There are 2 forms of the disease – acute and chronic. The acute form frequently includes fatal haemorrhagic diarrhoea typically seen in late fatteners and in gilts freshly introduced into the breeding herd (Porcine Haemorrhagic Enteropathy – PHE). The chronic form (Porcine Intestinal Adenomatosis – PIA) affects grow-finishing pigs and is associated with mild diarrhoea, weight loss and increased body weight variation. Subclinical Ileitis is a common chronic form of the disease. This form can be defined as an infection with Lawsonia intracellularis in the absence of clinical signs of the disease such as mortality and diarrhoea. Gross or microscopic lesions of Porcine Proliferative Enteropathy (PPE) are present but of insufficient severity to result in overt clinical signs.

Nevertheless, farms with subclinical presentation of Ileitis experience suboptimal growth, reduced feed intake and loss of homogenicity in body weight (Jacobson et al. 2003; Guedes 2004).