Assessment of post-vaccination immune response to peste des petits ruminants virus in small ruminants in the central and western regions of India

Abstract

The cross-sectional serosurvey for post-vaccination assessment of peste des petits ruminants (PPR) virus (PPRV) antibodies in sheep and goats was carried out in different states in the central and western regions of India after the implementation of vaccination under the PPR control programme. The serum samples (n = 4687) were collected from sheep (n = 1539) and goats (n = 3148) from August 2017 to March 2018 at various epidemiological units (n = 301) of the studied regions using a stratified random sampling method and PPR competitive ELISA kit was employed to detect PPRV antibodies. The results revealed 34, 21, 52, 74, 68, and 65% of prevalence of PPRV antibodies in small ruminants in Madhya Pradesh, Goa, Chhattisgarh, Maharashtra, Gujarat, and Rajasthan states, respectively, with a difference in seropositivity in sheep and goats across the states in sheep (p < 0.01) and goats (p < 0.01). Further, this serosurvey revealed that 60% of the epi-units (n = 185) had > 50% prevalence of post vaccination PPRV antibodies across states due to variations in vaccination rates and patterns. The vaccination coverage and the reported outbreaks varied between the states in the studied regions. Due to continuous vaccination under the control program, the reported PPR outbreaks have progressively declined in most of the studied states, and the PPR risk areas are confined to a few districts and sporadically, outbreaks are reported indicating the effectiveness of vaccination. These findings provide valuable information on potential PPRV episystems, and will assist with activities regarding intensive surveillance, vaccination, biosecurity, and modification of policy decisions towards designing and implementing control and eradication measures. Further, the present situation necessitates continuous mass vaccination and active surveillance programs to make these regions free from PPR in consonance with the PPR Global Control and Eradication Strategy under the PPR Global Eradication Program.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13337-022-00796-6.

Introduction

Peste des petits ruminants (PPR) is one of the important contagious World Organization of Animal Health (WOAH-formerly known as OIE) notifiable, transboundary viral diseases of sheep and goats, generally referred to as “goat plague or small ruminant plague” and is caused by the small ruminant morbillivirus (formerly known as PPR virus). The acute cases of the disease are clinically manifested by pyrexia, depression, off-feed, discharges from the oculo-nasal orifices, ulcerative necrotizing stomatitis, enteritis, and bronchopneumonia with the offensive foul breath [5, 11]. PPR affects the productivity of small ruminants in endemic countries and causes substantial economic losses to livestock farmers [15]. Therefore, the FAO and WOAH jointly launched the PPR Global Control and Eradication Strategy (GCES) plan for controlling and eradicating PPR under the Global Eradication program (GEP) [21]. The PPR vaccination program needs to be considered a public good as it benefits millions of small, marginal, and landless farmers and countries need to proactively plan and invest funds in the program to control and eradicate the PPRV globally by 2030, as envisaged by the WOAH and FAO [16].

India practiced focused PPR vaccination (vaccination limited to the place of the outbreaks to contain the disease spread) using the indigenously developed homologous PPR live attenuated vaccine (ICAR-IVRI-PPR vaccine Sungri-96 strain) in some of the states since 2002 [20]. Further, a strategic mass vaccination programme under the national PPR control programme (PPR-CP) was implemented in 2011 to mitigate the disease burden even before the global strategies were planned [6]. In the first phase of PPR-CP, all Southern Peninsular India region states and Union territories (UTs), including Maharashtra and Goa (http://www.dahd.nic.in), were proposed, whereas in the second phase of PPR-CP in the financial year (FY) 2014–2015 the remaining states and UTs were planned [6]. Nevertheless, neither systematic surveillance nor sero-monitoring was commenced to assess the effectiveness of the vaccination.

