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Worms belong to the animal kingdom. There are three types of worms (flatworms, roundworms [nematodes], and rotifers) found in water. For the most part, they dwell in the bed of material at the bottom of lakes and streams. There they do important work as scavengers. The rotifers are the only organisms in this category at or near the surface. They live primarily in stagnant freshwater. The eggs and larvae of various intestinal worms found in man and warm-blooded animals pollute the water at times. They do not generally cause widespread infection for several reasons: they are relatively few in number and are so large they can be filtered out of the water with comparative ease. The typical size of parasitic worms or helminths, such as flukes, tapeworms, hookworms, Ascaris, pin­worms, trichina worms, and filaria worms is 30-50 microns in diameter.


A basic classification in the Protista kingdom is that group of microscopic animal-like protists known as protozoa. These one-celled organisms live mainly in water either at or near the surface or at great depths in the oceans. Many live as parasites in the bodies of men and animals. Like other organisms, protozoa can be classed as helpful or injurious. Sometimes drinking water becomes infested with certain protozoa which are not disease-producing. When present, they give the water a fishy taste and odor. Some protozoa are aerobic, that is, they exist only where free oxygen is available. Some exist where no free oxygen is available. Others can be either aerobic or anaerobic.

Note: One important group of protozoa are those which commonly form cysts. These protozoans have somewhat bladder-like sacs or vesicles which form a resistant protective wall when they find themselves in unfavorable surroundings. On entering more favorable surroundings (such as the body of a warm-blooded animal), the cyst abandons this wall and dwells in the bloodstream of the animal. One of the most common of these cysts carries the waterborne disease, amoebic dysentery, for which there is no universal cure at this time. The protists Giardia lamblia and cryptosporidium are a cause of Acute Gastrointestinal Illness (AGI), which is the most frequently diagnosed waterborne disease in the United States. The symptoms of the AGI illness, giardiasis, may include severe dehydration, weight loss, and fatigue. Giardiasis can persist for several months or longer. Giardiasis is usually associated with unfiltered surface water that has not been disinfected sufficiently to kill or inactivate the protozoan cysts. Fortunately, these cysts, being typically 2 to 50 microns in diameter, are much larger than bacteria and can be removed from water by fine filtration.


Nematodes belong to the worm family. They are commonly called roundworms. Nematodes have long, cylindrical bodies which have no internal segments. Interestingly enough, those nematodes which are found in the bodies of men and warm-blooded animals are large enough to be visible to the naked eye. Those living in fresh water and in the soil are microscopic. Nematodes can be a problem in drinking water because they impart objectionable tastes and odors to water. They are also under suspicion of being carriers of the type of disease-bearing bacteria found in the intestines of warm-blooded animals, though studies show, however, this possibility is somewhat remote. Nematodes are apt to be found in municipal waters derived from surface supplies. (--> Next)

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