Five employees at Disneyland, California have been diagnosed with measles, bringing the total number of cases in the outbreak up to 53.
All workers who have come into contact with the five have been asked to show vaccination records or do a blood test, the Los Angeles Times reports.
Those who have not been vaccinated have been asked to go on paid leave until their health status can be confirmed.
Earlier this month, nine cases of measles were confirmed in two California-based theme parks, and in Utah from people who had visited the resorts between Dec. 17 and 20.
Since then, the disease has spread across three other states and to Mexico.