Lymphatic filariasis in the Karonga district of northern Malawi: a prevalence survey.

In Malawi, two main foci of lymphatic filariasis (LF) are known to exist: one in the south, in the Shire valley, and the other in the north, along the Songwe River, on the border with Tanzania. There have been no formal surveys in the Songwe area since the 1960s but an opportunity arose in 2000-2001 to map LF in this area, in the context of a leprosy survey that formed part of the follow-up of a large leprosy and tuberculosis vaccine trial. Overall 687 immunochromatographic (ICT) tests were carried out. Wuchereria bancrofti antigenaemia was found in > 25% of adults in each of the 12 villages sampled (four in the Songwe area and eight in the rest of the Karonga district), with village prevalences varying from 28%-58%. Of the 685 adult male residents of the Songwe area who were each given full-body clinical examinations, 80 (11.7%) were identified as cases of hydrocele. Lymphoedema was found in seven (1.0%) of these adult males and in 29 (3.7%) of the 769 adult female residents of the Songwe area who were also examined. Microfilariae were detected in 33 (30.8%) of the 107 thick smears of night-blood samples that were made from individuals with positive ICT cards. The W. bancrofti infection focus in Karonga district is therefore wider than was previously known. This has important implications for the implementation and eventual impact of LF-control activities in this area.

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.