Effective vaccination strategies for intensive rearing lambs and kids against PPR

Introduction

Peste des petits ruminants (PPR) is a highly contagious viral disease and characterized by fever, anorexia, conjunctivitis, nasal discharge, dyspnoea, pneumonia, erosive stomatitis and gastroenteritis (Sen et al., 2010) [15] . The PPR viral disease is caused by small ruminant morbillivirus (SRMV) belonging to the Morbillivirus genus under the family Paramyxoviridae (Gibbs et al., 1979) [11] . Further, the lineage IV of SRMV is mainly endemic in Middle East and Asia (Banyard et al., 2010) [4] among four distinct lineages (Shaila et al., 1996; Dhar et al., 2002) [16,9] . Currently, the vaccination is the only mainstay tool to control the spread of SRMV by producing long-lasting protective immunity. The vaccination of dam during pregnancy helps in efficient transfer of passive maternal antibody to offspring through colostrum thereby protects lambs and kids against SRMV during its early life. Moreover, the age at which the offspring can effectively be immunized is proportional to the amount of maternal antibody acquired from the dam (Bodjo et al., 2006) [6] . In the offspring, there is a window of susceptibility (WOS) period from several days to weeks in which the maternal antibodies recede to a low level to allow a vaccine to work, and also increase the chance of contract of disease (Vono et al., 2019) [22] . Further, the earlier findings recommended the optimum time for vaccination in offspring was at 4 th month of age (MOA) based on the decay maternal antibody level (Balamurugan et al., 2012d) [3] . Interestingly, besides considering the maternal antibody level in offspring, the time of vaccination in dam is also an influential factor in determining the optimum vaccination time in offspring.

So, the present study was devised to estimate the decay of maternal antibody titre in serum of dams and offspring in monthly interval under three categories and thereby the optimum vaccination time would be determined to eradicate SRMV infection from the susceptible populations by covering up the WOS period.

Materials and Methods

Experimental animal

Experimental design

The 60 offsprings were grouped (20 in each group) into three categories based on the interval between dam's vaccination and delivery of lamb/kid i.e. the category I, II and III are the offspring born to 2, 6 and 12 months post vaccinated dam respectively. Further, 6 offsprings born to the unvaccinated dam were used as control animals.

Sampling

The whole blood samples were collected from the dam and its offspring in vacutainer tube (Table 1) and allowed to ooze-out the serum. Then, the serum samples were separated as per the standard procedures and same were tested to measure the percent inhibition value thereby to ascertain the protective maternal antibody titre against SRMV as described in cELISA kit method. The test sera samples showing more than 40 percent inhibition (PI) of mean OD values of the monoclonal antibody control (Cm) wells were taken as positive for PPR antibodies and protective against SRMV provided other control wells fall within the range as per standard values (Singh et al., 2004) [20] .

Statistical analysis

Antibody titre data was summarized as geometric mean titres with 95% confidence intervals. The protective antibody titre was assessed using Kaplan-Meier survival analysis and the survival curves among the different category of animals were compared using the Log-rank (Mantel-Cox) test. Pairwise comparisons between groups were also performed using Log-rank (Mantel-Cox) test. All the statistical analyses were performed using GraphPad prism statistical software version 5.01, (GraphPad, San Diego, California, USA).

Results

Antibody titre as Percent Inhibition (PI) value in serum samples of dam

Results showed that all the 50 dam animals maintained the protective antibody titre above the threshold PI value 40% and the Geometric mean ranged between 52 to 73% against SRMV till 5 th Month of post lambing/kidding (MOL/K) as shown in Figure 1.

Antibody titre as PI value in serum samples of lambs and kids

Based on the Geometric mean of percent inhibition values of antibody against SRMV, the protective threshold value was maintained as 49 to 74% till third month of age (MOA) in category I followed by 48 to 64% till second MOA in category II and only 45% till first MOA in category III. Thereafter, the protective antibody titre in all three categories started declining progressively below the threshold value. Further, the PI value in control group was below the 40% threshold value in all the study interval period (Figure 2).

Protection rate of lambs and kids against SRMV

The protection rate was assessed based on threshold percent inhibition value at 40% in all three categories of animals (Table 2, Figure 3) 3 & Figure 4). Pairwise comparisons also demonstrated that category I animals maintained the protective antibody titres significantly longer than category II and III (Table 4).

