Transmission of Porcine reproductive and respiratory syndrome virus 1 to and from vaccinated pigs in a one-to-one model

Comparison of viraemia and nasal shedding after PRRSV-1 challenge following vaccination with three commercially available PRRS modified live virus vaccines

The effectiveness of three Porcine Reproductive and Respiratory Syndrome (PRRS) Modified Live Virus (MLV) vaccines against PRRSV viraemia and nasal shedding following experimental challenge was compared. The study comprised a negative control (T01), and three treatment groups (T02, T03 and T04) each vaccinated with a single dose of a commercial PRRS MLV vaccine, given in accordance with the vaccine's Summary of Product Characteristics (SPC). Pigs aged 21 days were vaccinated (day 0), challenged intranasally (day 28) with heterologous PRRSV-1-1 strain Olot/91, then monitored for PRRSV viraemia and nasal shedding for 12 days. After challenge, pigs were viraemic on fewer days in group T04 (0.67) than groups T01 (0.91), T02 (0.81) and T03 (0.97) (P < 0.0296). From day 34, inclusive, serum PRRSV titres were lower in group T04 than negative controls (P ≤ 0.0001) and groups T02 and T03 (P ≤ 0.0047); serum PRRSV titre Area Under the Curve (AUC) for group T04 (42.34) was lower than in T01 (65.49), T02 (60.67) and T03 (67.38) (P < 0.0100); pigs exhibited nasal shedding on fewer days in group T04 (0.40) than T01 (0.78), T02 (0.64) and T03 (0.56) (P < 0.0101); and nasal shedding AUC for group T04 (8.52) was lower than in groups T01 (23.59, P < 0.0001) and T02 (19.37, P = 0.0001). The ability of PRRS MLV vaccines to reduce the duration of viraemia and nasal shedding after intranasal challenge with a heterologous PRRSV-1-1 strain differ significantly.

Porcine Reproductive and Respiratory Syndrome Modified Live Virus Vaccine: A "Leaky" Vaccine with Debatable Efficacy and Safety

Porcine reproductive and respiratory syndrome (PRRS) caused by the PRRS virus (PRRSV) is one of the most economically important diseases, that has significantly impacted the global pork industry for over three decades, since it was first recognized in the United States in the late 1980s. Attributed to the PRRSV extensive genetic and antigenic variation and rapid mutability and evolution, nearly worldwide epidemics have been sustained by a set of emerging and re-emerging virus strains. Since the first modified live virus (MLV) vaccine was commercially available, it has been widely used for more than 20 years, for preventing and controlling PRRS. On the one hand, MLV can induce a protective immune response against homologous viruses by lightening the clinical signs of pigs and reducing the virus transmission in the affected herd, as well as helping to cost-effectively increase the production performance on pig farms affected by heterologous viruses. On the other hand, MLV can still replicate in the host, inducing viremia and virus shedding, and it fails to confer sterilizing immunity against PRRSV infection, that may accelerate viral mutation or recombination to adapt the host and to escape from the immune response, raising the risk of reversion to virulence. The unsatisfied heterologous cross-protection and safety issue of MLV are two debatable characterizations, which raise the concerns that whether it is necessary or valuable to use this leaky vaccine to protect the field viruses with a high probability of being heterologous. To provide better insights into the immune protection and safety related to MLV, recent advances and opinions on PRRSV attenuation, protection efficacy, immunosuppression, recombination, and reversion to virulence are reviewed here, hoping to give a more comprehensive recognition on MLV and to motivate scientific inspiration on novel strategies and approaches of developing the next generation of PRRS vaccine.

New insights about vaccine effectiveness: Impact of attenuated PRRS-strain vaccination on heterologous strain transmission

Vaccination is the main tool for controlling infectious diseases in livestock. Yet current vaccines only provide partial protection raising concerns about vaccine effectiveness in the field. Two successive transmission trials were performed involving 52 pigs to evaluate the effectiveness of a Porcine Reproductive and Respiratory Syndrome (PRRS) vaccinal strain candidate against horizontal transmission of a virulent heterologous strain. PRRS virus, above the specified limit of detection, was observed in serum and nasal secretions for all but one pig (the exception only tested positive for serum), indicating that vaccination did not protect pigs from becoming infected and shedding the heterologous strain. However, vaccination delayed the onset of viraemia, reduced the duration of shedding and significantly decreased viral load throughout infection. Serum antibody profiles indicated that 4 out of 13 (31%) vaccinates in one trial had no serological response (NSR). A Bayesian epidemiological model was fitted to the data to assess the impact of vaccination and presence of NSRs on PRRS virus transmission dynamics. Despite little evidence for reduction in the transmission rate, vaccinated animals were on average slower to become infectious, experienced a shorter infectious period and recovered faster. The overall PRRSV transmission potential, represented by the reproductive ratio R₀ was lower for the vaccinated animals, although there was substantial overlap in the credibility intervals for both groups. Model selection suggests that transmission parameters of vaccinated pigs with NSR were more similar to those of unvaccinated animals. The presence of NSRs in a population, however, seemed to only marginally affect the transmission dynamics. The results suggest that even when vaccination can't prevent infection, it can still have beneficial impacts on the transmission dynamics and contribute to reducing a herd's R₀. However, biosecurity and other measures need to be considered to decrease contact rates and lower R₀ below 1.

