For a tiny organism, Giardia can cause a whole lot of trouble.

Giardia lamblia is the name of a protozoa or microorganism that has adapted to life in the guts of other animals.

When it lives in humans, Giardia can cause giardiasis, a nasty mixture of cramps, nausea and diarrhea commonly known as "beaver fever."

The kind that affects humans is not the only type of Giardia, says Pat Roach, Water Contaminants Scientist with the federal Department of Indian Affairs and Northern Development.

"There's a whole whack of different Giardia out there."

Giardia are specialists, Roach says. Of the four accepted subspecies of the microorganism, one infects reptiles, one infects birds, one is common in rodents, and the fourth makes its home in humans and other large mammals.

Normally, between five and ten percent of the human population is infected with giardiasis at any given time, but not everyone gets sick. Eight out of ten people show no major symptoms, says Roach, so they don't realize they are carrying the parasite. However, they can still infect others.

"If you get a microscopic amount under one fingernail, that's enough to infect the whole family," he says. "And some people react hugely."

Giardia didn't always live in the guts of other animals, Roach says. It's one of the world's oldest surviving protozoa, or single-celled animals.

"Giardia was around before there were intestines to infect," says Roach. "It's perhaps a couple of hundred million years old."

At some point in its history, Giardia adapted to a life in the intestinal tracts of other animals. The active form, the trophozoite, lives in the small intestine of its animal host. The trophozoites multiply rapidly and some enter the large intestine where they form a cyst that is excreted in the host animal's faeces.

The cyst is a tough, protective package that allows the protozoa to survive until it finds another animal gut to infect. Giardia cysts can survive for weeks in cold water, and they're resistant but not immune to treatment by chlorine, iodine and other chemicals. Fortunately for humans, they can be killed by boiling the water and they can be filtered out.

If the Giardia cysts survive until they are swallowed by a new host, the acid in the host's intestinal tract causes them to excyst or "hatch" out of the cysts and take up residence -- and the cycle of infection begins again.

Although not everyone is affected strongly by Giardia, it can be dangerous. In Third World countries, where many people are already ill or malnourished, it can be a killer, Roach says. Even if the disease is not fatal, it can stunt children's growth and weaken adults.

Generally, antibiotics will cure a case of giardiasis. However, some strains of the protozoa are harder to get rid of than others and produce more serious symptoms.

"There are some particularly nasty strains," says Roach. "Especially from Nepal, it seems."

Humans aren't the only animals to be affected by Giardia. Beavers commonly carry the parasite but don't seem to be strongly affected by giardiasis. However, dogs can show the same symptoms as humans, and puppies are particularly susceptible.

"In fact it causes quite a bit of weight loss in domestic animals," Roach says.

Giardia cysts could be in any untreated surface water, the scientist says, even in the Yukon where the water is safer than in most parts of the world. And it doesn't take much to cause illness. As few as 10 microscopic cysts can trigger a case of giardiasis.

For more information about Giardia, giardiasis, and how to protect yourself, contact the Department of Indian Affairs and Northern Development in Whitehorse at 667-3139 or pick up a Giardia Fact Sheet from the Yukon Conservation Society.