The "you are cheating on me and trying to poison me" delusion is pretty common among those with DD. I did not get involved with this forum until shortly after I left my DD husband over 10 years ago. I found the forum helpful by seeing how similar the situations were - made it clearly a real syndrome or disease. Like if you had never had a runny nose and a cough, then you did, and for the first time you heard of this "cold" thing, and then you aren't alone.
But what I learned is not what you want to hear. DD does not go away. The DD person does not get any insight into their broken thought process. After I left him, if I had a nickle for every time someone said, "someday he will realize what he has lost and be sorry" I'd be rich today. I do not say a person is suffering with delusional disorder, because people with DD don't suffer. They think they have super powers. They can see what no one else can see. It is all so clear to them. My ex did go to a psychiatrist, did take anti-psychotic drugs, but to this day he believes he was being treated for insomnia. The drugs just blunted his reactions to his delusions, they did nothing to do away with them. The side effects of those drugs are real and serious and all they did was allow us to live together without the constant insistence that I confess and acknowledge he has been right all along, I am an adulterer and have tried to kill him. So he went off the drugs and I left and I have never regretted my decision.
My only advice is to leave. If you have small children, do all you can to get full custody. If they are older, talk to them, perhaps find a psychiatrist who can explain the condition to them so they understand to not engage with their mother when she says crazy stuff. I was upset with my ex's psychiatrist when he finally told me, a year and a half after my ex had been treating with him, that this condition essentially has no cure. That the drugs do not treat the delusions, just his reaction to them. That seems to be great for a while, outwardly, it seems the delusions are gone, but they aren't. The doctor also told me that if my ex were to re-marry, his delusions would transfer to his new wife, and that is exactly what happened. She divorced him too, but not until much damage, emotionally and financially, had been done.
So take care of yourself and your children. Don't worry about your wife. She is not suffering. DD is so much worse for the family than for the person who has it. I am not saying it is impossible for someone with DD to get treatment and get some insight into their broken thoughts, but that that is so rare that if you are the spouse, especially if you have children, that is not the outcome to bet your life on. You and your children deserve a life without constant stress and turmoil.