Eradication of Porcine Reproductive and Respiratory Syndrome virus in 125 herds in "Pays de la Loire" (France)

Abstract - In the "Pays de la Loire" region, PRRS was first detected in November 1992. The large involvement of farmers in the control measures implemented after the first outbreaks has contributed to maintain an infection prevalence lower than 2%. These measures were defined and approved by all the members of the regional pig industry, under the aegis of the Regional Sanitary Defence Confederation (FRGDS). Total and partial depopulations have proved to be effective means for controlling the spread of PRRS virus within and between herds. The purpose of this presentation is to describe the results of an eradication plan for PRRSV infection carried out on 125 herds. In a first attempt to eradicate PRRSV infection, eradication was conducted by total depopulation of several farms which represented a high risk of PRRSV contamination for other farms: the culled farms were for example, nucleus farms selling breeders, or infected farms located near nucleus herds. In these farms, all breeding and fattening pigs were progressively slaughtered. The rest of the pigs (e.g. growing and finishing pigs) were either moved off the site or grown until the end of the fattening period. When emptied, the farms were cleaned and disinfected. Three weeks after disinfection, the farms were repopulated with pigs originating from a PRRSV-free herd. Eradication of PRRS infection was then confirmed by two negative serological controls at two months of interval. From 1995 on, elimination of PRRSV from infected farms was also attempted through a partial depopulation strategy. A first condition had to be established before partial depopulation could start. This condition was the absence of PRRSV circulation within the breeding animals. Virus circulation was assessed by a serological survey carried out two times a year in each farm having breeding pigs. This survey included 20 to 30 breeding animals of different ages in different farm compartments, 5 nursery pigs (8-10 weeks of age), 5 growing and 5 finishing pigs (3-4 and 5 months of age, respectively). When the replacement gilts and the young sows were found to be serologically negative after two consecutive controls, then partial depopulation was initiated. The protocol was as follows: if the growing-finishing pigs were serologically positive, then the corresponding premises were progressively emptied, cleaned and disinfected. Thereafter, the positive sows of the same farm were progressively eliminated once their offspring were weaned. Their weaned piglets were then reared in the cleaned premises as before. In grower-finisher units, PRRS infection had not spread in all the buildings. In a few cases, partial depopulation was only undertaken in the infected premises. After 5 years of a control programme in the "Pays de la Loire" region, the PRRS virus has now been eliminated from the 125 herds involved in the programme, among 150 herds detected infected. For 99 of the herds, this was performed by the total depopulation method, applied in 22 farrow-to-finishing farms (1709 sows), 10 farrow-to-weaning farms (873 sows) and 67 growing-to-finishing farms (38 161 pigs). The partial method concerned: Among the 18 farrow-to-finishing herds undergoing a partial depopulation, six farms emptied the nurseries and the grower-finisher units. In the last 12 herds, only the positive sows were eliminated, the grower-finisher units becoming negative without any intervention. All partial depopulations were successful. Six finishing herds and one farrow-to-weaning herd, out of 99 totally depopulated herds, became infected a second time, 8 months after culling. These re-infections were presumably the consequences of the introduction of infected pigs or contamination by a nearby infection. Five of these seven farms were cleaned again. This study showed that eradication of PRRS in herds is feasible through either total or partial depopulation. The method of total depopulation is safe and fast albeit expensive. Partial depopulation makes it possible to eliminate only a part of the animals, hence allowing to preserve the overall genetic improvement of the farm. However, the success of partial depopulation requires that virus circulation among the sows be broken-down. This epidemiological situation is not found in all endemically infected farms. Twenty-one breeding herds thus remained infected in the "Pays de la Loire" region because of persistent virus circulation in breeding pigs. This report also demonstrates how useful can be the serological tools for monitoring virus circulation and for subsequently controlling the efficacy of eradication programmes.