Lyme Disease Surveillance Using a Random Sample Methodology, Allegheny County, 2015

BACKGROUND: Lyme disease is the most commonly reported vector-borne illness in the United States. Health departments need laboratory and clinical information to classify cases but receive mostly electronic laboratory reports of Lyme disease without clinical information. In 2014, the Allegheny County Health Department (ACHD) followed up on all Lyme disease reports, verifying 822 confirmed and probable cases among 1455 individual reports. Lacking the capacity to investigate every reported case beyond 2014, ACHD adopted a Lyme disease case estimation methodology developed in New York State and also used in Minnesota. When this methodology was applied to 2014 data, it yielded an estimate that was not significantly different from the actual case count. In 2015, ACHD used only the case estimation methodology.

METHODS:  In 2015, an ACHD epidemiologist randomly selected a 20% sample of laboratory reports each quarter for follow up, excluding reports of erythema migrans (EM) which were considered confirmed cases and not included in the sampling frame.  Most laboratory reports selected lacked physician contact information. ACHD staff made numerous phone calls to obtain valid fax numbers for providers. Providers were then sent a one-page form and asked to provide demographic and clinical information. Cases were then classified as confirmed, probable, suspect, or not a case.  When case investigations are complete, confirmed and probable case counts will be multiplied by five and provider reports of EM added to provide an estimate of the true case count.  

RESULTS: During the first three quarters of 2015, ACHD received 1671 reports of Lyme disease.  Of these,112 were noted to have erythema migrans (EM) and counted as confirmed cases.  Of the remaining 1559 lab reports, 312 (20%) were randomly selected for investigation.  Final results of case investigation and case estimates for all of 2015 will be available in March of 2016.  

CONCLUSIONS:   Allegheny County is the only county in Pennsylvania to have applied a random sample methodology to Lyme disease surveillance. Traditional Lyme disease surveillance requires resource intensive follow-up, especially in endemic areas. Case estimation methodology reduces the workload of health department staff while providing reasonable estimates of incidence. While acknowledging that this method makes comparison between counties and states or trends over time difficult to interpret given that estimates are not accepted by CDC, constraints on public health staff time make the random sample methodology an appropriate option for future Lyme disease surveillance in Allegheny County.