MADRID—Spanish authorities, ignoring protests by animal-rights activists, killed the dog of a nursing aide infected with Ebola.

Excalibur, a 12-year-old rescue dog, had been left at home alone after Teresa Romero tested positive for the virus and was quarantined in a hospital on Monday. Ms. Romero’s husband, Javier Limón, checked into the same isolation ward for observation, but only after leaving enough food and water to sustain the animal for weeks. The couple has no children.

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