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.     

Elevated serum haptoglobin in pigs infected with porcine reproductive and respiratory syndrome virus.

We examined the two acute phase proteins, alpha (alpha)-1 acid glycoprotein (AGP) and haptoglobin (HP), in serum of pigs following experimental porcine reproductive and respiratory syndrome (PRRS) virus infection. Increased levels of serum HP, but not AGP, were observed from 7 to 21 days post-inoculation in the infected pigs. Furthermore, serum IL-6 increased in the infected pigs, but TNF-alpha did not. The increase of serum IL-6 in pigs following PRRS virus infection may induce production of HP. Also, in the field investigation, serum HP in pigs was dramatically increased after exposure to the PRRS virus.     

Effect of vaccination with a modified-live porcine reproductive and respiratory syndrome virus vaccine on dynamics of homologous viral infection in pigs

OBJECTIVE: To determine effects of vaccination protocols with modified-live porcine reproductive and respiratory syndrome virus (PRRSV) vaccine on persistence and transmission of virus in pigs infected with a homologous isolate and determine clinical and virologic responses following heterologous viral challenge. ANIMALS: Four hundred forty 6- to 8-week-old PRRSV-na?ve pigs. PROCEDURES: Pigs were allocated into 5 groups. Groups A to D were inoculated with wild-type PRRSV VR2332. Group A (positive control pigs) received PRRSV only. Groups B, C, and D received modified-live PRRSV vaccine (1, 2, or 3 doses). Group E served as a negative control group. To evaluate viral transmission, sentinel pigs were introduced into each group at intervals from 37 to 67, 67 to 97, and 97 to 127 days postinoculation (DPI). To evaluate persistence, pigs were euthanized at 37, 67, 97, or 127 DPI. To assess clinical and virologic response after challenge, selected pigs from each group were inoculated at 98 DPI with a heterologous isolate (PRRSV MN-184). RESULTS: Mass vaccination significantly reduced the number of persistently infected pigs at 127 DPI. Vaccination did not eliminate wild-type PRRSV; administration of 2 or 3 doses of modified-live virus vaccine reduced viral shedding after 97 DPI. Previous exposure to wild-type and vaccine virus reduced clinical signs and enhanced growth following heterologous challenge but did not prevent infection. CONCLUSIONS AND CLINICAL RELEVANCE: Findings suggest that therapeutic vaccination may help to reduce economic losses of PRRSV caused by infection; further studies to define the role of modified-live virus vaccines in control-eradication programs are needed.     

Respiratory function and pulmonary lesions in pigs infected with porcine reproductive and respiratory syndrome virus

Pulmonary dysfunction was evaluated in pigs infected with porcine reproductive and respiratory syndrome virus (PRRSV, isolate VR-2332) and compared to clinical and pathological findings. Infected pigs developed fever, reduced appetite, respiratory distress and dullness at 9 days post-inoculation (dpi). Non-invasive pulmonary function tests using impulse oscillometry and rebreathing of test gases (He, CO) revealed peripheral airway obstruction, reduced lung compliance and reduced lung CO-transfer factor. PRRSV-induced pulmonary dysfunction was most marked at 9–18 dpi and was accompanied by a significantly increased respiratory rate and decreased tidal volume. Expiration was affected more than inspiration. On histopathological examination, multifocal areas of interstitial pneumonia (more severe and extensive at 10 dpi than 21 dpi) were identified as a possible structural basis for reduced lung compliance and gas exchange disturbances.     

Experimental reproduction of porcine respiratory disease complex in pigs inoculated porcine reproductive and respiratory syndrome virus and Mycoplasma hyopneumoniae and followed by inoculation with porcine circovirus type 2

The aim of this study was to reproduce severe pneumonic lesions, similar to those during naturally-occurring porcine respiratory disease complex, in pigs dually inoculated with porcine reproductive and respiratory syndrome virus and Mycoplasma hyopneumoniae at 6 weeks of age, followed by inoculation with porcine circovirus type 2 at two weeks after. Time and sequence of infection with three pathogens mirror Asian field conditions. Microscopically, interstitial pneumonia and peribronchiolar lymphoid hyperplasia are considered the most characteristic lung lesions in infected pigs. The results of the present study demonstrate that inoculation of pigs with these three pathogens can lead to severe interstitial pneumonia with peribronchial or peribronchiolar lymphoid hyperplasia and fibrosis.     

