To maintain hygiene and prevent the spread of parasites in a dog's sandbox, the sand should be cleaned at least once a week by removing feces and other debris. Additionally, the entire sandbox should be replaced with fresh sand every six months. Regular cleaning is crucial to prevent the buildup of bacteria and parasites that can thrive in soiled sand. Using a cover for the sandbox when it's not in use can also help maintain cleanliness by keeping out wildlife and reducing contamination from external sources.