New TB cases occur rapidly among close contacts of people with pulmonary TB

The close contacts of people with active pulmonary tuberculosis (TB) have high rates of the disease, according to US and Canadian research published in The Journal of Infectious Diseases. Almost one in twenty close contacts contracted TB, with three-quarters of diagnoses made within three months of the diagnosis of the index patient.

“We show that recently exposed close contacts have very high rates of tuberculosis,” write the authors. “These findings emphasize the importance of performing contact investigations immediately after identifying index cases, as a public health measure for detecting new cases of active tuberculosis and taking steps to interrupt transmission.”

The close contacts of people with active pulmonary TB are at high risk of developing the infection. But the research examining rates and timing of TB among the close contacts of people with TB was largely conducted 50 or more years ago.

To better inform modern TB prevention initiatives, investigators from the US and Canada conducted a prospective study examining rates and timing of active TB diagnosis among the close contacts of people recently diagnosed with culture-positive pulmonary TB.

Close contacts were defined as individuals who shared airspace with an individual with pulmonary TB for at least 15 hours a week, or for at least 180 hours or more during the period when the person was infectious. The contacts were screened as soon as possible after they were identified, and again 10 to 12 weeks after their last exposure to the patient.

The contacts were cross-matched with TB registries to see if they too were diagnosed with the disease, follow-up lasting four years. Enrolment took place between 2002 and 2006 and all the contacts were at least 15 years of age.

The study population consisted of 718 people with active pulmonary TB and 4490 close contacts. Tuberculosis was identified in 158 contacts (4%). Children aged five years and younger represented 31% of all close contacts. The majority of contacts (80%) with TB were diagnosed after the index patient.

Half of TB cases among contacts were diagnosed within a month of the index patient, with 75% diagnosed within three months and 92% within a year.

“More than half of all cases of tuberculosis among contacts in our study were diagnosed by 1 month and three quarters by 3 months after index case treatment initiation, suggesting that the opportunity to prevent additional cases of tuberculosis in contacts was more time limited than previously recognized,” comment the authors. “These findings have important implications for the optimal timing, expected yield, and prevention goals of contact investigations.”

Contacts aged five years and younger had a significantly higher risk (p < 0.001) of TB than other age groups.

Rates of active disease were significantly lower (p < 0.001) among contacts with latent TB who initiated and completed an appropriate course of TB therapy.

“Our prospective study provides important new information on the risk of tuberculosis over time in recent contacts to infectious tuberculosis patients,” conclude the researchers. “Our findings support the important role of contact investigation as a means of identifying and treating new cases of active tuberculosis among contacts, and underscore the importance of rapid screening and initiation of treatment for latent tuberculosis infection.”