The seroprevalence studies from different geographical areas under varying agro-climatic conditions may help to devise effective and, appropriate control strategies. Further, data analysis on outbreaks and cases from different geographical areas may help devise effective vaccination and control strategies to prevent disease incursion and acquire disease-free status [11] by implementing active surveillance and intensive monitoring program. Different serosurvey on the prevalence of PPRV antibodies in small ruminants has also been reported from various regions in the endemic countries including India [1, 2, 12, 13]. In our recent studies, a systematic seroepidemiological survey for PPR, especially the prevalence of PPRV antibodies at the epidemiological unit (epi-unit) level for the state (s), has been conducted in a few regions [7–10]. Therefore, the present cross-sectional serosurvey was undertaken for post-vaccination assessment of the status at the epi-units level in the different states of central and western regions of India, where the vaccination programme was being implemented under ongoing strategic PPR-CP and effect of vaccination on eliminating PPRV infection in small ruminants.

Materials and methods

Study region

Central India consists of Chhattisgarh and Madhya Pradesh (MP) states, whereas Western India includes the states of Goa, Maharashtra, Gujarat, Rajasthan, and the union territory (UT) of Dadra and Nagar Haveli and Daman and Diu. These regions were selected as the PPR-CP first and second phases initiated in these states in 2011 and 2014, respectively (http://www.dahd.nic.in/) [6] and the reported outbreaks in some of these states have been substantially reduced [11, 6]. Dadra and Nagar Haveli and Daman and Diu were excluded due to the small geographical area, meager animal population, and no report of PPR outbreaks (http://www.dahd.nic.in/) [11].

Sampling design

Serosurvey was conducted from August 2017 to March 2018 to establish the prevalence of PPRV antibodies in different states of India’s central and western regions. The working hypothesis of the study was the homogeneous occurrence of PPRV antibodies in the epi-units in different states in the target population. The sampling size, design, and methodology were followed as previously described [8–10] and a stratified random sampling procedure was adopted to collect sera from different states’ epi-units in the studied region with a total sample size of 356. The maximum sample size of 1320 animal units [660 for each of the target (sheep or goats) species] was calculated for each state using an epi-calculator as described earlier [8–10].

Serum samples and outbreaks data

In each epi-unit, sheep and goat sera were collected as per the sampling design through the All India Coordinated Research Project on Animal Disease Monitoring and Surveillance (AICRP on ADMAS), a collaborating center of the Indian Council of Agricultural Research-National Institute of Veterinary Epidemiology and Disease Informatics (ICAR-NIVEDI) in the respective states. The sampled villages in the states of the studied regions are depicted in GIS Map (Fig. 1) based on their geo-coordinates using QGIS Software 2.18.6 version. The collected samples were labeled and transported to ICAR-NIVEDI, Bengaluru in an ice-cool box, and upon receiving, the samples were stored at -20 °C until further use. The passive surveillance outbreaks data on PPR is available in the National Animal Diseases Referral Expert System (NADRES) database of the ICAR-NIVEDI [11], and the available PPR vaccination details for the FY from 1st April and 31st March of the next year were obtained from the studied state animal husbandry departments for different FY and were used for analysis for assessing the effect of vaccination.

Fig. 1.

The surveyed epi-units (villages) locations are depicted (as a dot) in the GIS Map of the studied states in central and western regions of India

ELISA and statistical analysis

According to the indigenous PPR competitive ELISA kit protocol [18], the collected sera were tested to detect PPRV-specific antibodies. The antibodies were measured in percentage inhibition (PI) and a sample with a PI of ≥ 40% was considered positive. The seroprevalence estimation and chi-squared test (χ²) were carried out in Microsoft Office Excel 2016 to estimate prevalence and understand the association of PPRV antibodies in sheep and goats across states and districts in the studied region.

