I've never bought cat grass already grown so I don't know if that could be the cause but I would be surprised if it was. I do grow my own from seeds I buy on ebay so I know it is safe.

Like Grace said, you are going to get them treated so don't worry. Also, Scout was treated with Revolution the day I brought her home. I took her to the vet and she had fleas and earmites so they gave her a dose in the office but she still had tapeworms a few months later. My point, I don't think Revolution really kills tapeworms. I'm not a fan of the stuff, Revolution, because that one dose on Scout made her hair fall out and burned her skin. About $150 later after tests and treatments she finally grew hair back.

Anyway, a dewormer will take care of if quickly so don't fret. Do make sure you clean and vacuum up all the "rice" so they don't accidently reinjest it and start the cycle over again. That was my vets advice.