I adopted a cat a few months ago also, she was also very defensive and hissed for a week or two. They may take some time to get used to a new environment, especially if it came into SPCA as a stray. My cat, pumpkin spice, hasn't hissed in weeks and seemed to acclimate well into her new home.
This may be a question you want to directly ask to them, this page is for people to talk about the SPCA location itself. To answer your question, some cats just need more time to get acquainted with your place. If this behavior persists more than a month, you should probably worry but in the meantime talk to other cat owners, maybe ones who can actually visit, how you should adjust your approach to this cat and any changes you can make. For the most part I don't think there's much to worry about though. Best of luck!
Yes.
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