Lawsonia intracellularis, a slender, curved, Gram-negative rod, has not been grown in cell-free media. It is a microaerophilic, obligate, intracellular pathogen which is aetiologically implicated in proliferative enteropathy of pigs and foals and other animals. Some degree of host species adaptation occurs. Lawsonia intracellularis grows intracellularly in pig enterocytes and infected animals excrete small numbers in their faeces. It has been recovered from the tissues of foals and other animals affected with proliferative enteropathy. Subclinically affected pigs and foals and wildlife which share their environment with clinically affected animals may have a role in perpetuating infection on farms.

Infection of enterocytes with L. intracellularis induces proliferation of these cells with the development of adenomatous and inflammatory lesions in the terminal ileum, caecum and colon. The exact mechanism of infection has not been established but the organism apparently attaches and enters epithelial cells followed by release from the endosome with multiplication taking place in the cell cytoplasm. Gnotobiotic pigs, which are devoid of intestinal flora, do not develop the disease when dosed with L. intracellularis unless they are pre-dosed with porcine intestinal flora. Common intestinal organisms probably provide the appropriate microenvironmental conditions required for the colonization and proliferation of L. intracellularis. In addition, active proliferation and differentiation of crypt cells, as occurs at weaning, appears to be associated with lesion production.

Clinical signs, which occur most frequently in weaned pigs from 6 to 20 weeks of age, range from chronic intermittent diarrhoea with reduction in weight gain to acute haemorrhagic enteropathy; the latter syndrome is observed more commonly in young adult animals. Although sudden deaths may occur in severely affected pigs, most animals with the milder form of the disease recover without treatment. Lesions in the ileum, caecum and colon include thickening of the wall, mucosal necrosis and, in severe cases, clotted blood in the lumen. Enlargement of the mesenteric lymph nodes is a feature of the disease. In foals, clinical signs are observed after weaning and include rapid weight loss with diarrhoea and colic, depression, fever and subcutaneous ventral oedema.

Lawsonia intracellularis can be demonstrated in faeces or ileal mucosa by immunofluorescence or by PCR. Serological tests include indirect fluorescent antibody tests, ELISA and immunoperoxidase monolayer assays.

Antimicrobial agents such as tylosin or tiamulin may be used therapeutically in feed or water. A live attenuated vaccine for use in pigs is now available in many countries and is effective in reducing clinical signs of disease.