Molecular epidemiology of tuberculosis

Abstract

Tuberculosis (TB) is a global epidemic, with one third of the world's population infected, at least 9 million new active cases per year and about 2 million deaths per year. Without adequate chemotherapy treatment TB may be a mortal disease.

In countries like Sweden the mortality was dramatically reduced about half a century ago when modern TB drugs where introduced. These gains are however seriously jeopardized by the now emerging multidrug resistant (MDR) and extensively drug-resistant (XDR) TB. In Sweden the majority of patients with TB are immigrant from countries with a high incidence of TB. The incidence among the Swedish-born population has continued to decrease while it has increased among the foreign-borns. In the West African country Guinea-Bissau TB is a common disease where the incidence is believed to be further increased by the epidemic of the human immunodeficiency virus (HIV). Since independence from Portugal in 1974, Guinea-Bissau has experienced considerable political and military turmoil. In 1998 the country and especially the capital Bissau suffered a civilian war that after a peace agreement in 1999 demanded a challenging rebuilding of the country. The national TB control programme has regrettably been having difficulties in its function.

Different genotypes of Mycobacterium tuberculosis complex (MTC) predominate in different geographical regions of the world and strain-to-strain variations may have important consequences for instance when it comes to transmissibility. Future diagnostics, drugs and vaccines are affected by these strain variations and it is therefore of great importance to establish the whole spectrum of strains of the MTC world wide. Despite the high prevalence of TB in Africa, relatively little is known about the MTC genetic diversity on this continent. This study phylogenetically and epidemiologically characterized MTC isolates obtained from TB patients in Sweden and Guinea-Bissau using the two molecular techniques Restriction Fragment Length Polymorphism (RFLP) and spacer oligonucleotide typing (spoligotyping). The work was performed with the view to understand species and strain diversity as well as transmission patterns.

In Sweden, 400 drug-resistant isolates were collected during the years 1994-2005. The great majority (87%) of the patients was foreign-born and originated from several regions, with a clear dominance for Africa, in particular the Horn of Africa. It was found that the T lineage was predominant and that the isolates of the foreign-born patients to a large extent reflected genotypes common in their country of origin. One large outbreak of isoniazid resistant TB was identified, at the time (2005) involving 96 patients, mainly from the Horn of Africa. This outbreak represents one of the largest outbreaks of TB ever reported and was an important warning signal to the Swedish authorities. This study showed that molecular epidemiological typing is a powerful tool to monitor and identify chains of transmission which indicate deficiencies in national TB control programs. It was our strong recommendation that Swedish health authorities should integrate full Directly Observed Treatment Short course (DOTS) in the Swedish national TB control program.

In Guinea-Bissau we reinvestigated the molecular epidemiology of strains with regards to the phylogeography of M. africanum in West Africa. Altogether, 418 isolates collected at three different points in time were investigated. The main (47%) lineage present was the AFRI (M. africanum) lineage which included the genotypically closely related but phenotypically heterogenic Guinea-Bissau family of strains. This family of strains appeared to be decreasing over time and may be out conquered by other more successful lineages such as the Beijing and Cameroon family of strains.