Improving the management procedures in farms infected with the Porcine Reproductive and Respiratory Syndrome virus using PDP models

Pig meat production need to be built up in the future due to the increase of the human population worldwide. To address this challenge, there is plenty of room for improvement in terms of pig production efficiency that could be severely hampered by the presence of diseases. In this sense, Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) is one of the most costly disease present in industrial pork production in Europe and North America. We have developed a model to analyze the effect of different management procedures to control this important virus in different epidemiological scenarios. Our results clearly suggest that no cross-fostering during lactation and the maintaining of litter integrity significantly decrease the number of sick and dead animals during the rearing period compared to scenarios where cross-fostering and no litter integrity are practiced. These results highlight the relevance of different management strategies to control PRRSV and quantify the effect of limiting cross-fostering and avoiding mixing animals from different litters in PRRSV positive farms to optimize animal production. Our findings will allow pig farmers to apply these management procedures to control this disease under field conditions in a very cost-effective way.

Safety and long-lasting immunity of the combined administration of a modified-live virus vaccine against porcine reproductive and respiratory syndrome virus 1 and an inactivated vaccine against porcine parvovirus and Erysipelothrix rhusiopathiae in breeding pigs

BACKGROUND

In the field, vaccination schedules based on modified-live virus (MLV) vaccines administered twice in gilts and every three to four months in sows are commonly used to immunize breeding herds against porcine reproductive and respiratory virus (PRRSV). Breeding sows are repeatedly vaccinated against several other agents. Thus, the combined administration of vaccines for their simultaneous use can simplify such complex immunization schedules. Here, we evaluated the safety and long-term immunity of the authorized combined administration of a PRRSV MLV vaccine and an inactivated vaccine against porcine parvovirus (PPV) and Erysipelothrix rhusiopathiae for their simultaneous use.Six-month-old naïve healthy gilts were vaccinated at day 0 and revaccinated at days 21 and 147, mimicking the abovementioned vaccination schedule. Systemic and local reactions, as well as body temperature, were measured. The excretion of PRRSV1 MLV was evaluated in oral fluids. Humoral responses against the three antigens were measured by ELISA. For PRRSV, homologous neutralizing antibodies (NAs) and homologous and heterologous cell-mediated immunity (CMI) were also assessed.

RESULTS

The combined administration of the tested vaccines, applied according to the manufacturer's instructions, was safe based on all evaluated parameters. Overall, we detected antibodies against PPV and PRRSV in all vaccinated pigs already after the first vaccination, whereas antibodies against E. rhusiopathiae were observed in all animals after revaccination. After subsequent revaccinations, we observed boosts for the humoral response for PPV at days 28 and 154 and at day 154 for E. rhusiopathiae. No boosts were detected during the experiment by PRRSV ELISA. In all vaccinated animals, homologous NAs against MLV were already detected before revaccination (day 21). After revaccination, there was a boost with mean titres of homologous NAs remaining constant thereafter. Concerning CMI, PRRSV-specific IFN-γ-secreting cells were already detected at day 21 for all evaluated strains and we observed boosts for all PRRSV1 strains after revaccination and recall revaccination.

CONCLUSIONS

We showed that the combined administration of tested vaccines described here using a vaccination schedule against PRRSV commonly implemented for breeding pigs in the field is safe and induces long-lasting humoral and cellular immunity against PRRSV, PPV, and E. rhusiopathiae.

