Hello I am Dr. Joey. Thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 15 years of experience. I look forward to working with you.
Prawns are certainly not toxic, but they will not help with the crystals. They are also not a balanced diet, if they are the only thing fed. In cases when we cannot get a cat to eat the prescription diet (the Urinary SO is wonderful), then I usually recommend find a good high quality cat food (canned is preferred to help increase water intake and flush those crystals out), then we add a supplement to the food, depending on the type of crystal present.
If he has high struvite crystals, these can be primary (not caused by something else), but they also can occur secondary to a urinary tract infection. Therefore, it is important that you discussed either treating a potential urinary tract infection or at least submitting his urine to the lab for culture to rule out an underlying bacterial infection. If there is no infection, then addressing this with diet (Urinary SO or Hill's c/d) OR adding a supplement to his food to increase the pH of his urine is helpful.
As a side note if you are giving a cranberry supplement which is fine in most cases of urinary problems, please be sure there is no vitamin C; this adversely changes the pH of the urine and will make getting crystals under control even harder.
Please let me know your questions.
If you are interested in going to route of finding a good cat food and then adding a supplement, your vet will need to provide the supplement since they are prescription.
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