Giardia is spread through the feces of infected carriers. In outdoor slang you may hear Giardia being referred to as, "beaver fever." This is because beavers are excellent hosts for spreading Giardia. The sad reality is that the main source of infection in the backcountry has been man. Improper procedures for human waste disposal in the backcountry by infected carriers spread Giardia into the ecosystem, which infect other animals; primarily beavers but a short list of carriers include deer, muskrat, coyotes, and feral pigs. As a matter of fact, health officials in the United States estimate that 4% to 20% of the total population is unknowing carriers of Giardia cysts.
By ingesting either the active trophozoite or a dormant cyst a human can be infected. The trophozoite is the active, amoeba like form of Giardia that is vastly larger than a virus or bacteria. The amoebas attach themselves to the walls of the small intestine where they feed off of the host carrier. Although they have not been known to do major damage or move to other parts of the body, severe infections can block the passage of nutrients into the body resulting in severe and rapid weight loss. Eventually the amoebas get further into the digestive tract and encase themselves in a cyst to assure their survival when they leave the body. Medical science isn't sure exactly how long a cyst can survive, but they do know it is at least two months, and extreme hot or cold doesn't seem to affect them. If the cyst is ingested the digestive fluids eat away the shell releasing the trophozoite and the cycle of infection is repeated.
Giardia cysts seem to be prolific when it comes to finding water. In National Forests where pit toilets are present, the rate of infection and contamination is much higher than remote parts of the United States. Even well water can be infected if the well is located to close to the leech field of a pit toilet or poorly designed. Just one single Giardia trophozoite can produce one million cysts in just ten days!
If a hiker were to ingest a single cyst, unless they were in incredibly poor health they wouldn't even know it. Doctors aren't exactly sure how or why Giardia makes us sick, but they do know you need to ingest at least ten trophozoites or cysts before you will get ill, and a case of Giardiasis.
Generally a person suffering from Giardiasis will show symptoms ten to twenty-one days after ingestion, but this time period can be shorter or longer. Infection can last from four days to four weeks, or even longer. Symptoms include oily, foul smelling diarrhea, a feeling of weakness and being run down, abdominal cramps, unexplained weight loss and nausea. More severe cases can bring on anorexia, vomiting and fever.
Because so many outdoor illnesses manifest themselves with flu like symptoms you should see a doctor if you become ill within 30 days of an outdoors trip. Especially if you have flu like symptoms, are feeling weak, nauseated, rundown or have diarrhea. Not only can this be an indication of Giardiasis or other intestinal infections, other illnesses like Lyme disease start off with similar flu like symptoms. Today there are about four different medications that can treat Giardiasis. They are all inexpensive, readily available, and have a limited number of side effects.
The key to avoiding Giardia and other water borne pathogens is know your water source and treat it accordingly. Boiling water is the cheapest, fastest, and most effective way to assure it safe from biological pathogens (but not chemicals). By following some common sense advice you can have a great and safe time while in the outdoors and not suffer the symptoms of beaver fever later.