Experimental reproduction of severe disease in CD/CD pigs concurrently infected with type 2 porcine circovirus and porcine reproductive and respiratory syndrome virus

Three-week-old cesarean-derived colostrum-deprived (CD/CD) pigs were inoculated with porcine circovirus type 2 (PCV2, n = 19), porcine reproductive and respiratory syndrome virus (PRRSV, n = 13), concurrent PCV2 and PRRSV (PCV2/PRRSV, n = 17), or a sham inoculum (n = 12) to compare the independent and combined effects of these agents. Necropsies were performed at 7, 10, 14, 21, 35, and 49 days postinoculation (dpi) or when pigs became moribund. By 10 dpi, PCV2/PRRSV-inoculated pigs had severe dyspnea, lethargy, and occasional icterus; after 10 dpi, mortality in this group was 10/11 (91%), and all PCV2/ PRRSV-inoculated pigs were dead by 20 dpi. PCV2-inoculated pigs developed lethargy and sporadic icterus, and 8/19 (42%) developed exudative epidermitis; mortality was 5/19 (26%). PRRSV-inoculated pigs developed dyspnea and mild lethargy that resolved by 28 dpi. Microscopic lesions consistent with postweaning multisystemic wasting syndrome (PMWS) were present in both PCV2- and PCV2/PRRSV-inoculated pigs and included lymphoid depletion, necrotizing hepatitis, mild necrotizing bronchiolitis, and infiltrates of macrophages that occasionally contained basophilic intracytoplasmic inclusion bodies in lymphoid and other tissues. PCV2/ PRRSV-inoculated pigs also had severe proliferative interstitial pneumonia and more consistent hepatic lesions. The most severe lesions contained the greatest number of PCV2 antigen-containing cells. PRRSV-inoculated pigs had moderate proliferative interstitial pneumonia but did not develop bronchiolar or hepatic lesions or lymphoid depletion. All groups remained seronegative to porcine parvovirus. The results indicate that 1) PCV2 coinfection increases the severity of PRRSV-induced interstitial pneumonia in CD/CD pigs and 2) PCV2 but not PRRSV induces the lymphoid depletion, granulomatous inflammation, and necrotizing hepatitis characteristic of PMWS.     

Characterization of homologous and heterologous adaptive immune responses in porcine reproductive and respiratory syndrome virus infection

The present study characterized the homologous and heterologous immune response in type-I porcine reproductive and respiratory syndrome virus (PRRSV) infection. Two experiments were conducted: in experiment 1, eight pigs were inoculated with PRRSV strain 3262 and 84 days post-inoculation (dpi) they were challenged with either strain 3262 or strain 3267 and followed for the next 14 days (98 dpi). In experiment 2, eight pigs were inoculated with strain 3267 and challenged at 84 dpi as above. Clinical course, viremia, humoral response (neutralizing and non-neutralizing antibodies, NA) and virus-specific IFN-γ responses (ELISPOT) were evaluated all throughout the study. Serum levels of IL-1, IL-6, IL-8, TNF-α and TGF-β were determined (ELISA) after the second challenge. In experiment 1 primo-inoculation with strain 3262 induced viremia of ≤ 28 days, low titres of homologous NA but strong IFN-γ responses. In contrast, strain 3267 induced longer viremias (up to 56 days), higher NA titres (≤ 6 log₂) and lower IFN-γ responses. Inoculation with 3267 produced higher serum IL-8 levels. After the re-challenge at 84 dpi, pigs in experiment 1 developed mostly a one week viremia regardless of the strain used. In experiment 2, neither the homologous nor the heterologous challenge resulted in detectable viremia although PRRSV was present in tonsils of some animals. Homologous re-inoculation with 3267 produced elevated TGF-β levels in serum for 7–14 days but this did not occur with the heterologous re-inoculation. In conclusion, inoculation with different PRRSV strains result in different virological and immunological outcomes and in different degrees of homologous and heterologous protection.     

猪繁殖与呼吸综合征病毒和致病性沙门氏菌的混合感染

2003年9～11月某猪场发生母猪持续性繁殖障碍和7日龄仔猪呼吸困难、寒颤、下痢、体温升高和实质性器官出血为主要特征的疾病。3头发病仔猪的病变组织用RT-PCR检测。均为猪繁殖与呼吸综合征病毒(PRRSV)阳性；由3头发病猪病料中所分离的细菌。可于普通琼脂、伊红美蓝和麦康凯琼脂培养基上生长，革兰氏染色阴性，形态与组织触片镜检及生化反应特性与沙门氏菌一致，可致死小鼠。仅对头孢唑啉呈中度敏感。结合该病的流行病学调查、临床症状、病理剖检，确诊为猪繁殖与呼吸综合征病毒(PRRSV)和致病性沙门氏菌混合感染。     

Antibody production and blastogenic response in pigs experimentally infected with porcine reproductive and respiratory syndrome virus.

