Molecular Diagnostics of Infectious Diseases

5 Amplification and detection methods (p. 53-54)

Stephen A. Bustin; Harald H. Kessler

The last century has witnessed extraordinary progress in the battle against deaths caused by infectious diseases. The implementation of universal vaccine programs to prevent childhood illnesses, the success of antibiotic and antiviral drugs to treat disease, together with improvements in hygiene, sanitation and diet have had dramatic effects: worldwide infant mortality is reduced by half, life expectancy has increased significantly, there is progress toward a malaria vaccine, which is scheduled to move into large-scale human trials and there has been significant progress towards the introduction of rationaldesign drugs.

However, the comforting thought that any threats posed by infectious diseases are a thing of the past, is a perilous delusion. Climate change, large-scale human migration and widespread use of nontherapeutic antimicrobial growth promoters are just a few factors that have led to a set of circumstances where infections now occur that are resistant to all current antibacterial options. This rising global health hazard is emphasized by the fact that infectious diseases are responsible for 25% of all human deaths worldwide. We are witnessing the re-emergence of diseases such as malaria and cholera, previously thought to be largely under control. Equally, there is a constant succession of newly identified infectious diseases caused by infectious agents that can jump from animals to humans, most recently exemplified by SARS or avian and swine influenzas. The extraordinary ability of pathogens to change and adapt has resulted in the emergence of multidrug-resistant strains of widespread infectious diseases such as tuberculosis (TB), which are virtually untreatable and often fatal. Furthermore, the decreased immune response due to HIV infection has lead to a resurgence of TB among millions in whom the disease has been dormant. HIV infection itself, first recognized in 1981, has caused a pandemic that is still in progress. The fragility of public health infrastructures is also characterized by the increasing incidence of nosocomial infections, exemplified by the emergence of Methicillin-resistant Staphylococcus aureus (MRSA) and diarrhea ascribed to Clostridium difficile. Taken together, these events emphasize the variability of infectious disease death rates, the uncertainty of disease emergence and the consequent continued threat to public health.

It is not surprising, then, that tremendous efforts are being made to improve the diagnostics of infectious diseases. The most important feature of an effective diagnostic assay is its capacity for the reliable identification of a targeted pathogen. At the same time, assays should be rapid, uncomplicated and capable of being carried out in the field (‘point-of-care’; POC). Unfortunately, the continued application of legacy assays entails that many ‘gold standard’ diagnostic technologies are slow, expensive, require highly skilled personnel, and are not practical for POC applications. Hence, there is enormous pressure to implement ongoing molecular scientific and technological advances in the development of improved disease surveillance and control systems.

The combination of molecular diagnostics with therapeutics constitutes a key component of integrated healthcare and molecular testing methods have become the strategy of choice for the detection of many infectious diseases. They can provide sensitive and reliable results, detect infectious pathogens both qualitatively and quantitatively and produce rapid test results enabling better and cheaper patient care. Although a qualitative measure of the infectious agent is often sufficient for the detection of many infectious diseases, there are several clinical conditions where the quantification of certain viruses is useful. This is true for measuring viral load in immunosuppressed patients or for monitoring of treatment response. Molecular testing can provide clinicians with faster test results, often measured in minutes, compared with traditional microbiological methods. Combined with a move towards fieldbased, automated, simpler and higher throughput technologies, this development promises to generate a wealth of data establishing organism numbers and types as well as information about virulence and resistance determinants that influence disease severity. This will result in better patient care, translating into improved patient outcome, shortened hospital stays, and reduced cost of antimicrobial treatment.