Utilizing Epidemiologic Analysis to Guide the Expansion of a Large Scale Fairfax County Tuberculosis Contact Investigation Among a Diverse Population

BACKGROUND:  Fairfax County is a highly diverse suburb of Washington, D.C., with a historically higher TB incidence rate when compared to the rest of the state of Virginia. In December 2012, an individual from a Fairfax County high school was diagnosed with active TB disease. In June 2013, two additional individuals from the same school were diagnosed with active TB disease, one of which was a low risk contact and PFGE match to the previous case. Upon review, a large scale contact investigation was conducted to screen and test students and staff at the high school. The investigation included three different contact rings, each broader than the last, with expansion primarily based on higher than expected levels of latent TB infection (LTBI) among the U.S. born contacts.

METHODS: In total, 2,277 students and staff were considered potentially exposed and therefore recommended for TB screening and testing. The expansion to broader contact rings was based on the epidemiological analysis of LTBI among contacts in that contact ring. Data was collected from 19 total points of testing operated by the Fairfax County Health Department and from results received from contacts' primary care provider’s screens.  Standard data was collected from each client and entered semi-daily into a TB access database. Epi Info 7 was utilized for data extraction and weekly analysis. Percent positive LTBIs were calculated and stratified by U.S. born and foreign born contacts. Odds ratios (OR) were also calculated for recent high risk foreign travel among similar stratified groups.

RESULTS:  To date, 1835 contacts were screened and tested and no new cases of active TB disease were identified.  When examining country of birth, 5.6% of U.S. born contacts exhibited LTBI while 28.8% of foreign born contacts exhibited LTBI. No significant difference of LTBI was noted between the three contact rings investigated.  Contacts with LTBI were 2.6 times more likely to have reported recent foreign travel than those who were not LTBI, and that result was statistically significant.

CONCLUSIONS:   The expansion of this investigation along different contact rings was based predominately on higher than expected levels of LTBI among the U.S. born contacts. This determination highlights the need for baseline LTBI prevalence rates among residents of a highly diverse population. Although it is known that some transmission did occur in the high school, the large number of foreign born contacts and reports of recent high risk foreign travel made the determination of the true level of transmission challenging in this investigation.