Where does the Giardia lamblia parasite come from and How is Giardia transmitted?

Antonie van Leeuwenhoek loved to look at stuff through his microscope.

One day in 1681, he was closely examining a sample of his own feces and described a parasite that was probably the one we now know as Giardia lamblia. It got its name, however, in 1859 from a biologist named Lambl.

It is the most frequent cause of nonbacterial diarrhea in North America, and you can get it by swallowing polluted water where you swim. Like Cryptosporidium, Giardia is protected when it’s not inside a host by a cyst, though it is not quite so resistant to chlorine as crypto is.

Conveniently (for the parasite, that is), human stomach acid is perfect for dissolving the cyst and allowing the trophozoite to “hatch,” after which it attaches itself to the wall of the intestine by using a sucker on its ventral surface. Then it reproduces asexually by binary fission.

Some researchers think that the parasite produces a toxin that causes the illness; there is another theory that the organism somehow obstructs the normal absorption that goes on in the intestines. There are various strains that can be distinguished by analysis of their DNA, but there doesn’t seem to be any consistent relationship between the kind of strain you get and the severity of your disease.

All you need is one cyst to make you sick, unlike bacteria, of which you can consume thousands before you get ill. Since kids get sick with Giardia more often than adults, it’s likely that infection confers immunity, but no one is quite sure. In addition to getting Giardia infection when swimming, you can also get it from food and from contact with feces, such as while diapering babies. (The incidence in day care centers is appalling, sometimes as high as 50%.) The disease is common in homosexual men, suggesting that sexual transmission is possible as well.

Swimming in polluted water is probably the most common way of getting this infection. Giardia is large enough to be filtered out of water systems, but this doesn’t prevent fecal contamination of recreational water. Metronidozole is the best treatment, but there is a small minority of sufferers, less than 4 percent, who remain symptomatic longer than two weeks and whose infections are quite difficult to treat.

Sometimes the chronic disease has serious consequences, specifically, a syndrome in which food can’t be absorbed properly, leading to severe weight loss. This chronic form, though rare, is more common in adults than children. Giardia infection can also be completely asymptomatic. In fact, about 7 percent of stool samples submitted for microscopic examination contain the cysts of the parasite, which makes it the most commonly observed flagellate in the intestinal tract.

The symptoms of Giardia infection are no fun. It usually begins with nausea and lack of appetite. Sometimes there is a slight fever and chills. Later, explosive, watery, foul-smelling diarrhea begins, accompanied by the passing of foul gas and, often, belching that has a bad taste. Cramps are common, too. Still, the only way to know for sure that it’s giardiasis is by seeing it in a stool sample, just as van Leeuwenhoek did more than three centuries ago.

The same rules about prevention of crypto in recreational water apply to Giardia. Since it’s transmitted in feces, the best way to prevent it is to keep feces out of the swimming water. Don’t swim with diarrhea, and don’t let your kids do so, either. Even if they don’t have diarrhea, washing little kids’ behinds before they go in is a good idea. While you’re at it, wash your own, too.

The CDC issues recommendations about what to do when there are fecal accidents in swimming pools (they call these “recreational water venues” to cover pools, water parks, wading pools, fountains, Jacuzzis, hot tubs, and any other place where people bathe together in chlorinated water).

There are detailed procedures for removing waste matter, raising the chlorination levels, backwashing filters and replacing filter media, keeping people out of the pool for varying periods of time depending on the incident (whether it is formed stool or diarrhea), measuring the results of the preventive measures taken, and maintaining a log of incidents that includes date and time of the event, formed stool or diarrhea, free available chlorine concentrations, pH levels, and so on. “Recreational water venues” that conscientiously follow these procedures are likely to be the safest places to swim.

What happens if the pool doesn’t follow the best maintenance procedures? Usually nothing, but then occasionally something very bad occurs. In August 1999, the Volusia County (Florida) Health Department received reports of three kids who had been infected with Shigella sonnei. S. sonnei is one member of a group of bacteria that cause gastrointestinal illness.

The disease, shigellosis, has the usual revolting symptoms, diarrhea, sometimes bloody, stomach cramps, and fever. Young kids are especially at risk for it, and sometimes their disease is so severe that it requires hospitalization for intravenous rehydration. In kids under two, it can cause seizures. There are about 8,000 cases reported every year, and since you can have it without symptoms or with very mild symptoms, there are probably many more infections than that. Although it can be treated with various antibiotics, it is starting to develop resistant strains; in any case, otherwise healthy people recover from mild cases without treatment. The use of antibiotics, therefore, should be limited to the most severe cases.

The only thing the three Florida children seemed to have in common is that they’d all been in an “interactive water fountain” that had opened on August 7 in a park near an ocean beach. (An interactive water fountain is one intended for playing in, as opposed to looking at.) The health department got very curious about this interactive fountain, so they interviewed a sample of 86 people who’d been playing in it. Thirty-eight of them, or 44 percent, had gotten sick, most of them with diarrhea, abdominal cramps, fever, and vomiting. Thirteen percent of them had bloody diarrhea. The people who got sick were an average age of eight years old, younger than the group that stayed well.

Then the health department took a look at the fountain itself. It worked by recirculating the water through a hypochlorite tablet chlorinization system with no filters. The problem with a tablet system like this is that the tablets wear out after about a week and are no longer effective.

The tablets hadn’t been changed since the park opened three weeks earlier, and no one had monitored the chlorine levels of the water. Diaper-aged kids loved this fountain, and they would routinely stand directly over the nozzles, where the spray did a nice job of washing their behinds. In other words, this fountain was a nearly perfect place for culturing and spreading germs, which it did with considerable efficiency. All you had to do was swallow a couple of mouthfuls of the water, and all but two of the people who got sick did exactly that.

While giardia, crypto, and shigella are the most common problems in swimming water, there are others, much rarer. One deserves mention if only because its life cycle is so bizarre and its effect on humans so deadly. There have been few cases of infection with a protozoan called Naegleria fowleri, but the ones that happen are truly horrifying.

Like giardia and crypto, Naegleria protects itself with a cyst when necessary, and then breaks out of the cyst to reproduce inside a host. It feeds on bacteria such as E. coli, a habit as adaptive as it is revolting. The protozoan enters the body through the nasal passages, typically when the person is swimming or bathing in hot springs or hot baths.

Inside the nasal passages, the protozoan comes out of its cyst and migrates to the olfactory nerves, eventually invading the brain. After about a week of incubation, the symptoms appear abruptly: rapidly progressive headaches, fever, nausea, vomiting, inflammation of the pharynx, lethargy, confusion, stiff neck, convulsions, and coma. Unless the illness is treated (amphotericin B has been used successfully), death can occur as fast as 24 hours after the first symptoms appear, though sometimes it takes as long as two weeks.

The way to diagnose the illness is do a lumbar puncture and examine the cerebrospinal fluid for evidence of the parasite, but because the disease is so rapidly fatal, there’s often little or no time for this.

Although this distressing disease is rare, many people have antibodies to the parasite, so exposure and asymptomatic infection are probably fairly common. But we’re only telling you about this one to give you something to have bad dreams about. The fact is, you’re extremely unlikely ever to suffer from it.