Risk factors for infection of sow herds with porcine reproductive and respiratory syndrome (PRRS) virus

In 1992, the porcine reproductive and respiratory syndrome virus (PRRSV) of European type (PRRSV-EU) was introduced in Denmark. By 1996, the virus had spread to approximately 25% of the Danish herds. In January 1996, a modified-live vaccine based on the American type of the virus (PRRSV-US) was used in replacement boars for Danish artificial insemination (AI) centres and from July 1996, the vaccine was used in PRRSV-EU infected herds for prevention of disease. Soon after vaccine introduction, PRRSV non-infected herds experienced outbreaks of disease due to infection with PRRSV-US. In this study, we investigated the risk factors (biosecurity level, animals, exposure from PRRSV-US-infected neighbour herds, semen, herd size, pig density and herd density) for infection with PRRSV-US in a cohort of 1071 sow herds; we used a nested case-control study. The retrospective observation period lasted from June 1996 (when they all were non-infected) to October 1997. Seventy-three non-vaccinated, closed sow herds became infected with the vaccine strain during this period. Each case herd was matched with two control herds from the cohort (controls had not been infected at the time of infection in the case herds). The data were analysed using a Cox-regression model. The hazard of infection increased significantly with exposure from PRRSV-US-infected neighbouring herds, purchase of animals from herds incubating PRRSV-US infection, increasing herd size and purchase of semen from boars at PRRSV-US-infected AI centres. The results are consistent with the modified-live vaccine strain spread to other herds by trade with animals and semen and by neighbour (area) transmission. We suggest that virus spread by aerosols was a frequent mode of transmission.     

猪繁殖和呼吸综合征油乳剂灭活苗的研制

使用猪场自行分离的PRRSV,进行细胞转瓶培养,收获病毒液后灭活,乳化制成油乳苗,各项指标检测均合格,并且免疫试验效果较好.     

猪繁殖与呼吸综合征的血清学调查

猪繁殖与呼吸综合征 ( PRRS)又称蓝耳病或神秘病 ,是近年来国际上最引人注目的一种新猪病 ,病原为猪繁殖呼吸综合征病毒 ( PRRSV)。本病于1 987年最早在美国暴发 ,1 990年冬传入欧洲 ,并很快在欧洲各国暴发和流行 ,现在 PRRS已在全世界范围内流行。中国最早于 1 996年检测猪群血清抗体阳性 ,并从可疑病猪群中分离了 PRRS病毒之后 ,有不少省市相继报道了该病 ,但福建省未见报道。PRRS不仅引起呼吸道感染、仔猪死亡、母猪繁殖障碍等直接损失 ,还会导致免疫抑制而继发多种其它疾病 ,给养猪业带来巨大的经济损失。 1 998年下半年以来 ,…     

Sow performance in an endemically porcine reproductive and respiratory syndrome (PRRS)-infected farm after sow vaccination with an attenuated PRRS vaccine

The objective of this field study was to evaluate in an endemically porcine reproductive and respiratory syndrome (PRRS) virus-infected farm the reproductive performance of sows after their vaccination with a PRRS attenuated vaccine. In a farrow-to-finish pig farm with history of endemic PRRS virus infection, a total of 200 gilts and sows were used. They were divided in 2 groups of 100 animals. The first group was used as untreated controls, while the animals of the second group were vaccinated against PRRS virus using the attenuated Porcilis PRRS vaccine (Intervet International, The Netherlands) based on European strain. All health and reproductive parameters were recorded from the time of vaccination up to next weaning. No adverse systemic or local reactions or side effects relative to vaccination were noted. Compared to controls, vaccinated sows showed significantly improved farrowing rate (89% versus 78%) and a tendency for fewer returns to oestrus, particularly those at irregular intervals. Fewer sows farrowed prematurely and showed post-partum dysgalactia syndrome, but more live pigs were born and weaned in each litter after vaccination. It was concluded that vaccination of sows with Porcilis PRRS attenuated vaccine in farms with endemic PRRSV infection has beneficial effects on their health and fertility.     

