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Hi there and thank you for your question. My name is ***** ***** I am a licensed, internship-trained veterinarian with 10 years experience in general medicine and emergency and critical care medicine and I would be happy to assist you. Please be patient as there may be slight delays in responses as I research your pet's issue.
Sometimes this can be a sign of a medical issue. If they feel "weakness" or sickness, they do not want to seem weak due to a pack mentality, so they will growl and hiss to appear strong. I would see your vet for an exam and possible blood work to rule out a medical cause for this. If that is all normal, I would buy a Feliway diffuser from Amazon.com or a pet store. It is something you plug into a wall socket and it diffuses a pheromone that calms and soothes cats and can prevent this sort of agitation/aggression. If those things do not work, then you may have to consider medication for the aggressor- Prozac is a common one, and can be given in flavored oral liquid or transdermal cream in the ear flap.
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