The threat of peste des petits ruminants: progress in vaccine development for disease control

Peste des petits ruminants (PPR) is a highly contagious animal disease caused by a virus in the genus Morbillivirus, family Paramyxoviridae. This infection is responsible for high morbidity and mortality in sheep and goats and in some small wild ruminant species. The huge number of small ruminants, which are reared in the endemic areas makes PPR a serious disease threatening the livelihood of poor farmers. Taking advantage of the closely relationship between rinderpest and PPR viruses, the attenuated rinderpest vaccine was used in the control of PPR. It is now replaced by the homologous attenuated PPR vaccine. Unfortunately, animals that have received this vaccine cannot be distinguished serologically from infected animals. With the advent of DNA recombinant technology, efforts are being made to develop effective PPR marker vaccines to enable such differentiation and which would allow countries to implement both vaccination and disease surveillance programmes at the same time.     

Comparative efficacy of various chemical stabilizers on the thermostability of a live-attenuated peste des petits ruminants (PPR) vaccine

Thermostability of a live-attenuated peste des petits ruminants (PPR) vaccine recently developed at Indian Veterinary Research Institute was studied using conventional lyophilization conditions. A total of four stabilizers viz., lactalbumin hydrolysate–sucrose (LS), Weybridge medium (WBM), buffered gelatin-sorbitol (BUGS) and trehalose dihydrate (TD) were used to prepare the lyophilized vaccine. The study revealed that the PPR vaccine lyophilized with either LS or TD is more stable than rest of the stabilizers having an expiry period of at least 45 days (so far studied) at 4 °C, 15–19 days at 25 °C and 1–2 days at 37 °C. However, at a temperature of 45 °C, BUGS had a marginal superiority, although lasted for few hours, followed by TD and LS with respect to shelf-life, LS and TD with respect to half-life. On the basis of half-life also LS followed by TD appeared superior at a temperature of 4, 25 and 37 °C. Reconstitution of vaccine with distilled water or 1 M MgSO₄ or 0.85% NaCl maintained the required virus titre (2.5 log₁₀ TCID₅₀ per dose) up to 8 h at 37 °C and 7 h at 45 °C. Among the three diluents, 1 M MgSO₄ appeared to be the better diluent for reconstitution of lyophilized PPR vaccine, as the loss on dilution was lowest and maintain the required virus titre for a longer period. Investigation suggests for using LS as stabilizer for lyophilization and 1 M MgSO₄ as vaccine diluent for the newly developed PPR vaccine.     

The detection of antibodies against peste des petits ruminants virus in cattle, sheep and goats and the possible implications to rinderpest control programmes.

Monoclonal antibody-based competitive ELISA (C-ELISA) have been used for the specific measurement of antibodies to both rinderpest and peste des petits ruminants (PPR) viruses in cattle, sheep and goats. Examination of serum samples from sheep and goats in Gambia, before and after vaccination with rinderpest vaccine, suggested that antibodies to PPR virus could prevent an immune response to the rinderpest vaccine. Cattle sera from Nigeria and Ghana showed a high prevalence of antibody against PPR virus which may explain the difficulty experienced in some countries in achieving high post-vaccination immunity levels against rinderpest. Because antibodies against PPR virus are both cross-neutralizing and cross-protective against rinderpest virus further vaccination in the presence of antibodies against PPR virus may be a waste of national resources. This paper presents serological evidence for the transmission of PPR virus from sheep and goats to cattle and highlights the need to include PPR serology in the sero-monitoring programme to give a better indication of national herd immunity.     

Scalable culture systems using different cell lines for the production of Peste des Petits ruminants vaccine

Peste des Petits ruminants (PPR) is considered as one of the major constraints to the productivity of small ruminants in Africa and Asian countries. Currently PPR control is done by vaccination with an attenuated PPR strain (Nigeria 75/1) produced in monolayers of Vero cells grown in roller bottles or static flasks. This work focuses on the production of a PPR vaccine strain using stirred conditions as an advanced option for process scale-up. Non-porous microcarriers (Cytodex-1) were used to support Vero cell growth in suspension cultures. The use of Ex-Cell medium could improve cell specific productivities obtained with standard serum containing medium, independently of the type of system used, i.e. static as well as suspension stirred cultures. As an alternative, several cell lines adapted to grow as single cells in suspension (CHO-K1, BHK-21A and 293) and another anchorage-dependent (MRC-5) were evaluated in their capacity to produce a PPR vaccine. BHK-21A and 293 cells grown as single-cell suspension in serum free medium were both suited to produce PPR vaccine with productivities similar to Vero cells, namely 10(6)TCID(50)/mL. However, for the 293 cells, these results were only obtained 2-3 days later. CHO-K1 and MRC-5 cells have shown not to be suitable to adequately produce this virus. These results provide further insights into the feasibility of applying microcarrier cell culture technology to produce PPR vaccine in Vero cells as well as in the alternative use of single-cell suspension cultures of BHK-21A, significantly simplifying the existing production process.     

