Evidence for divergence of restriction fragment length polymorphism patterns following in vivo replication of porcine reproductive and respiratory syndrome virus

OBJECTIVE: To determine stability of the restriction fragment length polymorphism (RFLP) pattern of a porcine reproductive and respiratory syndrome vaccine virus and patterns of other viral strains as they replicate in pigs. SAMPLE POPULATION: Field samples of porcine reproductive and respiratory syndrome virus (PRRSV) and samples from 2 weaned pigs, 2 nursery-age pigs, and 5 gilts experimentally infected with PRRSV. PROCEDURE: PRRSV was isolated from field samples, experimentally infected pigs, or pigs that were in contact with experimentally infected pigs. For each virus, RNA was isolated from infected cells, and RFLP patterns were determined. RESULTS: 61% of field samples had 2-5-2 RFLP patterns characteristic of the vaccine virus, 32% had field virus RFLP patterns, and 7% had intermediate RFLP patterns that indicated a virus with a close relationship to the vaccine virus. Viruses isolated from experimentally infected pigs had no change in RFLP patterns after up to 13 weeks of in vivo replication and transmission to contact pigs. CONCLUSIONS AND CLINICAL RELEVANCE: RFLP patterns distinguish the vaccine and field strains of PRRSV; however, as the vaccine virus spreads among a swine population, the RFLP pattern can change to a related intermediate pattern. A glycine at residue 151 of open reading frame 5 is another marker for the vaccine virus; this glycine is rapidly lost and eventually replaced with arginine as the vaccine virus replicates in pigs.     

Comparative safety and efficacy of attenuated single-strain and multi-strain vaccines for porcine reproductive and respiratory syndrome

The objective of this study was to compare the efficacy and safety of single-strain and multi-strain vaccines for the prevention of the respiratory facet of porcine reproductive and respiratory syndrome. The study comprised six groups of pigs (A through F, eight pigs per group). At the beginning of the study (Day 0) Groups C and D were vaccinated with a single-strain vaccine, and Groups E and F were vaccinated with a multi-strain vaccine. The multi-strain vaccine contained five attenuated strains of PRRSV including the strain used as the single-strain vaccine. On Day 28 Groups B (nonvaccinated/challenged control), D, and F were challenged with a highly virulent field strain of PRRSV that was unrelated to any of the strains used for vaccination. Group A was kept as a nonvaccinated/nonchallenged control. On Day 42 all pigs were necropsied. Their lungs and lymph nodes were examined for virus-induced changes. Serum samples obtained at weekly intervals during the study and lung lavage fluids obtained at necropsy were tested for the presence and titer of PRRSV. Serum samples were also tested for antibody. The presence and severity of clinical signs and lesions were the primary means by which vaccine efficacy and safety were evaluated. Both vaccines provided a high level of protective immunity to challenge. However, appreciable lymph node enlargement in pigs vaccinated with multi-strain vaccine, with or without subsequent challenge, raised a question as to its safety. Collectively these results indicate that both single-strain and multi-strain attenuated PRRSV vaccines can be effective immunogens, but additional studies in regard to safety are needed before multi-strain vaccines can be recommended for routine field use.     

Comparative evaluation of PRRS virus infection in vaccinated and naïve pigs

The purpose of this study was to evaluate the time-course of the immune response to a field Porcine Respiratory and Reproductive Syndrome virus (PRRSV) strain in PRRS-naïve, untreated pigs, as well as in four groups of age and breed-matched pigs injected with a live attenuated PRRS vaccine, its adjuvant, an inactivated PRRS vaccine and an irrelevant, inactivated Porcine Circovirus type 2 (PCV2) vaccine, respectively. PRRSV infection was confirmed in all groups by PCR and antibody assays. The antibody response measured by ELISA took place earlier in pigs injected with the live attenuated vaccine, which also developed a much stronger serum-neutralizing antibody response to the vaccine strain. Yet, no clear protection was evidenced in terms of viremia against the field virus strain, which showed 11.1% nucleotide divergence in ORF7 from the vaccine strain. In vitro, the interferon (IFN)-γ response to PRRSV was almost absent on PVD 60 in all groups under study, whereas the prevalence of interleukin (IL)-10 responses to PRRSV was fairly high in PCV2-vaccinated animals, only. Results indicate that distinct patterns of immune response to a field PRRSV strain can be recognized in PRRS-vaccinated and naïve pigs, which probably underlies fundamental differences in the development and differentiation of PRRSV-specific immune effector cells.     

