A few corrections/extra information:
In the post above it may appear that I thought that all parasitic worms are "water born", this is not correct. Some of this type of worm are spread via soil and plants (especially grass) that have been "fertilised" by animal droppings and are particulary prevalent in areas where mass grazing takes place.
I also should have made more clear that when I was talking about Herdsman and the Gobi desert, I should have included Caravans, and the transportation of Herds to "market", because the Gobi is not ideal pasture being that it is a desert.
I should have also mentioned that there are a some Venomous snakes that occur in Mongolia like the "Mongolian pit viper"* or the Adder (aka Common Viper)**
I have also thought about other "real" creatures that have been assigned "magic" abilities, in particular the Aye Aye:
"the aye-aye is often viewed as a harbinger of evil and killed on sight. Others believe, if one points its narrowest finger at someone, they are marked for death. Some say the appearance of an aye-aye in a village predicts the death of a villager, and the only way to prevent this is to kill it. The Sakalava people go so far as to claim aye-ayes sneak into houses through the thatched roofs and murder the sleeping occupants by using their middle finger to puncture the victim's aorta.[8]"***
I still believe that the MDW is based on large parasitic worms, but on reflection I now think that it is a confabulation of various animals including venomous snakes.
I understand that "Vipers" have been know to "Spit Venom":
"Other spitting species
Certain cobras, especially some Asian species that are not regarded as spitting cobras, do sometimes spit to some extent, though not as effectively as the spitters. Even some of the Viperidae have been reported to spit occasionally"****
(Pit vipers and Adders both belong to the Viperidae family and are present in Mongolia)
It was not until relatively recently that Whales were recognised as not being "Fish" due in part to their appearance and habitat (as in: they live in the seas like fishes).
I have to wonder if people didn't believe that Parasitic worms were not "snakes" and therefore posses some of their characteristics (in this case the ability to spit venom)
*Gloydius halys - Wikipedia
**Vipera berus - Wikipedia
***Aye-aye - Wikipedia
****Spitting cobra - Wikipedia
