Every wilderness aficionado should be familiar with the terms "backpacker’s diarrhea" and "beaver fever," both nicknames for infection with the protozoan parasite Giardia lamblia.  Plenty of outdoor users have experienced this unpleasant and sometimes debilitating gastrointestinal illness.  What is this parasite, how much of a threat does it actually pose, and what should be done about it?  If any of these questions have popped into your head, read on!

Life Cycle

Giardia lamblia is a protozoan (single-celled) parasite that lives in the intestines of animals and humans and is shed in the feces of infected individuals.  The parasite exists in the external environment in cyst form, which protects it and allows it to survive outside the body for long periods of time.  Once it is ingested and passes into the small intestine, it excysts and emerges as a trophozoite (the active feeding phase of the protozoan life cycle), and causes the symptoms of gastrointestinal illness sometimes seen in infected individuals.  As the trophozoites pass through the intestines and move toward the colon, they encyst and eventually exit the body as infective cysts(1).

Transmission

Because the cysts are shed in the feces of infected individuals, transmission of the parasite occurs from ingesting fecally contaminated food or water, from exposure to fecally contaminated environmental sources—wilderness rivers, lakes, and streams—and from person-to-person by the fecal-oral route(1).

Clinical Presentation

The incubation period for Giardia is 6 to 15 days, after which time symptoms of giardiasis generally appear.  Infection is often completely asymptomatic, but can be characterized by explosive, watery, fowl-smelling diarrhea combined with abdominal cramps, bloating, fatigue, weight loss, flatulence, anorexia, and nausea.  “Rotten egg burps” that smell like hydrogen sulfide are a typical symptom, and are as unpleasant for the infected individual as for those around him. Some times the illness resolves on its own, but it may become chronic in a few individuals(1). 

Diagnosis

Definitive diagnosis of Giardia infection generally relies on direct visualization of the organisms (either cysts or trophozoites) in stool specimens.  Diagnosis may prove challenging, however, as symptoms of infection may precede excretion of the organisms in stool, and excretion during infection may be erratic.  Multiple stool specimens may need to be examined.  Newer enzyme-linked immunosorbent assay (ELISA) techniques that detect Giardia antigens in stool specimens are reliable and are available as commercial kits(1).

Risk

Surveillance for Giardia in wilderness areas and national parks conducted in the 1980s detected low numbers of Giardia cysts in many sample sites, generating much publicity and causing the National Park System (NPS) and the United States Forest Service (USFS) to recommend that all backcountry water be treated before consumption(2).  Despite its notorious reputation and its status as the most commonly diagnosed intestinal parasite in North America(3), many questions remain about the real risk that Giardia poses to wilderness users.  Surveillance for Giardia in wilderness water sources throughout the country is sporadic and inconsistent, making evaluation of the baseline levels of the parasite in any particular water source impossible.  Additionally, surveillance for other waterborne pathogens, such as Cryptosporidium, that have a clinical presentation similar to Giardia, are virtually nonexistent.  Therefore, the true risk that Giardia—and other waterborne pathogens—pose is not well established.

One perspective held by some government agencies (NPS and USFS) is that giardiasis is a significant threat and that drinking untreated water from wilderness sources should be considered hazardous (4).  At the other end of the spectrum, some investigators, physicians, and wilderness experts believe the threat is mostly hype.  One study goes so far as to claim: “Neither health department surveillance nor the medical literature supports the widely held perception that giardiasis is a significant risk to backpackers in the United States.  In some respects, this situation resembles (the threat to beachgoers of) a shark attack: an extraordinarily rare event to which the public and press have devoted inappropriate attention(5).  The true threat falls somewhere between these two extremes.

While it is possible that the risk of contracting Giardia infection from ingesting water from wilderness sources may be less than its reputation would have us believe, many other possible waterborne pathogens may also be present in the water and may cause gastrointestinal illness that mimics giardiasis.  Such pathogens include: Cryptosporidium, Camplylobacter, Salmonella, Shigella, Yersinia, Aeromonas, Clostridia, and some strains of Escherichia coli(4).

