Peste des petits ruminants virus infection potentially facilitates bluetongue virus infection in sheep

Introduction

Peste des petits ruminants virus (PPRV) is an important viral pathogen of sheep and goats that can cause respiratory, gastrointestinal, and systemic disease in small ruminants (Kumar et al., 2014). However, PPRV infection may also occur in a subclinical or mild form, particularly in partially immune or less susceptible animals, potentially resulting in alterations in host immune function (Sultana et al., 2022) [7] . Such subclinical infections may not be readily recognized clinically but could influence the susceptibility of affected animals to other infectious agents. Bluetongue virus (BTV), an orbivirus transmitted primarily by Culicoides midges, is widely distributed among ruminants including sheep, goat, cattle and buffalo in India (Subhadra et al., 2023). However, overt disease and clinical signs are prominent in sheep, while other infected ruminants remain disease free. Although BTV infection can cause severe clinical disease in sheep, many infections are often subclinical, depending on the virus serotype, host susceptibility, and immune status (Subhadra et al., 2023). Concurrent infections or underlying immunological disturbances may therefore influence the outcome of BTV infection (Bhanuprakash et al., 2008, Caporale et al., 2014) [1,3,6] . The immunomodulatory effects associated with PPRV infection raise the possibility that even subclinical PPRV infection could predispose sheep to infection with other pathogens, including BTV (Bhanuprakash et al., 2008; Brodie et al., 1998) [1,2] . Detection of BTV infection at a higher frequency among PPRV-infected animals may thus indicate a possible association between the two infections. However, seropositivity or concurrent detection alone cannot establish PPRV as a causal predisposing factor, as factors such as vector exposure, age, management practices, geographic location, vaccination status, and other co-infections may also contribute. Investigating the association between subclinical or clinical PPRV infection and BTV infection could therefore provide insights into potential virus-host interactions and the epizootology of BTV in sheep populations. In this study, serum samples from sheep obtained during November, 2018 in Ottapidaram taluk of in southern Indian district of Thootukudi in Tamil Nadu, were analysed to study the epizootology of PPRV and BTV infections.

Material and Methods

Samples 45 serum samples were separated from blood obtained from sheep during November, 2018 in three locations, Ottudanpatti (n=17), Kandasamypuram, (n=14) and Akkanayakanpatti (n=14), in Ottapidaram taluk of in southern Indian district of Thootukudi in Tamil Nadu and stored in -20ºC until further processing.

Competitive ELISA for anti PPRV antibodies and anti BTV antibodies ID Screen® PPR Competitive ELISA kit and ID Screen® Bluetongue Competitive ELISA kit, manufactured by Innovative Diagnostics, Germany was used for screening of anti PPRV and anti BTV antibodies in the sheep serum samples, respectively, according the manufacturer's instructions. An OD value of less than 50% of negative sample (S/N%) is considered seropositive for PPRV, while an OD value of less than 40% of negative sample (S/N%) is considered seropositive for BTV.

Statistical Analysis

GraphPad Prism 8.4.3 was used for the statistical analysis. Owing to the small sample size of 45, Fisher's exact test was chosen to determine whether there was a statistically significant association between PPRV seropositivity and BTV seropositivity. Baptista-Pike method was used to determine the exact confidence-interval for the odds ratio of PPRV seropositivity as a risk factor for BTV seropositivity. Koopman asymptotic score method was used to calculate the 95% confidence interval (CI) for estimating the risk ratio/relative risk of BTV seropositivity in PPRV seropositive sheep.

Results

Sample collection

The serum were collected from three sheep flocks in the taluk of Ottapidaram in Thootukudi district during monsoon season in November 2018. The sheep flocks were not vaccinated against BTV or PPRV. Additionally, the flocks included four to five goats which served as a "leader goats" during grazing in pastures. The cohabitation of the goats and sheep is a potential source of PPRV infection to sheep as goats are more susceptible to PPRV infection. Since, all the sheep included in this study were raised in the same taluk of Thoothukudi district, it was assumed that seasonal effects, vector exposure, PPRV and BTV exposure history were equal.

Serological status

In the 45 serum samples, 30 sheep were seropositive for PPRV antibodies and 15 were seronegative for PPRV (Table 1 and Figure 1). Among the PPRV seropositive sheep, 23 were also seropositive for BTV antibodies, while 7 were seronegative for BTV. Among the PPRV seronegative sheep, 6 and 9 sheep were seropositive and seronegative for BTV, respectively. The BTV antibody positive status was 76.67% (23/30) in PPRV antibody positive sheep and 40% (6/15) in PPRV antibody negative sheep.

Table 1:

The distribution frequencies of the samples based on their serological status for anti PPRV and anti BTV antibodies. The percentage of each category compared to total number of samples is given in brackets.

Total samples = 45 BTV seropositive BTV seronegative PPRV seropositive 23 (51.11%) 7 (15.56%) PPRV seronegative 6 (13.33%) 9 (20.00%)

The distribution frequencies of the samples based on their serological status for anti PPRV and anti BTV antibodies.

Statistical tests of association of PPRV seropositivity with BTV seropositivity

Association estimated by two sided Fisher's exact test was p = 0.0227 (<0.05) indicated that PPRV seropositivity is significantly associated with BTV seropositivity (Table 2).

The odds ratio of a PPRV positive antibody being BTV positive was estimated at 4.929 with at 95% confidence interval of 1.322 to17.33. The relative risk of PPRV antibody positive sheep being BTV positive was 1.917 with a 95% confidence interval of 1.113 to 3.951. [2,3,4,5,8] . This study, though with small sample size, indicates a strong association between seropositivity of PPRV and BTV in sheep with equal exposure to PPRV from cohabiting goats, BTV from vectors and seasonal factors in a shared geographical area. This study warrants further detailed investigation with temporal clinical, virological and serological parameters to establish the potential of PPRV infection in sheep as a predisposing factor for BTV infection and influence on clinical outcome.

References

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