Isolation of porcine reproductive and respiratory syndrome (PRRS) virus in a Danish swine herd and experimental infection of pregnant gilts with the virus

The first case of porcine reproductive and respiratory syndrome (PRRS) in Denmark was diagnosed in March 1992 by the detection of specific antibodies against PRRS virus in serum samples originating from sows in a herd located on the island of Als. Subsequently, PRRS virus was isolated from a 200-sow farrow-to-finish herd with clinical signs consistent with PRRS. The virus was isolated by inoculation of pleural fluid from a stillborn piglet onto porcine pulmonary alveolar macrophages. The isolate was identified as PRRS virus by staining with a specific antiserum. By electron microscopy, the virus particle was found to be spherical and enveloped, measuring 45–55 nm in diameter and containing a 30–35 nm nucleocapsid. Only minor antigenic differences were found between the Danish and a Dutch isolate. Following intranasal inoculation of 3 pregnant gilts with the Danish isolate transplacental infection was demonstrated by the re-isolation of PRRS virus from approximately 45% of the piglets from the experimentally infected gilts. However, the experimental infection produced no significant reproductive disorders or other clinical signs. At autopsy, histopathological examination revealed slight interstitial pneumonia in a few piglets.     

Performance of fattening pigs in a farm infected with both porcine reproductive and respiratory syndrome (PRRS) virus and porcine circovirus type 2 following sow and piglet vaccination with an attenuated PRRS vaccine

The purpose of the study was to investigate whether, on farms with both post-weaning multisystemic wasting syndrome (PMWS) and porcine reproductive and respiratory syndrome (PRRS), the PRRS vaccination of sows and their fattening pigs protects against these syndromes. In a farrow-to-finish pig farm with a history of PRRS and PMWS, 200 gilts and sows were allocated to one of two groups of equal size. The first group (C-sow group) was used as untreated controls, while the animals of the second group (V-sow group) were vaccinated with live Porcilis PRRS vaccine. At the next weaning, all piglets of half the sows of the C sow group were vaccinated once at 35 days of age with the vaccine (CV group), while the offspring of the other half of the unvaccinated sows were left unvaccinated (CC group). Similarly, the offspring of half the sows of the V sow group were vaccinated (VV group), while those of the other half of the vaccinated sows were left unvaccinated (VC group). No significant differences in morbidity were observed between the groups during the nursery and finishing phases, while morbidity in the growers was significantly reduced in the CV- and VV-groups (P < 0.05). Growers' mortality was significantly reduced after piglet vaccination when compared with unvaccinated pigs of unvaccinated dams (P < 0.05). Average daily gain and feed conversion ratio were significantly improved in vaccinated piglets compared with those in the unvaccinated groups (P < 0.05).     

Effect of challenge dose and route on porcine reproductive and respiratory syndrome virus (PRRSV) infection in young swine

Porcine reproductive and respiratory syndrome virus (PRRSV) is perceived to be highly infectious because of the rapid spread of the virus through populations of domestic swine throughout the world. However, no information has been published on the minimum infectious dose of PRRSV and the effect of challenge dose on clinical response. In this experiment, ten groups of pigs (n = 3 per group) were inoculated with one of five different quantities (10(1)-10(5) fluorescent foci units per millilitre) of PRRSV (isolate ISU-P) by either intramuscular or intranasal routes. Clinical signs and body temperature were monitored for 21 days. Serum was collected periodically throughout the study period to monitor the presence of virus in serum and the early immune response of pigs. A 2-mL inoculum containing 10(1) fluorescent foci units of virus per millilitre was found sufficient to achieve infection by either route. Time to onset of clinical signs was highly associated with challenge dose (P < 0.01), regardless of route of exposure. However, no dose- or route-dependent differences in the severity of clinical manifestation were observed. No significant differences in the time of onset or degree of humoural immune response to PRRSV infection were observed between different treatment groups. However, intramuscular exposure appeared to induce a more uniform antibody response compared to intranasal exposure. These results confirmed that PRRSV is highly infectious; a fact that should be taken into consideration when designing strategies for the prevention and control of PRRSV.     

Pathogenesis of in utero infection in porcine fetuses with porcine reproductive and respiratory syndrome virus.

Porcine fetuses were exposed in utero to porcine reproductive and respiratory syndrome virus (PRRSV) at stages of gestation ranging from 34 to 85 days and examined 17 to 31 days later to determine the effect of gestational age on fetal susceptibility. For each of the 8 litters tested during the study, all of the fetuses of 1 horn of the uterus were exposed to virus by intraamniotic injection; those of the other horn were exposed similarly to a sham inoculum that consisted of sterile cell culture medium. Viral infectivity titers associated with fetal tissues collected at necropsy indicated that, regardless of gestational age, the virus had replicated in fetuses exposed intraamniotically. In addition, virus had also spread and replicated in sham-inoculated littermates in 3 litters. On the basis of these findings it appears that there may be little or no temporal difference in fetal susceptibility to infection with PRRSV. If so, the lack of early fetal death as a commonly recognized feature of naturally occurring cases of PRRS may be due to a greater resistance of early gestational fetuses to the lethal effects of PRRSV, as suggested by this study, and/or a greater likelihood of transplacental infection during late gestation.     

