An on-farm study of the epidemiology of Actinobacillus pleuropneumoniae infection in pigs as part of a vaccine efficacy trial

Thirty cohort pigs were followed from birth to slaughter to study epidemiol ogical patterns of porcine pleuropneumonia caused by Actinobacillus pleurop neumoniae. The study was conducted within a larger 380-animal study of vacc ination against Mycoplasma hyopneumoniae and A. pleuropneumoniae in a 340-s ow farrow-to-finish piggery with 4-month weaning, operating a continuous sy stem of intensive production in the North Island of New Zealand. The cohort pigs were randomly allocated into two equal groups: vaccinated and control . Pigs in the first group were vaccinated at 2 and 4 weeks of age with both M. hyopneumoniae vaccine and A. pleuropneumoniae vaccine at separate vacci nation sites. A series of nasal swabs was taken at 4, 8, 10, 11, 12, 14, 16 and 18 weeks of age. Each swab was streaked onto the surface of a selectiv e medium on the farm and the plates were immediately transported to a labor atory and incubated at 37 degrees C for 5 days. After the trial, pigs were slaughtered at an average of 132 days of age, lungs were examined and sampl es taken for bacteriological culture and isolation. Thirty-five out of 256 samples produced haemolytic colonies which were Gram-negative, V-factor-dep endent and positive to the CAMP test. A. pleuropneumoniae was first isolate d at 4 weeks of age from one vaccinated pig. This finding suggests that pig lets became infected in the farrowing pen and the source of infection might be a carrier sow. The interval-specific cumulative incidence of A. pleurop neumoniae infection reached a maximum of 54% and 40% at ii weeks of age in the vaccinated and control groups, respectively. Infection status of the li tter is considered to be a factor influencing morbidity in infected herds d uring weaner and grower periods. Our results suggest that simultaneous vacc ination with M. hyopneumoniae and A. pleuropneumoniae vaccines at 2 and 4 w eeks of age might lessen the prevalence but cannot absolutely prevent A. pl europneumoniae infection during the weaner or grower-finisher periods. (C) 1999 Elsevier Science B.V. All rights reserved.