Why disease due to Mycobacterium tuberculosis is less common than expected in HIV-positive patients in Edinburgh

Risk factors for TB and HIV coinfection in Scotland, 2001 to 2010

The number of patients with tuberculosis (TB) increased steadily in Scotland between 2005 and 2010. Human immunodeficiency virus (HIV) infection has been a contributory factor to increases in TB in a number of comparable industrialised countries. This study investigated the extent of, and risk factors for, TB and HIV coinfection in Scotland from 2001 to 2010. Patients with TB in the national TB database were linked to those in the national HIV database using probabilistic data linkage. Patient records were anonymised to maintain confidentiality. From 2001 to 2010, 106/4, 097 (2.6%, 95% CI: 2.1 to 3.1) TB patients matched with HIV patients, equating to a 10-year incidence of 2.1 cases per million population. Patients with both TB and HIV were more often born outside the United Kingdom,were of black African ethnicity, had refugee status and had extra-thoracic lymph node involvement or cryptic/disseminated TB disease. Individuals with TB and HIV coinfection were younger and symptomatic for a shorter time before their diagnosis of TB, compared with TB patients without HIV. TB and HIV coinfection was relatively uncommon in Scotland in the study period. Clinicians should recognise the potential for HIV infection among TB patients and the importance of offering an HIV test to all TB patients.

Tuberculosis risk varies with the duration of HIV infection: a prospective study of European drug users with known date of HIV seroconversion

BACKGROUND

It is not known whether the risk of active tuberculosis disease varies with the length of time that individuals are infected with HIV.

OBJECTIVE

To study how, independently of CD4 T cell count, the risk of tuberculosis varies with the duration of HIV infection.

METHODS

Using Poisson regression analysis, the incidence of and risk factors for tuberculosis were studied in 683 injecting drug users (IDU) with a documented date of HIV seroconversion followed in seven cohorts in six European countries until 1998.

RESULTS

Overall incidence was 11.5/1000 person-years. Adjusted for CD4 T cell count and geographic region, the risk ratio (RR) for tuberculosis (both pulmonary and extrapulmonary), compared with the first 3 years of HIV infection, was 2.8 for years 4 to 6 of HIV infection [95% confidence interval (CI), 1.3-6.3], 1.2 for year 7 to 9 (95% CI, 0.3-4.2) and 4.6 after 9 years (95% CI, 1.4-15.0). The adjusted RR for geographic region was 13.1 (95% CI, 4.3-40.0) for Amsterdam and 15.8 (95% CI, 4.8-52.0) for the Valencian region of Spain compared with all other sites combined.

CONCLUSION

The risk of tuberculosis is increased relatively early in HIV infection (year 4 to 6) and also later (after year 9) with possibly a relatively silent period between. As expected, IDU in Southern Europe have a substantially higher risk of tuberculosis than IDU in Northern and Central Europe. Amsterdam forms an exception for Northern Europe, with very high incidence rates.

Impact of HIV infection on tuberculosis

Tuberculosis is increasing in prevalence in many countries and is now the leading infectious cause of death world wide, being responsible for three million deaths annually. Infection with HIV, likewise increasing in prevalence, has emerged as the most important predisposing factor for developing overt tuberculosis in people co-infected with Mycobacterium tuberculosis. Owing to the widespread geographical overlap of these two infections, it is estimated that in 1999, HIV related tuberculosis will reach one million cases and will cause 30% of the expected 2.5 million AIDS related deaths. Tuberculosis in HIV infected individuals may have unusual clinical features and can cause diagnostic difficulties. Despite the effectiveness of modern short course treatment, the mortality of HIV related tuberculosis during and after treatment remains high, and this may be due to other HIV related infections. The "cursed duet" of infection with both HIV and M tuberculosis is generating a threat to human health of unparalleled proportions which, if not taken seriously by health workers and decision makers, could become totally unmanageable.

