4.3.4 Where does the chronic form of infection come from?

The chronic form of the disease has been reproduced many times in weaned pigs of various ages, given an oral challenge with Lawsonia intracellularis. The incubation period of 2 to 3 weeks in most animals enables a slow build-up of disease in the group, which can occur over a month or more after the disease is first intro­duced. In some infected pigs however, there can be large numbers of Lawsonia intracellularis in the faeces, causing rapid exposure of all of the pigs in the group, once one of them becomes infected.

Picture 4.3.4 a (by H. Voets)
The chronic form is especially common on traditional, single-site, farrow-to-finish farms with all the farm buildings on one property. This is because there is a simple flow of pigs and infected faeces around the farm, passing the disease from one pig to the next. An infection with lawsonia intracellularis can spread quit easily on this type of farm.

In most of these farms, the chronic and sub-clinical forms occur when the maternal anti-body levels that pigs normally derive from their mother start to reduce a few days after weaning. These antibodies reduce to a level where the pigs become susceptible to a Lawsonia intracellularis infection. This occurs after weaning at 5 or 6 weeks of age. In many pig farms, this coin­cides with the time when pigs are mixed in the farm nursery area and in the early growing period. After the incubation period, some pigs develop disease, start to excrete the bacteria and spread it to other pigs (see Figure 4.3.4 a).

Figure 4.3.4 a
Typical cross-sectional serological ELISA profile of single site farms.