Experimental infection of SPF piglets with porcine reproductive and respiratory syndrome (PRRS) viruses isolated from two farms

Porcine reproductive and respiratory syndrome virus in Ontario, Canada 1999 to 2010: genetic diversity and restriction fragment length polymorphisms

Classification of Ontario porcine reproductive and respiratory syndrome virus (PRRSV) field isolates (n = 505) from 1999 to 2010, based on a global type 2 PRRSV ORF5 phylogenetic framework, revealed genetic diversity comparable to PRRSV in the USA, with sequences assigned to five of nine lineages (1, 2, 5, 8 and 9). Importantly, the tree topology indicated a Canadian ancestry for the highly virulent MN184-related strains that first emerged in 2001 in the USA. Mapping of the RFLP patterns onto the phylogenetic tree revealed numerous examples of different RFLP patterns located within the same phylogenetic cluster. Statistical analysis showed occurrences where similar RFLP patterns masked diverse genetic distances and instances of close genetic proximity with divergent RFLP patterns. Collectively, extensive genetic diversity prevails in type 2 PRRSV in one region of the North American swine industry, and it is not described adequately by RFLP typing, which might have value in differentiating strains at the local farm level.

Experimental dual infection of specific pathogen-free pigs with porcine reproductive and respiratory syndrome virus and pseudorabies virus

Twenty 6-week-old specific pathogen-free pigs were divided into four groups. On day 0 of the experiment, PRRSV-PRV (n = 6) and PRRSV (n = 4) groups were intranasally inoculated with porcine reproductive and respiratory syndrome virus (PRRSV) (10(5.6) TCID50). On day 7, the PRRSV-PRV and PRV (n = 6) groups were intranasally inoculated with pseudorabies virus (PRV) (10(3.6) TCID50). Control pigs (n = 4) were kept as uninoculated negative controls. Half of the pigs in each group were euthanized and necropsied on day 14 or 21. Clinical signs such as depression and anorexia were observed in the PRRSV-PRV and PRV groups after inoculation with PRV. Although febrile response was observed after virus inoculations, the duration of that response was prolonged in the PRRSV-PRV group compared with the other groups. The lungs in the PRRSV-PRV group failed to collapse and were mottled or diffusely tan and red, whereas the lungs of the pigs in the other groups were grossly normal. Histopathologically, interstitial pneumonia was present in all PRRSV-inoculated pigs, but the pneumonic lesions were more severe in the PRRSV-PRV group. Mean PRRSV titres of tonsil and lung in the PRRSV-PRV group were significantly (P < 0.05) higher than that in the PRRSV group on day 21. These results indicate that dual infection with PRRSV and PRV increased clinical signs and pneumonic lesions in pigs infected with both viruses, as compared to pigs infected with PRRSV or PRV only, at least in the present experimental conditions.

Experimental reinfection with homologous porcine reproductive and respiratory syndrome virus in SPF pigs

The present examination was conducted to determine if the pigs infected with one strain of porcine reproductive and respiratory syndrome virus (PRRSV) would be protected against a subsequent homologous virus challenge. Sixteen 4-week-old SPF pigs were assigned to 2 experimental groups A and B. The pigs in group A were inoculated with 10(6.5) TCID50 of PRRSV by intranasal route. On 77 days post-inoculation (PI), pigs in groups A and B were similarly inoculated with same virus. After the secondary inoculation, the pigs in group A didn't show any clinical sign including pyrexia and reduction of white blood cell (WBC) number. Viremia was detected only on 3 days PI with low virus titer and any virus was not recovered from serum and tissues at the time of necropsy on 14 or 28 days PI. In contrast, pigs in group B showed pyrexia for 14 days and reduction of WBC number on 3 days PI. Viremia was detected between 3 and 28 days PI, and virus was isolated from several tissues of all pigs. These results indicate that previous exposure to PRRSV can prevent development of clinical signs and reduce virus proliferation in pigs after subsequent infection with the homologous PRRSV.

Experimental infection of maternally immune pigs with porcine reproductive and respiratory syndrome (PRRS) virus

To investigate the influence of maternal antibody to porcine reproductive and respiratory syndrome (PRRS) virus infection, the following examination was done using conventional and SPF pigs. Ten 17-day-old conventional pigs with maternal antibody against PRRS virus and 6 44-day-old SPF pigs seronegative were inoculated intranasally with 10(5.0) TCID50 of PRRS virus. Two conventional and 4 SPF pigs were served as non-inoculated control. In conventional pigs, coughing and febrile response were observed after inoculation, and mean rate of weight gain reduced. One of the inoculated conventional pigs died on post-inoculation-day (PID) 28 and Haemophilus parasuis was isolated from the lung. Although febrile response was also observed in the inoculated SPF pigs, reduction in weight gain rate was not recognized. Virus was isolated from all the sera of inoculated conventional and SPF pigs except one conventional pig between PID 7 and 49, and between PID 7 and 28, respectively. Onset of viremia in the several conventional pigs delayed. Virus was isolated from the tissues of the 5 conventional pigs on PID 65 and from the tissues of the dead pig. On the other hand, virus was not isolated from the tissues of non-inoculated conventional pigs, and inoculated and non-inoculated SPF pigs. At the virus inoculation, antibodies by the indirect fluorescent antibody (IFA) assay against PRRS virus were detected in the sera of conventional pigs with antibody titers of 1:20. Antibody titers gradually decreased after inoculation and rose from PID 21 or 28 and were between 1:160 and 1:640 on PID 63. Virus neutralization (VN) antibody titers were 1:2 or 1:4 at the inoculation and gradually decreased. Apparent rise in VN antibody titer was not observed after the inoculation. In the sera of control pigs, both antibody titers gradually decreased and did not rise. In the sera of the SPF pigs, antibodies by the IFA assay were first detected on PID 7 or 14. The titers of antibodies rose and reached their maximum with 1:320 to 1:2,560 on PID 21 to 35. VN antibodies were first detected in PID 42 to 56 and thereafter, the titers ranged between 1:1 to 1:4. Control SPF pigs were free of antibody throughout the examination. Antigenic variability was not recognized between the inoculated and recovered viruses by the VN test. The prolonged duration of viremia and virus isolation from the tissues on PID 65 in conventional pigs with low maternal antibody might support the present of antibody-dependent enhancement activity of PRRS virus infection.

