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Archived Comments for: Long term follow-up of drug resistant and drug susceptible tuberculosis contacts in a Low incidence setting

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  1. Transmission of tuberculosis from patients with drug resistant disease: type of contact, IPT and smear status of source cases matter.

    Eduardo Hernandez-Garduno, Universidad Autónoma del Estado de México

    10 December 2012

    This study showed higher rates of TST positivity and disease in close contacts exposed to TB cases with multidrug resistant tuberculosis (MDR-TB) as compared to contacts exposed to drug-sensitive cases. Exposure to isoniazid mono-resistance (HMR-TB) cases was associated with TST positivity only. However there are three major limitations to consider in this study: 1) The study included two types of close contacts: household contacts i.e. those sharing the same house with the source case and type 1 i.e. contacts sharing airspace with the source case for >4 hours per week. The exact time of exposure and closeness to the source case were not ascertained in the study because this information is not routinely captured in the BCCDC TB registry. According to these definitions it is likely that the degree of exposure was different in both groups. For instance a type 1 contact seated one row behind the source case on a plane during a 5 hour flight would be less exposed and likely will have less risk of infection and disease than a household contacts who has always lived in the same house with the source case. An indirect way to assess whether exposure was different between household contacts and type 1 contacts would be to determine if there was any difference with regard to rates of TST positivity and disease in both types of contacts. A statistically significant difference in the rates would indicate the presence of selection bias compromising the validity of the study particularly if type of contact data was unbalanced between the three study groups (DS-TB, HMR-TB and MDR-TB); 2) The study didn't mention whether or not the contacts received IPT, those receiving treatment would be favored and have less risk of disease and 3) Contacts' TST positivity was associated with sources' smear positivity but was secondary disease also associated with sources' smear positivity? The risk of disease is higher if contacts are exposed to smear positive cases with pulmonary disease as compared to those exposed to smear negative disease. If this was the case in this study, then another selection bias was likely present. Therefore, even with the limitation of a small sample size, the analyses of four groups of untreated (no-IPT) contacts by type of contact and smear status of sources (household contact/source AFB+, household contact/source AFB-, type 1 contact/source AFB+, type 1 contact /source AFB-) would have indicated whether exposure to MDR-TB or HDR-RB cases was indeed associated with higher TB transmission rates. As the authors stated, "data from traditional contact tracing studies note variable proportions of tuberculin skin test (TST) positivity and TB disease in MDR-TB contacts (references 13-20 in the study)" indicating that TB transmission from MDR-TB cases to contact is inconsistent and controversial. Future epidemiological studies with larger sample size are needed to clearly establish whether exposure to MDR-TB is associated with increased TB transmission in the community. Particular attention and detail should be observed when it comes to controlling by the aforementioned factors and by determinants and co-morbidities of TB infection as stated in the study.

    Competing interests

    none declared