Use of recombinant ApxIV in serodiagnosis of Actinobacillus pleuropneumoniae infections, development and prevalidation of the ApxIV ELISA.

Actinobacillus pleuropneumoniae is the etiological agent of porcine pleuropneumonia, which causes worldwide severe losses in pig farming. The virulence of the 15 serotypes of A. pleuropneumoniae is mainly determined by the three major RTX toxins ApxI, ApxII and ApxIII, which are secreted by the different serotypes in various combinations. A fourth RTX toxin, ApxIV, is produced by all 15 serotypes only during infection of pigs, but not under in vitro conditions. Pigs infected with A. pleuropneumoniae show specific antibodies directed against ApxIV. In contrast, antibodies against the other three toxins ApxI, ApxII and ApxIII are also found in pigs free of A. pleuropneumoniae. The antibodies to the three latter might result from other, less pathogenic Actinobacillus species such as A. rossii and A. suis. We used a recombinant protein based on the N'-terminal part of ApxIV to serologically detect A. pleuropneumoniae infections in pigs by immunoblot analysis. The analysis of sera of experimentally infected pigs revealed that ApxIV-immunoblots detected A. pleuropneumoniae infections in the second to third week post infection. We developed an indirect ELISA based on the purified recombinant N'-terminal moiety of ApxIV. The analysis of sera from pigs that were experimentally or naturally infected by A. pleuropneumoniae, and of sera of pigs that were free of A. pleuropneumoniae, revealed that the ELISA had a specificity of 100% and a sensitivity of 93.8%. The pre-validation study of the ApxIV-ELISA revealed that the latter was able to detect A. pleuropneumoniae-positive herds, even when clinical and pathological signs of porcine pleuropneumonia were not evident. Pigs vaccinated with a subunit vaccine Porcilis App were serologically negative in the ApxIV-ELISA.

Patterns of exposure to Lawsonia intracellularis infection on European pig farms.

A serological investigation was made of the patterns of exposure of pigs to Lawsonia intracellularis, the causative agent of proliferative enteropathy (ileitis), on farms in France and Spain. Blood samples from groups of adult female pigs in breeding programmes and from postweaning pigs were monitored, the latter every month for five months, by a L. intracellularis-specific immunofluorescence seroassay. Four of 33 farms monitored in France (12 per cent) and three of 29 farms monitored in Spain (10.3 per cent) remained free of clinical signs and seronegative throughout the study. The postweaning pigs on all of the remaining French farms and on 20 of the 26 remaining Spanish farms had a pattern of infection characterised by seroconversion in the grower period, generally between eight and 16 weeks of age. The seroprevalence in these groups ranged from 8 to 20 per cent. On all of these farms at least 15 per cent of the breeding females tested were seropositive, and the farms were under similar management systems, with a continuous flow of pigs or between buildings on one site, so-called 'one site, farrow-to-finish'. On the six remaining Spanish farms, under two management groups, a multiple-site system was used, with the piglets being separated from the adults at weaning and moved to a separate location. On three of these farms, the pattern of infection was characterised by seroconversion later in the finisher period, at between 16 and 20 weeks of age, and none of the breeding females was seropositive. On the three other multiple-site farms the pattern of infection resembled that on the one-site farms. On all of the farms, the seroconversion of groups of pigs was frequently associated with clinical or subclinical signs of ileitis.