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Welcome! Thanks for asking your question. My name is***** will do my best to help you today. First I need to ask you a few questions so that I can be well informed and give you the best advice.
1- So basically, both, Rosie and Millie are losing their voice, correct?
I am sorry that Millie and Rosie are having this problem. Please give me 5-10 minutes to type your answer, thank you.
Laura, what you are describing is consistent with laryngitis. This is very similar to us when we yell in a sporting event or concert and we loose our voice for several days. In this case, your cats may have been meowing/hissing a little too much at the kitten. Usually, with rest of the vocal cords, it will restore the voice within 1-2 weeks. It is also important to remember that there could be an underlying medical disorder like laryngeal paralysis, tumors of the larynx, infections, etc. However, with a history of a new kitten and happening in two cats I am very inclined to believe this is related to hissing at the kitten.
I would recommend you do your best placing them in an area where they are not meowing or hissing for the next week or so. If no improvement within the next couple of weeks I would have them examined by the family veterinarian for a hands on examination. If you notice any other symptoms develop like a nasal discharge, anorexia, lethargy, etc. then have them checked immediately. Otherwise, I would separate them from the kitten to give their vocal cords time to rest.
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There is really nothing over the counter that you can give them that will any significant therapeutic benefit, just need time and rest. I am sorry.
If this was caused by an underlying medical problem then you would treat the primary cause. For example, if they have an infection they would start the patient on antibiotics. But, in this case I believe rest is the way to go to rest the vocal cords.