Jenny “presented” two weeks later. We were spending our last night in a trekking lodge in the Everest region when she was overcome with severe diarrhoea and the classic sign of burping “rotten eggs” (fortunately the lodge had indoor toilet facilities, even if they were rather basic). I made the diagnosis on the smell alone!!
Normally the diagnosis is made by microscopic examination of stool samples looking for the cysts of the parasite. A single positive makes the diagnosis, although most laboratories would look at 3 samples over a period of days before saying that a patient doesn’t have giardiasis. With recent advances in diagnosis, PCR can be used to automatically detect the genetics of the parasite without needing a skilled Biomedical Scientist to recognise and identify the microorganism. However PCR is expensive. It is also possible to diagnose giardiasis by examining duodenal fluid collected either by endoscopy or through a string test (a small capsule with string attached is swallowed by the patient, the capsule dissolves releasing more string into the duodenum that soaks up fluid before being withdrawn back through the gastrointestinal tract and out of the mouth). As you can imagine, neither of these duodenal sampling techniques are particularly popular with patients!
There are two different treatment regimens for treating giardiasis with Metronidazole:
• Conventional treatment - PO Metronidazole 500mg BD 7-10 days
• Short course treatment - PO Metronidazole 2g OD 3 days
Jenny took short course as we needed to get on an aeroplane for our journey home. However, although initially feeling better she was soon experiencing symptoms again. In my experience, over 10 years later, I see more treatment failures with the short course therapy so I now usually recommend conventional treatment as first line. Sorry Jenny you should have seen a “proper” doctor not your husband...who was only a junior Microbiology Registrar at the time!
I have had quite a few patients with giardiasis who have failed to respond to treatment with Metronidazole, in whom I have had to recommend other antibiotic choices. But before trying another antibiotic there are few things to consider first.
Confirm the diagnosis
Recheck a stool sample to confirm that the problem is giardiasis and not another pathogen. It is important to remember that other causes of gastrointestinal infection are also faecal-orally transmitted, so the patient could possibly have other pathogens as well as giardiasis.
Check for compliance
Make sure that the patient has actually taken their medication. Metronidazole is contraindicated with alcohol. Some patients would rather stop their medication before the end of the course, rather than give up the drink. This is uncommon but it does happen so it’s worth checking.
Look for other cases or asymptomatic carriers
Other household members may have been exposed to G. lamblia and may be re-infecting the patient. G. lamblia can be carried asymptomatically so if anyone else in the household has the parasite, treat them to try and interrupt the cycle of cross infection.
Retreat with conventional therapy
If the patient had short course Metronidazole therapy (2g OD 3 days) then try retreating with conventional Metronidazole therapy (500mg BD 7-10 days). This is usually enough to clear the infection. Repeat a stool sample a week after the symptoms resolve to make sure it really has cleared.
Consider another antibiotic therapy
The traditional second line and subsequent therapies in the UK are given below. Current recommendations are that you work through the list in order. Jenny was discussed with are infectious diseases specialists in the UK and prescribed Albendazole; it is worth remembering that these specialists can help with particularly difficult cases. Treatments can include:
• PO Tinidazole 2g OD (same class of antibiotic as Metronidazole although said to be more effective in treating giardiasis)
• PO Nitazoxanide 500mg BD 3 days
• PO Albendazole 400mg OD 5 days
Many of these alternative treatments are not kept as stock in most pharmacies, so the patient may have to wait for them to be ordered especially for them. Based upon my own experience (I have had 10 years of the “Giardia story” from my wife!), I tend to see patients who fail treatment with Metronidazole also fail with Tinidazole, and this may be because they are the same class of antibiotic.
I have never seen a patient fail to clear Giardia with all of the above agents but the patients can still have problems with altered bowel habit for many months afterwards. Reassure them, it does settle eventually. So in your practise, do you favour short or conventional Metronidazole therapy? Let me know.
NB No Jenny’s were harmed during the writing of this blog...Jenny made a full recovery.