suppression is associated with a reduced risk of tuberculosis (TB) transmission
to household contacts, according to research published in the online edition of
Clinical Infectious Diseases.
Researchers in Lima, Peru, compared the risk of TB transmission to household
contacts of people newly diagnosed with TB according to HIV infection status,
and for the people with HIV, according to CD4 cell count. People living with HIV who had a CD4 cell count below 250 cells/mm3 were 50% less
likely to transmit TB to contacts than people who did not have HIV.
suggest that HIV-induced immunosuppression reduces the risk of TB transmission
from HIV co-infected index cases within households,” write the authors.
“Possible biological explanations are that immunosuppressed patients are
infectious for shorter periods of time and that the reduced TB bacillary load
and tissue destruction typical of late stage HIV reduced the likelihood of
Globally, TB is a
leading cause of death among people with HIV, and several studies have shown
that HIV is contributing to worsening TB epidemics in a number of settings.
However, the impact of HIV co-infection on the transmission of TB to household
contacts is unclear. Some studies have shown that HIV co-infection status has
no impact on the risk of transmission, whereas others have shown that people with HIV are less likely to pass on TB.
A possible reason
for the inconsistency in study results is that many of the existing studies did
not take into account the CD4 cell counts of the HIV-positive patients. Immune
suppression has been associated with reduced TB infectiousness.
Taking this factor
into account, investigators in Lima designed an observational study, comparing
rates of TB transmission to household contacts according to HIV infection
status, and, for the people with HIV according to immune suppression (CD4
cell count below vs above 250 cells/mm3).
All the index
patients had sputum smear-positive TB and were diagnosed with the infection between
September 2009 and August 2012.
household contacts and those with TB were identified using tuberculin
skin tests. Contacts with previous active TB or a positive tuberculin skin test
population comprised 1608 index patients (3% HIV positive) with active TB, and
4841 household contacts (2% HIV positive).
The prevalence of
TB in household contacts was 45% in adults and 23% in children.
associated with a positive tuberculin skin test among household contacts included
three or more BCG vaccinations scars, being related to the index patients and
living in an apartment or sub-standard accommodation. These factors remained
significant in multivariate analysis.
This analysis also
showed that household contacts of people with HIV with CD4 cell counts
below 250 cells/mm3 were half as likely to have TB compared
to contacts of HIV-negative TB patients (RR = 0.49; 95% CI, 0.24-0.96).
This finding was
unaltered when the investigators controlled for sputum smear status, delay in
initiating TB therapy, and cavitary disease status.
People living with HIV who had a CD4 cell count above 250 cell/mm3 were as likely to
transmit TB to household contacts as HIV-negative patients (RR = 0.90; 95% CI, 0.55-1.47).
No child under the
age of 15 who was exposed to an HIV-positive index patient with a CD4 cell count
below 250 cells/mm3 was infected with TB, compared to 22% of those
in contact with HIV-negative patients. The risk of TB infection for children
exposed to a non-immune suppressed HIV-positive individual was not significantly
different to the risk for children in contact with HIV-negative patients.
offer three explanations for the reduced infectiousness of TB patients with low CD4
- Shorter period of
infectiousness, as immune suppression is associated with more rapid diagnosis
or progression to death.
- Increased likelihood of being
sputum smear negative and therefore less infectious.
- Lower risk of having lung
tissue damage. The extent of damage has been associated with infectiousness in
“We found that
[household contacts] exposed to HIV-positive TB patients whose CD4 counts were
< 250 cells/mm3 had half the risk of TB infection compared to
[household contacts] exposed to HIV-negative index cases,” comment the