Black, parasitic, leech-like creatures approximately seven inches in length exist in a physical dimension parallel to our own. Normally we would be unaware of their presence since they inhabit a dimensional phase in which we do not interact, however the Sodan cloaking device enabled several of the creatures to cross over into our phase. The native dimension of the parasites and the technology of the cloaking device may have had some similarity to the Ancient inter-dimensional device that made insect-like creatures visible in our dimension.
The Sodan cloaking device produces an energy field that accesses a pocket of subspace around the wearer and makes him invisible by putting him slightly out of phase with our normal dimension of space-time. The device emits a significant amount of radiation which repels inter-dimensional creatures such as the leech-like parasites, however when the SGC acquired the cloaking devices after the slaughter of the Sodan, they assumed that the radiation was a coincidental byproduct of the technology, and they made modifications to shield the radioactive emissions in order to make the devices safer for human use. As a result, inter-dimensional parasites that were caught within the dimensional field of the cloaking device were able to randomly bleed through and cross into our dimension. The parasites are able to crawl and to leap a distance of several feet, and after entering our phase, they burrowed into the bodies of other indigenous animals and began changing the host animal's DNA until it mutated into a large, carnivorous predator.
The SGC had begun to use the Sodan cloaking devices for off-world missions that required inconspicuous covert observation, and one device stolen from Area 51 by members of the Trust was used covertly on Earth. Soon after these missions began, horrific monstrous creatures began appearing on the worlds where the modified cloaking devices had been used. Several deaths caused by animal attacks were reported on P9J-333 and P2R-866, and on Earth several hunters were killed by what was believed to be a grizzly bear in the woods near O'Neill's cabin in Silver Creek, Minnesota. One of the mutated creatures on P9J-333 was killed and returned to the SGC where Dr. Redden performed an examination, and she found the parasite still alive in the carcass of the dead animal. Dr. Redden determined that the dead animal had originally been a shy docile herbivore, a harmless mammal similar in appearance to an oppossum or a dog about 2 to 3 feet tall. Once the parasite had entered the host, it had lodged itself inside the animal's stomach wall, altering the host's DNA and causing a rapid and extensive mutation resulting in a large ferocious carnivore that had killed several people on the planet.
Because the parasite gives off a unique radioactive energy signature that is still detectable after it has entered a host, SG-1 and support teams were able to track and kill the mutated predators both off-world and on Earth. The creatures were remarkably resilient, requiring many rounds fired from multiple weapons simultaneously to bring them down. Two of the mutated creatures were killed in the woods near Silver Creek, Minnesota, and a radioactive perimeter was maintained around the park for another week in case additional creatures were present.
Cross Reference: Alternate Dimension, Inter-Dimensional Device, Inter-Dimensional Insects, Invisibility, P9J-333, Dr. Redden, Silver Creek, Sodan Cloak, Wade Stokes, Trust
Episode Reference: Uninvited