The zoo said it does not know how the lions became infected with COVID-19.
Though the Denver Zoo has vaccinated some of its most susceptible animals such as gorillas and otters against COVID-19, none of the lions nor the two infected tigers are vaccinated.
The zoo said it is planning to vaccinate all of its lions and tigers when more doses of the animal-specific Zoetis COVID-19 vaccine become available. The vaccine has been authorized for experimental use on a case-by-case basis by the U.S. Department of Agriculture.