Cryptosporidium parvum and Giardia sp. are recognized as severe pathogens capable of producing life threatening disease in animals and humans. Both parasites produce resistant stages that are passed with the feces of infected hosts. Transmission occurs either by the direct fecal–oral route or through ingestion of contaminated food or water.
Giardiosis is the most frequently diagnosed waterborne human disease in the USA. This fact, coupled with large-scale outbreaks of infections with waterborne Cryptosporidium sp., has focused attention on the identification of sources of contamination for these parasites in the environment.
Giardia duodenalis is a ubiquitous protozoan parasite that has emerged as a significant opportunistic human pathogen.
The estimated overall point prevalence of G. duodenalis was 19.1% in cattle...
http://www.biomedcentral.com/1746-6148/5/37This study suggests one out of five cattle (including those who range free in Sierra) host some form of Giardia. This coincides with lower but similar Giardia percentages found in other stock livestock – mules – 7 – 10%.
Deer are also suspect carriers with findings showing 3 - 7.1% of the deer population acting as Giardia hosts but still nothing compared to the high percentages another study found Giardia in cattle (approximately 6–82%) in North America.
http://vdi.sagepub.com/content/11/1/65.full.pdfBeaver are also known hosts but even though water creatures, their numbers are small in most lower Sierra environments.
Conclusions: Giardia cannot survive freezing temperatures.
“Evidence shows that freezing destroys cysts. Like cryptosporidia oocysts, Giardia cysts cannot maintain their shell integrity through freeze/thaw cycles”
http://www.atticacows.com/documentView.asp?docID=1542. As our Sierra reaches below 0º for months on end, either the Giardia scare is a myth or something man induced is assisting in spreading the disease. Cattle and mules (and their affinity to drop their biscuits near water) seem the most likely suspects.