Seroprevalence of peste des petits ruminants in sheep and goats in Eastern India

Abstract

The seroprevalence study of peste des petits ruminants (PPR) in small ruminants in Bihar and Odisha states in the Eastern region of India was carried out. A total of 1836 serum samples were collected from sheep (n = 648) and goats (n = 1188) from various epidemiological units (n = 112) in these states by a two-stage sampling plan during April 2017–March 2018. These samples were tested for the detection of virus antibodies by PPR competitive ELISA kit. The results revealed that the seroprevalence of PPR in sheep and goats in Bihar and Odisha states was 30.91% and 54.20%, respectively. Further, the chi-square analysis showed that the association exists between the presence of PPR virus antibodies in the goats (χ² = 93.28, p < 0.01) and between the states (χ² = 82.61, p < 0.01). This cross-sectional serosurvey also infers that the sheep and goats in most of the epi-units (n = 87) had < 70% of PPR virus antibodies prevalence. This warrants the intensive continuous mass vaccination program for a few more years to achieve the desired level of population immunity (epidemiological units protection level) and active surveillance to make these states free from PPR in the Eastern region of India.

Electronic supplementary material

The online version of this article (10.1007/s13337-020-00574-2) contains supplementary material, which is available to authorized users.

Peste des petits ruminants (PPR), otherwise known as ‘Plague of small ruminants, is an acute, contagious, notifiable, economically important transboundary animal disease of sheep and goats and caused by the Small ruminants morbillivirus (SRMV), (formerly known as PPR virus-PPRV) (https://ictvonline.org/virusTaxonomy.asp). Clinically, PPR is characterized by high fever, discharges from eyes and nasal orifices, oral ulcers, gastroenteritis, diarrhea and bronchopneumonia [1]. PPR coincides with considerable morbidity and mortality in sheep and goats and causes a serious menace to the national economy of the enzootic countries [2] with severe socio-economic implications to livestock owners. The disease in sheep and goats is reported to cause an estimated economic loss of USD 1.45–2.1 billion annually in Africa, the Middle East and parts of Asia [3]. In India, an annual estimated loss of INR 8058.8 million due to PPR in sheep and goats has been reported [2]. Currently, a global agreement was extended to eradicate PPR with the adoption of a PPR Global Control and Eradication Strategy (GCES) to make the globe free by 2030 [3]. In this direction, the Food and Agriculture Organization and World Organization for Animal Health jointly initiated an international strategic plan for global control and eradication program of PPR into action for the period 2017–2021 [3].

In India, sheep and goats are the “Any Time Money-ATM” of the poor landless, marginal, and small landholder’s farmers and they generate income and employment for their livelihood. PPR is one of the main restraints in augmenting productivity of small ruminants particularly affecting the poor farmers' economy [1]. The disease reporting, understanding of the disease epidemiology, strong support of diagnostics and surveillance and vaccination of the animals are highly imperative for the effective control of disease, besides strategic plan and infrastructure facilities. India practiced focused vaccination (vaccination limited to the place of the outbreak with the radius of 3–10 km to contain the disease spread) in PPR outbreak places since 2002 [4] and in program mode (mass vaccination covering the entire small ruminants population above age of 4 months old and subsequent biannual vaccination of naïve young population) since 2011/2014 in some of the states even before the global strategy was scheduled [5]. During 2010–2011, Government of India (https://www.dahd.nic.in) implemented a national control program on PPR (PPR-CP) [5], in which the vaccination was covered in South peninsular India in the first phase and the remaining states and union territories were included in the second phase from 2014 to 2015 [5]. In India, several outbreaks of PPR in small ruminants have been reported in spite of the vaccinations, and further several outbreaks have not been recorded properly, owing to inadequate animal disease surveillance and reporting systems. Nevertheless, the reported outbreaks have been brought under control and benefits outweigh the cost of the vaccination program [6]. Moreover, neither systematic surveillance nor sero-monitoring was undertaken to assess the effectiveness of the vaccination in a particular state or region or zone.