Effects of challenge with a virulent genotype II strain of porcine reproductive and respiratory syndrome virus on piglets vaccinated with an attenuated genotype I strain vaccine

Porcine reproductive and respiratory syndrome virus (PRRSV) is endemic in most parts of Asia, where genotype I and II strains of diverse virulence may coexist. This study evaluated the outcome of infection with a highly virulent Asian genotype II PRRSV isolate in piglets vaccinated with a genotype I vaccine. Twenty-one 3-week-old piglets were divided in three groups: Pigs in group V (n=8) were vaccinated with an attenuated genotype I commercial PRRSV vaccine, while pigs in group U (n=8) and a control group (group C; n=5) were unvaccinated; 6 weeks later, pigs in groups V and U were challenged intranasally with a highly virulent strain of genotype II PRRSV (1×10(5) 50% tissue culture infectious doses/mL), while pigs in group C received a placebo. Over a period of 21 days after challenge, vaccinated pigs had significantly lower mortality (0/8 versus 2/8), fewer days of fever, a lower frequency of catarrhal bronchopneumonia, higher weight gains (13.4 versus 6.6 kg) and lower levels of viraemia compared to unvaccinated challenged pigs. Immunisation with a genotype I attenuated PRRSV vaccine provided partial protection against challenge with a highly virulent genotype II strain.     

猪繁殖与呼吸综合征病毒与副猪嗜血杆菌混合感染的诊断

为确诊广东省阳江市某规模化猪场（存栏800头母猪）保育猪发病死亡的原因,本试验对从该发病猪场采集的3份肺脏、肝脏、脾脏临床样品进行细菌学检测及药敏试验,采用PCR/RT-PCR检测临床样品中猪伪狂犬病病毒（PRV）、猪瘟病毒（CSFV）、猪繁殖与呼吸综合征病毒（PRRSV）、猪圆环病毒2型（PCV2）、猪圆环病毒3型（PCV3）和猪肺炎支原体等病原。对特异性扩增的3株PRRSV的ORF5基因产物进行序列测定,与VR2332、HuN4、JXA1、CH-1a等代表毒株进行核苷酸序列同源性分析,并构建系统进化树。结果表明,试验分离鉴定出1株副猪嗜血杆菌（Hps）,对7种临床常用药如阿莫西林、头孢拉定等均有较强的敏感性。同源性比对结果表明,3株PRRSV （LJW1、LJW2和LJW3）ORF5基因核苷酸同源性为99.3%~99.8%,与欧洲型代表毒株Lelystad核苷酸同源性为64.0%~64.2%,与HP-PRRSV毒株JXA1、HuN4、CH-1a和TJ核苷酸同源性较高,分别为99.2%~99.5%、99.0%~99.3%、94.5%~94.9%和98.8%~99.2%;与中国河南和广西分离的HP-PRRSV毒株HeNzm1-16和GXLZ05-2015核苷酸同源性较高,分别为99.3%~99.7%和99.2%~99.7%,与美洲型经典疫苗株MLV、美洲型标准株NC、美洲型经典株VR2332核苷酸同源性较低,分别为88.5%~88.8%、85.2%~85.5%和82.3%~82.6%。PRRSV ORF5基因系统进化树分析表明,3株PRRSV均属于美洲型毒株,与国内HP-PRRSV代表毒株JXA1、HuN4和TJ等处于同一分支,亲缘关系较近。本研究揭示了该场保育猪发病病原,并从分子水平上明确了分离的3株PRRSV与不同代表毒株的亲缘关系,为弱毒疫苗的合理选择使用和综合防控PRRSV提供了参考依据。     

猪繁殖与呼吸综合征基因标记疫苗株NP49-ELISA鉴别诊断方法的建立

猪繁殖与呼吸综合征病毒（Porcine reproductive and respiratory syndrome virus,PRRSV）rHN4-△25＋NP49株是新型基因标记弱毒疫苗候选株,为了配合对该基因标记疫苗免疫猪血清抗体的鉴别诊断,本研究以标记基因编码的NP49（新城疫病毒NP蛋白C末端49个氨基酸,即NP49）多肽作为包被抗原,建立标记疫苗免疫猪血清中NP49抗体的ELISA鉴别诊断方法。通过对NP49-ELISA工作条件的优化,结果显示NP49-ELISA的多肽抗原最适包被浓度为500ng／孔,被检血清最佳稀释度为1：40。利用ROC曲线法确定S／P临界值为0．2,该方法批内与批间重复试验的变异系数均小于10％,与几种常见猪病阳性血清无交叉反应,具有较好重复性和特异性。本研究建立的NP49．ELISA鉴别诊断方法为猪繁殖与呼吸综合征新型基因标记疫苗的临床应用提供了有力保障。     

