Friday, 6 May 2011

Advances in the Detection of Aspergillus Developing Drug Resistance

Researchers here at the University of Manchester and the National Aspergillosis Centre have developed a sophisticated method of identifying strains of Aspergillus that are likely to be resistant to many of the antifungal drugs currently available.
Traditionally testing is done by growing samples of the isolates under test and then treating them with ever increasing doses of antifungal - there is a video giving an explanation of this process by Senior Clinical Scientist Nicola Duddy who works at the National Aspergillosis Centre

Notes taken at the meeting

This procedure takes several days and cannot be carried out if nothing grows out of a clinical specimen - something that happens regularly (10-50% failure rate).

The research at the National Aspergillosis Centre uses rapid molecular techniques (PCR) to detect tiny amounts of DNA in clinical samples, whether or not the sample contains fungi that will grow out in culture, thus removing the requirement to grow out a sample prior to testing, saving many days. DNA is detected in 96% of clinical samples taken from patients with confirmed invasive pulmonary disease (IPA) whereas there is a success rate of 91% when attempting to grow cultures from these clinical specimens - a significant  improvement but hardly a step change in detection rates.

The PCR technique looks much more impressive when we look at its ability to detect fungal DNA in other types of aspergillosis.  Allergic Bronchopulmonary Aspergillosis (ABPA) and Chronic Pulmonary Aspergillosis (CPA) are both forms of chronic infection that are treated with long term courses of antifungals - situations where the development of resistance is quite likely over time so it would be of great benefit to the patient if we could detect resistance as early as possible. Once resistance is established these patients may well deteriorate if not switched to a new antifungal.

The culture technique identified no Aspergillus in ABPA patients  and only 16.7% of CPA patients had a positive Aspergillus culture in this experiment. In stark contrast the new PCR technique identified the presence of Aspergillus in 79% of ABPA patients and 71% of CPA patients - a huge increase. This is tempered slightly by the control part of the experiment that shows Aspergillus detected in 36% of uninfected people so in one sense half of those detected in ABPA & CPA may be 'false positive' though this can be explained by the fact that all of use are breathing in Aspergillus in the air all of the time - these 'false positives' are likely to be genuine positive results.

Perhaps more importantly when markers for resistance to antifungals were looked at using the PCR technique it was found that amongst those clinical specimens that had NOT given a positive culture (and would thus have been completely missed using traditional techniques) 50 (CPA) to 75% (ABPA) showed signs of resistant populations building up.

Although these experiments were carried out on quite small populations there are quite clear signs that this technique has the potential to allow us to identify resistance in many more clinical samples than was previously possible, and identify them much more quickly than before. More work is necessary but this is most probably a significant advance in the treatment of several different types of aspergillosis.

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