Bottlenecks in the transmission of porcine reproductive and respiratory syndrome virus (PRRSV1) to naïve pigs and the quasi-species variation of the virus during infection in vaccinated pigs

This paper describes the results of two experiments regarding porcine reproductive and respiratory syndrome virus (PRRSV1): the first one studied the existence of bottlenecks in an experimental one-to-one model of transmission in pigs; while the second analysed the differences between viral quasi-species in vaccinated pigs that developed shorter or longer viraemias after natural challenge. Serum samples, as well as the initial inoculum, were deep-sequenced and a viral quasi-species was constructed per sample. For the first experiment, the results consistently reported a reduction in the quasi-species diversity after a transmission event, pointing to the existence of bottlenecks during PRRSV1 transmission. However, despite the identified preferred and un-preferred transmitted variants not being randomly distributed along the virus genome, it was not possible to identify any variant producing a structural change in any viral protein. In contrast, the mutations identified in GP2, nsp9 and M of the second experiment pointed to changes in the amino acid charges and the viral RNA-dependent RNA polymerase structure. The fact that the affected proteins are known targets of the immunity against PRRSV, plus the differential level of neutralizing antibodies present in pigs developing short or long viraemias, suggests that the immune response selected those changes.

Analysis of the genetic diversity and mRNA expression level in porcine reproductive and respiratory syndrome virus vaccinated pigs that developed short or long viremias after challenge

Porcine reproductive and respiratory syndrome virus (PRRSv) infection alters the host’s cellular and humoral immune response. Immunity against PRRSv is multigenic and vary between individuals. The aim of the present study was to compare several genes that encode for molecules involved in the immune response between two groups of vaccinated pigs that experienced short or long viremic periods after PRRSv challenge. These analyses include the sequencing of four SLA Class I, two Class II allele groups, and CD163, plus the analysis by quantitative realtime qRT-PCR of the constitutive expression of TLR2, TLR3, TLR4, TLR7, TLR8 and TLR9 mRNA and other molecules in peripheral blood mononuclear cells.

A Multiplex RT-PCR Assay to Detect and Discriminate Porcine Reproductive and Respiratory Syndrome Viruses in Clinical Specimens

Outbreaks of highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) have led to large economic losses in China. The attenuated vaccine (HP-PRRSV JXA1-R) was used to control HP-PRRSV. However, in recent years, co-infection with classical PRRSV (C-PRRSV), HP-PRRSV, and/or HP-PRRSV JXA1-R has been increasing in China, resulting in a significant impact on PRRSV diagnostics and management. To facilitate rapid discrimination of HP-PRRSV JXA1-R from HP-PRRSV and C-PRRSV, a multiplex RT-PCR assay for the visual detection of HP-PRRSV JXA1-R, HP-PRRSV, and C-PRRSV was established and evaluated with reference PRRSV strains and clinical samples. Primer specificities were evaluated with RNA/DNA extracted from 10 viral strains, and our results revealed that the primers had a high specificity for PRRSV. The assay sensitivity was 24 copies/μL for PRRSVs. A total of 516 serum samples were identified, of which 12.21% (63/516) were HP-PRRSV-positive, 2.33% (12/516) were HP-PRRSV JXA1-R-positive, and 1.16% (6/516) were C-PRRSV-positive, respectively, which was completely consistent with the sequencing method. The high specificity, sensitivity, and reliability of the multiplex RT-PCR assay described in this study indicate that it is useful for the rapid and differential diagnosis of HP-PRRSV JXA1-R, HP-PRRSV, and C-PRRSV.

Comparison of protocols for the analysis of type 1 porcine reproductive and respiratory syndrome virus by RT-PCR using oral fluids

The detection of porcine reproductive and respiratory syndrome virus (PRRSV) in oral fluids (OF) by quantitative real-time polymerase chain reaction (qRT-PCR) is gaining increasing popularity. However, the different steps leading to a result have not been extensively evaluated. The aim of the present study was to examine the effect on the performance of qRT-PCR with different sampling materials, conditions of storage of the OF, the need for centrifuging OF, as well as to compare RNA extraction methods and PCR mixes. For the assays, pen-based oral fluids were used, which were pooled and spiked in a serial dilution (up to genotype 10⁰ TCID₅₀/mL) of type 1 PRRSV isolate 3267. Centrifugation at 15,000g for 15min resulted in an increase in sensitivity (1-2 PCR cycles) that was significant (P<0.05) at the lowest dilution tested. The TRIzol and MagMAX RNA extraction methods gave the maximum sensitivity, lowest threshold cycle (Ct), at equivalent virus concentrations. The AgPath-ID One-Step RT-PCR Kit PCR mix reagents were more sensitive for the detection of PRRSV using a purified plasmid as standard, but LSI VetMAX PRRSV EU/NA PRRSV reagents resulted in a slightly better sensitivity with OF (p<0.05). The present results may be useful to standardize protocols for optimizing detection of type 1 PRRSV in OF by qRT-PCR.