Results

The results revealed that 11.1, 36.4, 82.1, 69.3, and 70.7% of samples in sheep and 37.0, 52.8, 69.4, 67.5, and 58.9% in goats were positive for PPRV antibodies in MP, Chhattisgarh, Maharashtra, Gujarat, and Rajasthan states, respectively and 20.8% of samples in goats in Goa state with an overall prevalence of 57.97% (goats-53% and Sheep-68%) in the studied regions. Further, the overall prevalence of PPRV antibodies in small ruminants was 34, 21, 52, 74 68, and 65% in MP, Goa, Chhattisgarh, Maharashtra, Gujarat, and Rajasthan states, respectively. State-wise details of samples tested and their percentage prevalence of PPRV antibodies are presented in Table 1. The variation in prevalence levels of antibodies in different epi-units in different states was also observed (Fig. 2A). The serosurvey also revealed that 60% of the epi-units (n = 185) had > 50% post-vaccination PPRV antibodies varied across states, due to variation in vaccination rates and patterns. Further, as per WOAH guidelines, the desired (> 70% prevalence of PPRV antibodies) level was observed only in 12 out of 60 epi-units screened from 33 districts in MP, and 13 out of 53 epi-units from 11 districts in Chhattisgarh state. In the Western region, > 70% prevalence of PPRV antibodies in small ruminants across various epi-units in Maharashtra, (26 out of 43 epi-units from 17 districts); in Gujarat (33 out of 60 epi-units from 17 districts), in Rajasthan, (31 out of 60 epi-units from 18 districts) and in Goa (1 of 25 epi-units from 2 districts) was observed. The district-wise details of the prevalence level of PPRV antibodies in small ruminants (Fig. 2B) in various epi-units in different states are provided as supplementary tables (Tables S1 to S6).

Table 1.

Prevalence of the PPR virus antibodies in small ruminants in central and western regions of India

Name of the State	No. of Tehsil/Block	No. of Village/Epi-Unit	No. of serum samples screened			No. of the samples positive in ELISA			Seroprevalence % status in Epi-units (No.)				Prevalence of PPRV antibodies (%) (CI- 95%)
			Sheep	Goats	Total	Sheep	Goats	Total	< 37	37–50	50–70	70–100	Sheep	Goats	Total
Madhya Pradesh	55	60	72	660	732	8	244	252	35	3	10	12	11.11 (6–20)	36.96 (33–41)	34.43 (31–38)
Chhattisgarh	44	53	44	580	624	16	306	322	15	9	16	13	36.36 (24–51)	52.76 (49–57)	51.60 (48–56)
Goa	10	25	–	260	260	–	54	54	18	2	4	1	–	20.77 (16–26)	20.77 (16–26)
Maharashtra	41	43	223	386	609	183	268	451	3	6	8	26	82.06 (76–86)	69.43 (65–74)	74.06 (70–77)
Gujarat	51	60	576	637	1213	399	430	829	9	–	18	33	69.27 (65–73)	67.5 (64–71)	68.34 (66–71)
Rajasthan	51	60	624	625	1249	441	368	809	13	3	13	31	70.67 (66–73)	58.88 (54–62)	64.77 (62–67)
Total	252	301	1539	3148	4687	1047	1670	2717	93	23	69	116	68.03 (65–70)	53.05 (51–55)	57.97 (57–59)

Chi square value (sheep-χ² = 150.14, p < 0.01; goats-χ² = 280.85, p < 0.01; species χ² = 95.22 p < 0.01)

Fig. 2.

Percent positivity levels of PPR virus antibodies distribution in the studied states—A. Epi-units, B. Districts

Further, the chi-squared test revealed an association between the antibodies positivity across the studied states in sheep (χ² = 150.14, p < 0.01) and goats (χ² = 280.85, p < 0.01). In the central region, in MP, PPRV antibodies in goats were associated across various districts (χ² = 85.58; p < 0.01), and age (χ² = 13.38, p < 0.01), whereas the sex of animals was not associated (χ² = 0.15, p > 0.10). Similarly, in Chhattisgarh, PPRV antibodies in goats were significantly associated across the districts (χ² = 73.97, p < 0.01). In Western region, PPRV antibodies association was observed in small ruminants across various districts in Maharashtra, (goats χ² = 19.96, p < 0.05 & sheep χ² = 75.04, p < 0.01), in Gujarat (sheep-χ² = 52.13, p < 0.01 & goats χ² = 89.22, p < 0.01), in Rajasthan, (sheep-χ² = 87.58, p < 0.01 & goats-χ² = 94.46, p < 0.01) and in Goa (goats-χ² = 4.54, p < 0.05).

