Tuberculosis and gender: exploring the patterns in a case control study in Malawi.

BACKGROUND: In many populations there is an excess of tuberculosis in young women and older men. We explored possible explanations for these patterns, concentrating on human immunodeficiency virus (HIV) status, pregnancy, smoking, cooking smoke exposure, contact with tuberculosis cases within the household or outside, and gender differences in health service usage and diagnostic delay. DESIGN: Case control study in Karonga District, Malawi. METHODS: Cases were new tuberculosis patients with bacteriological or histological evidence of tuberculosis. Controls were selected in the community using field-based random sampling. RESULTS: The study included 598 tuberculosis cases and 992 controls, with an excess of tuberculosis in young females and older males. This was more marked in HIV-positive individuals. HIV infection was a similarly strong risk factor for tuberculosis in both men and women. Tuberculosis was associated with having a family or household contact with tuberculosis for both men and women. For women, but not men, contacts outside the close family and household were also a risk factor for tuberculosis. Tuberculosis was not associated with current or recent pregnancy, or with smoking or smoke exposure. There were no differences between men and women in health service usage or delay. CONCLUSIONS: In this population, HIV infection and contacts with known tuberculosis patients are important determinants of the gender distribution of cases.

Linkage analysis of susceptibility to leprosy type using an IBD regression method.

Leprosy is a chronic disease caused by infection with Mycobacterium leprae, which is manifested across a wide clinical spectrum. There is evidence that susceptibility both to leprosy per se and to the clinical type of leprosy is influenced by host genetic factors. This paper describes the application of an identity by descent regression search for genetic determinants of leprosy type among families from Karonga District, Northern Malawi. Suggestive evidence was found for linkage to leprosy type on chr 21q22 (P<0.001). The methodological implications of the approach and the findings are discussed.

Comparison of two versus three smears in identifying culture-positive tuberculosis patients in a rural African setting with high HIV prevalence.

SETTING: Karonga district, northern Malawi. OBJECTIVE: To compare the sensitivity and specificity of two versus three smears for the diagnosis of pulmonary tuberculosis in a setting with high HIV prevalence. DESIGN: A total of 1992 pulmonary tuberculosis suspects with three sputum smears taken over a 2-7 day period and at least one culture result were studied. Smears were auramine stained and examined using fluorescence microscopy, and positives were confirmed with Ziehl-Neelsen staining and light microscopy. Cultures were set up on Löwenstein-Jensen media. True negative and positive status was defined on the basis of culture. The sensitivity, specificity, and positive and negative predictive values of two and three smears were compared. RESULTS: Compared to culture, the sensitivity, specificity, and positive and negative predictive values of three smears were 70%, 98%, 92%, and 92%, respectively. Restriction to the first two smears gave similar results. Of those detected as smear-positive using three smears, at least 97% would have been detected by two. Among those with HIV serology results available, the sensitivity of two smears for detecting culture-positive tuberculosis was identical to that using three. CONCLUSION: In this setting, using fluorescence and light microscopy, collecting two smears rather than three would only marginally reduce sensitivity and would slightly improve the specificity of diagnosis of tuberculosis; this is unaffected by HIV status. The potential for improving specificity is important because of the costs of misdiagnosis. In practice, both sensitivity and specificity may be increased due to the time saved by examining two rather than three smears.

Environmental mycobacteria in northern Malawi: implications for the epidemiology of tuberculosis and leprosy.

More than 36000 individuals living in rural Malawi were skin tested with antigens derived from 12 different species of environmental mycobacteria. Most were simultaneously tested with RT23 tuberculin, and all were followed up for both tuberculosis and leprosy incidence. Skin test results indicated widespread sensitivity to the environmental antigens, in particular to Mycobacterium scrofulaceum, M. intracellulare and one strain of M. fortuitum. Individuals with evidence of exposure to 'fast growers' (i.e. with induration to antigens from fast growers which exceeded their sensitivity to tuberculin), but not those exposed to 'slow growers', were at reduced risk of contracting both tuberculosis and leprosy, compared to individuals whose indurations to the environmental antigen were less than that to tuberculin. This evidence for cross protection from natural exposure to certain environmental mycobacteria may explain geographic distributions of mycobacterial disease and has important implications for the mechanisms and measurement of protection by mycobacterial vaccines.

