Vaccine 22 (2004) 4183–4190 Impact of genetic diversity of European-type porcine reproductive and respiratory syndrome virus strains on vaccine efficacy G. Labarque a , K. Van Reeth a , H. Nauwynck a,∗ , C. Drexler b , S. Van Gucht a , M. Pensaert a a Laboratory of Virology, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, 9820 Merelbeke, Belgium b Virological Research Department, Intervet International BV, P.O. Box 31, 5830 AA Boxmeer, The Netherlands Received 12 February 2004; received in revised form 4 May 2004; accepted 12 May 2004 Abstract The aim of this study was to find out how efficiently pigs that are vaccinated with an attenuated porcine reproductive and respiratory syndrome virus (PRRSV) vaccine based on a virus from the Lelystad cluster are protected against a European wild-type strain from the same or another genetic cluster. Two experiments were performed. In each experiment, 5-week-old PRRSV-seronegative pigs were vaccinated intramuscularly with 10 4.5 TCID 50 of a commercial vaccine based on a European virus strain from the Lelystad cluster. Non-vaccinated pigs were included as controls. At 5, 9, 15, 20, 28, 35 and 42 days post vaccination (PV), broncho-alveolar lavage (BAL) fluids and blood were collected to determine vaccine virus quantities. Forty-nine days PV, pigs were challenged intranasally with 10 6.0 TCID 50 of a European wild-type strain, belonging either to the Lelystad cluster (98% nucleotide identity in ORF5 with vaccine strain) (experiment A) or to an Italian cluster (84% nucleotide identity in ORF5 with vaccine strain) (experiment B). At 5, 9, 15, 20 and 27 days post challenge (PC), BAL fluids and blood were collected to determine virus quantities. Vaccine virus was first detected in BAL fluids and blood at 5 days PV and reached highest quantities between 9 and 15 days PV. One pig was positive in its BAL fluid until 42 days PV. After challenge, virus was isolated from BAL fluids and blood of all non-vaccinated control pigs. All vaccinated pigs challenged with the Lelystad strain remained negative for virus, while virus was present in BAL fluids and blood of all vaccinated pigs after challenge with the Italian strain. Mean virus titres of the vaccinated pigs challenged with the Italian strain were significantly lower than those of the non-vaccinated control pigs (P < 0.05) at 9, 15 and 20 days PC. Thus, the genetic diversity within European-type PRRSV may affect the efficacy of the current European-type vaccines. © 2004 Elsevier Ltd. All rights reserved. Keywords: Pig; Porcine reproductive and respiratory syndrome virus; Efficacy 1. Introduction Porcine reproductive and respiratory syndrome virus (PRRSV), an arterivirus, is the causative agent of a syndrome in pigs, which was first observed in the United States in 1987 [11] and appeared in Western Europe in 1990 [25]. The first introduction of PRRSV in the susceptible swine population was characterized by acute outbreaks of late-term reproduc- tive failure in breeding pigs, pre-weaning mortality in suck- ling pigs, and respiratory disease in suckling, nursery, and fattening pigs. Even though the clinical picture is similar ∗ Corresponding author. Tel.: +32 9 264 73 73; fax: +32 9 264 74 95. E-mail address: hans.nauwynck@ugent.be (H. Nauwynck). on the two continents and the syndrome emerged on both continents within a time period of only 3 years, European and North American PRRSV isolates are surprisingly dif- ferent, both genetically and antigenically [10,40]. Therefore, PRRSV isolates were classified into two distinct genotypes, namely the European and the North American genotype [18,20]. A high degree of variability has been demonstrated within the North American genotype [10,17,26]. While early studies suggested that a low degree of genetic and antigenic variability might exist among European PRRSV isolates [5,16,31], later studies in some countries, such as Russia, Czech Republic, Denmark, Italy, Austria, Poland, Lithua- nia, and Spain have reported a high divergence of Euro- pean PRRSV isolates [1,3,6–9,19,27,30]. Recent studies have 0264-410X/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vaccine.2004.05.008