Incident TB Infections
There were 54 new clinical TB cases identified after HAART initiation giving an overall TB incidence rate of 4.5 per 100 py (95% CI: 3.3 to 5.8). There were 3 cases of incident TB in the group with prevalent TB at the time of HAART initiation. Early incident TB was 3-fold higher (11.5/100 py; 95% CI: 7.1 to 17.5) compared with late incident TB (3.2/100 py, 95% CI: 2.2 to 4.5; IRR, 3.6; 95% CI: 2.0 to 6.4; P < 0.001) (Fig. 2). Diagnosis of incident PTB (N = 29) was made through sputum smear (n = 7/29), chest x-ray (n = 19/29), sputum culture (n = 1/3), and clinical grounds only (n = 2/29). Diagnosis of incident EPTB (n = 23) was made through diagnostic radiology (n = 13/19), pleural tap (n = 1/1), histology from biopsy specimen (n = 1/1), clinical grounds only (n = 5/23), and unknown (n = 3). Two patients with incident TB had both PTB and EPTB, which was diagnosed by diagnostic radiology.
Risk Factors for Early and Late Incident TB
The rate of early incident TB was almost 2-fold higher among female; 13.3/100 py; (95% CI: 7.6 to 21.3) compared with male patients; 7.3/100 py; (95% CI: 2.0 to 18.8; IRR, 1.8; 95% CI: 0.6 to 7.4; P = 0.21), and almost 5-fold higher among patients aged between 24 and 34 years (17.8/100 py, 95% CI: 10.0 to 29.4) compared with ≥35 years (3.8/100 py, 95% CI: 0.8 to 11.1; IRR, 4.7; 95% CI: 1.3 to 25.2; P = 0.01) (Table 3). However, interaction between gender and age was not statistically significant (P = 0.10). No other factors were found to be associated with incident TB.
Baseline immune status of patients did not impact overall TB incidence rates; patients with CD4+ counts of <50 cells per cubic millimeter had TB incidence rates of 5.3/100 py (95% CI: 2.7 to 9.3) compared with 4.9/100 py (95% CI: 2.4 to 9.0) in patients with CD4+ counts of >200 cells per cubic millimeter (P = 0.81). Additionally, baseline immune status did not impact rates of incident TB.
TB incidence rates 3 months post-HAART initiation among patients with CD4+ counts of <50, between 50 and 200, and >200 cells per cubic millimeter were 4.6 (95% CI: 2.1 to 8.8), 2.4 (95% CI: 1.3 to 4.0), and 4.7 (95% CI: 2.0 to 9.2) per 100 py (Table 2).
Impact on Therapeutic Outcomes
There were 51 incident cases of TB among patients with no prevalent TB at baseline (26 PTB, 25 EPTB), and 3 incident cases of PTB among patients with prevalent TB at baseline. There was one case of multidrug-resistant TB in this cohort. Patients with incident PTB and EPTB had similar median baseline CD4+ counts, 106 versus 129 cells per cubic millimeter (P = 0.57), and baseline log viral loads, 5.2 versus 5.0 (P = 0.19); and similar mean CD4+ count increases (P = 0.17) and log viral load decreases (P = 0.16) from baseline to 12 months post-HAART initiation. The median (IQR) months on HAART to a new episode of TB was 3.7 (0.8–10.9) and 5.2 (1.6–10.8) in the PTB and EPTB groups, respectively. Median CD4+ count change from baseline to 12 months post-HAART initiation was significantly higher in patients experiencing early incident 167 (IQR, 141–235) versus late incident 76 (IQR, 32–187) cellsper cubic millimeter TB (P = 0.03) (see Table S1, Supplemental Digital Content, http://links.lww.com/QAI/A486).
There were 57 deaths in this cohort with an overall mortality rate of 4.3 per 100 py (95% CI: 3.3 to 5.6). There were 7 deaths among patients with baseline prevalent TB, mortality rate of 2.7 (95% CI: 1.1 to 5.6) per 100 py compared with 50 deaths among 796 patients with no baseline prevalent TB, mortality rate of 4.7 (95% CI: 3.5 to 6.3) per 100 py (P = 0.20) of follow-up. Patients with at least 1 episode of TB (either history, prevalent or incident TB) had a lower mortality rate 3.4 (95% CI: 2.0 to 5.4) compared with patients with no TB, 4.5 (95% CI: 3.1 to 6.3) per 100 py (IRR, 0.8; 95% CI: 0.5 to 1.4; P = 0.42) of follow-up. There was 1 death each in the early and late incident TB groups. The causes of death for 29 of 57 patients was known, and this included PTB (n = 4), EPTB (n = 5), complications from severe gastroenteritis (n = 6), trauma (n = 2), natural causes (n = 5), meningitis-unknown cause (n = 2), lower respiratory tract infection (n = 2), cerebrovascular accident (n = 1), intracranial lesion (n = 1), and complications from hyperlactatemia (n = 1). Causes of death were not available in 28 patients as many of them died at home.
