According to Dr Sharon Hubbs-Kreft, a certified Natural Health Consultant and Certified Herbalist (http://en.allexperts.com/q/Alternative-Medicine-Pets-3721/Breathing-problems-cat.htm) Hawthorn Berries are fine for cats and helpful for Restrictive Cardiomyopathy and related breathing problems.
I have also read that the berries are fine for dogs too.
[at the above-mentioned site it is possible to ask questions, though I don't know how long you may have to wait for an answer.]
Any more info. I find out I'll let you know.
Hawthorn berries....I should know that because I deal in herbs! However, I'm not sure as some herbs which are fine for humans can be a bad idea for dogs.
Hang on a moment, and I'll go away and try to find out.