Category Archives: History of Microbiology

“Old rules for old tests”

800px-Mantoux_tuberculin_skin_test

I have been looking recently at the various merits of Mantoux testing versus the newer Interferon Gamma Release Assays (IGRAs) for the diagnosis of latent TB infection. Whilst the IGRAs (such as T -spot and Quantiferon Gold) are not perfect by any stretch of the imagination they still seem to have difficulty replacing the long entrenched Mantoux test.

The Mantoux test, which has been in existence for well over a hundred years suffers from various problems. False negatives due to immunocompromise, false positives after BCG, inter-observer variability in measuring the results, and the logistics of administration, just for starters.

One wonders if Mantoux were a new test invented today, whether a test with so many deficiencies and subjectivity would get anywhere near the commercial market. The validation requirements for new tests as stipulated by accreditation agencies are much stricter now than they were 100 years ago, 50 years ago, or even 20 years ago.

I think we are applying old rules for old tests to a certain extent. Mantoux testing is an institution, a tradition, and is what a lot of us are used to. However because it was acceptable testing in previous generations does not mean it is acceptable by today’s standards.

I don’t think IGRAs will need to improve too much more before Mantoux testing ends up as a historical test, and PPD is kept in museums and not laboratories….

Michael

TB or not TB – that is the question?

As usual my delving into family history has prompted a post … I’ve been researching my Great-great-Uncle who was killed in action at Somme, France whilst serving as a WW1 ANZAC and discovered that his father, my great-great-grandfather died from (and I shall quote from the Marlborough newspaper that it was printed in) … “that dread disease consumption“.

Now you may consider TB to be a third world, old-fashioned type of disease and to a degree you would be right however TB remains the second greatest killer globally (HIV/AIDS is number one) due to a single infectious agent.  In 2013, 9 million people fell ill with tuberculosis and 1.5 million died as a result – 480,000 of these cases were MDR-TB (multi drug resistant).  There is an upside to all this in that 37 million lives were saved due to effective diagnosis and treatment between 2000-2013 and the number falling ill to the disease each year is declining albeit slowly.  New Zealand statistics show 305 cases nationwide during 2014 and this number has been relatively stable over the past five years.

MDR-TB is defined as those strains that are resistant to at least Isoniazid and Rifampicin (the two most powerful and standard first line drugs for treatment of TB).   XDR-TB are strains that are extensively drug resistant and are defined as MDR-TB with additional resistance to any fluoroquinolone and at least one of the second line agents (Amikacin, Capreomycin or Kanamycin).  Of the 480,000 MDR-TB cases in 2013 about 9.0% of these were determined to be XDR-TB.  Within New Zealand the rate of resistance is much lower, on average only 1-2% of isolates each year showing this level of resistance.  In the past 10 years there have been 33 cases of MDR-TB in good ol’ NZ and all but two of these cases were born overseas where it has been assumed they contracted it – 29 of these 31 cases were born in an Asian country.  Only one case of XDR-TB has ever been identified in New Zealand, this was in 2010.  I think this is one time were our geographical isolation from a large part of the world is to our benefit.

Tuberculosis was a disease that the WHO considered dropping from their watchlist in the 1960’s/1970’s due to it’s decline however it made a huge resurgence in the 1990’s with the number of HIV/AIDS cases increasing and is certainly a disease which we cannot afford to ignore.

You can view a copy of the 2014 WHO global TB report here.

Nitpicking

I’ve been doing some reading lately, mainly on the subject of World War I and it naturally brings to mind topics for this website ….. this time “lice”.

Why are we not able to eradicate parasites such as lice?  Surely if the little blighters do not have a host to live off then they die therefore minus the hosts and we shouldn’t have issues of lice, yes?

I guess it is the age old problem of compliance … there is always a host somewhere who is not willing or able to get rid of them and therefore continues to spread them to those who spend many hours and much money to de-louse their children (mainly) from these wriggly little pests.  I am referring in general to head lice which seem to be a fact of life throughout most kids journey through primary school however with the very popular “selfie” causing a surge in cases in older age groups also.  It is the sort of nuisance that if everyone cared enough to treat and eradicate then we should be able to get rid of ……

The head louse ( Pediculus humanus capitis) is essentially a harmless pest.  It is a wingless insect which spends its entire life on the head of a host feeding off small amounts of blood.  They cannot fly or jump and do not transmit disease although they can be responsible for secondary infections of the skin due to scratching.  They are simply a nuisance and one that should be able to be eliminated from our society.

Body lice (Pediculus humanus corporis and sometimes Pediculus humanus humanus), common during the Great War, are far more dangerous due to their potential to transmit  diseases such as typhus and trench fever.  The two species are physically very similar, almost indistinguishable, but do not interbreed however they have been known to under laboratory conditions.  Again a parasitic pest that should be able to be eliminated with adequate hygiene practises.

If the world can eradicate smallpox (Variola virus – declared obsolete by the WHO in 1979) then why can we not do the same with our Pediculus friends?  Just as we needed the “buy in” of people to get vaccinated against smallpox we should be able to get their “buy in” to de-louse.