Patterns and implications of naturally acquired immune responses to environmental and tuberculous mycobacterial antigens in northern Malawi.

Interferon (IFN)-gamma responsiveness to 12 purified protein derivative (PPD) and new tuberculin antigens from 9 species of mycobacteria was assessed, using a whole blood assay, in 616 young adults living in northern Malawi, where Mycobacterium bovis bacille Calmette-Guérin (BCG) vaccination provides no protection against pulmonary tuberculosis. The prevalence of IFN-gamma responsiveness was highest for PPDs of M. avium, M. intracellulare, and M. scrofulaceum (the MAIS complex). Correlations between responsiveness paralleled genetic relatedness of the mycobacterial species. A randomized, controlled trial was carried out, to assess the increase in IFN-gamma responsiveness to M. tuberculosis PPD that can be attributed to M. bovis BCG vaccination. The BCG-attributable increase in IFN-gamma response to M. tuberculosis PPD was greater for individuals with low initial responsiveness to MAIS antigens than for those with high initial responsiveness. Although not statistically significant, the trend is consistent with the hypothesis that prior exposure to environmental mycobacteria interferes with immune responses to BCG vaccination.

Relationship between IFN-gamma and skin test responsiveness to Mycobacterium tuberculosis PPD in healthy, non-BCG-vaccinated young adults in Northern Malawi.

SETTING: Rural northern Malawi, where vaccination with BCG Glaxo (1077) provides protection against leprosy but not against pulmonary tuberculosis. OBJECTIVE: To evaluate the patterns of responsiveness to purified protein derivative of Mycobacterium tuberculosis (PPD) in terms of delayed type hypersensitivity (DTH) and interferon-gamma (IFN-gamma) production. DESIGN: IFN-gamma was measured in 6 day whole blood cultures diluted 1 in 10, stimulated with PPD RT48, and the results compared to the DTH response to PPD RT23. A total of 633 individuals aged 12 to 28 years, without prior BCG vaccination, were recruited. RESULTS: Overall, 63% of subjects made a positive IFN-gamma response (defined as >62 pg/ml), and 37% gave a DTH induration of >5 mm. A strong correlation between skin test and IFN-gamma responses was observed, although with interesting exceptions: 13/270 individuals with zero DTH showed IFN-gamma responses >500 pg/ml, and 7/53 individuals with >10 mm induration showed IFN-gamma responses < or = 62 pg/ml. The prevalence of skin test responsiveness increased with age, and was higher among older males than females; age-sex patterns were less clear for IFN-gamma production. CONCLUSION: The 6 day IFN-gamma response to PPD correlates well with Mantoux skin test induration. The discordant individuals may represent important subsets in terms of protective immunity and risk of clinical tuberculosis.

Trends in antituberculosis drug resistance in Karonga District, Malawi, 1986-1998.

SETTING: Karonga District, Malawi. OBJECTIVES: To examine long term trends in initial and acquired resistance to antituberculosis drugs in a rural area of Africa. DESIGN: Monitoring of all patients with culture-confirmed tuberculosis 1986-1998. RESULTS: Initial drug resistance results were available for 1121 patients. The proportion resistant to any of the first line drugs (streptomycin, isoniazid, rifampicin or ethambutol) was 9.6%, and to isoniazid 7.2%. Initial resistance to at least isoniazid and rifampicin (multidrug resistance) was seen in only six patients. No initial resistance to ethambutol was found. There was no significant change in initial drug resistance over time. Overall, 22/120 (18%) patients with previous treatment were resistant to at least one drug; only one had multidrug resistance. Acquired resistance decreased over the period of the study. There were no associations between age, sex or human immunodeficiency virus (HIV) status and initial or acquired drug resistance. CONCLUSIONS: Changes in acquired resistance may reflect the recent performance of a control programme more quickly than those in initial resistance. It is encouraging that acquired resistance decreased and levels of multidrug resistance were low despite more than a decade of use of rifampicin. The lack of association between HIV and drug resistance confirms findings elsewhere in Africa.

Tuberculosis: associations with HIV and socioeconomic status in rural Malawi.

