Learning about gastroenteritis

(Inside Aotea, December 2010)

Experiencing a bout of gastroenteritis is never pleasant. Mackenzie Nicol, our Microbiology team leader, explains how we diagnose it to help doctors get sufferers back on their feet again.

Gastroenteritis is an inflammation of the gastrointestinal tract, including the stomach and the small intestine. The cause can be bacteria, viruses, parasites, or factors such as diet or stress that disrupt normal gastrointestinal function.

Laboratory testing is generally not required as most episodes of gastroenteritis are self limiting. This means you will get better in a very short time. Your doctor will be aware of the indications for testing.

What we’re looking for

Common bacterial culprits include Salmonella, Campylobacter, Shigella, Clostridia, E. coli and Yersinia. Foods contaminated with these bacteria often cause food poisoning. Viruses are the commonest cause of gastroenteritis which is why doctors don’t prescribe antibiotics for every case, so our role is to try and find treatable causes.

We’re lucky in that there are only a couple of parasites to look out for in New Zealand — cryptosporidium and giardia. If you’ve recently travelled overseas, though, it is important to let your doctor know, as there may be other organisms we need to consider.

Identifying the offending organisms

Diagnostic tests for gastrointestinal infection require a specimen of faece.

For bacterial infections, we attempt to grow the pathogens by placing a small piece of the specimen on an agar plate. The plates are incubated and over the next few days we look to see if any pathogens have grown.

We detect some viruses using a chemical based test called an immunoassay.

Parasitic infections can also be detected using an immunoassay test; or by direct observation under a microscope.

In all cases, the results will usually be ready for your doctor three days after we receive your specimen.

If you still have symptoms at that stage your doctor may prescribe specific treatment for your illness.