Source case finding in San Diego, California, rarely detects the source for children with tuberculosis (TB) infection or disease. One third of all pediatric TB isolates in San Diego are Mycobacterium bovis, a strain associated with raw dairy products. This study was conducted to determine risk factors for TB infection in San Diego.
Case-control study of children ≤5 years old screened for TB as part of routine health care visit. Asymptomatic children with a positive (≥10 mm) Mantoux skin test (TST) were matched by age to 1 to 2 children with negative TST from the same clinic. We assessed risk factors for TB infection through parental interview and chart review.
A total of 62 cases and 97 controls were enrolled. Eleven cases and 25 controls were excluded from analysis because of previous positive skin tests. Compared with controls, cases were more likely to have received BCG vaccine (73% vs 7%, odds ratio [OR] 44), to be foreign born (35% vs 11%, OR 4.3), and to have eaten raw milk or cheese (21% vs 8%, OR 3.76). The median time between the most recent previous TST and the current test was 12 months for cases and 25 months for controls. Other factors associated with a positive TST included foreign travel, staying in a home while out of the country, and having a relative with a positive TST. There was no association between contact with a known TB case. In a multivariable model, receipt of BCG, contact with a relative with a positive TST, and having a previous TST within the past year were independently associated with TB infection.
We identified several new or reemerging associations with positive TST including cross border travel, staying in a foreign home, and eating raw dairy products. The strong associations with BCG receipt and more recent previous TST may represent falsely positive reactions, booster phenomena, or may be markers for a population that is truly at greater risk for TB infection. Unlike studies conducted in nonborder areas, we found no association between positive TB skin tests and contact with a TB case or a foreign visitor. Efforts to control pediatric TB in San Diego need to address local risk factors including consumption of unpasteurized dairy products and cross-border travel. The interpretation of a positive TST in a young child in San Diego who has received BCG is problematic.