A Centers for Disease Control and Prevention–led study links spillover from small mammals such as voles and squirrels to cases of borealpox in five adults and one child in Alaska from 2020 to 2023.

Borealpox virus (BRPV; formerly Alaskapox virus) is an orthopoxvirus (OPXV) first found in 2015 in a woman living near Fairbanks, in Alaska’s interior. The infection was identified as a novel OPXV, but the source was unidentified.

Published yesterday in Clinical Infectious Diseases, the study involved patient or parent interviews, trapping 176 wild small mammals at six sites for OPXV testing, and phylogenetic analyses of viral DNA sequences to reconstruct their evolution.

Nearly all OPXVs are zoonotic viruses that infect mammals, with rodents often the primary animal reservoir. “Human population immunity to OPXVs is waning globally after the eradication of smallpox, which is likely a reason why OPXVs are being identified with increasing frequency globally,” the investigators wrote.

Immune-compromised patient died

Five of the infected patients had one or more lesions and lymph node swelling and later recovered, most after receiving antibiotics. The other patient, an older man with a weakened immune system, was hospitalized and died despite receiving experimental OPXV medications. 

Better understanding BRPV ecology might help develop more focused prevention measures in addition to standard recommendations to prevent zoonotic infections, such as practicing hand hygiene and avoiding contact with wild animals.

One patient reported vaccination against smallpox, and all had contact with domestic animals, many of which hunted small mammals. One patient’s dog tested positive for borealpox. 

No patients had traveled outside of Alaska, and no evidence of person-to-person spread was found. All BRPV genomes were nearly 100% identical to that of the virus isolated from the 2015 patient.

Several small-mammal species had BRPV DNA and evidence of past OPXV infection in their blood. Genetic distance and phylogenetic analyses pointed to multiple animal-to-human spillover events.

“Better understanding BRPV ecology might help develop more focused prevention measures in addition to standard recommendations to prevent zoonotic infections, such as practicing hand hygiene and avoiding contact with wild animals, including taking measures to keep small mammals out of buildings,” the authors wrote.

They called for research into BRPV’s geographic range in small mammals in northern regions.