Vaccine 29 (2011) 1731–1735 Contents lists available at ScienceDirect Vaccine journal homepage: www.elsevier.com/locate/vaccine Effect of vaccination of pigs against experimental infection with high and low virulence Mycoplasma hyopneumoniae strains I. Villarreal a,∗ , D. Maes a , K. Vranckx b , D. Calus b , F. Pasmans b , F. Haesebrouck b a Department of Reproduction, Obstetrics and Herd Health, Ghent University, Faculty of Veterinary Medicine, Salisburylaan 133, B-9820 Merelbeke, Belgium b Department of Pathology, Bacteriology and Avian Diseases, Ghent University, Faculty of Veterinary Medicine, Salisburylaan 133, B-9820 Merelbeke, Belgium article info Article history: Received 7 October 2010 Received in revised form 23 December 2010 Accepted 5 January 2011 Available online 13 January 2011 Keywords: M. hyopneumoniae Vaccination Efficacy Virulence Strain abstract This study investigated the infection pattern and lung lesion development in pigs caused by a low and highly virulent Mycoplasma hyopneumoniae strain at 4 and 8 weeks (w) post infection (PI). It also deter- mined the efficacy of a commercial inactivated whole-cell vaccine against infection with each one of these M. hyopneumoniae strains. Ninety piglets free of M. hyopneumoniae were selected, and 40 of them were randomly vaccinated during their first week of life. At weaning, all piglets were allocated to 10 different groups and housed in pens with absolute filters. At 4 weeks of age, pigs were inoculated intra- tracheally with either a highly virulent M. hyopneumoniae strain, a low virulent strain or with sterile culture medium. Half of all animals were euthanized at 4w PI, while the remaining half was euthanized at 8w PI. Coughing was assessed daily, and lung lesions, immunofluorescence (IF), bacteriological analysis and nested PCR were assessed after necropsy. It was demonstrated that contrary to the highly virulent strain, the low virulent strain required more than 4 weeks PI (commonly accepted as the standard infec- tion model) to reach maximum clinical symptoms. Vaccination significantly reduced clinical symptoms, macroscopic and microscopic lung lesions in pigs infected with the highly virulent strain. This effect was more pronounced at 4 than at 8 weeks PI. Protective efficacy was also observed in pigs infected with the low virulent strain, but the effect was less pronounced than on the highly virulent strain. © 2011 Elsevier Ltd. All rights reserved. 1. Introduction Mycoplasma hyopneumoniae (M. hyopneumoniae) is the causative agent of enzootic pneumonia in swine, a disease with worldwide economic impact in intensive pig production [10,25] and characterized by chronic non-productive coughing, retarded growth rate and poor feed conversion ratio [18]. It is well known that factors such as management practices and housing conditions influence the infection pattern and the severity of the disease in pig herds [9]. However, also the virulence of M. hyopneumoniae strains has been demonstrated to determine the clinical course of the infection [28]. Experimental infection with a highly virulent M. hyopneumoniae strain was associated with severe clinical signs and lung lesions 4 weeks post infection (PI), whereas clinical symptoms and lung lesions were milder following infection with low virulent strains [12,28]. In most M. hyopneumoniae experimental infection studies, pigs are euthanized at 4 weeks PI, based on the fact that macroscopic lung lesions reach their maximum size 2–4 weeks after inoculation [6,30]. This model has been widely accepted as the standard infec- ∗ Corresponding author. Tel.: +32 9 264 75 40; fax: +32 9 264 75 34. E-mail address: ikaukiel@gmail.com (I. Villarreal). tion model for this pathogen. However, recent experiments showed that, depending on the strain, the most severe clinical symptoms and lung lesions are sometimes seen later than 4 weeks after infec- tion [29]. As a result, the standard infection model might not be suitable for studying all types of M. hyopneumoniae strains, as some of them might require more than 4 weeks PI to fully develop symp- toms and lesions in the infected pigs. Vaccination is frequently practiced to control M. hyopneumo- niae infections in pig herds worldwide. Although vaccination with the currently available bacterin vaccines is not able to prevent transmission nor establishment of the microorganism in the lungs [12,26], numerous studies have shown that it reduces the clinical symptoms and lung lesions, improves performance, and leads to an economic benefit for the pig producers [10,14,19]. M. hyopneumo- niae is, however, a highly heterogenous species not only with regard to virulence [12,28], but also at genetic, antigenic and proteomic level [1,16,21]. Most of the currently available bacterin vaccines are based on an adjuvanted suspension of inactivated M. hyopneumo- niae organisms, and it is not clear whether the protective effect of vaccination may vary depending on the strain causing the infection. Therefore, the aims of this study were twofold. First, in order to assess the suitability of the standard infection model, the present study investigated the infection pattern and lung lesion develop- ment in pigs infected by low and highly virulent M. hyopneumoniae 0264-410X/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vaccine.2011.01.002