Water Filters and Giardia

Kyoto: Hiking from Kibune to Kurama

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Giardiasis occurs worldwide. In the United States, Giardia is the parasitemost commonly identified in stool specimens submitted to statelaboratories for parasitologic examination. From 1977 through 1979,approximately 4% of 1 million stool specimens submitted to statelaboratories were positive for Giardia (6). Other surveys havedemonstrated Giardia prevalence rates ranging from 1 to 20% depending onthe location and ages of persons studied. Giardiasis ranks among the top20 infectious diseases that ca use the greatest morbidity in Africa, Asia,and Latin America (7); it has been estimated that about 2 millioninfections occur per year in these regions (8).

People who are at highest risk for acquiring a Giardia infection in theUnited States may be placed into five major categories:

1) People in cities whose drinking water originates from streams or riversand whose water treatment process does not include filtration, orfiltration is ineffective because of malfunctioning equipment. 2)Hikers/campers/outdoorspeople. 3) International travelers 4) Children whoattend day-care centers, day-care center staff, and parents and siblingsof children infected in day-care centers. 5) Homosexual men.

People in categories 1, 2, and 3 have in common the same general source ofinfections, i.e., they acquire Giardia from fecally contaminated drinkingwater. The city resident usually becomes infected because the municipalwater treatment process does not include a filter that is necessary tophysically remove the parasite from the water. The number of people in theUnited States at risk (i.e., the number who receive municipal drinkingwater from unfiltered surface water) is estimated to be 20 million. International travelers may also acquire the parasite from improperly treatedmunicipal waters in cities or villages in other parts of the world,particularly in developing countries. In Eurasia, only travelers toLeningrad appear to be at increased risk. In prospective studies, 88% ofU.S. and 35% of Finnish travelers to Leningrad who had negative stooltests for Giardia on departure to the Soviet Union developed symptoms ofgiardiasis and had positive tests for Giardia after they returned home(10,11). With the exception of visitors to Leningrad, however, Giardia has not beenimplicated as a major cause of traveler's diarrhea. The parasite has beendetected in fewer than 2% of travelers who develop diarrhea. Hikers andcampers risk infection every time they drink untreated raw water from a stream or river.

Persons in categories 4 and 5 become exposed through more direct contactwith feces of an infected person, e.g., exposure to soiled diapers of aninfected child (day-care center-associated cases), or through direct orindirect anal-oral sexual practices in the case of homosexual men.

Although community waterborne outbreaks of giardiasis have received thegreatest publicity in the United States during the past decade, about halfof the Giardia cases discussed with staff of the Centers for DiseaseControl in the past 2 to 3 years have a day-care center exposure as the most likely source of infection. Numerousoutbreaks of Giardia in day-care centers have been reported in recentyears. Infection rates for children in day-care center outbreaks range>from 21 to 44% in the United states and from 8 to 27% in Canada (12,13,14,15,16,17). The highest infection ratesare usually observed in children who wear diapers (l to 3 years of age).In one study of 18 randomly selected day care centers in Atlanta (CDCunpublished data), 10% of diapered children were found infected.Transmission from this age group to older children, day-care staff, andhousehold contacts is also common. About 20% of parents caring for aninfected child will come infected.

It is important that local health officials and managers of water utilitycompanies realize that sources of Giardia infection other than municipaldrinking water exist. Armed with this knowledge, they are less likely tomake a quick (and sometimes wrong) assumption that a cluster of recentlydiagnosed cases in a city is related to municipal drinking water. Ofcourse, drinking water must not be ruled out as a source of infection whena larger than expected number of cases are recognized in a community, but the possibility that the cases are associated with a day-care centeroutbreak, drinking untreated stream water, or international travel shouldalso be entertained.

Parasite Biology

To understand the finer aspects of Giardia transmission and the strategiesfor control, one must become familiar with several aspects of theparasite's biology. Two forms of the parasite exist: a trophozoite and acyst, both of which are much larger than bacteria (see Figure 1).Trophozoites live in the upper small intestine where they attach to theintestinal wall by means of a disc-shaped suction pad on their ventralsurface. Trophozoites actively feed and reproduce at this location. Atsome time during the trophozoite's life, it releases its hold on the bowel wall and floatsin the fecal stream through the intestine. As it makes this journey, itundergoes a morphologic transformation into an egglike structure called acyst. The cyst, which is about 6 t o 9 micrometers in diameter x 8 to 12micrometers (1/100 millimeter) in length, has a thick exterior wall thatprotects the parasite against the harsh elements that it will encounteroutside the body. This cyst form of the parasite is infectious for other people or animals. Most people become infected either directly byhand-to-mouth transfer of cysts from the feces of an infected individual,or indirectly by drinking feces-contaminated water. Less common modes oftransmission included ingestion of fecally contaminated food andhand-to-mouth transfer of cysts after touching a fecally contaminatedsurface. After the cyst is swallowed, the trophozoite is liberated throughthe action of stomach acid and digestive enzymes and becomes establishedin the small intestine.

Although infection after the ingestion of only one Giardia cyst istheoretically possible, the minimum number of cysts shown to infect ahuman under experimental conditions is ten (18). Trophozoites divide bybinary fission about every 12 hours. What this means in practical terms that if a person swallowed only a single cyst,reproduction at this rate would result in more than 1 million parasites 10days later, and 1 billion parasites by day 15.

The exact mechanism by which Giardia causes illness is not yet wellunderstood, but is not necessarily related to the number of organismspresent. Nearly all of the symptoms, however, are related to dysfunctionof the gastrointestinal tract. The parasite rarely invades other parts ofthe body, such as the gall bladder or pancreatic ducts. Intestinalinfection does not result in permanent damage.