My outdoor cat has a big belly, is losing weight, and lethargic. Worms?

Thanks for using Petco Pet Education Center, formerly Petcoach. I agree that a heavy worm burden can cause what you’re describing. In fact, it is the most likely reason. You can try an over the counter cat dewormer from a pet store. Follow the directions exactly in terms of amount and number of treatments. Hopefully, that’s all it takes to get her feeling better.

Unfortunately, there are other (much less common) things that can cause large belly, weight loss, and lethargy including: FIP, heart failure, liver failure, cancer, protein problems, etc. So, if the dewormer doesn’t help, you may have to decide if you’d want to take her to the vet for further testing or if you’d like her humanely euthanized, etc.

Best wishes and thank you so much for taking care of this stray!