The prevalence of PPRV antibodies in animals in the field conditions [7] indicates sub-clinical or inapparent infection, as vaccination is irregular and restricted in the enzootic settings of underdeveloped and developing countries in the world. Moreover, the presence of antibodies also indicated in situations, where the animal was exposed to the virus naturally and recovered or animal’s immune response to the vaccine. Studying the seroprevalence from various geographical areas under different agro-climatic conditions may be helpful in devising effective appropriate control strategies [8]. Additionally, a systematic seroepidemiological survey for PPR epidemiological units (epi-units) for the state or zone or region has not been conducted except for a few studies [9, 10]. Therefore, in the present study cross-sectional serosurvey is being applied to establish the seroprevalence of PPR at epi-units’ levels in small ruminants in the Bihar and Odisha states at a given period of time in the Eastern region of India. This will help in knowing the effect of the vaccination program and its strategies for the control of the disease.

Eastern India region is consisting of states of Bihar, Jharkhand, West Bengal, and Odisha and the outbreaks are being reported in this region (NADRES report, https://nivedi.res.in/). In this study, Bihar and Odisha states were selected to assess the status of PPR seroprevalence in sheep and goat populations. In the Indian context, the village is distinct and considered as the epidemiological unit (epi-unit) in the studied states. As per the recent release of provisional key results of the 20th Livestock Census, 2019, India has 148.88 million goats and 74.26 million sheep during 2019 (https://dahd.nic.in/division/provisional-key-results-20th-livestock-census accessed on 25th October 2019). The small ruminants’ population has increased by 10.14% goats and 14.13% sheep when compared to the 19th Livestock Census, 2012. The list of villages in each state having more than 200 sheep and goats (with inclusion and exclusion criteria) was shortlisted, which accounted for the sampling frame. A cross-sectional study was conducted between April 2017 and March 2018 with the null hypothesis (H0) of the independent presence of virus antibodies in sheep and goats across the districts in the studied states and between the states.

To study the prevalence of PPRV antibodies, a two-stage sampling method was employed. Accordingly, the animal unit-level prevalence of 30% was considered as per GCES guidelines [3] and the prevalence of PPR before the implementation of vaccination in India [11]. The village (cluster) level prevalence was set as 5% as reported earlier [2, 6] to determine the sample size at disintegrated levels separately for Bihar and Odisha state. The sample size of primary epi-units and secondary animal units were determined using a two-stage sampling methodology based on epi-calculator (https://www.nivedi.res.in/Nadres_v2/Epical/stratified/random_sampling.php.). Thus, a total of 62 village units and a maximum sample of 1364 animal units [682 for each of the sheep or goats] to be sampled for each state was arrived with a collection of a maximum of 11 samples from each species in each of the epi-unit was determined by the hypergeometric distribution [3] and the epi-units in each state were allocated randomly by R software (https://www.r-project.org/) using sampling frame.

In each epi-unit, animal serum samples were collected randomly through All India Coordinated Research Project on Animal Disease Monitoring and Surveillance (AICRP on ADMAS), a collaborating center of ICAR-NIVEDI, in the respective states. In the village, where only either goats or sheep species reared, samples of either species were collected. The collected sera were labeled and transported to the Institute in an ice-cool box and upon receipt, the samples were stored at − 20 °C until further use. All the serum samples were tested by competitive ELISA [12] for the detection of PPRV specific antibodies, which were measured in terms of percentage inhibition (PI) according to Singh [12] protocol and samples with a PI of ≥ 40% were considered as a positive.

The chi-squared test (χ²) was carried out in MS-office Excel 2016 as per the described method [13] to understand the association (not independent) of PPRV antibodies in sheep and goats between the states and goats across the districts in the state. Further, annual growth rate (GR) of prevalence of PPRV antibodies in the state was assessed to predict the number of years of vaccination required to achieve desired 70% prevalence level of PPRV antibodies level [3] using mathematical formula [GR = [(b − a)/a] × 100/N] as described earlier [10]. Additionally, the number of years considered for growth assessment was three, due to the vaccinated populations would have turned over by then as sheep and goat typical lifespan is 3 years. Further, keeping in view the turnover of the sheep and goat populations, the calculated growth was discounted by 30% each year [11]. Therefore, generally, the population immunity will be only around 70–75% at the point of prevalence study.