In a rural HIV treatment programme in a TB endemic setting, we found unacceptably high TB incidence rates irrespective of baseline immunologic status of patients at HAART initiation, or duration on HAART. We document TB incidence rates of 4.5/100 py of follow-up among HIV-infected patients receiving community-based HAART care in a rural setting. This incidence is higher than reports from other sub-Saharan countries and most other high TB burden settings2,21,22 and nearly 10-fold higher than reported among HIV-negative patients from a setting with similar TB notification rates.23 Thus, we provide further evidence for continued susceptibility of HAART-accessing patients to TB infection in a hyper-endemic TB setting sustained over 2 years post-HAART initiation. This continued susceptibility to TB despite HAART likely points to ongoing community-level and possibly nosocomial TB transmission.
We demonstrate an alarmingly high and consistent TB incidence (Table 2) among all baseline CD4+ count strata during study follow-up. The data show an inverse relationship between time on HAART and TB incidence, corroborating past studies, and likely because of greater TB-specific immune restoration with time spent on HAART.6,24–26 Furthermore, we highlight the need for close clinical observation in the first few months post-HAART initiation by demonstrating highest rates of incident TB, likely because of “unmasked” infection, in the first 3 months post-HAART initiation. Additionally, the continued high rate of new TB infections even at 24 months post-HAART initiation is in keeping with published reports showing that, despite its decline with time on HAART, TB incidence among a HAART-accessing HIV population remains higher than in the general HIV-uninfected population.27 We speculate that the high TB incidence rates observed in this study may be because of either impaired restoration of TB specific immunity when patients are severely immune compromised (baseline CD4+ counts ≤200 cells/mm3) at HAART initiation, or because of high ongoing community level TB transmission. The incidence rates observed are most likely because of a combination of both these factors. Interestingly, although women carry a disproportionate burden of HIV in sub-Saharan Africa, and despite finding a 2-fold higher rate of early incident TB in women compared with men, further analysis stratified by age and gender showed no statistically significant difference in risk for either early or late incident TB. Notwithstanding lower pre-HAART CD4+ counts among patients who developed early incident TB, therapeutic outcomes among patients at 12 months post-HAART initiation was similar in all groups.
In contrast to our study, published literature demonstrates a far more substantial time-dependant reduction in TB incidence among patients on HAART.28–37 These studies report highest TB incidence during the first 3 months of HAART10 with a progressive reduction of all forms of TB during the first year of follow-up from 5.77/100 to 2.23/100 py.38 Published meta-analysis data from developed country cohorts estimate a comparable effect of HAART on TB incidence despite differences in background risk of Mycobacterium tuberculosis infection. These data provide an estimated TB incidence of 3 cases per 1000 py among patients accessing HAART, 10-fold lower compared with the TB incidence rates we found.10 Findings similar to ours were reported in only 1 other study, conducted in a densely populated urban informal settlement with an HIV seroprevalence of 28% and TB notification rate of >1000/100,000 population. In this study, TB incidence was reduced from 22.1 to 4.5/100 py among patients with a median baseline CD4+ count of 96 cells per cubic millimeter (IQR, 46–156), after approximately 3 years of HAART.39
The vast majority of TB episodes among patients with a previous TB history occurred in the 5-year period immediately before HAART initiation, a likely clinical feature of symptomatic HIV disease. We demonstrated a TB prevalence rate of 17.9% among HIV-infected patients initiating HAART, lower than previous reports from a South African community-based HAART program.39 Other published studies from resource-limited settings report a high TB prevalence at HAART enrollment25,39 primarily among patients with CD4+ count <50 cells per cubic millimeter. However, data from this study show high rates of prevalent TB especially in lower baseline CD4+ count strata and not only among those severely immune compromised.40 Among patients enrolling on HAART, 1 in 4 with CD4+ counts of <100 cells per cubic millimeter and 1 in 7 patients with CD4+ counts of ≥100 cells per cubic millimeter had active TB. It is important to note, however, that excluding active TB among HIV-infected patients is usually complex owing to high rates of smear-negative TB and difficulty of diagnosing asymptomatic or subclinical TB, common among patients accessing HAART.41,42
Unsurprisingly, there was no statistically significant difference in mortality rates among cases with known TB at baseline compared with those without. Mortality studies among HIV-infected patients conducted in this setting have repeatedly demonstrated high rates of undiagnosed TB responsible for as much as 79% of all deaths in HIV-infected patients.43,44 Mortality rates were similar among patients with new episodes of EPTB as compared with those with new episodes of PTB; 2.3 (95% CI: 0.1 to 13.1) versus 2.1 (95% CI: 0 to 11.6) per 100 py of follow-up. Interestingly, the mortality rate at 24 months in this study among patients with and without prevalent TB at baseline was very different to 5-year mortality rates of 4.84 and 2.62 per 100 py of follow-up among prevalent TB and TB-free patients initiating antiretroviral therapy in Cape Town, South Africa.45
Although it may be likely that most cases of early incident TB was due to immune reconstitution inflammatory syndrome (IRIS), it remains unclear as to what proportion of incident TB was due to “unmasking” versus “paradoxical” TB IRIS versus new TB infections. It is well understood that IRIS embodies an interpretive crisis because its clinical diagnosis is similar to TB treatment failure, drug toxicities, TB relapse, or new AIDS-defining illness,46 and cannot be diagnosed by available tests.47 Although consensus case definitions for TB IRIS have been formulated,20 the translation of these guidelines to the bedside remains poor. It is likely that this study underestimated rates of TB IRIS due to the poor ability to diagnose this phenomenon. In addition, in the absence of chest x-rays, minimally symptomatic or asymptomatic pulmonary disease may be incorrectly attributed to HIV or to other opportunistic infections.
