Parasite concerns frequently come up in regards to feeding a raw diet. Raw meat can and does carry parasites, this is fairly common knowledge. However, freezing meat for a MINIMUM of FOUR WEEKS at a temperature of -18 to -20 Celsius (0 to -4° Fahrenheit) is sufficient to kill almost all species of meat-borne parasite (known exceptions noted as follows).
Of note, the FDA recommends keeping home freezers at about -18 Celsius (0° Fahrenheit) for proper food storage. Thus, as long as your home freezer is working properly you should be able to reach adequately low temperatures for safely freezing your meat.
There are however, a few known exceptions that resist freezing even for the duration noted above:
Trichinella spiralis (causes Trichinosis):
Trichinella is very hardy in the cold, and some strains, particularly in the Arctic, are resistant to freezing. Common hosts for trichinella include pigs and boars, wild rodents, bears, walrus, and other wild carnivores (including panthers). Market pork meat used to be a common carrier of Trichinella, because pigs are known to eat almost anything – including infected rats and rat feces. However, since previous outbreaks the USDA/FDA have cracked down VERY tightly on the pork industry. Pork is now one of the most tightly regulated and inspected meats on the market, and all USDA/FDA regulated/inspected pork sources are very closely monitored for trichinella, making the risk of trichinella in human grade pork meat very low.
Studies have demonstrated that freezing as described above will inactivate trichinella strains in domestically raised pork. However, some concern remains that wild strains, particularly in cold/Northern regions, may be more resistant. Studies found that trichinella in bear and walrus remained infectious even after freezing for 20 months! As such, we do NOT recommend feeding raw meat from: wild pork/boar, home-raised pork/boar not regulated by the FDA, wild rodents, bear, walrus, or any wild omnivore or carnivore.
Trematodes:
Another exception is two studied species of Trematodes. The first, clonorchis sinensis (Chinese or oriental liver fluke), is found in snails, shrimp, and freshwater fish in Eastern Asia (orea, China, Taiwan, and northern Vietnam) and Eastern Russia. This parasite may resist freezing and per one study by Franssen et al (2019) “Only 20 days of freezing at −12 °C or 3 days of freezing at −20 °C followed by thawing and another freeze treatment for 4 days at −20 °C eliminated infectivity.”
The other Trematode found to be resistant to freezing was Heterophyes metacercariae, a minute intestinal fluke, found in snails, brackishwater fish, and birds/mammals that consume said fish, in the regions of Egypt, the Middle East, and Far East. While freezing reduced, it it not eliminate the infectivity of this organism.
For this reason, we advise against feeding snails, as well as fish sourced from endemic areas (China, Taiwan, northern Vietnam, Eastern Russia, Egypt, Middle and Far East). If you choose to feed raw fish, it is advisable to freeze for a period of 3-7 days, thaw, and then re-freeze for a minimum of one more week (preferably longer).
Sources:
Heterophyiasis. The Center for Disease Control and Prevention website. Accessed January 30, 2021. https://www.cdc.gov/dpdx/heterophyiasis/index.html
Kaplan, K.(2020). New freeze-resistant Trichinella species discovered. The USDA Agricultural Research Service website. Published May 6, 2020. Accessed January 30, 2021. https://www.ars.usda.gov/news-events/news/research-news/2020/new-freeze-resistant-trichinella-species-discovered/?fbclid=IwAR0GQFChR2sxSJ5Wcij4XQp5C1d3OfrhAUQVCJoV18FXSHg0drQEl1frkTo
Lacour, S., Heckmann, A., Mace, P., Grasset-Chevillot, A., Zanella, G., Vallee, I., Kapel, C., Boireau, P. (2013). Freeze-tolerance of Trichinella muscle larvae in experimentally infected wild boars. Veterinary Parasitology. 194 (2-4): 175-178.