 A study showed that high levels of nutrients used in farming and ranching activities fuel parasite infections that have caused frog deformities in ponds and lakes across North America . The study showed increased levels of nitrogen and phosphorus cause sharp hikes in the abundance of trematodes , and that the parasites subsequently form cysts in the developing limbs of tadpoles causing missing limbs , extra limbs and other severe malformations including five or six extra or even no limbs . 