Tuberculosis (TB) is associated with human immunodeficiency virus (HIV) infection, increasing age and male sex, but less is known about other risk factors in developing countries. As part of the Karonga Prevention Study in northern Malawi, we conducted a retrospective cohort study in the general population to assess risk factors for the development of TB. Individuals were identified in 1986-89 and TB cases diagnosed up to 1996 were included. TB was confirmed in 62/11,059 (0.56%) HIV negative individuals and 7/182 (3.9%) HIV positive individuals (relative risk 7.1, 95% confidence interval 3.2-15.7). This association was little altered by adjustment for age, sex or socioeconomic factors. The risk of TB was higher in those aged over 30 years than in younger individuals, in men than in women, in those engaged in occupations other than farming than in subsistence farmers, in those living in households with burnt brick dwellings than in those with less well built dwellings, and in those with some schooling than in those with none. These associations persisted after adjusting for age, sex, HIV status and population density. The absolute risks of TB were low in this study due to the passive follow-up and strict diagnostic criteria. The relative risk with HIV was of a similar magnitude to that measured elsewhere. Increased risks of TB with age and in men are expected. Associations with measures of higher socioeconomic status were unexpected. They may reflect a greater likelihood of diagnosis in this group.

BCG scars in northern Malawi: sensitivity and repeatability of scar reading, and factors affecting scar size.

SETTING: Karonga district, northern Malawi. OBJECTIVE: To assess the sensitivity and repeatability of BCG scar reading, and factors affecting scar size. DESIGN: Follow-up of individuals aged > 3 months who were recruited into a BCG vaccine trial (1986-1989), and of infants vaccinated in health centres (1989-1991), who were examined for presence and size of BCG scars in subsequent years. All examinations were carried out blind of information on true vaccination status or the results of previous examinations. RESULTS: For trial individuals who were considered scar negative at recruitment and received BCG, the sensitivity of scar reading was > or = 93%, repeatability was > or = 94% for those < 60 years old at vaccination, and only around 1% were assessed as having > 1 BCG scar post-vaccination. For infants vaccinated when < 1 month old in health centres, the proportion who still had recognisable scars 4 years later was < 80%. Scars were larger in individuals with a prior BCG vaccination, and for those aged 15-59 at vaccination the scars were approximately 1 mm larger for males than for females. CONCLUSIONS: A BCG scar is a highly sensitive and repeatable indicator of vaccination status when the vaccine is properly handled, delivered appropriately, and given at over 3 months of age, but not for vaccinations given within 1 month of birth. Given that most vaccinations in the world are given soon after birth, this low sensitivity will lead to both vaccine coverage and vaccine efficacy being underestimated in studies in which vaccination status is inferred from the presence/absence of a distinctive BCG scar. Age-sex patterns identified for scar size show important similarities to those found with skin test responses to tuberculin.

Measurement and determinants of tuberculosis outcome in Karonga District, Malawi.

Evaluation of disease outcome is central to the assessment of tuberculosis (TB) control programmes. In the study reported in this article we examined the factors influencing the measurement of outcome, survival rates during and after treatment, smear conversion rates, and relapse rates for patients diagnosed with TB in a rural area of Malawi between 1986 and mid-1994. Patients with less certain diagnoses of TB were more likely to die than those with confirmed TB, both among those who were seropositive and those who were seronegative to human immunodeficiency virus (HIV). The mortality rate among smear-positive patients with a separate culture-positive specimen was half that of patients with no such diagnostic confirmation. Patients not registered by the Ministry of Health had much higher mortality and default rates than did registered patients. Among smear-positive patients, HIV serostatus was the most important influence on mortality both during and after treatment (crude hazard ratios (95% confidence intervals) = 5.6 (3.0-10) and 7.7 (3.4-17), resp.), but HIV serostatus did not influence smear conversion rates. The initial degree of smear positivity influenced smear conversion rates, but not mortality rates. No significant predictors of relapse were identified. Unless considerable care is taken to include all TB patients, and to exclude nontuberculous patients, recorded TB outcome statistics are difficult to interpret and may be misleading. In populations with high rates of HIV infection, TB target cure rates of 85% are unrealistic. When new interventions are assessed it cannot be assumed that factors which influence the smear conversion rate will also influence the mortality rate.

