The definition of Sapience is that of wisdom or sagacity.
Wisdom in so far as being the property of an individual, is roughly the ability to identify the situation you are in, and respond more or less correctly to that situation.
Darwin provides an interesting and pragmatic definition of correct. Any creature that fails to identify and react correctly dies. This is the law of natural selection.
It has a corollary in more advanced life forms called the law of sexual selection. ie. Its not just enough to have survived the environment you must also successfully convince a mate to reproduce with you and succeed at that too.
So a thing has Sapience if it correctly perceives and responds to its environment, and to be correct this thing must have managed to survive the law of natural selection long enough to have successfully passed the law of sexual selection.
Therefor, any disease that kills, sterilises, or otherwise disables the individual from reproduction/mating can be considered a disease that renders any individual non-sapient.
Across a population most diseases will have these effects, some individuals will manage to survive without becoming non-sapient, but most won't. It is only recent medical history that has changed the status quo.
Perhaps not quite what you meant
That being said, perhaps you meant a reduction the range of human behaviours to approximate those behaviours much more inline with our primate, or mammalian cousins. Those cousins being quite well adjusted, intelligent individuals capable of wisely acting in their environments, even though they don't understand offices.
To say that a Human can be reduced to a specific form of primate or mammalian intelligence is quite probably rude. Presuming the human mind could be transplanted into an avatar of appropriate scale, a full functioning human would not survive as a rat, or as a horse. They might, be able to survive as an ape... but they would likely be a very stupid ape as judged by their contemporaries.
Perhaps what you meant is for a human to have removed from them apparently human behaviours, and traits. As a quick list:
- The ability to make complex plans involving alternate futures, and precise outcomes.
- The ability to control fine motor actions, particularly around their hands
- Free will, or at least the apparent ability to choose against some set of personally held values.
Most higher order human behaviour is contained in the Cerebral Cortex particularly the frontal lobes, the pre-motor cortex and supplementary motor cortex. As a nod toward Stargate, Brocca's area exists in the frontal lobes of the cerebral cortex.
These lobes are largely responsible for our ability to imagine scenarios and construct and execute complex non-instinctual motions. When they are disrupted the ability to plan, the ability to be dexterous, and the ability to move with free will are hampered or removed.
Just to note, many complex motions overtime become somewhat instinctual, like bad habits. If those cortical regions above were disrupted, these habits will likely still occur when instinctively activated.
There are a number of neurological conditions/diseases that can affect these areas.
Alzheimer's Disease and Dementia slowly erode the things learned over a lifetime. Those who suffer from it are still physically skilled at whatever actions they learned early in life such as riding bicycles, walking, etc... But lose the higher order skills to plan, or learn new motor actions. This allows older exploratory, and predator defense systems to dictate behaviour explaining why they explore, and react defensively to many novel situations.
Parkinson's disease acts the motor cortex itself. Progressively rendering the individual incapable of intentional movement. The disease also attacks the somatic system, which deals with involuntary actions such as breathing, digesting, and heart beats. So not quite what you are looking for.
Huntington's Disease is another candidate. It too works by attacking motor-control systems, often leaving the individual conscious but an unmoving prisoner in their own bodies.
Technically speaking it is quite possible to remove most of the human from a human through disease.
It is certainly possible via other techniques such as removing portions of the brain, or chemically with drugs (anesthetics temporarily disrupt the function of the cerebral cortex).
However there aren't many diseases that would cause this, primarily because such a disease would for all intents and purposes kill the person. A fast acting disease would almost certainly guarantee the individuals death, those around them being confused, or completely understanding the situation would probably kill out of fear. So such diseases tend to be slow, and occur later in life (when most individuals would have already succeeded at sexual selection).
That being said, a hypothetical disease could cause these effects and somehow stop before too much damage had occurred.
By functioning like Huntington's/Parkinson's and attacking the motor functions, particularly fine motor functions. Potentially it could give a shambling gait to their walk, and interfere enough with grasping, that only crude tool use is possible (ie bashing rocks together but not threading a needle).
By functioning like Alzheimer's and attacking the personality/experience/planning regions the individual could loose significant portions of their verbal memories becoming more child-like, and non-verbal (unable to speak though they can make guttural sounds), and their effective liquid IQ would be significantly reduced as the damage inhibits learning. Although they will retain much of their crystallised IQ (the physical actions, and behavioural modes learnt previously) thus they might be able to fix a watch, without being able to explain that they can, and being completely mystified while they do it, then follow the butterfly through town, across the train tracks, and onto the roof.
The last ingredient is the wild aspect. The disease could simultaneously hyper-activate the predator systems and predator avoidance systems. These are much older portions of the brain, and will have a great influence over exhibited behaviour.
The predator system would induce active hunting, regardless of hunger. This would ensure that the individual actively sought prey.
The predator avoidance system would make the individual high strung/flighty. This would cause the individual to engage violently, or actively flee.
Given the drive to hunt, and the propensity to attack/flee with extreme action, the individual would be very inclined to attack, and if unsuccessful run away much like a wild predator would.
So all up, the disease would mildly damage the fine motor control structures to produce uncoordinated movements, attack the frontal cortex reducing fluid IQ preventing behavioural learning, and also hyper-activate the predator, and predator-avoidance systems so as to make the individual aggressive, and committed to violence or escape.