Water Filters and Giardia

Kyoto: Hiking from Kibune to Kurama

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Data reported to the CDC indicate that Giardia is the most frequentlyidentified cause of diarrheal outbreaks associated with drinking water inthe United States. The remainder of this article will be devoted towaterborne transmission of Giardia. Waterbo rne epidemics of giardiasisare a relatively frequent occurrence. In 1983, for example, Giardia wasidentified as the cause of diarrhea in 68% of waterborne outbreaks inwhich the causal agent was identified (19). From 1965 to 1982, more than50 waterborne outbreaks were reported (20). In 1984, about 250,000 peoplein Pennsylvania were advised to boil drinking water for 6 months becauseof Giardia-contaminated water. Many of the municipal waterborne outbreaksof Giardia have been subjected to intense study to determine their cause.Several general conclusions can be made from data obtained in thosestudies. Waterborne transmission of Giardia in the United States usuallyoccurs in mountainous regions where community drinking water is obtainedfrom clear running streams, is chlorinated but is not filtered beforedistribution. Although mountain streams appear to be clean, fecalcontamination upstream by human residents or visitors, as well as byGiardia-infected animals such as beavers, has been well documented. It isworth emphasizing that water obtained from deep wells is an unlikelysource of Giardia because of the natural filtration of water as itpercolates through the soil to reach underground cisterns. Well-watersources that pose the greatest risk of fecal contamination are those that are poorly constructed or improperlylocated. A few outbreaks have occurred in towns that included filtrationin the water treatment process, but the filtration was not effective inremoving Giardia cysts because of defects in filter construction, poormaintenance of the filter media, or inadequate pretreatment of the waterbefore it was filtered. Occasional outbreaks have also occurred because ofaccidental cross-connections between water and sewerage systems.

One can conclude from these data that two major ingredients are necessaryfor waterborne outbreak. First, there must be Giardia cysts in untreatedsource water and, second, the water purification process must either failto kill or fail to remove Giardia cysts from the water.

Although beavers are often blamed for contaminating water with Giardiacysts, it seems unlikely that they are responsible for introducing theparasite into new areas. It is far more likely that they are also victims:Giardia cysts may be carried in untreated human sewage discharged intothe water by small-town sewage disposal plants or originate from cabintoilets that drain directly into streams and rivers. Backpackers, campers,and sports enthusiasts may also deposit Giardia-contaminated feces in theen vironment that are subsequently washed into streams by rain. In supportof this concept is a growing amount of data that indicate a higher Giardiainfection rate in beavers living downstream from U.S. National Forestcampgrounds compared with a near zero rate of infection in beavers livingin more remote areas.

Although beavers may be unwitting victims in the Giardia story, they stillplay an important part in the transmission scheme, because they can (andprobably do) serve as amplifying hosts. An amplifying host is one that iseasy to infect, serves as a good habitat for the parasite to reproduce,and, in the case of Giardia, returns millions of cysts to the water forevery one ingested. Beavers are especially important in this regardbecause they tend to defecate in or very near the water, which ensuresthat most of the Giardia cysts excreted are returned to the water

The contribution of other animals to waterborne outbreaks of Giardia isless clear. Muskrats (another semiaquatic animal) have been found inseveral parts of the United States to have high infection rates (30 to40%) (2l). Recent studies have shown that m uskrats can be infected withGiardia cysts obtained from humans and beavers. Occasional Giardiainfections have been reported in coyotes, deer, elk, cattle, dogs, andcats, but not in horses and sheep, encountered in mountainous regions ofthe United States. Naturally occurring Giardia infections have not beenfound in most other wild animals (bear, nutria, rabbit, squirrel, badger,marmot, skunk, ferret, porcupine, mink, raccoon, river otter, bobcat,lynx, moose, bighorn sheep) (22).