It is lucky (for the purposes of diagnosis) that the symptoms of deadly nightshade poisoning are so pronounced. This will give owners ample time to get their pet to a veterinary clinic so that they can receive medical attention.
Vomiting
After ingesting the deadly nightshade, the cat will quickly begin to exhibit the classic signs of nausea (pacing, irritability, and gagging) before starting to vomit. There will typically be no visible change in color of the vomit itself, but there may be a small amount of blood present due to irritation of the digestive tract. Vomiting can be a major contributor to dehydration, especially when paired with the other symptoms of deadly nightshade. For this reason, owners should make sure that their cat has ready access to water in order to replenish some of the lost fluids.
Diarrhea
The cat may also begin to defecate uncontrollably as the body attempts to purge the deadly nightshade toxins from its system. Owners may notice a small amount of blood in the resultant feces. The main danger posed by diarrhea is the risk of dehydration. The liquid content of the expelled feces will quickly drain the body of the water that is vital to its ability to function. Keeping the cat hydrated in the wake of a bout of diarrhea is very important to ensuring its health.
Pupil Dilation
A tell-tale sign of deadly nightshade poisoning is an involuntary widening of the pupil. Also noticeable will be a markedly reduced pupillary response to light, with the pupil remaining wide even when exposed to bright lights.
Drooling
Excessive salivation is also an effect of ingesting belladonna. Although not necessarily an issue by itself, extended periods of drooling can, in conjunction with vomiting and diarrhea, cause a rapid loss of fluid from the body and lead to dehydration.
Inactivity
Deadly nightshade poisoning can additionally cause the cat to appear very subdued and apathetic in its behavior. This is partly due to exhaustion from the aforementioned symptoms, but is also due to general disorientation and lack of coordination. Owners may also notice a degree of muscular weakness and loss of motor function in affected cats.
Slow Heart Rate
Deadly nightshade can have a direct effect on the cardiovascular system. This will typically manifest as a slow heartbeat, but can also result in audibly labored breathing.