Full Terms 15: 16-20 PRCV infections on farms which are situated several kilometres apart and which apply strict sanitary measures (7,13). Thishas resulted in a rapid spread of PRCV since itsappearance in 1984. From 1987 on, it has been extremely difficultto find Belgian swine farms which have not hada PRCV infection (13, 14). On closed breeding-fattening farms, PRCV infections regularly occur in pigs younger than 3 months. The infections are observed especially during winter and early spring.It is not clear if the virus persists on these farmsor is newly introduced with each wave of infection (15). The increased prevalence of PRCV in the Belgian swine population has been accompanied bya decrease in the number of diagnosed TGE-outbreaks, suggesting thatimmunity against the widespread PRCV affects the behaviour ofTGEV or that TGEV itself has been changed. However, it is still unclear whether the presence of PRCV results ina decreased prevalence of TGEV infections or only causesa milder course so that the infections are no longer clinically manifest. Differential serological diagnosis between PRCVand TGEV infections is not possible with the classical sero-neutralization test, since infections with PRCV or TGEV induce antibodies which neutralized both viruses to thesame titre. However, a competitive inhibition ELISA has been developed which permits serological differentiation between PRCVand TGEV infections. This assay is performed using a non-neutralizing monoclonal antibody directed againstan epitope on the spike protein (S) of TGEV, which is absenton PRCV (4). Pigs infected with TGEV develop antibodies against this differentialepitope, and these antibodies are detected with the competitiveinhibition ELISA (3). The purpose of the present serological study was to obtain insight into the prevalence of PRCV and ofTGEV. First, the incidence of PRCV and TGEV infections was determined by a serological survey of slaughterhousesows in February 1990 and by a serological examination of fattening pigson 33 farms. The latter study was performed between February 1989 and February 1990. Second, during the longitudinal serological study on the 33 farms it was examined whether theoccurrence of PRCV infections is influenced by the location of thefarm in a dense swine population, the herd size and theseason. Third, it was investigated whether PRCV can persist on farms. MATERIAL AND METHODS Origin of the sera I. Serological survey of slaughterhousesows. The prevalence of PRCV and TGEV infectionswas determined by using sera from 160 sows, collected at slaughter in 1990. The sows originated from 81 different farms, namely 49 closed breeding-fattening farms, 23 breeding farms and 9 farmsof which the type was unknown. These farmswere located in the provinces of East-Flanders and Antwerp. Betweenone and five samples of sera were collected per farm. 1 6 T H E V E T E RIN A R Y Q U A R T E R L Y ,V O L 15.N o .1.M A R C H ,19 9 3 0 0 Downloaded by University of Wisconsin Oshkosh at 09:21 11 November 2015 ORIGINAL PAPERS 2. Prevalence of PRCV and TGEV antibodiesin fattening swine on 33 farms : influence of the swine population density, the herd size and the season The prevalence of PRCV or TGEV infections on 33 farms was determined serologically.Eleven of the farms were located within an area of 4 km accommodatinga total of 18 farms and were designated farms ina densely populated area. Twenty-two farms were located in areas with not more than one to four farms per 12.5 km and were designated widely dispersed farms. This distinction allowedus to determine the influence of the swine population densityon the occurrence of PRCV or TGEV infections. The 22 widelydispersed farms were all breeding-fattening farms. The number ofsows and fattening swine on these farms are given in table 1. The neighbouring farms, which are situated within a distance of 0 to 0.5, 0.5 to 1 and 1 to 2 km, respectively, of the widely dispersed farms, are also presented. Of the 11 farms located in a dense swine population, 5 were breeding farms with the number ofsows varying between 45 and 105 and the number of 11- to 20-week-old pigs, between 20 to 150, 5were closed breeding-fattening farms (in which pigs or sows from other farms were not introduced) with 30 to 300 sows and 190 to 1450 fattening swine, and 1 was an open breeding- fattening farm (11-week-old pigs from other farms were introduced) with 180sows and 1500 fattening swine. Four to eightsera were collected from 20- to 26-week-old swine on all 33 farms every 2 to 3 months between February 1989 and February 1990(Table 1). The longitudinal serological survey allowed us to examine the influence of the season on the occurrence of an infection. Table 1. Aone year serological study on the presence of PRCV-antibodies in 20- to 26-week-old fattenerson 22 breeding-fattening farms situated in areas with a low farm density. Farm Number Cionber Neogoounng tams, number of olman a ee4ance fattener. bows sncv-insucas