Can I catch shingles from someone who has chicken pox?

This is a frequently asked question and the answer is most definitely “no”. So why is the question asked so often? Probably because many people realise that there is a link between the two and that link is the varicella zoster virus.

This virus belongs to a group of viruses called the herpesviruses and it is a cousin of the virus that causes cold sores and genital herpes as well as the virus that causes glandular fever (infectious mononucleosis).

One of the important things about these viruses is that, after they cause an infection, they are not cleared from your body. The symptoms may disappear but the virus will remain dormant for the rest of your life. From time to time the virus can “reactivate” and you may experience symptoms. With herpes simplex (the cold sore virus), for example, this would take the form of another cold sore. Many people find that there is a “trigger” for these recurrences such as stress or excessive exposure to sunlight.

Back to varicella zoster virus. When you first encounter this virus, usually as a child, this will result in chicken-pox. You recover, but the virus remains with you. Later on in life, the virus may reactivate and when it does it you don’t have a second bout of chicken pox, you get shingles. So, same virus but two quite different infections.

This means that if you haven’t had chicken pox, you can’t get shingles. And you can’t catch shingles from someone with chicken-pox – you “catch” shingles from yourself. However, its worth remembering that you can catch chicken-pox from a person with shingles.

The bacterium which tells you that you might have cancer

 

If a patient is admitted to hospital with signs of a serious infection it is usual to take samples of blood to culture in the microbiology lab. This will detect the presence of bacteria in the blood – a condition known as bacteraemia.

Recently we diagnosed bacteraemia caused by Streptococcus bovis (also known as Streptococcus gallolyticus) in a patient. We spoke to the patient’s consultant and explained to her that there is an association between Streptococcus bovis bacteraemia and bowel cancer. Sometimes the patient may not have cancer but, instead, will have a polyp in the large bowel which has the potential to become cancerous if not removed.

The patient will now have further investigations in the form of a colonoscopy to see if there is anything amiss in the large bowel.

The reasons for the link between Streptococcus bovis and cancer or pre-cancerous changes in the bowel are not fully understood but, whatever the explanation, we are grudgingly grateful to this bacterium when it acts an early warning system for malignancy.

When is a positive lab test really positive?

On a pregnant woman’s first visit to the antenatal clinic it is routine practice to send a blood specimen to the microbiology lab to test for blood borne viruses, such as hepatitis B and HIV as these can be passed on to the baby around the time of birth. We recently had a mum-to-be whose blood specimen tested positive for HIV, a result which would be devastating at a time of joy and anticipation. So how did we approach this highly sensitive issue?

Before we answer this question it’s worth looking at how we interpret test results in the lab, in particular two features of the test used: sensitivity and the specificity.

The sensitivity of a test is a measure of its ability to identify correctly the infection in the patients who are tested. No test is 100% sensitive and this means that there will always be some patients who have the disease which is not picked up by the test.

Specificity is the capacity of a test to correctly exclude those patients who genuinely don’t have a particular infection or disease. Again, no test is 100% specific. So, there will be some patients who test positive for an infection but don’t actually have the infection – these tests results are called “false positives”.

Although we can never achieve 100% sensitivity or specificity, many current tests have sensitivities and specificities of 98 or 99%. This is pretty good, but sensitivity and specificity isn’t the whole story. We also need to take into account how common an infection is. If it’s very common, then the positive predictive value of a test is high. The positive predictive value is a good way of estimating the probability that a positive test really is positive and that the patient does, indeed, have the infection.

The positive predictive value is obtained by dividing the number of true positive results by the number of all positive results (that is, true positives + false positives).

HIV infection in the area served by my hospital is rare, so the chance of this positive result reflecting that our expectant mum truly has HIV are actually quite low, even though we use a test with good sensitivity and specificity. If, however, I were working in a hospital in sub-Saharan Africa where HIV is common then the chance of her being HIV-positive would, sadly, be high.

Where did this leave our patient? Recognising that this could be a false-positive, we sent the blood to another lab, which tested for HIV using a number of other tests. These were all negative. Our test was, indeed, a false-positive meaning, fortunately, the patient didn’t have HIV after all. So we have to be cautious in interpreting results of the tests we do in the lab as “positive” may not really mean “positive”.