Which organism is commonly found in water of fast-moving streams and mountain lakes?

• A. Giardia Intestinalis Cyst
• B. Giardia Intestinalis
• C. Trichomonas vaginalis
• D. Plasmodium species

Answer: (B)

Giardia intestinalis is a waterborne protozoan parasite that causes giardiasis. In natural, cold freshwater like fast-moving streams and mountain lakes, the infectious form found in water is the cyst, which is highly resistant and can survive environmental conditions long enough to be ingested with drinking water. When these cysts reach the small intestine, they convert to trophozoites and establish infection. The organism most typically associated with contamination of such waters is Giardia intestinalis, which is why this choice is the best answer. Trichomonas vaginalis is primarily a sexually transmitted parasite and not a waterborne contaminant of streams, and Plasmodium species are malaria parasites transmitted by mosquitoes, not organisms free-living in stream water.