Outdoor workers beware: Reported cases of Lyme disease have more than doubled since 1991 when Lyme became a "nationally notifiable disease," according to a recent report from the Centers for Disease Control and Prevention (CDC).

The findings indicate that 93 percent of reported cases were concentrated in 10 states: Connecticut, Delaware, Maryland, Massachusetts, Minnesota, New Jersey, New York, Pennsylvania, Rhode Island and Wisconsin.

The rise in cases is likely the result of both a true increase in the frequency of the disease as well as better recognition and reporting due to enhanced detection of cases, according to CDC.

Approximately 20,000 cases of Lyme disease are reported each year. The infected ticks, mosquitoes and fleas that carry the disease are most active in the summer months.

Early symptoms of infection include fever, headache, fatigue and a characteristic skin rash. Left untreated, infection can spread to joints, the heart and the nervous system, according to CDC. Proactive steps include daily self-examination for ticks, use of repellent containing 20 percent or more of DEET and selective use of insecticides that target ticks.