PIP: Measurement of treatment outcome is central to tuberculosis control programs. A study conducted in Malawi's rural Karonga District in 1986-94 examined factors influencing the measurement of outcome: survival rates during and after treatment, smear conversion rates, and relapse rates for patients diagnosed with tuberculosis. Information was available on 1655 certain, probable, or possible tuberculosis patients. Overall, 22.5% of patients died before the end of treatment, 57.9% completed treatment and were discharged, 4.3% moved out of the district, and 15.3% defaulted or were lost to follow-up. 35% of HIV-positive patients, compared with 11% of HIV-negative patients, died before the end of treatment. Patients with uncertain tuberculosis diagnoses were more likely to die than those with certain diagnoses, regardless of their HIV serostatus. The mortality rate among smear-positive patients with a separate culture-positive specimen was half that of patients with no such diagnostic confirmation. Patients not registered by the Ministry of Health had substantially higher mortality and default rates than registered patients. HIV serostatus was the most important determinant of mortality both during and after treatment in smear-positive patients (crude hazards ratios, 5.6 and 7.7, respectively; 95% confidence intervals, 3.0-10 and 3.4-17, respectively), but HIV status did not influence smear conversion rates. The initial degree of smear positivity influenced smear conversion rates but not mortality rates. No significant predictors of relapse were identified. These findings indicate that tuberculosis outcome statistics may be misleading unless care is taken to include all tuberculosis patients and exclude nontubercular patients.

Household and dwelling contact as risk factors for leprosy in northern Malawi.

Data on household and dwelling contact with known leprosy cases were available on more than 80,000 initially disease-free individuals followed up during the 1980s in a rural district of northern Malawi. A total of 331 new cases of leprosy were diagnosed among them. Individuals recorded as living in household or dwelling contact with multibacillary patients at the start of follow-up were at approximately five- to eightfold increased risk of leprosy, respectively, compared with individuals not living in such households or dwellings. Individuals living in household or dwelling contact with paucibacillary cases were both at approximately twofold increased risk. The higher risk associated with multibacillary contact and the fact that dwelling contact entailed a greater risk than household contact if the association was with multibacillary, but not with paucibacillary, disease suggest that paucibacillary cases may not themselves be sources of transmission, but rather just markers that a household has had contact with some (outside) source of infection. When household contact was considered alone, the risks of disease were appreciably higher for younger than for older contacts and for male compared with female contacts. Despite the elevated risk of leprosy associated with household or dwelling contact, only 15% of all incidence cases arose among recognized household contacts. Given the dynamic nature of household membership and consequent misclassification of contact status, the true contribution to overall incidence of contact within household or dwelling settings is likely to be much higher than this, perhaps 30% or higher. Considering the predilection of males for infectious multibacillary forms of the disease, the transmission of Mycobacterium leprae at an early age, in particular to males, may be of particular importance for the persistence of leprosy in endemic communities. Although residential contact with a multibacillary case is the strongest known determinant of leprosy risk, the vast majority of such contacts never manifest disease, which indicates a crucial role for genetic and/or environmental factors in the transmission of M. leprae infection and/or the pathogenesis of clinical leprosy.

Polymerase chain reaction of nasal swabs from tuberculosis patients and their contacts.

Previous studies have found Mycobacterium leprae in nasal swabs from leprosy patients, their contacts, and persons living in endemic areas. It might be expected that M. tuberculosis would be present on nasal mucosa of pulmonary tuberculosis patients, but whether they can be detected in patients or contacts is unknown. We used the polymerase chain reaction (PCR) technique on nasal swabs from tuberculosis patients, contacts of tuberculosis patients, leprosy patients, and London controls to look for both M. tuberculosis and M. leprae. Swabs dipped in sputum specimens from smear-positive patients were used as positive controls. The PCRs were conducted in two independent laboratories. M. tuberculosis was detected in nasal swabs from 6/16 smear-positive tuberculosis patients and from 1/10 household contacts by one of the laboratories. All of the sputum swabs were positive for M. tuberculosis, and all of the London controls were negative. M. leprae were found in nasal swabs from 2/5 leprosy patients, but one laboratory also reported M. leprae in swabs from 4/21 tuberculosis patients and from one sputum specimen. The results show that M. tuberculosis can be found in the noses of some tuberculosis patients, and suggest that the bacilli also may be detected in some household contacts. The comparisons with M. leprae and between the two laboratories give further insights into the sensitivity and specificity of the technique.

