Mapping the seroprevalence of influenza D virus in Swedish dairy herds

Sammanfattning: Influenza D virus, IDV, is a member of the Orthomyxoviridae family and was discovered in North America, 2011. The virus was first found in swine but has since then been found in a broad range of hosts. Studies have shown that cattle appear to be the main reservoir. The epidemiology of IDV is not fully understood, but it appears to be transmitted through direct-contact and aerosols in cattle. Experimental studies on calves have shown that IDV can infect both the upper- and lower respiratory tract but only cause mild respiratory clinical signs. Histological examinations following experimental infection have shown that IDV mainly cases cause epithelial neutrophil infiltration and mild epithelial attenuation. Immunologically there is evidence suggesting that IDV can inhibit the immune response in the host. The threat posed by IDV is the link between it and bovine respiratory disease (BRD). Studies have found that IDV is often prevalent in cattle suffering from BRD and it has been suggested that IDV infections can promote the development of BRD. This study aimed to continue the surveillance and mapping of Influenza D among dairy cattle in Sweden where a former study left off to further explore the prevalence and dynamics of IDV in Sweden. To assess the situation, 338 bulk tank milk samples were collected and analysed with an in-house indirect ELISA. The results from the study suggest that the seroprevalence of IDV in Swedish dairy herds has increased since 2019. It also appears as the seroprevalence for IDV is much higher in southern Sweden. Based on the statistical analysis performed, larger herds seem to be more likely to be positive. However, the difference in mean PP-values was not significantly different between each herd size category. This could be an indication that there are certain age groups in the herds that have elevated antibody levels.