Giardia lamblia is the other common protozoan parasite that infests drinking water.
During the 1990s, reports of giardiasis increased all over the country. In 1997–1998, the latest years for which the CDC has published statistics, New York State, including New York City, had the largest number of cases, 3,673.
But before you conclude from this that New York City is a dirty place, you should know that the rate of infection in New York, 20.3 cases per 100,000 population, was less than half that of the rate in Vermont, 42.3 cases per 100,000 population.
This probably won’t stop bottled water distributors from using pictures of Vermont scenery on their labels to associate their products with clean, cool New England mountain streams, and it won’t stop anyone from believing, contrary to observable fact, that New York City has dirty tap water. The highest rates of giardiasis, oddly, were among kids under five and people in their 30s. Most cases were reported during the summer and early fall, suggesting that swimming in polluted water has something to do with it.
G. lamblia, also called Giardia intestinalis, is a one-celled parasite that lives in the intestines of both animals and people. It has a life cycle that involves at one stage a kind of cyst, an outer shell that protects the organism while it is outside the host’s body.
There are two trophozoites inside each cyst, and in this encapsulated form it can exist for an extended period in the environment, long enough to find a new host in which it can reproduce. Both forms are shed in stool, but the cysts are found mostly in non-diarrheal feces, the trophozoites in diarrhea. The trophozoite has two nuclei and four pairs of flagellae, and its appearance under a microscope suggests a cartoon extraterrestrial.
When a cyst gets into a person’s intestines, the trophozoites “hatch” and begin multiplying by longitudinal fission, no sex is involved. They stay in the small intestines, either free-swimming or attached to the intestinal wall by a sucking disk. Then they pass into the colon, where some of them become encysted, ready to be sent out into the world in feces to find another host. When they are swallowed by an unsuspecting human, this revolting process starts all over again.
While some people can be infected with giardia and not have any symptoms at all, others get the usual signs of intestinal illness, diarrhea, stomach pains, and cramps. In the worst cases, the diarrhea can lead to dehydration and can actually be fatal, and the symptoms can last up to six weeks or even longer.
Everyone is at risk for this, child care workers, children in day care centers, travelers, hikers, campers, swimmers, anyone who drinks from a contaminated source or accidentally swallows contaminated water. Municipal water systems have on occasion become contaminated, causing wide outbreaks of disease. An outbreak occurred in New York State in June 1997 that affected 50 people.
Its source was a surface water supply that was chlorinated but not filtered. Giardia is somewhat resistant to chlorine, but not nearly as resistant as cryptosporidium. A beaver was found near a valve box in the reservoir, but it was never established whether the beaver was infected.
Treatment usually isn’t necessary, because the disease goes away by itself, but treating the illness may be advisable in young children who are persistently fatigued or losing weight. Medicine may also be required if several family members are ill, or if one member of the family is pregnant and therefore not able to take the most effective medicines.
Metronidazole is the drug of choice, and there are several other prescription drugs available as well. Giardia is larger than cryptosporidium, so the same water filters that are effective against crypto will also work for giardia.