Effect of orally administered Lactobacillus casei on porcine reproductive and respiratory syndrome (PRRS) virus vaccination in pigs

The purpose of this study was to determine whether intranasal/oral administration of probiotics can assist vaccination efficacy against an important swine pathogen, porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV). A controlled challenge trial was performed employing: (a) pigs vaccinated against PRRS and treated with a Lactobacillus casei, (b) pigs vaccinated against PRRS only, (c) pigs treated with L. casei only, and (d) pigs neither vaccinated against PRRS nor treated with L. casei. All pigs were challenged intranasally with a wild PRRSV strain. There was no difference in clinical signs or rectal temperature among the four groups. However, pigs that received L. casei gained significantly more weight than pigs that did not. Vaccinated pigs did not gain more weight than nonvaccinated pigs. Vaccinated groups had significantly fewer viraemic pigs on days post-challenge 4, 11 and 17 than nonvaccinated groups of pigs. There was no effect of probiotic on prevalence or duration of viraemia. Among viraemic pigs, there was no significant difference in mean log base(10) titer of PRRS virus among groups. These results suggest that orally administered L. casei does not affect immune response in such a way as to affect PRRS viraemia or nasal shedding. However, it still appears to provide significant benefit when administered during vaccination as indicated by the higher bodyweight gain following PRRS virus infection.     

Efficacy of Fostera PRRS modified live virus vaccine against a Canadian heterologous virulent field strain of porcine reproductive and respiratory syndrome virus

Vaccination is a useful option to control infection with porcine reproductive and respiratory syndrome virus (PRRSV), and several modified live-PRRSV vaccines have been developed. These vaccines have shown some efficacy in reducing the incidence and severity of clinical disease as well as the duration of viremia and virus shedding but have failed to provide sterilizing immunity. The efficacy of modified live-virus (MLV) vaccines is greater against a homologous strain compared with heterologous PRRSV strains. The objective of this study was to evaluate the efficacy of Fostera PRRS MLV vaccine in protecting against challenge with a heterologous field strain widely circulating in the swine herds of eastern Canada. Forty-six piglets were divided into 4 groups: nonvaccinated-nonchallenged; nonvaccinated-challenged; vaccinated-challenged; and vaccinated-nonchallenged. The animals were vaccinated at 23 d of age with Fostera PRRS and challenged 23 d later with a heterologous field strain of PRRSV (FMV12-1425619). Overall, the vaccine showed some beneficial effects in the challenged animals by reducing the severity of clinical signs and the viral load. A significant difference between nonvaccinated and vaccinated animals was detected for some parameters starting 11 to 13 d after challenge, which suggested that the cell-mediated immune response or other delayed responses could be more important than pre-existing PRRSV antibodies in vaccinated animals within the context of protection against heterologous strains.     

Genetic analysis of reproductive performance in sows during porcine reproductive and respiratory syndrome(PRRS) and porcine epidemic diarrhea(PED)outbreaks

Background: Porcine reproductive and respiratory syndrome(PRRS) is one of the most infectious swine diseases in the world, resulting in over 600 million dollars of economic loss in the USA alone. More recently, the USA swine industry has been having additional major economic losses due to the spread of porcine epidemic diarrhea(PED).However, information regarding the amount of genetic variation for response to diseases in reproductive sows is still very limited. The objectives of this study were to identify periods of infection with of PRRS virus(PRRSV) and/or PED virus(PEDV), and to estimate the impact their impact on the phenotypic and genetic reproductive performance of commercial sows.Results: Disease(PRRS or PED) was significant(P 0.05) for all traits analyzed except for total piglets born.Heritability estimates for traits during Clean(without any disease), PRRS, and PED ranged from 0.01(number of mummies; Clean and PED) to 0.41(abortion; PED). Genetic correlations between traits within disease statuses ranged from-0.99(proportion born dead with number weaned; PRRS) to 0.99(number born dead with born alive;Clean). Within trait, between disease statuses, estimates ranged from-0.17(number weaned between PRRS and PED) to 0.99(abortion between Clean and PRRS).Conclusion: Results indicate that selection for improved performance during PRRS and PED in commercial sows is possible and would not negatively impact performance in Clean environments.     

