In 1995, Minnesota middle school students sampled frogs in a nearby pond and found that half of them were deformed, and it was discovered soon after that this was a widespread occurrence. A total of 8 frog and toad species were affected by this. Approximately 0.2% of frogs found in the area were deformed from 1973 to 1993, and that percentage increased to 2.3% by 1996. The cause of this deformity was found to be a species of trematode, Ribeiroia ondatrae, which are flatworm parasites that cause deformities and sometimes death in frogs. Trematode larvae in frogs cause cysts. If the cysts are in a developing tissue, it can cause a deformity; for example, if the cysts are found in a tissue that is developing legs, the cysts can cause extra or truncated legs. The occurrence of deformities in the frogs in the area had increased due to runoff of Atrazine, an herbicide (which is banned in the European Union and other developed countries) which kills plants in a pond, causing them to decompose and ultimately cause eutrophication of the pond. This eutrophication of ponds led to more snails in which trematodes begin their lives. The trematodes then move from the snails into the frogs, causing the deformities.