4.3.6 Where does the acute form of infection come from?

This form of the disease has also now been reproduced by oral challenge of pigs with Lawsonia intracellularis. These studies suggest that the acute form is more likely to develop in “older” pigs that are at least one to several months after weaning. Important is that pigs have not been previously exposed to the dis­ease and that these naïve pigs are also exposed to a relatively high oral challenge dose of Lawsonia intracellularis. Young pigs exposed to a low or moderate level of infection are much more likely to develop the chronic form of the disease, not the acute form.

One way that pigs can remain naïve until they are older is if they receive continuous antibiotic medication programs that reduce early exposure to Lawsonia. In controlled challenge studies with groups of pigs that had received oral antibiotics until they were 18 weeks old, it was found that these pigs were still highly susceptible and many developed severe acute haemorrhagic proliferative enteropathy after cessation of medication with antibiotics and an oral challenge with Lawsonia intracellularis was made. Natural outbreaks of the acute form of the disease may partly reflect changes in the use of antibiotics.

Separation of age groups in modern pig farms is an other factor that may be important for the incidence of acute haemorrhagic proliferative enteropathy. This can have marked benefits for reduction of pneumonias, but seems to affect the timing of development of the immunity of pigs to proliferative enteropathy.

In modern farms, especially the so-called“multi-site” farms, the breeding herd is very clean and may be well separated from the growing pigs. In some of these farms it has been found that the breeding pigs are negative for Lawsonia intracellularis. This is in contrast to single site farms, where up to 30% of the breed­ing herd may be infected.

This results in a first Lawsonia intracellularis exposure of these clean breeding animals at a later age in the finishing period. The delay may be until these pigs are 4 or 5 months old, making them much more susceptible to the acute form of disease.

Once a pig becomes affected, its haemorrhagic faeces contains very large numbers of Lawsonia intracellularis causing rapid spreading of bacteria and infection to the rest of the group. If a smear is made of the haemorrhagic faeces, then large numbers of Lawsonia bacteria become clearly visible using specific or unspecific staining methods.