The seroprevalence of 30.91% and 54.20% was observed in small ruminants in Bihar and Odisha states, respectively. State-wise details of samples tested for the PPRV antibodies and their prevalence are presented in Table 1. Odisha state is located in the eastern coastal part, surrounded by the states of West Bengal, Jharkhand, Chhattisgarh, and Andhra Pradesh, and the state has 6.51 million goats and 1.5 million sheep population. The state practiced ‘focused vaccination’ from 2004 as and when required for control of the outbreaks. In consonance with PPR-CP, the state implemented a mass vaccination during the year 2016 with the vaccination coverage of 44.8% in 2015–2016; 67.8% in 2016–2017 and 24.9% in 2017–2018 as per record available. On screening of 1299 serum samples, 704 samples were found to be positive for virus antibodies with 49.38% seroprevalence in sheep and 58.99% in goats with association (χ² = 138.50, p < 0.01) across the districts (Supplementary Table S2), as regular vaccination is being practiced in all the districts of Odisha as per PPR-CP plan. Further, the Chi-squared test analysis revealed that the presence of the PPRV antibodies in goats (χ² = 93.28, p < 0.01) and between states (χ² = 82.61, p < 0.01) were associated. The result was concurrent with a previous study where, the seroprevalence of 48.42% in small ruminants with 44.7% in sheep and 51.21% in goats has been reported using district-wise random samples [14]. Further, on the comparison, an increase in the prevalence of PPRV antibodies to the tune of six percent in the population indicates the effects of the implemented vaccination program since 2015 regularly to maintain the population immunity level in the state.

Table 1.

State-wise details of small ruminants population, serum samples tested for PPR virus antibodies, seroprevalence status and calculated Annual growth rate for accomplishing the desired percentage of antibodies prevalence in Eastern India

State name	Population as per the 19th Livestock Censusa (20th Livestock Censusa)		No. of the tehsil /block	No. of the village /epi-unit	No. of the samples screened			No. of the samples positive in ELISA			Seroprevalence % status in epi-units (No.)			Percentage prevalence of PPRV antibodies (CI-95%)
	Sheep	Goats			Sheep	Goats	Total	Sheep	Goats	Total	< 30	30–70	> 70	Sheep	Goats	Total
Biharb	0.23M (0.21M)	12.15M (12.82 )	50	52	–	537	537	–	166	166	31	16	5	–	30.91 (27–35)	30.91 (27–35)
Odisha	1.58M (1.27M)	6.51M (6.39M)	56	60	648	651	1299	320	384	704	10	30	20	49.38 (46–53)	58.99 (55–63)	54.20 (51–57)
Total	1.81M (1.49M)	18.66M 19.21M)	106	112	648	1188	1836	320	550	870	41	46	25	49.38 (46–53)	46.3 (43–49)	47.39 (45–50)

State name	Calculated annual growth rate for accomplishing the desired percentage of antibodies prevalenceb
	2017–2018 In the Present study	2018–2019	2019–2020	2020–2021	2021–2022
Biharb	–	–	–	–	–
Odisha	54.20	56.76	75.58	94.10	113.22

Chi square value (states—χ² = 82.61*** p < 0.01) (goats—χ² = 93.28***p < 0.01) ***Significance

^aM million; as per the recent release of provisional key results of the 20th Livestock Census, 2019, India has 148.88 million goats and 74.26 million sheep during 2019 (https://dahd.nic.in/division/provisional-key-results-20th-livestock-census accessed on 25th October 2019)

^bFor Bihar state annual growth rate could not be carried out due to the prevalence of PPRV antibodies during 2017–2018 was 30%, which is the base level seroprevalence employed for the calculation