A goat poxvirus-vectored peste-des-petits-ruminants vaccine induces long-lasting neutralization antibody to high levels in goats and sheep

Recombinant capripoxvirus (CPV) is a promising candidate differentiating infected from vaccinated animals (DIVA) vaccine against peste-des-petits-ruminants (PPR). In order for recombinant CPV to be successfully used in the field, there should exist dependable indicators for quality control of vaccine products, surveillance and vaccination evaluation. Viral neutralization antibody (VNA) is correlated to protection against PPR and is a technically feasible indicator for this purpose. The immunogenicity of this vectored vaccine in goats and sheep, however, has not been fully evaluated. In this study, we generated two recombinant CPV viruses, rCPV-PPRVH and rCPV-PPRVF, that express PPR virus (PPRV) glycoproteins H and F, respectively. Vaccination studies with different dosages of recombinant viruses showed that rCPV-PPRVH was a more potent inducer of PPRV VNA than rCPV-PPRVF. One dose of rCPV-PPRVH was enough to seroconvert 80% of immunized sheep. A second dose induced significantly higher PPRV VNA titers. There was no significant difference in PPRV VNA responses between goats and sheep. Subcutaneous inoculation also induced a significant PPRV VNA response. PPRV VNA could be detected for over 6 months in more than 80% of vaccinated goats and sheep. Boost vaccination at 6-month intervals induced significant re-boost efficacy of PPRV VNA in goats and sheep. More over, two doses of rCPV-PPRVH could completely overcome the interference caused by pre-existing immunity to the CPV vaccine backbone in animals. Vaccination with rCPV-PPRVH also protected goats from virulent CPV challenge. Our results demonstrate that VNA can serve as a dependent indicator for effective vaccination and immune protection of animals in the field. The recombinant CPV vaccine used in our studies could be a practical and useful candidate DIVA vaccine in countries where PPR newly emerges or where stamp-out plans are yet to be implemented.     

The livestock vaccine supply chain: Why it matters and how it can help eradicate peste des petits Ruminants,based on findings in Karamoja,Uganda

Understanding factors that hinder vaccination, including logistical and social constraints, is critical to finding the most effective approach for the global eradication of peste des petits ruminants (PPR). Vaccination projects should analyze the supply chain and take it into consideration when planning and creating a vaccination strategy. Adequate supply chain management of the PPR vaccine could lead to reduced cost, increased availability, and the construction of a data platform for other livestock vaccines. Integrating the supply chain of PPR vaccine with other veterinary or health commodities could reduce cost, as well as increase uptake. The use of a thermostable vaccine could potentially have a positive impact on the eradication of PPR in remote areas, such as the Karamoja subregion in Uganda, as it did with rinderpest across Sub Saharan Africa. In terms of vaccine delivery, the use of community animal health workers (CAHWs) could be beneficial in certain areas, such as the Karamoja subregion of Uganda, by alleviating supply chain constraints in the last-mile delivery, as well as increasing coverage and uptake. A gendered approach to livestock vaccines should also be considered, as decision-making power regarding livestock vaccination is gendered in many various contexts. The PPR eradication strategy—as well as other livestock vaccination programs—would be more effective and efficient if the supply chain management were considered as a key component in the process and efforts tailored, accordingly.     

A thermostable presentation of the live,attenuated peste des petits ruminants vaccine in use in Africa and Asia

The research objective was to develop a thermostable vaccine against peste des petits ruminants (PPR), a morbilliviral disease of small ruminants targeted for eradication that is a major constraint on the livelihoods of the rural poor throughout much of Africa and Asia. Although existing PPR vaccines provide life-long immunity, they require continuous refrigeration. This limits their utility in developing countries. Methods for the lyophilization of a related morbillivirus, rinderpest (RP), resulted in vaccine that could be used in the field for up to 30 days without refrigeration which was a major contribution to the global eradication of RP completed in 2011. The present research applied the rinderpest lyophilization method to the attenuated Nigeria 75/1 PPR vaccine strain, and measured thermostability in accelerated stability tests (AST) at 37 °C. The shelf-life of the vaccine was determined as the time a vial retained the minimum dose required as a 25-dose presentation at the specified temperature. A lactalbumin hydrolysate and sucrose (LS) stabilizer was compared to stabilizers based on trehalose. PPR vaccine produced using the Xerovac drying method was compared to vaccine produced using the rinderpest lyophilization method in AST. LS vaccine was evaluated in AST at 37, 45 and 56 °C and an Arrhenius plot was constructed for estimation of stability at temperatures not tested. Vaccines produced using LS and the rinderpest method of lyophilization were the most stable. The shelf-life of the Xerovac preparation was 22.2 days at 37 °C. The three LS vaccine batches had shelf-lives at 37 °C of 177.6, 105.0 and 148.9 days, respectively, at 37 °C. At 56 °C, the shelf-life was 13.7 days. The projected half-life at 25 °C was 1.3 years. This is sufficient thermostability for use without a cold chain for up to 30 days which will greatly facilitate the delivery of vaccination in the global eradication of PPR.     

