Worldwide Prevalence of Small Ruminant Lentiviruses in Sheep: A Systematic Review and Meta-Analysis

Abstract

Small Ruminant Lentiviruses (SRLV) are highly prevalent retroviruses with significant genetic diversity and antigenic heterogeneity that cause a progressive wasting disease of sheep called Maedi-visna. This work provides a systematic review and meta-analysis of the last 40 years (1981–2020) of scientific publications on SRLV individual and flock prevalence. Fifty-eight publications and 314 studies were included. Most articles used a single diagnostic test to estimate prevalence (77.6%), whereas articles using three or more tests were scarce (6.9%). Serological tests are more frequently used than direct methods and ELISA has progressively replaced AGID over the last decades. SRLV infection in sheep is widespread across the world, with Europe showing the highest individual prevalence (40.9%) and being the geographical area in which most studies have been performed. Africa, Asia, and North America show values between 16.7% to 21.8% at the individual level. South and Central America show the lowest individual SRLV prevalence (1.7%). There was a strong positive correlation between individual and flock prevalence (ρ = 0.728; p ≤ 0.001). Despite the global importance of small ruminants, the coverage of knowledge on SRLV prevalence is patchy and inconsistent. There is a lack of a gold standard method and a defined sampling strategy among countries and continents.

Evaluation of Serological Methods and a New Real-Time Nested PCR for Small Ruminant Lentiviruses

“…In order to evaluate the performance of each test in the absence of a generally accepted gold standard, the SRLV true positive status was identified as a composite reference standard as recommended by others [13,16,[26][27][28]. For that purpose, the composite concordance of the serological results across the different screening tests was analyzed, as shown in the Venn diagram (Figure 1).…”

“…Based on these results, we arbitrarily defined the condition of a positive reaction with at least two screening ELISAs as a composite standard rule for serological truth, complemented with real-time PCR as an additional, independent truth standard. In order to evaluate the performance of each test in the absence of a generally accepted gold standard, the SRLV true positive status was identified as a composite reference standard as recommended by others [13,16,[26][27][28]. For that purpose, the composite concordance of the serological results across the different screening tests was analyzed, as shown in the Venn diagram (Figure 1).…”

“…A major drawback of serology is the high genetic variability of the SRLVs, translating into high antigenic diversity [14,15]. Therefore, the serological diagnosis of SRLV remains quite challenging [7,12,16] in the absence of a "gold standard" [17]. The high genomic variability also complicates the design of tools for the molecular detection of SRLV.…”

Gene Expression Profiling Reveals New Pathways and Genes Associated with Visna/Maedi Viral Disease

“…Animals become infected when young via the oral and respiratory routes, and afterwards the infection progresses slowly and clinical manifestations appear mainly in adults. SRLV infection is widespread across the world, having the highest individual prevalence (40.9%) in Europe and a prevalence higher than 15% in Africa, Asia and North America [2]. The infection usually causes multi-organ failure, leading to serious diseases such as pneumonia, mastitis, arthritis, wasting and various neurological disorders [3].…”

Detection of aluminium hydroxide‐induced granulomas in sheep by computed tomography: A feasible approach for small ruminant lentiviruses diagnosis and research

“…Recently, macrophages within Al hydroxide‐induced granulomas have been identified as a small ruminant lentivirus (SRLV) replication site in naturally infected sheep (Echeverría, de Miguel, Asín, et al., 2020). SRLV infection is widespread in sheep all over the world, sometimes with remarkably high individual and flock seroprevalence (de Miguel et al., 2021). This multisystemic, chronic infection causes several types of production loses and limitations to trade of live animals (Echeverría, De Miguel, De Pablo‐Maiso, et al., 2020).…”