Tuberculosis prevention: where do we go from here?

PIP: The Karonga (Malawi) Prevention Trial revealed that repeat BCG vaccinations did not protect against pulmonary tuberculosis (TB) but appeared to provide some protection against glandular TB. They increased protection against leprosy. In fact, a single BCG vaccination conferred 50% protection against leprosy and a repeat BCG vaccination increased protection by another 50%. This trial's findings confirm the need for maintaining BCG vaccination programs in countries where leprosy is a public health problem, for individuals at high risk of leprosy (i.e., contacts of leprosy cases), and because BCG provides some protection against severe forms of TB (i.e., miliary disease and TB meningitis). An alternative TB vaccine needs to be developed, however. The protective efficacy of BCG against pulmonary TB is higher at latitudes far from the equator (80% in northern Europe vs. 0% in India and Malawi). It appears that the immunologic effects of environmental mycobacteria compromise BCG's protective effect against pulmonary TB. There is heterologous immunity between various mycobacterial infections. Low-level delayed-type hypersensitivity (DTH) to tuberculin in non-BCG vaccinated people reflects exposure to environmental mycobacteria. These people are at lower risk of TB than are people with either no DTH or strong DTH to tuberculin. Intradermal exposure to different mycobacteria provides varying degrees of protection against TB in guinea pigs. The warmer and the wetter the environment, the more widespread is colonization by mycobacteria. An area of future research is mapping the distribution of environmental mycobacteria, correlating it with the pattern of DTH responses to tuberculin, and then laboratory work to isolate relevant antigens of the mycobacteria. Another approach is identifying mycobacterial antigens that elicit protective immune responses in vitro so researchers can then identify which antigens and responses are associated with patterns of DTH known to reflect low risk of TB and which response patterns are elicited by BCG against leprosy but not TB antigens. New vaccines are not on the imminent horizon, however.^ieng

Testing for antibody to human immunodeficiency virus type 1 in a population in which mycobacterial diseases are endemic.

During a large epidemiologic study in the Karonga District of northern Malawi, serum samples from 139 patients with incident leprosy, 124 with newly diagnosed leprosy, 277 patients with incident tuberculosis, and 2296 controls were tested for antibodies to human immunodeficiency virus. Sera were tested according to a four-test protocol using two ELISAs and two particle agglutination assays. Overall, 188 samples were considered positive, 2634 were considered negative, and 14 were indeterminate. All 18 available positive specimens from leprosy patients, a random sample of 14 positive specimens from tuberculosis patients, and 15 positive specimens from controls were tested by Western blot. There was no evidence of substantial numbers of ELISA false-positives in any patient group or among controls.

Patterns of initial and acquired antituberculosis drug resistance in Karonga District, Malawi.

There is concern that drug-resistant tuberculosis is increasing and may be concentrated among HIV-positive patients. Little information is available from developing countries, where surveillance studies are often unable to distinguish resistance in previously untreated patients (initial resistance) from resistance acquired following drug therapy, and where information on the HIV status of the patients is rare. Initial resistance patterns reflect the strains being transmitted in the community. We have studied patterns of resistance in northern Malawi, where the Lepra Evaluation Project has been collecting data on drug resistance since 1986. Initial drug sensitivity results were available for 373 new cases of tuberculosis. Initial resistance to at least one drug was found in 44 of these patients (11.8%, 95% CI 8.5-15.1): 13 were resistant to streptomycin alone, 13 to isoniazid alone, and 17 to more than one drug. Only 3 patients showed initial rifampicin resistance-1 in isolation, 1 in combination with streptomycin, and 1 with triple resistance. Drug resistance was not related to age, sex, or HIV status of the patient and there was no evidence of any increase over the period studied. There was no evidence of geographic clustering of the resistant strains, or of any increased risk of resistant strains in households with previous tuberculosis cases. Acquired resistance during follow-up was found in 5 of 329 patients with documented initially fully sensitive strains. 5 patients with initial resistance seemed to show reversion to sensitivity. The absence of an increase in drug resistance, despite an increase in tuberculosis cases over the period, is encouraging for the control programme. It emphasises the need to collect information from many areas before assuming that increases in antituberculosis drug resistance are occurring worldwide.