2.2. Inclusion and Exclusion Criteria
The following criteria were applied for screening of the eligible articles:
The original research articles and case reports which documented the influenza viruses in swine in natural settings across the world were included in the analysis.
The experimental studies which did not report the natural cases were excluded from the analysis.
The reviews, letters, editorials, conference proceedings, and articles in a language other than English were not included in the analysis. Duplicate articles were also excluded from the analysis.
The eligible articles (n = 281) thus selected were included in the analysis for this systematic review.