The porcine reproductive and respiratory syndrome (PRRS) virus first entered the Pays de la Loire region in November 1992, with variable effects ranging from sub-clinical seroconversion to severe reproductive failure and piglet mortality, and significant reduction of daily weight gains in finishing pigs. An epidemiological survey was carried out in February 1993. Since the infection prevalence was low (11 infected out of 2310 herds), the pig population was of medium density and the eradication programme of Aujeszky's disease had been successful in the Pays de la Loire region, it was decided (in March 1993) to undertake a control programme for PRRS. In 1993, introduction of infected pigs was known to be the most frequent source by which PRRS virus entered a herd. In the absence of vaccination, this source of virus introduction was reduced by a control programme applied to all members of the regional pig industry, through the impetus of the leaders of the Regional Sanitary Defence Confederation (FRGDS). The control programme was applied on purchased animals (sows, boars, piglets), artificial insemination centres and other environmental factors (people, vehicles, materials, slurry,...). Moreover, pigs from many infected herds were slaughtered, Results showed that in a context of low prevalence and limited spreading to nearby herds, efficient control of animal movements limited the infection spread. At the end of 1993, the PRRS prevalence was 2.7% in the region. Two years after the first outbreak, the PRRS infection could be considered as controlled since 98% of the herds remained free. In order to maintain this low infected status, the control programme was renewed. From this study epidemiological investigations have raised two major initial sources of infection, the use of contaminated semen and the introduction of infected pigs. Around an infected herd, serological screening is still running to detect infection in nearby herds. (C) 1997 Elsevier Science B.V.
