Six Cases of Borealpox and Evidence of a Zoonotic Source: Alaska, 2020–2023

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Abstract
Background: Borealpox virus (previously known as Alaskapox virus) is an Orthopoxvirus species first identified in a patient living near Fairbanks, Alaska, in 2015; the source of the patient's infection was unknown. Six additional borealpox cases have been identified through 2023. Methods: We conducted interviews to ascertain travel history and potential exposures for the six patients, trapped small mammals for orthopoxvirus testing, and performed a phylogenetic analysis of viral DNA sequences. Results: Five patients had similar clinical illness, consisting of at least one lesion and, in most cases, lymphadenopathy. One patient with underlying immunosuppression experienced severe illness, hospitalization, and died despite receiving investigational orthopoxvirus therapeutics. One of the patients had a self-reported prior history of smallpox vaccination. None had traveled outside of Alaska, and there was no evidence of person-to-person transmission. Borealpox virus DNA and serologic evidence for past orthopoxvirus infection were detected in multiple small-mammal species. Genetic distance and phylogenetic analyses are compatible with multiple spillover events from animals to humans. Conclusions: Epidemiologic, ecologic, and phylogenetic evidence indicate that borealpox is a zoonotic disease, and current data suggest the reservoir is likely small mammals. Although most infections are mild and self-limiting, immunosuppressed persons may be at risk for severe disease. Further studies are needed to determine the geographic range of borealpox virus in small mammals throughout the Circumpolar North.

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