Testing programmes beginning in 2021 sampled thousands of wild and captive white-tailed deer in the United States and Canada. Antibody positivity and active infections were both common in some regions, in animals showing no obvious illness.
Sequencing showed the pattern was not a single spillover. Deer viruses grouped into many separate lineages, each traceable to a human introduction, after which the virus circulated among deer and accumulated its own mutations. Some variants persisted in deer populations after they had faded in people. At least one suspected deer-to-human transmission has been reported in Ontario, though such events appear rare.
How humans repeatedly transmitted the virus to a wary wild animal remains uncertain, with contaminated water, feeding sites and handling all proposed. So does the more consequential question: whether white-tailed deer constitute a durable animal reservoir capable of returning divergent variants to people, or whether the infections fade out over time.