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Effect of sow parity on the transmission of Lawsonia intracellularis to piglets

PORCINE proliferative enteropathy (PPE) may result in poor growth rate, diarrhoea and stunting, or may be manifested as sudden death or bloody diarrhoea in late finishing pigs or replacement breeding animals (Gebhart and Guedes 2001). In a serological survey in the USA, Bane and others (1997) found a 96 per cent seroprevalence for Lawsonia intracellularis, the causative agent of PPE. Bronsvoort and others (2001), Bona and Bilkei (2003) and Hagen and Bilkei (2003) found a strong association between the seropositivity to L intracellularis of grower-finisher pigs and the seropositivity and rearing conditions of their dams. In herds with endemic L intracellularis infection, the piglets of older sows (those of more than parity 3), but not those of first-parity females, are passively protected, predominantly by maternal immunoglobulin M (IgM) and immunoglobulin A (IgA), against infection with the pathogen (McOrist and Gebhart 1999). This short communication describes a study of the effects of age and parity of sows on the transmission of L intracellularis to their piglets. The study was performed in a 900-sow production unit in Slovakia between January and June 2003. The farm had previously experienced clinical PPE; indirect immunofluorescence antibody (IFA) testing confirmed the disease. Blood samples from 99 healthy late pregnant gilts and 98 sows of parity 3 to 5 were collected, and an IFA serum assay was used to detect antiL intracellularis IgG and IgM antibodies, as described by Knittel and others (1997). After the seropositivity of the pregnant animals had been determined, one newborn piglet from each of 25 randomly selected positive gilt litters and 25 parity 3 to 5 sow litters was repeatedly tested at two, five, eight, 11, 14, 17, 20, 23 and 26 weeks of age for seropositivity to L intracellularis. For the IFA testing, the coating antigen was a pure culture of L intracellularis strain N343. Both pig serum and the anti pig immunoglobulin were diluted 1:30 in phosphate-buffered saline before use (Knittel and others 1997). Samples that showed fluorescence were considered positive. All the offspring of IFA-positive females were seronegative at two weeks of age. The offspring of gilts showed seropositivity to L intracellularis from five until 26 weeks of age. During the same testing period the offspring of parity 3 to 5 sows showed lower IFA values, which declined more rapidly (Table 1). In this study, the serological status to L intracellularis of the piglets during the weaner and grower-finisher stages was strongly associated with the age/parity of their dams. Consistent with these results, Barna and Bilkei (2003) found that the seropositivity of the offspring of IFA-negative gilts was highest at five weeks of age (84 per cent), and declined gradually from week 11 to only 10 per cent at week 26. At the same time, the offspring of IFA-positive gilts showed lower and faster decaying seroprevalence, being 32 per cent at week 5 and 0 per cent from week 17 onwards. Consistent with the findings of Barna and Bilkei (2003), the present results indicate that older, seropositive sows either do not excrete sufficient organisms, or protect their offspring passively with maternal antibodies (Bronsvoort and others 2001). This suggests that there may be a greater resistance to L intracellularis with increasing age, or that there is a lower turnover of breeding animals in these herds, resulting in less contact between susceptible and already infected sows. Barna and Bilkei (2003) found no difference in the occurrence of diarrhoea between the offspring of L intracellularis seropositive and seronegative gilts. However, the mortality was 9 per cent among the offspring of seronegative gilts and only 5 per cent among the offspring of seropositive gilts. Average weight gains were higher among the offspring of seropositive gilts than seronegative gilts (68·9 g per day better in the nursery, and 127 g per day better in the growerfinisher unit). Unfortunately, in the present trial diarrhoea, mortality and daily gain were not evaluated. The present results suggest that the maternally derived passive immunity gave partial protection to the offspring of L intracellularis seropositive gilts against the development of long-term infection and reinfection. The longer-lasting seropositivity of the offspring of L intracellularis seronegative gilts (Barna and Bilkei 2003) suggests reinfection. L intracellularis can be present for weeks or months in proliferating crypt cells, but, without reinfection, faecal shedding and seropositivity can cease four weeks after challenge (Barna and Bilkei 2003). In the present study, the IFA assay was used to determine the prevalence and timing of exposure to L intracellularis rather than PCR, because it was likely that PCR would detect only clinically infected pigs or those shedding the organism. Although PCR is highly sensitive, only animals with active infection excrete sufficient organisms for detection by PCR. Seroconversion in growing pigs under field conditions showed that L intracellularis titres of 1:30 decayed within three to six weeks (Bona and Bilkei 2003, Hagen and Bilkei 2003). Once a pig has been infected, it becomes the most important source for further transmission of the agent, because of increased pig-to-pig contact in the nursery and in the grower-finisher area of a unit. Because clinical PPE is rarely observed in sows older than two years old (those of parity 3 and above), it is likely that some natural immunity exists. In older sows, specific cell-mediated and humoral immunity have been detected (McOrist and others 1999).