2006 Ridremont Ipvs

Abstract No: P.02-05

PRRS SEROLOGICAL PROFILES IN GROWING AND FATTENING PERIODS: ANALYSIS OF THE EFFECTS OF FARM SIZE AND LOCAL STOCK DENSITY

B Ridremont1, V Auvigne2 1

Intervet SA, BEAUCOUZE, France 2 Ekipaj, ST GREGOIRE, France

Introduction From February 2002 to June 2004, with Intervet support, more than 800 PRRS serological profiles have been performed in French herds on unvaccinated growing pigs by swine practitioners. A first analysis of all the “nonnegative” profiles, carried out prior to March 2003, revealed some differences between geographical areas (1). The differences were observed in piglets in three age groups : 5-7, 8-10 and 11-13 weeks of age. We inferred the hypothesis that structural herd factors influence PRRS virus circulation in growing pigs. In this paper, we consider two of these potential risk factors : - Herd size : because contacts between pigs increase with herd size. - Area contaminations : because transmissions by aerosol or by insects or other factors, are likely to be more frequent in areas of high stocking density. Material and Methods The study involved 285 PRRS serological profiles from samples taken from three age groups (5 to 7, 8 to 10, 11 to 13 weeks of age). These age ranges are referred to later as 6, 9 and 12 weeks. The profiles were extracted from a database of 750 serological profiles (all ‘non-negative’ profiles carried out by swine practitioners between February 2002 and June 2004). None of the growing pigs on these farms were vaccinated when sampled. The sera were analyzed by Elisa (98 % with Idexx kit). For each profile, the following parameters were provided by the UGPVB organization from the Infoporc database : farm size (number of reproductive animals, weaned piglets and fattening pigs), farm type (farrow-to-finish farm, finishing farm), stock density in the neighbourhood (number of pigs within a radius of 3 km, excluding the farm sampled). A total of 246 profiles were submitted to statistical analysis by logistical regression using Epiinfo® software. Results The mean number of sows per farm was 202 and 25 % of the profiles came from farms with at least 400 sows (above the average for Brittany). Table 1 Results by age group. Age category (weeks) 6 9 12 Positive batches (%) 31.7 25.2 40.2 Positive pigs (%) 36 26.8 38.8 Mean S/P ratio 1.0 1.3 1.8 (among pos. Idexx) An age group is regarded as positive, if more than half the pigs in the batch are positive (Table 1). For the 9- and 12-week old groups, there is no correlation between serological status of the batch and farm size.

However, for 6-week old pigs, the chances of a batch being positive is significantly higher in farms with more than 280 sows (Tables 2 and 3). There is no correlation with the local stock density in any age category. Positivity at 6 weeks of age, mostly reflecting the presence of maternally derived antibodies (MDA), does not explain the positivity at 12 weeks of age, which will be the result of infection. Table 2 6 week-old pigs -seropositivity risk factors (234 farms with sows). Odds-ratio [IC] p Number 50 to 150 1 [reference] sows 150 to 280 1.5 [0.7 - 3.2] 0.4 > 280 4.3 [2.1 - 8.9] 0.0001 Density < 4 pigs/ha 1 [reference] 4 to 8 1.2 [0.6 -.5] 0.6 pigs/ha > 8 pigs/ha 1.2 [0.6 - 2.4] 0.6 Table 3 Relationship between farm size and seropositivity at 6 weeks old. Number of sows 50 to 150 150 to > 280 0 280 %pos.batches 20.5 24.4 51.3 25.0 Nr profiles 80 75 79 12 Discussion This study considered risk factors for seroconversion on infected farms, not the risk of contamination in a negative herd. In the absence of systematic PCR or IgM tests in seropositive 6-week old piglets, it is quite difficult to discriminate between maternally derived antibodies and those following active infection prior to weaning. However, the Elisa S/P ratios of seropositive growing pigs are much lower at 6 weeks of age than at 12 weeks (Table 1). Furthermore there is no statistical relationship between the status at 6 and at 12 weeks of age. Antibodies at 6 weeks of age may well be essentially of maternal origin and the differences observed according to the size of farm could be caused by differences in the immune status of the sow herd. In conclusion, it seems impossible to attribute a particular risk of contamination according to a herd’s size or geographical location. Transmission which depends on proximity to others does not seem to play a major role in the level of virus circulation on an infected farm. Diagnosis at a farm level is always essential. Reference 1. Ridremont B., Auvigne V., Proceedings Journées AFMVP (2003), p 171.

Proceedings of the 19th IPVS Congress, Copenhagen, Denmark, 2006 · Volume 2

31