Parasitic twining is believed to be related to the process that results in conjoined twins, thus parasitic twins are also often referred to as asymmetrical conjoined twins or unequal conjoined twins. The direct cause and reason for parasitic twining is still somewhat unknown however the leading theory states the two separate embryos or fetuses are formed in uteri and one of the twins stops developing during gestation and results in a vestigial to a healthy, otherwise mostly fully-formed individual twin. These parasitic twins are defined as parasitic, rather than conjoined, by being incompletely formed or wholly dependent on the body functions of the host twin – know as the autosite.
The condition of parasitic twining does not have a standard appearance and the parasite can be attached at various locations on the body of the host twin.However, some standard classifications have come into regular medical terminology.
Just as conjoined twins united at the head are described as craniopagus or cephalopagus and the specific area of the head where the attachment occurs denotes the second half of the classification (for example, craniopagus occipitalis is the term for fusion in the occipital region and craniopagus parietalis is when the fusion is in the parietal region); craniopagus parasiticus is term for a parasitic head attached to the head of a more fully-developed twin. The most famous example of this case was the Two-Headed Boy of Bengal of 1783 however several other examples exist. Pare mentions and gives an illustration of a woman with a similar condition, having ‘two heads’ and there is another similar record of a Milanese girl who had two heads, one animated and the other quite dead. Upon her death and autopsy it was found that she also had two stomachs.
Anomalies and Curiosities of Medicine also mentions a woman in Bavaria in 1541 with two heads, one of which was deformed, who begged from door to door and the same text references a second naturalist who wrote of what seems the same Bavarian woman ‘of twenty-six with two heads, ‘one of which was comely and the other extremely ugly’.
Epigastric and Dipygus parasites are likely what most envision when they hear the words ‘parasitic twin’. The epigastric category describes the phenomenon of an incomplete twin, usually consisting of a stunted body attached at the lower abdomen often with a rudimentary head imbedded in the autosite’s abdomen. Epigastric parasites are acephalic-acardiac, without a brain and or a heart, and are therefore completely dependent on the autosite. Basically, this condition describes limbs dangling from the belly of a child or adult however, there have been cases of entire ‘dead’ bodies attached at the belly and also singular heads. Dipygus (double buttock) and also pygomelia (limbs attached to the buttocks) parasitic twining describes duplication of the lower extremities only. In dipygus two small pelvises are formed side-by side and the autosite has control over all four legs. Often, two sets of sexual organs are present and there have been cases of both sets being fully functional, able to both conceive and procreate.
Finally, fetus in fetu parasitic twining is the stuff of nightmares. Fetus in fetu involves the complete encapsulation of a parasitic twin within the body of the autosite. This means it is conceivable a person can go through life unaware that they carry within them the body of their sibling, a sibling that is literally feeding off of their body resources. Anomalies and Curiosities of Medicine presents many cases of fetus-in-fetu, some of which were not detected until the host began complaining of abdominal pains in adulthood as the twin grew. Often the twin is not discovered until after death as, in some cases, the twin remained dormant thought out the lifespan of the autosite. The placement of a fetus in fetu twin can be anywhere within the body of the autosite. At this moment, is it possible that you have small fetus ‘living’ in the grey matter of your brain – and that is no joke.