{16, 56, 75, 121} Infected pig tissue is homogenized and filtered to remove tissue debris and contaminating bacteria. The filtrate is then applied to partially grown...