Discussion

Peste des petits ruminants is an acute contagious viral disease and considered as an endemic disease among small ruminants in India (Singh, [18,19,17,10] and it is most severe in less than one year age animals (Constable et al., 2017) [8] . Hence, the protection of offspring against SRMV during the early life depends on the availability of maternally acquired immunity through colostrum. Moreover, the maternal antibodies influence the optimal time of vaccination in young animals. In the present study, experimental animals have been categorized into three and the serum samples of offspring and dam were tested by cELISA kit method. The results revealed that all the vaccinated dams maintained the protectiveness against SRMV beyond 365 days. This long lasting response is due to regular vaccination of animals at farm and our results were in accordance with earlier report that the flock mean titre remained above the protection threshold 12 months post vaccination (Awa et al., 2002) [2] and longer than two years in sheep and goat (Tatar and Kabakli et. al., 2006) [21] .

In the category-I, all the offsprings maintained above the threshold PI value till 2 nd MOA and even 90% in the 3 rd MOA. This extended protectiveness in offspring support the effective transfer of maternal antibody from the dam (vaccinated during pregnancy). Our result coincide with earlier report that 80% of 3-4 month old offspring born to dam vaccinated during pregnancy were maintained the PPR virus antibodies (Bidjeh et al., 1999) [5] . Similarly, vaccination of dam/adult before parturition or during pregnancy helps for efficient transfer of maternal immunity to their offspring (Hammami et. al [12,7,13] .

In the category-II, all the lambs and kids maintained above the threshold PI value only in the 1 st MOA. Thereafter, only 80% and 40% of offspring maintained the protectiveness in the 2 nd and 3 rd MOA respectively. It might be due to inadequate transfer of maternal immunity. This result was in accordance with Bodjo et al., (2006) [6] were 70% and 95% of lambs became negative for maternal antibody against SRMV at day 75 and 90 after birth. So, the results suggesting that the 60% lambs and kids born to 6 months post vaccinated dam are vulnerable to SRMV infection even in the 3 rd MOA itself.

In the category-III, only 75% of offsprings maintained the protectiveness till 1 st MOA and drastically reduced to 25% in the 2 nd MOA. Thereafter, all the offspring failed to maintain the threshold value and become susceptible to SRMV infection. Though the dam maintained the protectiveness beyond one year, the transfer of maternal antibody to young animals was poor and it might be due to the offsprings born one year after the dam's vaccination. Similarly, Alamerew et al., (2026) [1] reported that the highest seropositivity rate was observed in the ≤ 30 day age group, whereas the lowest was recorded in the 61-90 day age group of offsprings. Our results also inaccordance with Markus et al., (2019) [14] reported that more than 80% of the kids had lost its maternal immunity by 10 weeks (2.5 months) of age. So, the result suggested that 75% lambs and kids born to 12 month post vaccinated dam were vulnerable to SRMV infection even in the 2 nd MOA itself. Therefore, the effective vaccination time could be determined based on the magnitude and persistence of antibody titres, the proportion of animals maintaining protective titres over time, and the median duration of protection estimated from kaplan-meier survival analysis.

Conclusion

PPR is considered as a major constraint with a case fatality rate as high as 100% among small ruminant production. The young animals should be covered by vaccination against SRMV disease when the maternal antibody levels goes below the threshold PI value 40%. Therefore, the offsprings born to the dam vaccinated during pregnancy can be vaccinated early in the 4 th month of age, whereas the offspring born to the one year before vaccinated dam became vulnerable to SRMV infection soon after birth. Hence, these offsprings should be vaccinated as early as in the 2 nd month of age itself. Similarly, the offspring born to the dam vaccinated six month before lambing/kidding should be vaccinated early in third month of age to cover up the period of window of susceptibility against SRMV infection and to maintain at least 80% population with effective protective immunity to eliminate the SRMV virus circulation in the country.

Declaration Conflict of Interest

On behalf of all author declare that no conflict of interest with the publication

Ethics Statement

The animals used in this study were in accordance with the ethics of the TANUVAS institutional animal ethics committee (IAEC).

Acknowledgement

I gratefully acknowledge and thankful to the TANUVAS University for funding and support during entire study period.

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