Efficacy of vaccination with porcine reproductive and respiratory syndrome virus following challenges with field isolates in Japan

The objective of this study was to evaluate the influences of genetic and antigenic variations in field isolates of porcine reproductive and respiratory syndrome virus (PRRSV) on vaccine efficacy. Four-week-old pigs were vaccinated with a commercial modified live virus vaccine. Four weeks after vaccination, pigs in both the vaccinated group and the non-vaccinated group were challenged intranasally with 10(7) TCID(50) of PRRSV wt-11 (Experiment 1) or PRRSV wt-7 (Experiment 2). Based on genome sequencing of ORF5 and cross neutralization test results, PRRSV wt-11 is similar to the vaccine strain, whereas wt-7 is distinct from the vaccine strain. In the vaccinated challenged groups, clinical signs were less severe, the mean rate of weight gain was greater, and gross lung lesions were less severe when compared with the non-vaccinated challenged groups in both experiments. In Experiment 1, the virus was isolated from serum at 3 days post-challenge, and the mean virus titers in broncho-alveolar lavage fluids (BALF) and tissues were lower in pigs in the vaccinated challenged groups compared with those in the non-vaccinated challenged group. In Experiment 2, virus isolation from serum, BALF and tissues showed no significant differences between the groups. These results suggest that commercial PRRSV vaccine could be effective in reducing clinical disease following a challenge with field isolates of PRRSV. However, with regards to virological protection, the efficacy of the vaccine may be affected by the nature of the PRRSV isolates.     

Immunologic responses and reproductive outcomes following exposure to wild-type or attenuated porcine reproductive and respiratory syndrome virus in swine under field conditions

OBJECTIVE: To compare immunologic responses and reproductive outcomes in sows housed under field conditions following controlled exposure to a wild-type strain of porcine reproductive and respiratory syndrome virus (PRRSV strain WTV) or vaccination with a modified-live virus (MLV) vaccine. DESIGN: Randomized controlled trial. ANIMALS: 30 PRRSV-na?ve 10-week-old female pigs. PROCEDURE: Humoral and cell-mediated immune responses were monitored while pigs were held in isolation for 84 days after inoculation with the WTV strain (n = 10), inoculation with the WTV strain and 42 days later vaccination with a killed-virus vaccine (10), or vaccination with an MLV vaccine (10). Reproductive outcomes were measured after pigs were released into the farm herd. RESULTS: Inoculation with the WTV strain, regardless of whether a killed-virus vaccine was subsequently administered, elicited faster and more substantial production of strain-specific neutralizing antibodies, as well as a more rapid generation of interferon-gamma secreting cells, than did vaccination with the MLV vaccine. Despite the enhanced immune responses in pigs inoculated with the WTV strain, animals vaccinated with the MLV vaccine produced a mean of 2.45 more pigs than did sows exposed to the WTV strain, mainly because of a lower rate for failure to conceive. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that current assays of immunity to PRRSV correlate only imperfectly with degree of clinical protection and that the practice of controlled exposure of sows to a circulating PRRSV strain should be reconsidered in light of negative clinical outcomes.     

PRRS流行毒株与减毒活疫苗TJM F92株qPCR区分方法的建立与应用

 
为快速准确区分PRRS流行毒株与减毒活疫苗TJM F92株以及对流行毒株进行定量检测,按GenBank中发表的美洲型PRRSV SX 1分离株、弱毒疫苗株NSP2基因缺失部位的不同设计了一对特异性引物,通过RT PCR和体外转录的方法,构建了体外转录RNA作为标准品,并对反应条件和反应体系进行优化,旨在建立一种敏感性高、特异性强、重复性和稳定性良好的qPCR鉴别方法。结果表明,该方法最低能检测出1.0×101拷贝/μL的模板,敏感性比常规PCR高100倍;应用该方法对218份临床样本进行鉴别与定量,qPCR检出率较常规RT PCR高12.9个百分点。研究表明,qPCR鉴别方法的建立实现了对PRRS流行毒株与疫苗毒株的快速区分以及对流行毒株的定量检测,为PRRS的快速诊断提供了依据。     

