Thus, the vaccine strains Ingelvac PRRS and Amervac PRRS were transmitted to 100% and 88

Thus, the vaccine strains Ingelvac PRRS and Amervac PRRS were transmitted to 100% and 88.9% of the sentinel pigs, respectively, while vaccine strains Porcilis PRRS and Pyrsvac-183 were found in only 44.4% and 22.2% of in-contact pigs, respectively. CY-09 safe. However, some differences were found in virological parameters. Thus, neither Pyrsvac-183 nor Porcilis PRRS could be detected in porcine alveolar macrophage (PAM) cultures or CY-09 in lung sections used to determine PRRSV by immunohistochemistry, indicating that these viruses might have lost their ability to replicate in PAM. This inability to replicate in PAM might be related to the lower transmission rate and the delay in the onset of viremia observed in these groups Introduction Porcine Reproductive and Respiratory Syndrome (PRRS) is an economically significant disease of pigs that causes respiratory distress in piglets and reproductive failure in sows [1,2]. The causal agent, PRRS virus (PRRSV), is a small, enveloped, single-stranded positive-sense RNA virus of the family [3]. Although, in general, PRRS is usually clinically comparable in North America and Europe, the respective strains differ in virulence [4,5] and in antigenic [6,7] and genetic [8] properties. These differences have led to the classification of PRRSV isolates into two genotypes: type 1 that comprises viruses related to the European prototype Lelystad-virus and type 2 that includes viruses related to the American prototype strain VR-2332 [8]. The huge impact of PRRS in the swine industry has stimulated the development of various types of vaccines, including inactivated and modified-live virus (MLV) vaccines, for the control of the disease in both growing pigs and breeding females. MLV CY-09 vaccines based on type 1 and type 2 viruses were originally developed for the control of PRRS in growing pigs, although some of them are now registered for the control of the reproductive form of PRRS. However, the safety of these products has been questioned based on the results of some experimental studies and on field evidence. Thus, experimental studies carried out with Ingelvac PRRS MLV, a vaccine based on a type 2 CY-09 isolate, have exhibited that vaccine virus replicates in vaccinated pigs, causes detectable viremia, persists in the organism of vaccinates for weeks [9-11] and is shed by different routes causing the infection of sentinel pigs [12]. In addition, the virus can cross the placental barrier in pregnant sows infecting the developing fetuses [13] and can be transmitted to na?ve newborn piglets during lactation [14]. Even more, reversions to virulence have been suspected in the field based on the similarity between the vaccine strain and some strains that have caused clinical problems in areas where the vaccine has been used, regardless of whether the affected animals had been vaccinated or not [15,16]. Despite the knowledge in relation to the safety of type 2 Ingelvac PRRS MLV vaccine, not much information has been published about the safety of MLV vaccines based on type 1 PRRS viruses, even though they are frequently used for the control of the disease in several European countries. In fact, there are only a few reports that demonstrate that these vaccines replicate in the host causing viremia during variable periods of time both in growing pigs [17] and in breeding females [18], which can lead to transplacental contamination of fetuses [18]. However, no information is usually available regarding the ability of these vaccine strains of being shed and transmitted to in-contact pigs. Even more, the above mentioned studies have been carried out under different conditions, using different experimental models, i.e. growing pigs versus breeding females, and different experimental designs, with differences affecting not only the age of vaccinated pigs but also the quality and quantity of the parameters measured. Under these circumstances, the information available about the safety of different vaccines is only partial and direct comparison of the results between different experiments is not feasible, making it impossible to determine whether different vaccine strains differ in their safety properties. Consequently, the objective of the present study was to elucidate and compare the safety of all MLV vaccines commercially available in Europe under the same experimental conditions, measuring their ability to induce clinical signs and lung lesions, their replication capacity in the host, their ability to be shed by different routes and their transmissibility to na?ve pigs. Material and methods Animals and facilities One hundred and one hundred and twenty three-week-old crossbred conventional piglets seronegative for PRRSV, and Klf4 unfavorable by PCR for porcine circovirus type.