After being exposed to rabies, an unvaccinated dog must receive which immediate action?

The recommended immediate action for an unvaccinated dog exposed to rabies is to administer a rabies vaccine as a part of post-exposure prophylaxis. Rabies is a serious viral disease that can be fatal once clinical symptoms develop, and vaccination is the principal method for prevention following exposure. In such cases, administering a rabies booster vaccine as soon as possible helps to stimulate the dog's immune system to produce antibodies that can neutralize the virus before it takes hold.

This immediate vaccination is critical because the rabies virus has an incubation period, during which time it may not be detected in the animal, and by vaccinating the dog promptly, you can prevent the virus from progressing to stages where serious symptoms would develop.

The other choices—observation, testing, and annual vaccinations—do not provide the immediate intervention required after potential rabies exposure. Observing the dog for 30 days may not prevent the disease if it is already incubating. Testing for rabies typically occurs post-mortem once symptoms have manifested and is not a preventative measure. Annual vaccinations do not address the immediate risks following exposure; they are part of routine preventive care but insufficient for an exposed unvaccinated dog.