Further, the proportion of epi-units with > 70% of animals to be protected as per OIE guidelines was assessed for population immunity. In the present study, > 70% prevalence level of antibodies was observed in 20 out of 60 epi-units (covering the 56 blocks/Tehsils in 21 districts) tested in Odisha state (Supplementary Table S2). Further, the annual growth rate (GR) on the prevalence of PPRV antibodies in the Odisha state was assessed, if a regular mass vaccination program is being in practice for the control and eradication of the disease. The calculated year-wise annual GR by the mathematical model to achieve the desired level of 70% prevalence [3] of antibodies is tabulated (Table 1). Moreover, Fournié [15] estimated the obligatory immune population of > 37% in at least 71% of village populations to restrict the spread of the virus in an endemic setting by fitting a metapopulation simulating model for disease control. This estimate corresponded with the observed results of the current study in Odisha (71.7% of epi-units), which showed the restricted spread of the virus in the state, as there were no frequent outbreaks have been reported for the last three preceding years of the survey period.

Bihar state is situated in the middle of Indo-Gangetic plains of India and located between 24°–27° N latitude and 82°–88° E longitude and possesses 7.63% of India’s total goat population [16] and has 12.15 million goats and 0.23 million sheep population. The state has not implemented the vaccination program as per the adoption of PPR-CP during 2014–2015 and practiced only limited focus vaccination in the event of the outbreak to control the disease spread. On screening of the 537 serum samples, 166 goat samples were found to be positive for PPRV antibodies and were associated (χ² = 68.70, p < 0.01) across the districts (Supplementary Table S3), as the vaccination was not practiced in all the districts of the state as per PPR-CP program. The present result was concurrent with the earlier (2015–2016) report of 34.5% seroprevalence in goats [16]. In Bihar state, only 5 out of 52 epi-units (covering the 50 blocks in 24 districts) tested (Supplementary Table S3), have shown > 70% prevalence epi-unit level antibodies, as the state has not adopted mass vaccination practices and generally animals were migrating from neighboring states for trade purpose. Moreover, it will take a minimum of 4–5 years (2023–2024) to achieve desired 70% target levels of antibodies as envisaged in PPR-CP, because both of these studies indicate prevalence was at the base level of 30–33% seroprevalence [11]. For that, the intensive mass vaccination covering the entire population initially, subsequently bi-annual vaccination covering the naïve young ones needs to be adopted as control strategies [5], in which a post-vaccination immunity of 70% should achieve within 3 years of implementation of vacccination programme [3], after that, vaccination may be restricted to the area adjoining with borders or places of reported outbreaks and the animals in the migratory route and border areas. Nevertheless, the present study needs to be visualized with certain limitations, such as the collected sera over a period of a year, the host factors like breed, age, sex, etc. and the vaccination status of animals was not available for further multi-factorial analysis. Additionally, even though we have targeted 62 epi-units in each state as per the sampling plan, we could not able to achieve the target due to administrative or financial issues.

In conclusion, the present cross-sectional serosurvey provides information on the seroprevalence of PPR in small ruminants in the epi-units of Bihar and Odisha states, as the samples analyzed were a true representation of the finite target population. There was a variation in the seroprevalence levels among the tested states since the vaccination pattern was not uniform between the states. PPR-CP vaccination activities need to be continued for a few more years to achieve the desired epi-units protection levels in the studied states. The study also deduces that the small ruminants in the most of the epi-units were having less than 30% seroprevalence, which warrants the intensive vaccination and active surveillance programs to make PPR free states within the stipulated period as per national PPR control and eradication strategic plan 2025.

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Conflict of interest

Authors declare that no conflict of interest.

Ethical standards

The manuscript does not contain animal experimental trial. No ethical clearance is required for collecting small volumes of blood samples required for seroepidemiological studies, as per CPCSEA (Committee for the Purpose of Control and Supervision of Experiments on Animals) guidelines. Moreover, samples were collected by well-trained veterinarians with respect to animal welfare regulations.