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.