Molecular evidence for heterogeneity of the multiple-drug-resistant Mycobacterium tuberculosis population in Scotland (1990 to 1997)

Multiple-drug-resistant Mycobacterium tuberculosis (MDR-MTB) has been well studied in hospitals or health care institutions and in human immunodeficiency virus-infected populations. However, the characteristics of MDR-MTB in the community have not been well investigated. An understanding of its prevalence and circulation within the community will help to estimate the problem and optimize the strategies for control and prevention of its development and transmission. In this study, MDR-MTB isolates from Scotland collected between 1990 and 1997 were characterized, along with non-drug-resistant isolates. The results showed that they were genetically diverse, suggesting they were unrelated to each other and had probably evolved independently. Several new alleles of rpoB, katG, and ahpC were identified: rpoB codon 525 (ACC-->AAC; Thr525Asn); katG codon 128 (CGG-->CAG; Arg128Gln) and codon 291 (GCT-->CCT; Ala291Pro); and the ahpC synonymous substitution at codon 6 (ATT-->ATC). One of the MDR-MTB isolates from an Asian patient had an IS6110 restriction fragment length polymorphism pattern very similar to that of the MDR-MTB W strain and had the same drug resistance-related alleles but did not have any epidemiological connection with the W strains. Additionally, a cluster of M. tuberculosis isolates was identified in our collection of 715 clinical isolates; the isolates in this cluster had genetic backgrounds very similar to those of the W strains, one of which had already developed multiple drug resistances. The diverse population of MDR-MTB in Scotland, along with a low incidence of drug-resistant M. tuberculosis, has implications for the control of the organism and prevention of its spread.

Immunodeficiency in non-tuberculous mycobacterial disease

T-cell immunity was investigated in eight patients with non-tuberculous mycobacterial disease, to see whether impaired immune function might be the explanation for their infection. Cellular immune function was evaluated in vitro by measuring the proliferation of peripheral blood mononuclear cells in response to both non-specific mitogens (phytohaemagglutinin and pokeweed mitogen) and specific recall antigens (streptokinase-streptodornase and purified protein derivative from Mycobacterium tuberculosis), and in vivo, by measuring the skin test response to a panel of recall antigens. Functionally relevant T-lymphocyte sub-populations (CD4, CD8, activated CD3 and gamma/delta T-cells) were enumerated by two-colour flow cytometry. The results were compared with those for a group of patients with pulmonary tuberculosis, with groups of controls matched for age and smoking habit, and with a patient group receiving steroid treatment. The patients with non-tuberculous mycobacterial disease had poor or absent skin test responses; in vitro, their response to recall antigens was depressed, although their response to mitogens was normal. The patients had significantly raised levels of CD8 lymphocytes and activated T-cells, but lacked any circulating gamma/delta T-cells. There were also differences between the various control groups. In conclusion, this study demonstrates a deficiency in the cellular immune system of these patients, which is most readily detectable by skin testing, or by measuring lymphocyte proliferative responses to recall antigens. However, the study also shows changes in cellular immune responses in controls matched for age and smoking and in patients on steroid treatment, and underscores the need for matched controls. Further work needs to be done to ascertain whether the cellular immune deficiency is a cause of, or is caused by, the mycobacterial infections, and also to investigate the pathological significance of the alterations in T-cell sub-populations.

Increasing incidence of tuberculosis in the young and the elderly in Scotland

BACKGROUND

Since 1987 there has been an arrest in the previously established decline of tuberculosis notifications in Scotland. A study was undertaken to determine whether age contributed to this phenomenon.

METHODS

Notifications of tuberculosis in Scotland were quantified by year and age group for the years 1981-92 from national statistics supplied by the Information and Statistics Division. Population data were obtained from the 1981 and 1991 national censuses.

RESULTS

Age group analysis of pulmonary tuberculosis notifications showed that, in the 0-14 age group, incidence (per 10(5) population) decreased from 7.4 in 1981 to 2.6 in 1987, rising by an estimated 12.6% per annum to 3.7 in 1992. In the 65+ age group incidence declined from 30.1 in 1981 to 17.3 in 1988, and rose by an estimated 4.1% per annum to 22.2 in 1992. In the age groups 15-44 and 45-64 a continuous decrease in notification rate was seen over the period of the study.

CONCLUSIONS

The plateauing of the incidence of tuberculosis in Scotland is associated with significant increases since 1987 of tuberculosis in the young and elderly. Contributions from ethnic minorities and those infected with HIV are negligible. An ageing population over the decade, with the highest tuberculosis rates seen in the older age group, may explain these findings.