Seven five-week piglets were infected intranasally with 10(5) TCID50 of porcine reproductive and respiratory syndrome (PRRS) virus strain IAF.exp91. All virus-exposed pigs developed fever, labored abdominal breathing, conjunctivitis, and lymph node enlargement within the first 96 h postexposure (PE), which continued to d 10 to 14 PE. Two pigs that were necropsied at d 7 and 10 PE had diffuse interstitial pneumonitis, cardiopathy and lymphadenopathy. All 5 remaining pigs produced serum IgM and IgG antibodies against PRRS virus by 7 or 14 days PE, as demonstrated by indirect immunofluorescence. This corresponded with the capability of isolating the virus from serum d 7 to d 49 or d 63 PE. Low serum neutralizing antibody titers were detected in 3 of the virus-exposed pigs by 35 days PE. A transient episode of diminished proliferative response of peripheral blood lymphocytes to mitogens phytohemagglutinin (PHA) and concanavalin A (Con A) was observed in the virus-exposed pigs at d 3 PE. However, in vitro spontaneous uptake of [3H]-thymidine was significantly increased in lymphocyte cultures of the same pigs at d 7 or d 14 PE. These results suggest polyclonal activation of peripheral blood lymphocytes.     

Differential immunity in pigs with high and low responses to porcine reproductive and respiratory syndrome virus infection

One hundred Hampshire x Duroc cross-bred pigs (HD) and 100 NE Index line (I) pigs were infected with porcine reproductive and respiratory syndrome (PRRS) virus and evaluated for resistance/susceptibility. Controls (100/line) were uninfected littermates to the infected pigs. Viremia, change in weight (WTdelta), and rectal temperature at 0, 4, 7, and 14 d postinfection were recorded. Lung, bronchial lymph node (BLN), and blood tissue were collected at necropsy (14 d postinfection). The first principal component from principal component analyses of all variables was used to rank the pigs for phenotypic response to PRRS virus. Low responders (low PRRS burden) had high WTdelta, low viremia, and few lung lesions; high responders (high PRRS burden) had low WTdelta, high viremia, and many lesions. The RNA was extracted from lung and BLN tissue of the 7 highest and 7 lowest responders per line and from each of their littermates. Expression of 11 innate and T helper 1 immune markers was evaluated with cDNA in a 2 x 2 x 2 factorial design. Significant upregulation in lung, lymph, or both of infected pigs relative to controls occurred for all but one gene. Expression differences were greater in HD than I pigs. Significant downregulation for certain immune genes in low pigs, relative to littermate controls, was detected in lung and BLN, particularly in line I. Serum levels of the immune cytokines affirmed the gene expression differences. High preinfection serum levels of IL 8 were significantly associated with PRRS virus-resistant, low pigs. After infection, low expression of interferon gamma in cDNA and in serum was also correlated with PRRS virus resistance. Important genetic associations were revealed for fine mapping of candidate genes for PRRS virus resistance and determining the causative alleles.     

Analysis of constitutive cytokine expression by pigs infected in-utero with porcine reproductive and respiratory syndrome virus

To investigate cytokine alterations in pigs infected in-utero with porcine reproductive and respiratory syndrome virus (PRRSV), constitutive mRNA expression by peripheral blood mononuclear cells (PBMCs) was measured. PBMC from in-utero PRRSV-infected pigs displayed significantly increased IL-6, IL-10, and IFN-gamma mRNA expression at 0 and 14 days of age compared with age-matched control pigs. There were no significant differences in IL-2, IL-4, and IL-12 mRNA expression between in-utero PRRSV-infected and control pigs. However, the IL-10/IL-12 ratio was significantly increased in in-utero PRRSV-infected pigs at 0 and 14 days of age, suggesting the imbalance of IL-10 and IL-12 mRNA production. The abnormal mRNA expression of cytokines in in-utero PRRSV-infected pigs occurred concurrently with a significant decrease in the CD4(+)/CD8(+) T-cell ratio in peripheral blood. PRRSV was not isolated from the sera of pigs at 9 weeks of age that had been viremic at 0 and 14 days old. Delayed type hypersensitivity (DTH) responses to Tuberculin and analysis of cytokine mRNA expression by PBMC showed that cell-mediated immune response and cytokine message profiles in pigs infected in-utero with PRRSV had returned to levels similar to those of control pigs by 9 weeks of age. We conclude that in-utero infection with PRRSV results in significant alteration of cytokine mRNA expression that may cause transient immunomodulation. However, at 10 weeks of age the pigs' immune responses seemed to recover. This may help to understand the immunopathogenesis of in-utero PRRSV infection and the increased susceptibility to secondary bacterial pathogens in neonatal piglets.     