The observed vaccination coverage varied between the states in the studied regions, and the vaccination coverage percentage was 44, 37, and 4.0% in MP; 80, 71, and 80%, in Chhattisgarh; 61, 29, and 79% in Gujarat and 15, 51, and 74% in Maharashtra during FY 2016–2017, 2017–2018 and 2018–2019, respectively, with the required vaccination coverage in Rajasthan and no vaccination in Goa state during these periods. Further, based on the passive surveillance official data, the total number of reported PPR outbreaks for the last five FY from 2015 to 2016 to 2020–2021 was 12, 119, 20, 44, and four (4) for MP, Maharashtra, Gujarat, Rajasthan, and Goa states, respectively. In 2020, 39 outbreaks were reported from 15 districts in the studied regions, of which Maharashtra had the highest (33 outbreaks from nine districts) and the lowest two outbreaks from either one or two districts of other states. The state-wise details of the number of vaccinations carried out in small ruminants’ population were depicted in Fig. 3, along with the FY reported outbreaks in each of the studied states. In the states where continuous vaccination is being adopted, disease outbreaks are being reported sporadically. Further, a decline in the number of reported year-wise outbreaks was depicted in these states since disease reports and PPR were restricted to only a few districts in 2020 (Fig. 4).

Fig. 3.

Trend of reported outbreaks in the states of Madhya Pradesh (A), Chhattisgarh (B), Maharashtra (C), Goa (D), Gujarat (E), and Rajasthan (F) from the financial year 2015–2016 to 2020–2021 versus the doses of vaccine used (in million) in the vaccination program during the preceding years against the small ruminant population (outbreaks in number , vaccination in million )

Fig. 4.

The reported outbreaks in different categories of endemic districts of the studied states with risk areas are depicted in six scales at different periods of analysis using QGIS-2.18 in the maps of the respective states. The endemicity of PPR outbreak categorization is based on the scale of the cumulative outbreaks that occurred in the districts per year in the given period of analysis and classified into different risk levels (if the outbreaks numbers in the district as 0—no risk, 1—very low, 2—low, 3—medium, 4—high and > 4 very high-risk districts). No official outbreaks have been reported in Chhattisgarh state since 2014; however, two outbreaks were observed, with 647 diagnosed and 272 death cases during 2010–2012 as per published report (Govindaraj et al. 16)

Discussion

Understanding PPRV episystems (hotspots” or “niches” means where PPRV circulates or infiltrates from animal movement in many ways and the virus can move into or out of and persist in a population) supports policymakers in taking appropriate decisions towards control and eradication of PPR. This study necessitates intensive mass vaccination [11] by the state animal husbandry and veterinary services to eliminate the PPRV infection. In this direction, the Department of Animal Husbandry and Dairying (DAHD), GoI prepared the National Strategic Plan for PPR Eradication (NPPE) in the lines of the National Rinderpest Eradication Programme (NPRE) to eradicate PPR by 2030 in consonance with the PPR GCES [11].

This serosurvey revealed that the small ruminants in the epi-units had different percentage prevalence of PPRV antibodies with varying levels of post-vaccination immune response to PPRV across districts and states, likely due to variations in vaccination rates and patterns. The present study results corroborate the different prevalence studies on seroprevalence after PPR vaccination in small ruminants reported from different countries, including India. Further, the observed significant PPRV antibodies association across the district within a state might be due to the variation in the vaccination practices among the districts, as stated earlier [6, 7]. In the Western region, as per the 20^th Livestock census, Gujarat and Rajasthan states have 4.86 and 20.84 million goats and 1.78 and 7.9 million sheep, respectively. Gujarat state initiated the mass PPR vaccination program as per the PPR-CP plan for the FY 2016–2017, whereas Rajasthan followed focused vaccination from 2002 to 2003 [20] till 2013–2014 and adopted vaccination campaigns from the FY 2015–2016 to keep the disease under control with the required vaccination coverage for the last 2 years. Similarly, Maharashtra (the state has 10.6 million goats and 2.68 million sheep) initiated focused vaccination during FY 2002–2003 [20] and in the program mode since 2011. However, extensive vaccination was initiated during 2^nd phase of the PPR-CP during 2015–2016. Even though the state had desired overall protective antibody immunity levels of > 70% in 2017–2018, the outbreaks have been reported in some pockets of the state regularly. The vaccination coverage was 14.5, 51.1, 73.7, 26.3, and 3.5% during FY 2016–2017; 2017–2018; 2018–2019; 2019–2020, and 2020–2021, respectively, with the reported outbreaks of 8, 5, 25, 27, and 33 mainly from three to four endemic districts of Maharashtra state. Moreover, the rise in the number of outbreaks since 2019 could be due to a decrease in the percentage of vaccination coverage in the population, as well as due to better reporting mechanisms, especially after implementation of  the control program.

