1. Introduction
A scoping review is an approach to identifying knowledge gaps, scope a body of literature, clarify concepts, or to investigate research conduct (Munn et al., 2018). Astroviruses (AstVs) belong to theAstroviridae family, and are positive-sense, single-stranded RNA viruses with a genome that varies from 6.4 to 7.3 kb. AstVs are detected in many mammals and birds (Maclachlan et al., 2016) and have been associated with the enteric disease as well as found in asymptomatic animals (Luo et al., 2011; Brnić et al., 2014). In recent years, AstVs have been detected in association with neurologic symptoms in pigs in swine herds of the United States and Europe (Arruda et al., 2017; Matias et al., 2019; Rawal et al., 2019a; Rawal et al., 2019b; Rawal et al., 2020; Boros et al., 2017) cattle (Li et al., 2013), mink (Blomström et al., 2010), and humans (Quan et al., 2010). Astroviruses are found to be species-specific and until now there are no reports of interspecies transmission.
In pigs showing diarrhea, porcine astroviruses (PoAstVs) were first reported about 40 years ago (Bridger et al., 1980). The first molecular characterization of PoAstV was conducted in 2001 (Jonassen et al., 2001). PoAstVs have been reported globally (Benedictis et al., 2011) and are divided into at least five distinct lineages from PoAstV1 to PoAsV5 (Luo, Laurin, & Shan, 2011; Xiao et al., 2013). Based on this background, it is of interest to know if PoAstV is causally related to disease conditions in swine. It is possible that the organism might cause opportunistic secondary infection, or it might be generally present and therefore just incidental. To address how causation can be established several approaches are available. One classical approach is using the Bradford Hill’s guidelines (Höfler et al., 2005) which have been successfully applied to investigate causes and risk factors of several infectious and non-infectious diseases (Awadh et al., 2017; Frank et al., 2016; Waddell et al., 2016). By evaluating the current body of literature and how it would or would not support a claim of causation, it is possible to identify gaps in the literature and direct resources to fill those gaps.
Therefore, the primary objective of this study was to summarize the studies reporting the frequency of detection, associated clinical presentations, and type of samples and diagnostic tools used for the detection of porcine astroviruses. The secondary objective was to summarize the body of knowledge in regard to ability of PoAstVs to cause disease in swine, using the Bradford Hill framework and identify potential gaps in the current body of work. The manuscript is organized as follows: a description of the approach to identifying and characterizing the relevant literature, a descriptive summary of major findings reported in the literature, and a discussion of how the types of studies conducted fit into the Bradford Hill framework, and conclusions that can be drawn.