Evaluation of the Minnesota Blastomycosis Surveillance System for Human and Veterinary Cases

BACKGROUND:  Blastomycosis is caused by the fungus Blastomyces dermatitidis, found in moist soils along waterways in the Midwest. Transmission of blastomycosis occurs by inhalation of the spores, and the disease generally manifests as acute pulmonary disease. Human and veterinary cases are reportable to the Minnesota Department of Health (MDH). Canine cases are more numerous than human cases, and interviewing the owners of infected pets helps define blastomycosis-endemic areas. There were a total of 475 human cases (range, 22-49 cases annually) and 1,045 veterinary cases (range, 52-99 cases annually) in Minnesota during 1999-2013. A formal evaluation of the blastomycosis surveillance system’s overall usefulness and effectiveness was conducted.

METHODS:  This surveillance system was evaluated using the 2001 Centers for Disease Control and Prevention Updated Guidelines for Evaluating Public Health Surveillance Systems. Requirements for blastomycosis reporting and case-patient follow up were assessed, and the system’s performance was evaluated based on its simplicity, flexibility, data quality, acceptability, test sensitivity, representativeness, timeliness, stability, and overall usefulness. The overall system, as well as each attribute, was rated using a 5 point scale (excellent, very good, good, fair, and poor). Recommendations were given to improve the system.

RESULTS:  MDH’s blastomycosis surveillance system is a flexible and stable system that provides high quality data. It is capable of receiving reports from health care providers (HCPs), infection control preventionists (ICPs), and veterinarians in multiple formats. MDH staff contact HCPs and veterinarians to obtain clinical and demographic data before interviewing case-patients or pet owners for symptom and exposure histories. The resulting data are complete and valid, and surveillance findings are available through statistics and maps on the MDH website. Since MDH’s surveillance system is passive and laboratory tests may be performed within veterinary offices, some veterinary cases may not be reported to MDH. Data quality would be classified as “very good” or “excellent” with minor administrative changes to ensure that a greater percentage of veterinary cases are captured. Because blastomycosis reporting is often delayed and interviewing is currently completed by one part-time student, there is a lapse in the time between diagnosis and interviewing, increasing the likelihood of recall bias and case-patient loss-to-follow-up.

CONCLUSIONS:  Overall, MDH’s blastomycosis surveillance system is a representative system that is capable of capturing geographic distribution, clustering of cases, and clinical characteristics of human and veterinary blastomycosis cases in Minnesota. Weaknesses of the current blastomycosis surveillance system have been identified, and recommendations have been made to improve effectiveness.