Evaluation of the presence of porcine reproductive and respiratory syndrome virus in pig meat and experimental transmission following oral exposure

A study was performed to evaluate the presence of porcine reproductive and respiratory syndrome virus (PRRSV) in pig meat collected at slaughterhouses and its potential transmission to pigs via pig meat. A total of 1039 blood samples were collected from pigs upon their arrival at the abattoir. The following day, meat samples (n = 1027) were collected from the carcasses of these same pigs. Samples originated from 2 Canadian slaughterhouses, 1 situated in the province of Quebec and the other situated in the province of Manitoba. Serum samples were tested for antibodies to PRRSV and both serum and meat samples were also tested for PRRSV nucleic acid by polymerase chain reaction (PCR). Seropositivity to PRRSV for all serum samples was 74.3%. Furthermore 45 (4.3%) of the total serum samples and 19 (1.9%) of the 1027 meat samples were positive for PRRSV by PCR. Sequence analysis of open reading frame (ORF) 5 performed on 15 of the 19 PRRSV strains identified in pig meat indicated that 9 were field strains and 6 were vaccine-like (98% to 99.7% nucleotide homology with the Ingelvac RespPRRS/Repro vaccine). One of these 6 strains presented an intermediate 2-6-2 restriction fragment length polymorphism (RFLP) cut pattern and the others showed the characteristic 2-5-2 RFLP pattern of the vaccine strain. All strains sequenced were determined to be North American strains. In only 1 of the 19 PRRSV-positive meat samples could PRRSV be isolated. To test the potential infectivity of meat samples containing residual PRRSV, 11 of the PCR-positive meat samples (weighing 1.05 to 1.8 kg) were each used in feeding experiments of 2 PRRSV antibody-negative specific pathogen-free pigs of 9 wk of age. Samples were cut into several pieces and fed to each pair of pigs on 2 consecutive days. Each pig pair was housed in a separate cubicle and serum samples were collected at -7, 0, 7, 14, and 20 to 21 days post exposure. Seven pig pairs were found to be infected by PRRSV following ingestion of meat samples, including meat samples containing vaccine-like virus, as judged by the demonstration of PRRSV antibodies and/or PRRSV nucleic acid in the serum. In summary, the present study indicated that low residual quantities of PRRSV may be found in a small percentage of pig meat collected at slaugtherhouses. Furthermore, when this meat was fed raw to pigs in the experimental setting designed, pigs could be infected by PRRSV.     

Experimental inoculation of late term pregnant sows with a field isolate of porcine reproductive and respiratory syndrome vaccine-derived virus.

The use of a live attenuated porcine reproductive and respiratory syndrome virus (PRRSV) vaccine in piglets has been associated with reproductive disorders in non-vaccinated sows. Vaccine-derived virus (VDV) has been isolated from foetuses, stillborn pigs, and dead piglets, indicating that the live vaccine spread from vaccinated piglets to non-vaccinated sows, and that the virus might be implicated in the severe reproductive problems observed. In the present study, one such VDV isolate was used to experimentally infect pregnant sows in the last trimester. The chosen isolate, which had more than 99.6% identity to the attenuated vaccine virus, originated from the lungs of a stillborn pig from a swine herd with a sudden high level of stillborn pigs and increased piglet mortality in the nursing period. Intranasal inoculation of sows with the virus isolate resulted in congenital infection, foetal death, and preweaning pig mortality. As such, the present study showed that vaccine-derived PRRSV can cause disease in swine consistent with PRRS.     

Failure of porcine reproductive and respiratory syndrome virus to replicate in porcine endothelial cell cultures.

Clinical, gross, and microscopic pathologic and immunohistochemical findings in pigs infected with porcine reproductive and respiratory syndrome virus (PRRSV) suggest that PRRSV may replicate in endothelial cells. Endothelial cell cultures from porcine aorta and pulmonary artery were tested for susceptibility to various strains of PRRSV. Cultures were identified as endothelium by light microscopy and immunohistochemical staining for P-selectin and von Willebrand factor. Five strains of PRRSV, i.e., the National Veterinary Services Laboratories, Ames, IA PRRSV strain 130-PDV and 4 field strains isolated from pneumonic lungs, failed to replicate in these porcine large-vessel endothelial cell cultures.     

