Proliferative enteropathies of pigs

Proliferative enteropathies of pigs

G H K LAWSON AND M C WILLIAMS

Introduction

Proliferative enteropathy (PE) is a pathological description of the underlying intestinal lesion that occurs in a number of clinical conditions of 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.1 Other animal species may be affected by proliferative enteropathy with intracelluar bacteria,14, 16, 19, 79, 106 while others may show proliferative enteropathy in the absence of intracellular bacteria.11 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.1

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). Older mature animals are affected by a condition of intestinal blood loss referred to as proliferative haemorrhagic enteropathy (PHE). Changes superimposed on PIA can result in either necrotic enteritis (NE) or regional ileitis (RI).

Proliferative enteropathy was first described in North America in 1931,4 and now probably occurs wherever domestic pigs are reared. The disease has been present in southern Africa for many decades, and a veterinary museum specimen of the ileum of a pig dating from the 1950s demonstrates classic RI. In 1978, an outbreak of PHE in sows in a large commercial piggery in the Free State Province of South Africa was investigated,108 and outbreaks of PHE were also diagnosed in piggeries in the Western Cape Province.110 In South Africa and probably elsewhere in southern Africa, the disease is still most commonly recognized as PHE, and occurs in large, intensive commercial units. Detailed investigations of its prevalence have not been reported.

Aetiology

The demonstration of bacteria within the cells of the lesions of PE97 initiated a search for this organism that proved confusing to all involved.46 The curved morphology of the intracellular bacteria indicated a possible relation with the genus Campylobacter and such bacteria could be recovered from the lesions.17, 24, 50, 104, 105 A variety of these organisms came to be associated with the disease, but none proved capable of reproducing the condition experimentally.6, 57, 89, 90 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.51, 58, 59 The new organism, an obligate intracellular bacterium that could be co-cultivated with rat enterocytes,47 belonged to a new genus and, after a tortuous process, was named Lawsonia intracellularis.60 The intracellular organism had been described as Campylobacter-like organisms (CLOs), intracellular organisms (IOs), or ileal symbiont intracellularis before the present name was approved.20

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 electronluscent zone. There are no unusual features within the bacterial cytoplasm, nor have bacterial appendages been described.20 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.95 The bacteria adopt a similar location in cultured cells.62

Since the development of bacterial genetics, bacteria that are difficult to cultivate or only grow in association with cells are often classified by means of the sequences of the genes coding for the 16S rDNA.107 This allows like bacteria to be grouped together without having to determine the numerous features that are used to classify free-living organisms. On the evidence of the base sequences of bacterial 16S rDNA and the GroEL amino acid sequences, L. intracellularis is grouped in the delta subdivision of the Proteobacteria, and is taxonomically isolated from other characterized bacteria.12, 20, 60

The bacterium can be grown in a number of cell lines, notably rat small intestinal cell line IEC-18 (ATCC CRL 1589) and INT-407 Henle intestinal cells (ATCC CCL 6).9, 35, 47 Infected pig tissue is homogenized and filtered to remove tissue debris and contaminating bacteria. The filtrate is then applied to partially grown monolayers of cells in the presence of vancomycin and neomycin in an atmosphere of nitrogen containing 6 to 8 per cent oxygen and 5 per cent carbon dioxide. The bacterium multiplies within the cells, which continue to divide. Heavily infected cells reach maximum numbers about seven to eight days after infection. Passage of the...

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