Dr Margaret Hannan outlines why the increasing incidence of TB cases in parts of north Dublin are of profound concern and the importance of early diagnosis and treatment.
A recent collaborative study between the Department of Medical Microbiology in the Mater hospital, Dublin, and the Departments of Infectious Disease and Geographic Medicine in Stanford University Medical School in the US, has shown that pulmonary tuberculosis (TB of the Lungs) is rising among young Irish-born males in pockets of inner city Dublin.
This rise in TB levels is caused by recent transmission of infection from one person to another and not by reactivation of old disease as was previously thought.
TB is now the second leading infectious disease killer in the world. The World health Organisation (WHO) estimates that approximately 1.7 billion people in the world are infected with TB (a third of the world's population), with 3 million new cases reported and 8 million deaths each year.
TB is an ancient disease that emerged as an epidemic in the 1600s. It began to decline as sanitation improved in the 19th century, and retreated further when effective therapy was developed in the 1950s. It was virtually forgotten until a recent resurgence in the US, and around the world, in the 1990s.
This resurgence was associated with the advent of the AIDS epidemic, a lessening of public health measures and the closing down of TB programmes.
At this time the world also became aware of the emergence of a new type of TB, one that was resistant to commonly used medicines, multi-drug resistant TB.
As a result of this global resurgence, the WHO declared TB a global emergency in 1993. The developing world still has levels of TB reminiscent of Europe in early 1900s.
War, famine population displacement and crowded living conditions have always been fertile ground for TB. Despite promised political commitment to control the growing pandemic, support for TB-control programmes has been lacking, and TB continued to exact its toll.
Transmission of TB occurs when someone who is infectious (has active disease) coughs the micro-organism into the air and someone nearby inhales it into their lungs. It is transmitted in the same way as the common cold though it is much less contagious.
Once inhaled into the lungs the infection can progress to active disease or, if the body's defences mechanisms are effective, the infection is walled-off and the infection does not progress to disease.
In this latter case the patient has "latent" TB, ie the infection is dormant and the patient has no symptoms of disease but the micro-organism still persists in the lung.
Latent TB becomes active as the patient ages or for some other reason such as when weakening of the immune system. 10 per cent of latent TB will eventually become active. Once TB is active whether from the initial infection or when latent TB has re-activated, there is a 50 per cent death rate if the infection is left untreated.
When the disease becomes active it damages the lungs primarily but can affect any organ in the body. The most severe form causes infection in the brain and can lead to rapid deterioration in a patient's health, and death if not treated immediately. The ability of TB to remain dormant in the lung makes this a very difficult disease to control.
In our recent Dublin study, new molecular diagnostic techniques were applied in the Mater to all mycobacterium tuberculosis (the scientific name for the TB microbe) cultures from patients over a period of five years.
These techniques allowed us to compare the DNA of mycobacterium tuberculosis strains to determine whether they are the same. By cutting genomic DNA at repeated sequences along the genome and then running the broken fragments on an agarose gel we can determine which strains are related and so determine which patient infected the other.
When we find large number of TB cases with identical strains in the same area, we have strong evidence there has been a recent outbreak of infection in one place at a given time.
Our study in North Dublin suggests 50 per cent of new cases have a "common source" of infection and 50 per cent reflect breakdown of latent disease. This common source has been identified through contact investigations at local pubs and schools in North Dublin, and those primarily affected are young Irish-born males.
They are not the usual suspects and do not reflect the population we would expect to have a high risk, eg homeless, immigrants, HIV infected or drug addicts, it appears to be primarily an Irish disease in these areas of inner city Dublin.
The new tools for TB diagnosis were adopted in New York and other US cities during the Multi-drug-resistant TB epidemic of the 1990s. The new techniques are an essential tool to TB programmes throughout Europe and North America.
Although the current figures for TB in Ireland had been dropping for the past few decades our figures are still double those in Northern Ireland and the gradual downward trend seems to be levelling off in recent years and rising in some parts of the country. There is concern that these rising levels may also be related to the increased number of immigrants from high TB prevalence countries entering the country and of further concern is that the type of TB coming from these countries is multi-drug resistant.
The potential exists therefore for the transmission of multi-drug-resistant TB across the Irish community in the same way as we have seen in the North Dublin inner city study if measures are not taken to control this infection.
If these potentially untreatable infections were to emerge, these patients would require long term (sometimes up to two years) respiratory isolation. Every effort should be taken to maintain long-term respiratory isolation units for TB patients in this country - now is not the time to cut back on TB services in Ireland.
The opening of a new fast-track TB laboratory was celebrated on World TB Day last month in the Mater and is a significant step toward improving control of TB in North Dublin. It provides a rapid real-time molecular diagnostic service to hospitals to facilitate early diagnosis of infection and so prevent unnecessary spread of the disease in our community.
Dr Margaret Hannan is a consultant clinical microbiologist and epidimiologist, at the Mater hospital, Dublin.