We acknowledge several limitations. This study was conducted in a hyper-endemic HIV and TB setting, which limits the generalizability of these findings to similar settings. Limited access to chest x-rays in this primary health care setting, especially for asymptomatic patients with low CD4+ counts may have led to significant underreporting of TB, which potentially limits the validity, and to a lesser extent the generalizability of the study. The reliance on TB clinical symptom screening, lack of use of a standardized algorithm for TB screening and diagnosis, together with limited access to TB microscopy services, further contributed to TB cases being missed. The findings from this study highlight the need for enhanced screening and the use of a diagnostic algorithm for TB in a setting with generalized TB and HIV co-epidemics. High rates of smear-negative TB and long delays in TB diagnosis may have led to the misclassification of prevalent TB cases as incident TB, especially for episodes that occurred in the first 3 months post-HAART initiation. Data used were routinely collected programmatic data, and missing data elements may have led to an underestimation of the actual burden of TB in this setting. Additionally, TB outcome data were not always readily available for complicated patients referred for hospitalization or for those who were lost to follow-up. In contrast to published literature, we report low IRIS estimates, which may be because of the retrospective nature of this study and to the lack of standardized clinical tools for diagnosing IRIS. Molecular strain typing would have been extremely useful in determining if cases of recurrent TB in this cohort were due to relapse or reinfection, particularly in patients with early incident TB and in the 3 cases with incident and recurrent TB, but given the lack of cost-effective but unavailable tools such as chest x-rays in this setting, such endeavors were unfortunately unrealistic. It is important to note that our health care facility implemented standard WHO TB infection control measures, and data for this study were collected before the programmatic implementation of IPT. The impact of IPT on TB prevalence and incidence in community-based HAART programmes remains to be explored, however, in this cohort it is important to note that starting patients on IPT with HAART may result in many patients with active TB disease having initiated IPT.
Our study on the prevalence and incidence of TB in a rural, community-based HIV treatment programme has described a disproportionately high TB burden in patients accessing HAART. Our findings highlight the urgent need to implement TB-preventive therapy and TB infection control practices in HAART programmes in endemic settings. TB incidence by baseline CD4+ count is highest among patients with CD4+ count of <50 cells per cubic millimeter40; however, we also observed high TB incidence rates at higher CD4+ counts. Therefore, despite apparent immunologic recovery among HAART-accessing HIV patients, high TB incidence is still a potent threat, possibly reflecting community and nosocomial TB transmission. The availability of point-of-care diagnostic assays such as GeneXpert that readily diagnose TB while patients queue for services will facilitate TB case finding before the start of HAART, thereby reducing TB-related morbidity and mortality commonly found among patients newly enrolled into HAART programmes in TB-endemic setting.
The authors gratefully acknowledge the contributions of the CAPRISA AIDS Treatment team for providing clinical care of study patients. The authors thank the KwaZulu-Natal Department of Health, the staff of the Umgungundlovu district health office, and the nurses at the Mafakhathini Primary Care Clinic for their professional support and for clinical care of patients. The mentorship and oversight provided by Professor Salim Abdool Karim and Quarraisha Abdool Karim of CAPRISA was invaluable, without which this project would not have been possible.
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HIV; TB; tuberculosis; HAART; Africa; KwaZulu-Natal; South Africa
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