Biological responses to porcine respiratory and reproductive syndrome virus in pigs of two genetic populations

One hundred pigs from the NE Index Line (NEI) and 100 Hampshire-Duroc cross pigs (HD) were inoculated intranasally with porcine respiratory and reproductive syndrome virus (PRRSV 97-7895 strain) at 26 d of age to determine whether genetic variation in response to PRRSV exists. An uninfected littermate to each infected pig served as a control. Pigs were from 163 dams and 83 sires. Body weight and rectal temperature were recorded, and blood samples were drawn from each pig on d 0 before inoculation and on d 4, 7, and 14 after inoculation. Pigs were sacrificed on d 14. Lung and bronchial lymph nodes were collected, placed in optimal cutting temperature compound, and frozen at -80 degrees C. The presence of PRRSV in serum and in lung tissue and bronchial lymph nodes was determined by isolation in cell culture. The presence of antibodies in serum collected on d 14 was determined by a commercial ELISA test. Lung tissue was examined microscopically and scored for incidence and severity of lesions (score of 1 to 3; 1 = no or few lesions, and 3 = severe interstitial pneumonia). Data were analyzed with a mixed model that included random sire and dam effects. The interaction of line x treatment was significant (P < 0.001) for weight change and rectal temperature. Un-infected HD pigs gained 0.67 kg more from d 0 to 14 and averaged 0.32 degrees C higher rectal temperature than uninfected NEI pigs (P < 0.001), whereas infected NEI pigs gained 0.34 kg more and had -0.54 degrees C lower temperature than infected HD pigs (P < 0.001). Viremic titer (cell culture infectious dose 50%/mL) was greater (P < 0.05) in HD than NEI at d 4 (10(4.52) vs. 10(4.22)), 7 (10(4.47) vs. 10(3.99)), and 14 (10(3.49) vs. 10(3.23)). Viral titer loads in lung (P = 0.11) and bronchial lymph nodes tended (P = 0.07) to be greater in HD than NEI pigs. Antibody signal-to-positive (S/P) ELISA ratios in infected pigs ranged from 0.18 to 3.38, and 88% had levels > or = 0.40, which is the positive threshold for this ELISA. The S/P range in uninfected pigs was 0 to 1.11, and 99% had levels < or = 0.40. Mean S/P ratio for infected pigs was 0.23 units higher in HD than in NEI (P < 0.001). The HD pigs had a greater incidence of interstitial pneumonia and 0.65 higher mean lesion scores than NEI pigs (P < 0.001). In summary, responses of pigs of the two lines to infection with PRRSV differed, indicating that underlying genetic variation existed.     

Experimental airborne transmission of porcine reproductive and respiratory syndrome virus and Bordetella bronchiseptica

Experiments were designed to determine if porcine reproductive and respiratory syndrome virus (PRRSV) or Bordetella bronchiseptica could be transmitted through indirect airborne contact. Three principal pigs were infected with PRRSV, B. bronchiseptica or both. Five days after the principal pigs were challenged, the three principal pigs and one direct-contact pig were placed into one isolation tent together, and three indirect-contact pigs were placed into another isolation tent which received its air supply from the first isolation tent. Airborne transmission of B. bronchiseptica occurred in 5/5 trials where B. bronchiseptica was the only agent used, and in 3/5 trials where the principal pigs were coinfected with both agents. Airborne transmission of PRRSV occurred in 4/5 trials where PRRSV was the only agent used, and in 2/5 trials where the principal pigs were coinfected with both agents. Thus, airborne transmission of both agents over short distances, such as within a barn, is probable.     

Commercial spray-dried porcine plasma does not transmit porcine circovirus type 2 in weaned pigs challenged with porcine reproductive and respiratory syndrome virus

The objective of this study was to evaluate if spray dried porcine plasma (SDPP) containing porcine circovirus type 2 (PCV2) genome supplemented in feed could transmit PCV2 to pigs challenged with porcine reproductive and respiratory syndrome virus (PRRSV). Twenty-three PRRSV-free pigs, non-viraemic for PCV2, were housed in bio-safety level 3 facilities and assigned to four groups in a 2×2 factorial design consisting of PRRSV challenge and a negative control. The diet contained 0 or 8kg SDPP per 100kg of feed. PRRSV challenge groups were inoculated intranasally with 2mL of a suspension containing 10(6) TCID(50)/mL PRRSV. The SDPP used in the study contained 7.56×10(5) PCV2 genome copies per gram. Dietary treatments were fed from 4days prior to PRRSV inoculation until 28days post-inoculation (PI). All challenged pigs developed PRRSV viraemia by day 3PI and PRRSV antibodies were detected in sera by day 14PI, with no difference between diet treatments. Neither PRRSV viraemia nor seroconversion was observed in non-challenged pigs. PCV2 was not detected in the serum of any pigs throughout the experimental period. SDPP containing the PCV2 genome supplemented in feed did not result in PCV2 transmission to either healthy or PRRSV-infected pigs under these experimental conditions.     