Effect of attenuated Erysipelothrix rhusiopathiae vaccine in pigs infected with porcine reproductive respiratory syndrome virus

Twenty 2nd specific pathogen-free pigs were divided into 4 groups: Group A were infected with porcine reproductive and respiratory syndrome (PRRS) virus at 6 weeks of age and treated with available swine erysipelas and swine fever combined vaccine (vaccinated) at 7 weeks of age; Group B were vaccinated at 7 weeks of age and infected with PRRS virus at 8 weeks of age; Group C were vaccinated at 7 weeks of age: Group D were neither vaccinated nor infected with PRRS virus. All pigs were challenged to Erysipelothrix rhusiopathiae C42 strain at 10 weeks of age. No clinical signs appeared after vaccination of group A and B pigs, thus confirming that the safety of the vaccine was not influenced by infection with PRRS virus. None of the pigs in Groups A and C developed erysipelas after challenge exposure to E. rhusiopathiae. In contrast, fever and/or urticaria appeared transiently in all pigs of Group B after challenge exposure. At the time of challenge exposure to E. rhusiopathiae, the PRRS virus titer was high in sera of Group B, but was low in those from Group A. However, vaccination of pigs with attenuated E. rhusiopathiae was effective in dual infection with PRRS virus and E. rhusiopathiae, because the clinical signs were milder and the E. rhusiopathiae strain was less recovered from these pigs compared to pigs of group D.

In vivo replication of porcine reproductive and respiratory syndrome virus in swine alveolar macrophages and change in the cell population in bronchoalveolar lavage fluid after infection

Replication of porcine reproductive and respiratory syndrome (PRRS) virus in swine alveolar macrophages (AM) and cell population in broncho-alveolar lavage fluid (BALF) obtained from PRRS virus-infected pigs were investigated. BALF samples were periodically collected from 6 pigs infected with PRRS virus and 3 non-inoculated control pigs by means of fiber-optic bronchoscope between post-inoculation day (PID) 0 and 56. The mean ratio of macrophages in BALF collected from infected group was 92.7 +/- 3.2% before inoculation and gradually decreased from PID 14. On the other hand, the ratio of lymphocytes was 4.8 +/- 3.2% before inoculation and increased from PID 21 and indicated 41.8 +/- 9.1% on PID 28. After that, they decreased gradually and that of macrophages correspondingly increased. The ratio of neutrophils maintained between 0.7% and 5.1%. The ratios of macrophages, lymphocytes and neutrophils collected from control group were almost stable through the examination. Intracellular PRRS virus antigens in AM were detected from PID 2 by indirect immunofluorescence assay (IIFA). PRRS virus was first isolated from BALF samples collected from inoculated group between PID 2 and 49. From serum, virus was isolated between PID 2 and 21. Antibodies in sera measured by IIFA to PRRS virus were first detected on PID 14 and the antibody titer rose to 1:640 or 1:1,280. The results suggested that PRRS virus replicates in swine AM for a relatively long period.

Effect of heparin on a haemagglutinin of porcine reproductive and respiratory syndrome virus

Heparin inhibited haemagglutination by porcine reproductive and respiratory syndrome virus (PRRSV) and by Aujesky's disease virus, but failed to inhibit haemagglutination by parainfluenza virus type 3. The minimal inhibitory concentration of heparin required to inhibit 8 HA U of PRRSV haemagglutinin ranged from 0.1 to 1 U ml-1. Mouse erythrocytes failed to combine with the haemagglutination inhibitory factor of heparin. However, mouse erythrocytes treated with heparinase had greatly reduced agglutinability by PRRSV. The formation of a haemagglutinin-heparin complex could be observed by sedimenting heparin with the haemagglutinin. All these findings suggest that a heparin-like molecule on the surface of mouse erythrocytes serves as the virus-cell receptor.

Early serodiagnosis of porcine reproductive and respiratory syndrome virus infection of pigs by detection of slow-reacting and complement-requiring neutralizing antibody

In order to evaluate and enhance the sensitivity of the neutralization (NT) test for detecting antibody in pigs infected with porcine reproductive and respiratory syndrome (PRRS) virus, the effect of altered incubation conditions and complement use on neutralizing (NT) antibody titer were investigated. Higher NT antibody titers were consistently obtained by addition of 20% guinea pig fresh serum to virus-serum mixtures in NT tests. Furthermore, the complement-requiring NT antibody titer increased in many serum samples when the virus-serum mixtures, rather than being incubated at 37 degrees C for 60 min, were incubated first at 4 degrees C for 48 hr and then with a complement at 37 degrees C for 60 min. The slow-reacting and complement-requiring NT antibody was detected as early as 8 days post-inoculation. It was detected in sera collected at 8 to 28 days post-inoculation and was sensitive to 2-mercaptoethanol treatment. Sera collected at 35 to 44 days post-inoculation contained 2-mercaptoethanol resistant NT antibodies. These results indicate that the modified NT test is useful for early serodiagnosis of PRRS virus infection through detection of higher NT antibody titers, and in detecting them earlier.