Interference in the vaccination of cattle against rinderpest virus by antibodies against peste des petits ruminants (PPR) virus

The ability of the attenuated vaccine 75/1 of peste des petits ruminants to interfere with rinderpest vaccination in cattle was investigated experimentally. Young cattle (93) were selected and tested as being negative for antibodies against RP or PPR viruses. These were vaccinated with the peste des petits ruminants attenuated vaccine strain PPR75/1. All animals produced specific antibodies against the peste des petits ruminants vaccine after one or two doses. The cattle were then vaccinated with attenuated rinderpest vaccine. Two months later, 88 of these animals were sampled and 21/88 were positive for antibodies specific for rinderpest. The 67 negative animals received a second rinderpest vaccine dose after which 31seroconverted. The 36 animals which failed to seroconvert were re-vaccinated, of these 28 seroconverted. This study highlights the interference by peste des petits ruminants vaccination, presumably through production of antibodies that cross react with the live rinderpest virus in the vaccine used. This interference is also observed after vaccination against rinderpest followed by subsequent administration of peste des petits ruminants vaccine.     

Protective efficacy of a single immunization with capripoxvirus-vectored recombinant peste des petits ruminants vaccines in presence of pre-existing immunity

Sheeppox, goatpox and peste des petits ruminants (PPR) are highly contagious ruminant diseases widely distributed in Africa, the Middle East and Asia. Capripoxvirus (CPV)-vectored recombinant PPR vaccines (rCPV-PPR vaccines), which have been developed and shown to protect against both Capripox (CP) and PPR, would be critical tools in the control of these important diseases. In most parts of the world, these disease distributions overlap each other leaving concerns about the potential impact that pre-existing immunity against either disease may have on the protective efficacy of these bivalent rCPV-PPR vaccines. Currently, this question has not been indisputably addressed. Therefore, we undertook this study, under experimental conditions designed for the context of mass vaccination campaigns of small ruminants, using the two CPV recombinants (Kenya sheep-1 (KS-1) strain-based constructs) developed previously in our laboratory. Pre-existing immunity was first induced by immunization either with an attenuated CPV vaccine strain (KS-1) or the attenuated PPRV vaccine strain (Nigeria 75/1) and animals were thereafter inoculated once subcutaneously with a mixture of CPV recombinants expressing either the hemagglutinin (H) or the fusion (F) protein gene of PPRV (10³ TCID₅₀/animal of each). Finally, these animals were challenged with a virulent CPV strain followed by a virulent PPRV strain 3 weeks later. Our study demonstrated full protection against CP for vaccinated animals with prior exposure to PPRV and a partial protection against PPR for vaccinated animals with prior exposure to CPV. The latter animals exhibited a mild clinical form of PPR and did not show any post-challenge anamnestic neutralizing antibody response against PPRV. The implications of these results are discussed herein and suggestions made for future research regarding the development of CPV-vectored vaccines.     

Development of a PPRV challenge model in goats and its use to assess the efficacy of a PPR vaccine

Peste des Petits Ruminants (PPR) is a severe disease of small ruminants and has high economic impacts in developing countries. Endemic in Africa, the Middle East and Asia, the disease is currently progressing with occurrences reported in North Africa, Turkey and in Georgia, and now threatens Europe. Much remains unknown about the infection dynamics, the virulence of the different strains and species/breed susceptibility. Robust experimental challenge models are needed to explore these fields and to confirm the efficacy of currently sold vaccines. We first assessed virulence of two PPR virus strains (CI89 and MA08) in Saanen goats. Whereas the MA08 strain led to classical severe clinical signs of PPR, the CI89 strain appeared to cause a mild disease in Saanen goats, highlighting the difference in virulence between strains in this animal model. We further demonstrated the importance of the inoculation route in the appearance of clinical signs and that ocular excretion is a better choice than blood for viral detection. After developing a robust challenge model, we assessed the efficacy of a vaccine (PPR-VAC®, BVI Botswana) against the MA08 strain and demonstrated that this vaccine blocked viral excretion and significantly reduced clinical signs. These results reinforce the paradigm that a strain from one lineage could protect against strains from other lineages.