In the central region, MP state has 11.0 and 0.324 million goats and sheep, respectively, and practiced focused vaccination from 2006 to 2007 and implemented PPR-CP with 100% vaccination in the first year, followed by 30% bi-annual vaccination of young animals for the subsequent 2 years since 2016–2017 [16]. Chhattisgarh is a contiguous state of MP with a similar socio-economic and livestock rearing pattern dominated by 4.19 million goats (95.5%) with only 0.18 million sheep and is carved from MP in 2000. The state implemented an annual PPR-Mass Vaccination Campaign (MVC) in the lines of the ‘Pulse Polio model’ through funding from Rashtriya Krishi Vikas Yojana (RKVY), GoI, during FY 2010–2011 for control of PPR [16] with the overall vaccination coverage from 61% in 2010–2011 to 91% in 2015–2016, with no incidence of PPR from FY 2013–2014 onwards [16, 6]. However, the limited primary survey showed the disease incidence level of only 0.8% during 2015–2016 after 6 years of PPR-MVC implementation [16], whereas in non-PPR CP implemented MP state, the reported incidence was 8 and 19.5% during FY 2008–2009 and 2015–2016, respectively [16, 3]. Even though it is impossible to differentiate the vaccinated and infected animals by employing the PPR c-ELISA kit [18], the present prevalence study in the non-outbreaks reported state of Chhattisgarh showed > 52% prevalence of PPRV antibodies, which indicates vaccination is being implemented in the target populations [7], as the baseline seroprevalence before the implementation of the vaccination varied from 32.4 to 46.1% [4, 19]. The strategic PPR-MVC has controlled PPR and it may eventually lead to complete control and eradication [11, 16, 6, 7]. Because an earlier conducted prevalence serosurvey also revealed only a 55% prevalence of PPRV antibodies and was significantly associated with age groups of animals [7]. This might be due to the replacement of the vaccinated population and the appearance of a 30–40% naïve population every year [17] due to high reproductive rates and fecundity of female goats, combined with the slaughtering of male goats at an early age for meat purpose [6, 17] and also the movement of animals from neighboring states. If considering the new-born young animals < 4 months old in the state have the passive immunity of ~ 25–30%, then the overall population immunity will go up to 80–85%, which is above the recommended level of >70% population immunity for the protection of the animals against PPR in the region as per the GCES guidelines [21].

The effect of vaccination and the yearly population immunity progress to vaccination depends on the vaccination coverage and the vaccine’s efficacy at the field level along with the replacement of the existing population with naïve animals. In the states where continuous vaccination is being adopted, disease outbreaks are being reported sporadically in a few pockets where mixing of the animals or gathering occurred (local market, transportation for trade, nomad migration) with a 90% reduction in the incidence of PPR [16, 9]. As no outbreaks have been reported in Chhattisgarh since 2012–2013, it might be due to the maintenance of the > 75% annual percentage of vaccination coverage over the last three FY (2016–2017; 2017–2018 and 2018–2019). Hence, the existing vaccination program needs to be revisited and vaccination may be restricted in areas adjoining state borders, migratory animals, animal markets, and check posts or places of reported sporadic outbreaks in the lines of the NPPE. Generally, 80–95% vaccination coverage of the targeted risk populations is required to achieve the desirable flock immunity to prevent the active transmission of the disease considering the other epidemiological factors for the control and eradication of PPR [22]. Further, to eliminate PPRV, the 70–80% proportion of epi-units with > 70% of animals should have protective antibodies to gain the desired population/herd immunity levels [21]; however, in the present study, none of the 25 epi-units tested in Goa (except Cudnem village) showed > 70% immune response to PPRV as the state has only 9446 goats and 662 sheep and has not adopted a regular vaccination; generally, animals come from neighboring states for trade and slaughtering purposes.