猪瘟兔化弱毒疫苗与我国近年猪瘟野毒的免疫保护相关性试验

２批猪瘟兔化毒睾丸细胞苗分别来自两家兽医生物药品厂，各分别免疫接种猪只，随后以５株猪瘟病毒野毒分别攻击，疫苗接种猪完全存活，对照猪完全死亡。此５株野毒分离自国内不同地区，并皆已经过细致的鉴定，以免疫荧光技术对猪扁桃体进行了活体检查，各对照猪材料上能在攻击后一周时出现病毒，而疫苗接种猪在整个观察期间未出现病毒，据此，显然可见猪瘟兔化毒细胞苗在预防国内猪瘟上极有效力     

Strain specificity of the immune response of pigs following vaccination with various strains of porcine reproductive and respiratory syndrome virus

The primary objective of the study was to determine strain specificity of the immune response of pigs following vaccination with selected strains of porcine reproductive and respiratory syndrome virus (PRRSV). The experimental design included five groups (I through V, six pigs per group) free of antibody for PRRSV at the beginning of the experiment (day 0). On day 0, groups III, IV, and V were vaccinated with attenuated versions of PRRSV strains 8, 9, and 14, respectively. On day 21, the immunity of group II (non-vaccinated/challenged controls) and groups III, IV, and V was challenged by exposing each pig to a composite of the virulent versions of these same three strains. On day 35, all pigs, including non-vaccinated/non-challenged pigs of group I, were necropsied. Lungs and selected lymph nodes were examined for lesions. Serum samples obtained at weekly intervals throughout the study and lung lavage fluids obtained at necropsy were tested for the presence of PRRSV and its strain identity. Before challenge the strain of PRRSV identified in the sera of vaccinated pigs was always that with which the particular pig had been vaccinated (i.e. homologous strain), whereas, with one exception, only heterologous strains were identified after challenge. This apparent strain exclusion as a result of vaccination was statistically significant (P = 0.004). The tendency for heterologous strains to predominate after challenge suggests that a pig's immune response to PRRSV has some degree of strain specificity. Whether this finding has any clinical relevance in regard to immunoprophylaxis remains to be determined.     

Comparative genomic analysis of five pairs of virulent parental/attenuated vaccine strains of PRRSV

Porcine reproductive and respiratory syndrome virus (PRRSV), the causative agent of porcine reproductive and respiratory syndrome, is responsible for serious disease in pigs resulting in substantial economic losses in the porcine industry. An attenuated vaccine strain, HuN4-F112, was obtained by passaging virulent PRRSV strain HuN4 on Marc-145 cells (for 112 passages), and the full-genomic sequence was determined. To understand the molecular basis of attenuation of PRRSV, we compared and analyzed the genomic sequences of HuN4/HuN4-F112, together with those of other four virulent parental/attenuated vaccine strains. Among the 19 PRRSV proteins, two (NSP6 and NSP8) were highly conserved, without any mutations and considered irrelative to attenuation. The mutation rates of envelope-associated structural proteins were obviously higher than those of most non-structural proteins. It is interesting that the gene of the smallest structural protein, E protein, had the highest mutation rate among all of the structural genes analyzed, and also harbored a highly variable region. Our results indicate that determinants of PRRSV attenuation are multigenic products of both non-structural and structural genes. To our knowledge, this is the first report showing that the envelope-associated structural proteins (including E and GP2-GP5 proteins) may play a significant role. These findings contribute towards our understanding of PRRSV attenuation and will provide an important clue for further study.     

