Vector-Borne and Zoonotic Diseases Program

Lyme Disease

Lyme disease is caused by the bacterium Borrelia burgdorferi and is transmitted to humans by the Black-Legged Tick (Ixodes scapularis) and the Western Black-Legged Tick (Ixodes pacificus). The Black-Legged Tick (formerly called the Deer Tick) is commonly found in the northeastern states and upper Great Lakes Region of the U.S. Most human cases of Lyme disease are reported in these regions; however, cases of Lyme disease have been reported in other states, as well. The Western Black-Legged Tick is found in California, Oregon Washington State and Arizona. These vector tick species generally occur in regions of higher humidity, but due to the fact that humidity levels vary throughout Arizona, humidity levels alone should not be used to determine and/or rule out the distribution and prevalence of the Western Black-Legged tick in Arizona.

A 1991-92 published medical research study, co-authored by a former ADHS Vector-borne and Zoonotic Diseases Program Manager, established the Hualapai Mountains in Mohave County, northwest Arizona as an endemic area for Western Black-Legged Ticks. In addition, the same published medical research study tentatively identified Borerelia burgdorferi, the bacterium that causes Lyme disease, in 4% of the Western Black-Legged Ticks collected during that study. However, the current distribution and prevalence of these ticks is unknown, as no further studies have been performed by ADHS since 1991.

Lyme disease is a reportable condition and has been since 1987, however ADHS statistics do not include those cases that health practitioners and/or laboratories fail to report to state and county health departments. All suspected cases that are reported are investigated by state and/or local health officials. All cases of Lyme disease in the State of Arizona that are included in ADHS statistics of reported cases thus far include only those cases that have been reported to and investigated by state and local health departments, which have met the epidemiological surveillance criteria set forth by CDC.

All cases of Lyme disease included in statistical data in Arizona thus far represent Arizona residents who have gotten infected with this disease while traveling in other states where Lyme disease is also endemic. To date, no cases of Lyme disease contracted in Arizona that have been reported to and investigated by state and local health departments have met the epidemiological surveillance criteria. A number of clinically-diagnosed cases have been reported to state and/or local health departments, but those cases have not met the epidemiologic surveillance criteria set forth by CDC.

Due to the fact that not all cases are reported to state and local health departments by health practitioners and some cases acquired in Arizona may not meet the epidemiologic surveillance criteria, the lack of statistically-counted cases of Lyme disease acquired in Arizona should not deter health practitioners from using their clinical judgment when rendering a clinical diagnosis of Lyme disease in patients reporting symptoms from tick bites acquired in Arizona.

CDC's recommendation to health practitioners is that the epidemiologic surveillance criteria are not intended to be used in the clinical diagnosis of Lyme disease. A clinical diagnosis of Lyme disease should be based on history and symptoms, as the epidemiologic surveillance criteria for confirming and statistically reporting cases of Lyme disease are to be utilized for surveillance purposes only.

For more information, visit the CDC website.