According to the Centers for Disease Control and Prevention, only 8 percent of individuals with diarrheal illness in the United States seek medical care(3). Thus, many cases of self-diagnosed giardiasis may have really been due to some other infectious agent. The protozoan parasite Cryptosporidium has emerged in recent decades as an important contaminant in drinking water, and studies suggest that it is the third most common intestinal pathogen worldwide(6).  In the United States, outbreaks of Cryptosporidium infection due to contamination of public water supplies have occurred on many occasions, including a Milwaukee, Wisconsin, outbreak in 1993 that produced 403,000 infections and was the largest waterborne-disease outbreak ever documented in the United States(7). In a smaller outbreak in western Georgia in 1987 13,000 individuals were infected(8).

Like Giardia cysts, Cryptosporidium oocysts are shed in high numbers in the feces of many mammals, including humans, and are frequently found in a variety of water sources.  In 1994, a Cryptosporidium outbreak in New Jersey involving 2,070 individuals was associated with recreational exposure to lake water(9).

Further, surveillance of water samples collected from six rivers in California and Washington in 1985 detected Cryptosporidium oocysts in all samples examined from all six of the rivers(10).  Given these findings of oocysts in recreational and wilderness water sources, it is likely that Cryptosporidium also represents an important cause of backpacker’s diarrhea.

Even if the risk of giardiasis is low, the risk of contracting backpacker’s diarrhea due to other infectious agents may be much higher.  The presence of Giardia, Cryptosporidium, and other waterborne pathogens in any given water source varies over time—they may be present one day and gone the next.  Further, the microbes may be present in some parts of a water source and not in others. It is impossible to be certain whether any source is free of contamination, so most experts recommend playing it safe and treating all wilderness water before ingestion.

The similarity of illness produced by Cryptosporidium to giardiasis is particularly significant because the parasite differs from Giardia in several important ways.  Giardia infection can be prevented by treating water with iodine, or by filtering or boiling water before drinking it, while Cryptosporidium oocysts are resistant to iodine treatment(11).  Further, giardiasis can be effectively treated with medications, while no treatment is effective for cryptosporidiosis, which can have fatal consequences for immunocompromised individuals unable to mount a sufficient immune response and recover from the infection(1).  Wilderness users and clinicians must be aware that Giardia may not be the only cause of backpacker’s diarrhea, and prevention and treatment measures must reflect this.

Prevention

The following is a list of preventative measures recommended by the Centers for Disease Control and Prevention to avoid infection with Giardia, Cryptosporidium, and other waterborne pathogens:

·         Avoid drinking untreated water from wilderness sources.

·         Treat water by boiling or with filtration, as iodine treatment will not kill Cryptosporidium oocysts.  (After filtration, iodine or chlorine is still needed to eliminate viruses, which are too small to be removed by filters.)

·         Avoid swallowing recreational water (while swimming, bathing, tooth brushing).

·         Practice good hygiene (frequent hand washing).

·         Avoid food that might be contaminated (wash fruits and vegetables with uncontaminated water).

Treatment

Although giardiasis often resolves on its own, symptoms can last for 2 to 6 weeks, occasionally even longer.  If giardiasis is suspected, individuals should consult a physician for treatment of the infection.  The medication that has most often been used in the past to treat giardiasis in the United States is the nitroimidazole,  Metronidazole (Flagyl) 250 mg three times a day for five days.  Tinidazole, another nitroimidazole, has been widely used in other parts of the world under the trade name Tinebah. Since May 2004 it has been available in the United States as Tindamax. The dosage is 4 capsules (2 gm) once only, so this drug does not require as prolonged avoidance of ethanol as does Flagyl. Both of these drugs are more than 90% efficacious.  Unfortunately, cryptosporidiosis is not so easily treated.  No effective medication currently exists.  Left untreated, the infection usually resolves in about 2 weeks in healthy individuals.  In immunocompromised patients, the infection often becomes chronic and life-threatening and must be monitored closely(1).

More information about water disinfection can be found on the CDC website at:

http://www.cdc.gov/ncidod/dpd/parasites/cryptosporidiosis/factsht_cryptosporidiosis.htm

Additional Information about Giardiasis and Cryptosporidiosis:

http://www.cdc.gov/ncidod/dpd/parasites/giardiasis/default.htm

http://www.cdc.gov/ncidod/diseases/submenus/sub_crypto.htm

Bibliography

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