2018年广东部分地区猪繁殖与呼吸综合征病毒流行毒株ORF5基因变异分析

为了解广东省猪繁殖与呼吸综合征病毒(PRRSV)流行毒株ORF5基因遗传变异情况,采用RT-PCR对2018年采自广东部分地区疑似患有PRRS的猪肺组织样品进行PRRSV ORF5基因扩增以及克隆测序,并进行生物信息学分析。结果表明,成功扩增出18株PRRSV流行毒株的ORF5基因片段。ORF5基因序列分析表明,18株PRRSV流行毒株ORF5基因核苷酸同源性为83.7%~99.8%,PRRSV流行毒株与参考毒株的同源性为62.1%~99.8%。基于ORF5基因的遗传进化树分析表明,18株PRRSV流行株均为美洲型毒株。其中,10株与以JXA1为代表的高致病性毒株亲缘较近,2株与新型高致病性毒株FZ16A相似;1株与以NT1为代表的疫苗返强毒株亲缘较近,1株与以R98为代表的疫苗毒株亲缘性较近,4株与广东新报道的GM2和QYYZ毒株亲缘性较近。DNA推导氨基酸序列分析表明,18株流行株的氨基酸序列与国内已报道的代表株相比发生不同程度的变异,GP5抗原表位上存在着差异。研究结果揭示了广东地区PRRSV有新型强毒株、重组毒株以及疫苗返强毒株的流行,提示养殖者谨慎、合理使用疫苗,防止疫苗毒株返强和毒株重组,为该地区防控PRRS提供参考。     

Heterogeneity of porcine reproductive and respiratory syndrome virus: implications for current vaccine efficacy and future vaccine development

Porcine reproductive and respiratory syndrome virus (PRRSV) continues to be a major problem to the pork industry worldwide. Increasing data indicate that PRRSV strains differ in virulence in infected pigs and are biologically, antigenically, and genetically heterogeneous. It is evident that the current vaccines, based on a single PRRSV strain, are not effective in protecting against infections with the genetically diverse field strains of PRRSV. The recent outbreaks of atypical or acute PRRS in vaccinated pigs have raised a serious concern about the efficacy of the current vaccines and provided the impetus for developing more effective vaccines. Special attention in this review is given to published work on antigenic, pathogenic and genetic variations of PRRSV and its potential implications for vaccine efficacy and development. Although there are ample data documenting the heterogeneous nature of PRRSV strains, information regarding how the heterogeneity is generated and what clinical impact it may have is very scarce. The observed heterogeneity will likely pose a major obstacle for effective prevention and control of PRRS. There remains an urgent need for fundamental research on this virus to understand the basic biology and the mechanism of heterogeneity and pathogenesis of PRRSV.     

Similarity of European porcine reproductive and respiratory syndrome virus strains to vaccine strain is not necessarily predictive of the degree of protective immunity conferred

The objective of this study was to determine the degree of protection conferred by a Lelystad-like modified live virus (MLV) vaccine against a heterologous wild-type porcine reproductive and respiratory syndrome virus (PRRSV) isolate of the same cluster. For this purpose, fourteen 3-week-old piglets were divided into three groups: Group A pigs were vaccinated with a modified live virus vaccine, Group B pigs were used as positive controls, and Group C pigs as negative controls. Twenty-eight days after the last dose of vaccine, all pigs in Groups A and B were inoculated with the Spanish PRRSV strain 5710. To evaluate efficacy, clinical signs were recorded and the presence of challenge virus was determined by virus isolation in blood samples and nasal swabs collected at various time points post-challenge (p.c.) and in tissue samples collected at necropsy 24 days p.c. After challenge, moderate clinical signs were observed in pigs from Groups A and B. In addition, all vaccinated pigs were viremic at least once, although viremia tended to be more sporadic in this group than in Group B pigs. PRRSV was detected in at least one tissue sample from four out of five pigs from Group A and in all pigs from Group B. The results indicate that the protection conferred by the MLV vaccine used in this study against a closely related virulent strain was only partial. The findings suggest that the degree of genetic homology of ORF5 between MLV vaccine and challenge isolate is not a good predictor of vaccine efficacy.     

高致病性PRRS活疫苗(HuN4-F112株)抗体对Ⅱ型不同亚群PRRSV的中和效果

为研究高致病性猪繁殖与呼吸综合征病毒(HP-PRRSV)活疫苗(HuN4-F112株)诱导的抗体对II型不同亚群PRRSV的中和作用,本研究将10头PRRSV抗原、抗体阴性的6周龄仔猪,每头仔猪肌肉注射1头份疫苗(106.0TCID50/mL),每周采血并分离血清,检测该血清对II型PRRSV第1、2、4亚群的代表病毒株勃林格PRRSV活疫苗(VR-2332株)、PRRSV活疫苗(CH-1R株)和HP-PRRSV活疫苗(HuN4-F112株)的中和效价。实验结果显示,仔猪免疫3周后开始产生针对HuN4-F112的中和抗体,11周至22周抗体水平达到高峰,抗体持续至少25周,但只有少数免疫猪在几个时间点的血清对VR-2332和CH-1R疫苗株具有血清交叉中和作用,而且中和效价较低。