Outbreaks of SARS-CoV-2 in naturally infected mink farms: Impact, transmission dynamics, genetic patterns, and environmental contamination

SARS-CoV-2 infection outbreaks in minks have serious implications associated with animal health and welfare, and public health. In two naturally infected mink farms (A and B) located in Greece, we investigated the outbreaks and assessed parameters associated with virus transmission, immunity, pathology, and environmental contamination. Symptoms ranged from anorexia and mild depression to respiratory signs of varying intensity. Although the farms were at different breeding stages, mortality was similarly high (8.4% and 10.0%). The viral strains belonged to lineages B.1.1.218 and B.1.1.305, possessing the mink-specific S-Y453F substitution. Lung histopathology identified necrosis of smooth muscle and connective tissue elements of vascular walls, and vasculitis as the main early key events of the acute SARS-CoV-2-induced broncho-interstitial pneumonia. Molecular investigation in two dead minks indicated a consistently higher (0.3-1.3 log(10) RNA copies/g) viral load in organs of the male mink compared to the female. In farm A, the infected farmers were responsible for the significant initial infection of 229 out of 1,000 handled minks, suggesting a very efficient human-to-mink transmission. Subsequent infections across the sheds wherein animals were being housed occurred due to airborne transmission. Based on a R-0 of 2.90 and a growth rate equal to 0.293, the generation time was estimated to be 3.6 days, indicative of the massive SARS-CoV-2 dispersal among minks. After the end of the outbreaks, a similar percentage of animals were immune in the two farms (93.0% and 93.3%), preventing further virus transmission whereas, viral RNA was detected in samples collected from shed surfaces and air. Consequently, strict biosecurity is imperative during the occurrence of clinical signs. Environmental viral load monitoring, in conjunction with NGS should be adopted in mink farm surveillance. The minimum proportion of minks that need to be immunized to avoid outbreaks in farms was calculated at 65.5%, which is important for future vaccination campaigns. Author summarySARS-CoV-2 infection outbreaks in minks were first reported on April 2020. As this condition is important for both minks and humans, we investigated natural infection outbreaks in 2 mink farms at different breeding stages. We observed a very efficient human-to-mink transmission in one of the farms, in which farmers infected a high number of minks due to blood samplings. In both farms, a wide range of symptoms was observed, followed by high mortality spanning a period of 22-23 days, indicative of massive spread among animals and between sheds due to airborne transmission. The immunity conferred after the end of the outbreak was equally very high in both farms (~93%) and epidemiological data suggest that the minimum proportion of immune minks for outbreak halting is 65.5%. The characterized SARS-CoV-2 strains possessed the mink-specific S-Y453F amino-acid substitution. Histopathological findings were suggestive of extensive lung vessel damage before the establishment of classical interstitial pneumonia lesions. We also detected SARS-CoV-2 RNA in dust and air samples after the resolution of the clinical signs. We believe that awareness should be raised for all people involved in mink handling during outbreaks, to minimize the possibilities of direct or airborne transmission through bio-aerosols or infected dust.