A modified immunoperoxidase assay for detection of antibody porcine reproductive and respiratory syndrome (PRRS) virus in swine sera

MARC-145 cell monolayers infected with PRRS virus were fixed in 3% neutral buffered formalin and embedded in paraffin. The sections were stained by avidin-biotin complex immunoperoxidase (ABC) method. Test sera were applied to sections as primary antibodies. The positive reactions were detected by ABC method and indirect immunofluorescent assay (IFA). There was good correlation between ABC and IFA, and the titers in ABC were higher than those in IFA. The present results indicate that the immunohistochemical staining is a useful test for the detection and quantitation of PRRS virus antibody in swine sera as well as IFA.     

Acute phase response in porcine reproductive and respiratory syndrome virus infection

This study was focused on the changes observed in the serum concentration of haptoglobin (Hp), C-reactive protein (CRP), serum amyloid A (SAA) and Pig-major acute protein (Pig-MAP), during experimental porcine reproductive and respiratory syndrome virus (PRRSV) infection and in their relationship with the expression of interleukin 1β (IL-1β), interleukin 6 (IL-6) and tumor necrosis factor alpha (TNF-α). Hp and Pig-MAP serum levels were increased at 10 dpi, but CRP and SAA showed a delayed and highly variable increase. All three proinflammatory cytokines were poorly expressed, and only a mild increase in IL-1β was observed at 7 dpi. The increased expression of Hp coincided with the light enhancement observed in both IL-6 and TNF-α, and might be related with an increased expression of IL-10. The low expression of TNF-α might point to a possible mechanism of viral evasion of host-immune response. This issue and the delayed expression of CRP and SAA should be taken into account in future studies about modulation of the immune response by PRRSV infection.     

Molecular characterization of recent Korean porcine reproductive and respiratory syndrome (PRRS) viruses and comparison to other Asian PRRS viruses

Twenty-eight PRRS viruses (PRRSVs) isolated from various pig farms in Korea between 2002 and 2003 were sequenced for open-reading frame (ORF) 5 and/or full-length genome and compared with numerous PRRSVs reported from North America, Europe and Asia. All Korean isolates examined were genetically of the North American genotype. The ORF5 sequence of one isolate was identical to Ingelvac PRRS MLV vaccine virus. ORF5 nucleotide sequence divergence of the remaining 27 Korean PRRSVs from VR-2332, the prototype of the North American PRRSV and parental strain of the MLV vaccine virus, ranged from 1.3% to 12.9%, which corresponded to 2.0% to 14.9% divergence at the amino acid level, raising a concern on the efficacy of the MLV vaccine. Phylogenetic analyses of ORF5 and/or full-length sequences revealed that the Korean PRRSVs formed a clade distinct from PRRSVs reported from other Asian countries (China, Taiwan, Japan, and Thailand). Our study demonstrated that PRRSVs of the North American genotype were introduced to the Korean swine population some time ago and have evolved independently from PRRSV in other Asian countries, suggesting that geographic separation might influence the molecular evolution of PRRSV. This should be taken into consideration when a national PRRS prevention and control policy for international trade is established.     

Persistence of porcine reproductive and respiratory syndrome virus infection in a swine operation.

A herd of Quebec seedstock pigs experienced in early 1992 a typical outbreak of porcine reproductive and respiratory syndrome (PRRS) associated with lesions of interstitial, proliferative and necrotizing pneumonia in weaned piglets. The nature of the infection was confirmed by serology using indirect immunofluorescence (IIF) and virus isolation in primary cultures of porcine alveolar macrophages (PAM). Farm production recovered after eight weeks of losses. In order to evaluate the persistence of infection in the herd, five SPF-piglets were introduced in two different sections of the PRRS-affected barn four months after the disappearance of clinical symptoms, and two others were placed in a neighboring building with apparently healthy farrow-to-finnish pigs. Clinical signs, body temperature, humoral immune response, virological and histopathological findings were recorded over a 42-day period. Clinical signs were evident in all of the sentinels and prolonged fever (> or = 40 degrees C) was recorded one day post-exposure (PE). Antibody titers to PRRS virus could be detected by IIF on PAM seven days PE, and reached 1:1024 by day 21 PE. Three of the sentinels developed significant virus neutralizing antibody titers (> 1:8 to < or = 1:128) by day 35 PE. In all cases, the virus could be isolated from the serum between day 7 and 42 PE. Thus, the virus and specific antibodies coexisted for several weeks. Lesions of interstitial pneumonia was demonstrated in few animals. In experimental inoculation studies, the viral strain isolated from the sentinel pigs produced severe reproductive disorders in two sows inoculated at 95 days of gestation.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.