Fournié [14] stated that viral spread could be prevented if the proportion of an immune small ruminant is kept permanently above 37% in at least 71% of the village population in an endemic setting by fitting a metapopulation simulating model for the control of the disease. Due to the high turnover of small ruminants, maintaining the fraction of immune animals above this threshold would require high vaccination coverage through regular/periodic vaccination campaigns. The estimate provided by Fournié [14] concurred with the observed results in Chhattisgarh, Maharashtra, Gujarat, and Rajasthan with 71.7, 93.0, 82.3, 77.0% of the epi-units, respectively implying restricted spread of PPRV infection in these states, and no frequent outbreaks [apart from experienced a few sporadic outbreaks in a few pockets were reported in the recent past in these states, as analysis showed a drastic reduction in the reported outbreaks and cases [16, 6].

Nevertheless, this study had certain limitations, such as the demographic data (age, sex) was available only for the MP state and hence the multi-factorial regression analysis could not be carried out for the entire studied regions. Further, only the reported outbreaks were considered for epidemiological analysis (population, age, and sex data were not available), and these outbreaks were likely under-reported due to a lack of foolproof machinery to collect the real-time disease situation in the field [11]. Generally, the success of disease control depends on various factors, including vaccination and the adoption of biosecurity measures. Based on the success stories of some of the states, an annual mass vaccination program for sheep and goats has been planned by the DAHD, GoI under the NPPE to eradicate PPR in consonance with the PPR GCES plan under PPR-GEP through annual strategic mass vaccination of animals till 2024, attaining desired targeted herd/flock immunity [annual vaccination in “pulse vaccination mode” covering 80–90% of the entire sheep and goats of > 4 months age-old in the pre-designated period for 2–3 years to reach 70–80% population immunity level in each of the states to eliminate PPR in the identified epi-zone/region along with sero-monitoring and surveillance] and stoppage of virus circulation through surveillance by 2025/26 and followed by freedom from PPRV infection to control and eliminate the PPR from India [11].

Conclusion

This serosurvey provides information on population immunity of post-vaccination assessment of PPR virus antibodies in sheep and goats in various epi-units of the studied regions. A difference in seropositivity in sheep and goats was observed across the states in sheep and goats in the studied regions. The sixty-percent of the epi-units had > 50% post-vaccination PPRV antibodies across states, likely due to variations in vaccination rates and patterns. Despite variation in the proportion of seropositive animals in these populations, PPR outbreaks have progressively declined in most of the studied states, and the PPR risk areas are confined to a few districts where mass vaccination has been implemented due to the effectiveness of the vaccine as well as the vaccination programme itself. These findings provide valuable information on potential PPRV episystems, and it will assist activities regarding intensive surveillance, vaccination, biosecurity, and modification of policy decisions towards designing and implementing control and eradication measures.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

BV and SSK carried out the laboratory test. KVK and GG analyzed the data and edited the manuscript. DM provided diagnostic support. KPS and DH designed the sample plan and managed the samples. KVK prepared a GIS map for the study area. VB designed the work with overall monitoring, analyzed the data, and drafted and edited the manuscript. PR and BRS provided guidance and support in research work. All authors drafted, read, and approved the final manuscript.

Data availability

Not Applicable.

Declarations

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval

The manuscript does not contain animal experimental trials. Well-trained veterinarians concerning animal welfare regulations collected samples.