Extrapulmonary tuberculosis, human immunodeficiency virus, and foreign birth in North Carolina, 1993 – 2006

Background The proportion of extrapulmonary tuberculosis (EPTB) reported in the United States has been gradually increasing. HIV infection and foreign birth are increasingly associated with tuberculosis and understanding their effect on the clinical presentation of tuberculosis is important. Methods Case-control study of 6,124 persons with tuberculosis reported to the North Carolina Division of Public health from January 1, 1993 to December 31, 2006. Multivariate logistic regression was used to obtain adjusted odds ratios measuring the associations of foreign birth region and US born race/ethnicity, by HIV status, with EPTB. Results Among all patients with tuberculosis, 1,366 (22.3%) had EPTB, 563 (9.2%) were HIV co-infected, and 1,299 (21.2%) were foreign born. Among HIV negative patients, EPTB was associated with being foreign born (adjusted ORs 1.36 to 5.09, depending on region of birth) and with being US born, Black/African American (OR 1.84; 95% CI 1.42, 2.39). Among HIV infected patients, EPTB was associated with being US born, Black/African American (OR 2.60; 95% CI 1.83, 3.71) and with foreign birth in the Americas (OR 5.12; 95% CI 2.84, 9.23). Conclusion Foreign born tuberculosis cases were more likely to have EPTB than US born tuberculosis cases, even in the absence of HIV infection. Increasing proportions of foreign born and HIV-attributable tuberculosis cases in the United States will likely result in a sustained burden of EPTB. Further research is needed to explore why the occurrence and type of EPTB differs by region of birth and whether host genetic and/or bacterial variation can explain these differences in EPTB.


Background
The incidence of tuberculosis (TB) in the United States (US) has been declining over the past decades except for a resurgence from 1985 to 1992 ( Figure 1) [1,2]. In 2006, the number of TB cases in the US reached an all-time low with 13,779 new cases, corresponding to an incidence of 4.6 cases/100,000 persons [3]. The decline in extrapulmonary TB (EPTB) has not been as great as for pulmonary TB (PTB). Consequently, the proportion of EPTB cases has increased from 13.5% of all reported TB cases in 1975 to 21.0% in 2006 ( Figure 1) [3][4][5][6]. This relative increase may be an underestimate due to recent changes in case definitions, as cases with concomitant pulmonary disease or miliary disease are now counted as PTB cases [3][4][5][6].
Several studies have observed that the proportion of EPTB is higher among HIV co-infected individuals [7][8][9][10][11] and foreign born immigrants [8,9,12,13]. The latter population currently accounts for over half of all TB cases in the US [3].
Few studies have quantified the independent effect of HIV and foreign birth on EPTB and none have analyzed their joint association with EPTB. In this study, we investigated the association of HIV and foreign birth location, both individually and jointly, with the occurrence of EPTB among reported TB cases in North Carolina from 1993 to 2006.

Methods
We analyzed all verified TB cases in the North Carolina TB registry that were reported to the Division of Public Health from January 1, 1993 through December 31, 2006. Cases were either laboratory, clinical, or provider verified using standard definitions from the Centers for Disease Control and Prevention (CDC) [3]. Demographic information, TB risk factors, disease presentation, and diagnostic and treatment information on each case were collected using the CDC's standardized Report of Verified Case of Tuberculosis form and entered into the North Carolina registry.

Variables used
The outcome of interest was EPTB, defined as any verified TB case whose site(s) of disease was not recorded as "Pulmonary". Individuals with concomitant PTB and EPTB were excluded due to insufficient numbers and because we aimed to analyze the effect of HIV and foreign birth region on exclusive EPTB.

Statistical Analysis
Crude odds ratios (OR) and ORs from logistic regression were used to measure the association between each exposure category and EPTB. A final model was derived using backward elimination [14,15]. Further interactions beyond HIV infection by race/ethnicity or birth region were not considered. All covariates were assessed for confounding. If the OR changed by more than 10% when a covariate was removed from the model, that covariate was considered a confounder and retained in the model [16]. Finally, a year variable was included in the model to account for increasing implementation of routine HIV testing during the study period.
All analyses were performed using SAS version 9.  The location of EPTB varied by HIV and foreign birth status (Table 3). The most common sites of disease were the pleura (25.1%) and cervical lymph nodes (15.7%). HIV co-infected individuals were more likely to have non-cervical lymph node or miliary disease compared to HIV uninfected individuals, while foreign born EPTB cases were more likely to have cervical lymphadenitis than US born cases. The site of EPTB also varied by region of birth ( Figure 2). Lymphatic TB accounted for 50% of EPTB in East Asia, 47% in India, 45% in Africa, 40% in Southeast Asia, and 35% in the Americas, but only 17% in Europe/ Middle East, and 21% in the US.