Immunopathogenesis of porcine reproductive and respiratory syndrome in the respiratory tract of pigs

Porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) impairs local pulmonary immune responses by damaging the mucociliary transport system, impairing the function of porcine alveolar macrophages and inducing apoptosis of immune cells. An imbalance between pro- and anti-inflammatory cytokines, including tumour necrosis factor-α and interleukin-10, in PRRS may impair the immune response of the lung. Pulmonary macrophage subpopulations have a range of susceptibilities to different PRRSV strains and different capacities to express cytokines. Infection with PRRSV decreases the bactericidal activity of macrophages, which increases susceptibility to secondary bacterial infections. PRRSV infection is associated with an increase in concentrations of haptoglobin, which may interact with the virus receptor (CD163) and induce the synthesis of anti-inflammatory mediators. The balance between pro- and anti-inflammatory cytokines modulates the expression of CD163, which may affect the pathogenicity and replication of the virus in different tissues. With the emergence of highly pathogenic PRRSV, there is a need for more information on the immunopathogenesis of different strains of PRRS, particularly to develop more effective vaccines.     

PRRSV感染猪体内多杀性巴氏杆菌的分离与血清型鉴定

应用常规方法和PCR技术,对来自安徽肥西、庐江、肥东、定远、桐城5个地区猪场检测为猪繁殖与呼吸综合征病毒(PRRSV)阳性的肺脏进行多杀性巴氏杆菌(Pm)的分离及其血清型鉴定。结果显示,49份病料中分离鉴定出7株Pm,且均属于A型,分离率为14.3%,高于其他细菌(副猪嗜血杆菌、猪链球菌)的检出率。表明猪繁殖与呼吸综合征(PRRS)患猪继发或混合感染Pm的比率较高,而且A型Pm是安徽省感染猪群中的主要血清型。     

Oral transmission of porcine reproductive and respiratory syndrome virus by muscle of experimentally infected pigs

The current study was performed to determine if porcine reproductive and respiratory syndrome virus (PRRSV) could be transmitted to pigs by feeding muscle tissue obtained from recently infected pigs. Muscle obtained from pigs infected with either a European strain (EU donor pigs) or American strain (US donor pigs) of PRRSV was fed to PRRSV-free receiver pigs. The donor pigs were slaughtered 11 days post-infection (dpi). PRRSV was detected by conventional virus isolation in muscle at 11 dpi from 7 of 12 EU donor pigs and 5 of 12 US donor pigs. In contrast to conventional virus isolation, all muscle samples from infected pigs were positive for viral nucleic acid by PCR, except for muscle from one animal infected with the American strain of PRRSV. Five hundred grams of raw semimembranosus muscle from each of the donor pigs was fed over a 2 days period (250 g per day) to each of two receiver pigs (48 receiver pigs). The receiver pigs were housed separately in five groups. One of the five groups was fed muscle obtained from US donor pigs that was also spiked with the American strain of PRRSV. Sentinel pigs were placed in-contact with the group of receiver pigs fed spiked muscle. All receiver pigs became viraemic by 6 days post-feeding (dpf). There was evidence of horizontal transmission with sentinel pigs, in-contact with receiver pigs, becoming viraemic. The study demonstrates that PRRSV could be infectious through the oral route via the feeding of meat obtained from recently infected pigs.     

猪繁殖与呼吸综合征病毒的研究进展

猪繁殖与呼吸综合征 (Porcine reproductiveand respiratory syndrome,PRRS)是 1 987年发现的一种新的猪病 ,它主要是引起母猪的繁殖障碍 ,如流产、早产、死胎、木乃伊胎和呼吸道症状的一种由猪繁殖与呼吸综合征病毒 (PRRSV)引起的接触性传染病。此病最初在美国首先被发现 ,随后相继在世界各国报道 ,我国台湾省也有疫情的报道。 1 995年我国首次从进口猪中检出 PRRS阳性猪 ,并从中分离到 PRRSV。 1 996年郭宝清等首次从国内 PRRS血清阳性猪群中分离到 PRRSV,从而证实了我国也有此病的流行[1] 。由于 PRRS主要是引起母猪的繁殖…