Diagnosis and treatment of latent tuberculosis are both difficult and uneconomical. The Monteux test based on skin reaction to mycobacterial protein components is seriously cross-reactive and could produce false positive and false negative results in case of 70% of the Indians. Similar difficulties could be encountered with Interferon – gamma test. The reasons for this diagnostic failure vis-à-vis latent TB could be due to many immunological confounders such as the past vaccination (with BCG) and exposure to environmental mycobacteria (there are 300 different saprophytic mycobacterial species which impact our immune system exposure). Moreover, other chronic pathogens such as Helicobacter pylori which inflict nearly 80% of all Indians in its benign form could also interfere with immunological testing for latent tuberculosis due to their bystander effects.
It is in-correct ethically to impose prophylactic therapy upon individuals who have no clinical disease and who might never get it in their lifetime. Anti TB drugs are hepatotoxic in long course and low dosage or short course could select out drug resistant bacteria. In my opinion, treatment of millions of putative, latent TB cases will inflict the government exchequer enormously and only the pharma companies who are pushing for a few years for the treatment of latent TB as a standard regimen will only benefit from it. It will shift focus from the management of clinically ailing patients which in itself is a mammoth task for a high burden country like India
As far as the India centric Lancet article on latent TB detection in prospective immigrants in the UK is concerned, it is merely sensationalization of the issue. Now a days, in the infection epidemiology arena, it has become a fashion in the West to project India as a source of infection. There could be some truth in the scenario given our high infection burden, however, the story is worst in case of countries of the world that are dubiously known for their highly virulent MDR and XDR strains such as South Africa, Russia and the countries of the former USSR. In fact UK should happily give immigrant status to Indians because it is proved already that the Indian strains of Mycobacteria are of ancestral type (genotype TbD1+) and the treatment success rates of up to 95% have been recorded under the DOTS program in India. These strains are theorized as 'shy' in terms of dissemination as compared to some of the very aggressive genotypes such as Beijing, Africa and Haarlem. India has so far not experienced any institutionalized outbreak as against the famous fatal outbreaks of New York and Kwazulu Natal. That means the Indian strains are less aggressive and controllable. Indians enjoy a distinct natural protection from latent TB due to the facts that their genetic makeup is different, their strains are different and their immune system is already primed due to a saprophytic antigenic background and/or by Helicobacter infection. UK should therefore not be worried for Indians. They should in fact be worried about the Pakistanis and Sri Lankans who do not have the ancestral strains of TB bacteria (TbD1+) in their countries and could proceed to full blown TB more rapidly than Indians. It will be interesting if the Health Protection Agency of the UK comes up with clear statistics on how many Indians, Pakistanis, and Sri Lankans have progressed to full blown TB per year after their arrival in UK and how many of them were infected with which type of strain and whether or not they were diabetics or living with HIV/AIDS.
There is however, one important angle to the conversion of Indians from latent to open TB cases while in the UK – it is Vitamin D, an important determinant of resistance or susceptibility to TB and which is synthesized naturally by the body when exposed to sunlight. Due to poor sunlight in UK, it is possible that deficiency of Vitamin D might build up especially in cases of vegetarian people who may not be able to maintain their dietary intake of vitamin D. UK Immigrant authorities should therefore, pay more attention to the food and lifestyle of these immigrants so that they keep their immune system stepped –up as they do back home in India.