Discussion
This study analyzed and quantified the individual and joint associations of HIV infection and foreign birth with EPTB while controlling for important confounders. We observed that foreign born TB cases were more likely to have exclusive EPTB than US born TB cases, even in the absence of HIV infection. HIV infection was also associated with exclusive EPTB above and beyond the effects of race/ethnicity or geographic region for Black/African Americans and foreign born cases from the Americas. We also found that the site of EPTB differed by region and HIV status. Lymphadenitis accounted for a disproportionate

Lymphatic
Other extrapulmonary* Pulmonary Others have also documented an association between EPTB and foreign birth. Wilbershied et al. observed ORs, adjusted for HIV infection and other covariates, ranging from 0.9 to 3.9 for the association of exclusive EPTB with foreign born populations in New York City [9]. They did not account for race/ethnicity among the US born cases, which is an important limitation given we found that the occurrence of EPTB may differ by race/ethnicity.  [8,9]. Onorato et al. reported that HIV infected persons were twice as likely to have EPTB as HIV uninfected persons [11], and Yang et al.
found that those with HIV infection had nearly 5 times the odds of EPTB than HIV uninfected TB cases [10]. These last two studies, however, included cases with concomitant PTB in the EPTB category. Concomitant pulmonary and extrapulmonary disease has been shown to be more common in HIV infected persons [7], which may explain the stronger association they observed.
While EPTB among HIV infected persons is related to immunosuppression, little is known about the factors associated with EPTB among foreign born populations. Some authors have speculated that this is an artifact of screening immigrants for PTB, which could inflate the proportion of EPTB by removing or treating those with PTB prior to immigration [18,19]. We feel this is unlikely, as such an affect would be seen mainly in the first year of immigration, while our and other data [9] suggest that EPTB occurs more often among immigrants who have been in the US more than 5 years. Other factors such as genetic variations in Mycobacterium tuberculosis and/or genetic variations in host immune response may account for the observations. Yang et al. found that TB patients infected with M. tuberculosis (MTB) containing a mutation in the phospholipase-C gene D had more than two times the odds of having extrathoracic TB (with or without thoracic involvement) compared to those with the wild type strain (OR 2.19; 95% CI 1.27, 3.76), while controlling for HIV infection, sex and race [20]. Other studies have found genetic polymorphisms in host immune responses that are associated with EPTB, including Manose-Binding Protein and TB meningitis [21], Interleuken (IL)-1β/IL-1R [22], IL-10 and IFN-γ [23], and NRAMP1 and pleural TB [24]. Fernando et al. found an association between a polymorphism in the P2X 7 gene and EPTB [25]. P2X 7 is a receptor expressed on macrophages which facilitates induction and death of MTB. Polymorphisms that inhibit Potential limitations of our study should be acknowledged. HIV testing was not done for 35.5% of the reported TB cases in our study. This has been noted in other studies analyzing surveillance data, with 30% to 50% of TB patients having unknown HIV status [9,10,26]. However, this is unlikely to affect validity and reliability of the data as noted in a recent analysis of California's TB registry [27]. While HIV testing and counseling is offered free of charge to TB patients at the health departments, incomplete implementation of routine HIV testing likely accounts for why many patients were not tested. The patient may not have been offered an HIV test due to perceived low risk by the clinician, or the patient may have refused testing when offered. Either of these scenarios would result in missing HIV data. In our study, 54% of patients not tested for HIV were at low risk for HIV (>44 years old with no reported substance abuse or homelessness). Nevertheless, testing improved during the study period with 92% of TB cases reported in 2006 receiving HIV testing (Figure 3).
Small numbers of HIV co-infected TB patients hindered our ability to analyze EPTB among foreign born persons with HIV infection, and the absence of CD4 count data precluded a more detailed analysis by level of immunosupression. This may be important as some studies have shown that EPTB, especially disseminated or meningeal disease, occurs more often in individuals with cell counts less than 200 cells/μl [28][29][30], even though other studies found no association between frequency of EPTB and decreasing CD4+ cell counts [31][32][33].

Conclusion
Extrapulmonary TB poses an important hurdle for the elimination of TB in the US. The proportion of EPTB will likely increase as foreign born and HIV attributable TB cases continue to rise in the US. Morbidity and mortality may be exacerbated in this group because of stigma, language, cultural, or immigration-related barriers to timely healthcare. Further research is needed to explore why the occurrence and type of EPTB differs by region of birth and whether host genetic and/or bacterial variation can explain these differences in EPTB.  Year Percent