Yesterday’s episode of e-tv’s 3rd Degree discussed the case of 60+ deaths in KZN resulting from infection by extremely drug resistant tuberculosis (XDR-TB). Apart from being yet another very black mark against SA’s health ministry (for its much-delayed reaction to timely and ample warnings), it is also an object lesson in just how critical a proper understanding of evolutionary processes can be.
But first, a little background information and a few comments: Tuberculosis is a contagious but curable disease of the lungs that worldwide currently kills about 1.7 million people a year. It spreads through the air, much like the common cold. The infective agent of TB is the Tubercole, or TB, Bacillus, which is a type of mycobacterium rather than a virus, as is the case with the common cold. At present, South-East Asia has the highest number of TB cases - about 33% of the world total - with an annual new infection rate of 182 per 100,000. However, sub-Saharan Africa’s annual new infection rate is almost twice as high at 356 per 100,000.
Treatment in the form of antibiotics was first developed about 50 years ago, and TB incidence, prevalence and mortality rates declined sharply, to the point where it was felt that TB was about to become a thing of the past, much like smallpox. Treatments were therefore not developed any further. However, the normal treatment regimen is lengthy, taking six to eight months to complete, and is often accompanied by unpleasant side effects. The discipline required to see the treatment to completion is often lacking, especially in poor countries and rural areas where TB is most common. The discipline problem is aggravated by the fact that improvements are noted soon after treatment begins, and patients wrongly assume that their full recovery requires no further medication. Some TB bacilli survive the curtailed treatment, and the patient is almost guaranteed to suffer a relapse in the near future.
The salient facts of the treatment, such as its length, the possibility of unpleasant side effects and the necessity for completing it, are carefully explained to patients at its commencement, so it is a little spurious of 3rd Degree to attempt to blame the Dept. of Health alone. At least some of the responsibility must be apportioned to the patients themselves. Also, 3rd Degree did not spell out the full facts about TB treatment, which are central to the whole issue. The emergence of mmultiple drug resistant TB (MDR-TB) and XDR-TB was barely given the attention it deserved: one medical doctor was permitted to say that they resulted from “sub-optimal treatment.”
Though truthful, that description is hardly complete, and explains nothing useful. The emergence of MDR-TB and XDR-TB is a direct consequence of incomplete treatment and evolutionary processes, and is exactly what evolutionary theory predicts will happen. In any population of TB bacilli, the available genetic variety will ensure that some bacilli are more resistant, and some less so, to the presence of a given antibiotic. The less susceptible ones will tend to survive, and pass on their antibiotic resistance to their offspring. Again, some of the new generation will be more, and some less, resistant, but on average, each new generation of bacilli will have a greater resistance than the previous one, provided that the antibiotic is present.
The antibiotic thus exerts a selection pressure on the bacilli, driving the entire bacillus population towards greater resistance. The few random genetic mutations that increase resistance in this regard - most will be neutral, and some negative - will be better adapted to their antibiotic environment and therefore survive preferentially, while the neutral and negative variations will quickly succumb to the antibiotic and die.
The way to cure TB then is to keep the TB bacillus population small by taking the appropriate medication for a lengthy period and allowing the body’s immune system to mop up the surviving bacilli. If treatment is stopped too early, the immune system will not be able to cope and the more resistant TB strain will flourish. As has frequently occurred, repeating this cycle of relapse and incomplete treatment will amount to a selective breeding exercise where the relevant TB strain is effectively bred to be resistant to different types and potencies of antibiotics.
It is doubtful that new antibiotics can be developed quickly enough that effectively treat MDR-TB and, especially, XDR-TB. So we’re basically left once again with the problem of implementing prevention and containment protocols, which, as has ever been the case, are considerably more problematic to administer than effective treatment management.
If nothing else, the above facts do, however, graphically illustrate the dangers and costs of ignoring good science.
'Luthon64
