The current general theory for the cause of Parkinson's Disease, at least for the Idopathic ("unknown cause") variety is that there is a genetic pre-disposition to the disease, and that, coupled with a "toxic event", trigger the development of PD. The so-called toxic event could be a severe blow to the head, a bout of pneumonia, exposure to agricultural chemicals etc.
In part, this came about because of a several studies involving a number of twins. In some cases both twins developed PD, but in other cases, only one did. If genetics was the only causative factor, then, went the theory, both twins should develop PD.
Then there were studies of extended families living in the same environment. Sometimes more than one of the family got PD, sometimes only the one. If environmental causes were the only causative factor, then more than one family should have developed PD
So there had to be an explanation that satisfied both genetic and environmental criteria, thus the proposition of "genetic predisposition" PLUS "toxic event". There is no substantive proof of this theory.
However, when we consider other types of PD, such as juvenile onset (about 15 to 20) and early onset (about 20 to 40), there has been evideice that there are autosomal (non-sex) chromosomes involved, both dominant (one copy only of the altered gene in each cell is sufficient to cause the disorder) and recessive (two copies of the gene in each cell have been altered).
Several genes have been found to be involved in PD, including the GBA, LRRK2, PARK2, PARK7, PINK1, SNCA, SNCAIP, and UCHL1 genes.
Among other factors, the herbicides 2,4-D and 2,4,5-T have been implicated in causing PD.
But before we get too far ahead of ourselves in attributing one cause or another, we must ask ourselves the question "What causative agents were around when Dr. James Parkinson (1755-1824), first described the disease in Western medical terms?"
Not to mention the Ayurveda, the Indian medical doctrine which described PD about 5000BC. Then there is the Chinese text "Huang di nei jing su wen" about 500BC, the Ancient Greeks Homer 8BC and Erasistratus , 310-250BC, and several such as Celsus, Galen, Dioscorides and several more. It is even said to be mentioned in the Bible.
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