When purity doesn’t run deep

Tasmania is a great place, with vast areas of beautiful, unspoilt wilderness. And, just as some television commercials would have you believe, you’d expect the water there to be pure and unspoilt too.

Unfortunately, appearances can be deceiving. A microscopic parasite, Giardia duodenalis, is very common in Tasmanian waters. And it’s a major cause of diarrhea, or specifically, giardiasis.

Giardia is a genus of protozoa, single-celled organisms that live in animal digestive systems. Their life cycle goes through a couple of phases: the form that moves into and reproduces aexually in the small intestine is called the trophozoite. They’re also the ones that cause diarrhea, which moves the trophozoites into the large intestine where they form cysts, which are then excreted in faeces. These cysts make their way into the waterways, are again ingested by animals and, once they reach the stomach, release the trophozoites and start all over again.

The cysts are quite hardy, able to survive in the acidic environment of the stomach. They can also live for several months outside the body, performing better in colder water. Which is why they do so well in Tasmania, where they’re endemic in the animal population. A study published in 1998 found Giardia infection in 20% of dogs and cats, and up to 62% in native bandicoots.

Digitally-colourised, scanning electron micrograph of a Giardia protozoan from a rat’s intestine, showing the thread-like flagella that it uses to move (click to embiggen)
Digitally-colourised, scanning electron micrograph of a Giardia protozoan from a rat’s intestine, showing the thread-like flagella that it uses to move (Image by Dr Stan Erlandsen and Dr Dennis Feely, Center for Disease Control, via Wikimedia Commons)
Fortunately, the fact that they prefer cold water means it’s fairly easy to avoid infection. Simply boiling the water kills the cysts, and any remaining trophozoites, making the water safe to drink.

Just don’t sip directly from a mountain stream, no matter how pure it appears.

Kettlewell JS, Bettiol SS, Davies N, Milstein T & Goldsmid JM 1998, “Epidemiology of giardiasis in Tasmania: a potential risk to residents and visitors”, Journal of Travel Medicine, vol. 5, no. 3, pp. 127-30 (PDF, 390 KB)

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