I was getting ready to post a message about email technology, but now I can't do that, but maybe I can help anyway.
I think the letter may be seen as "fake" and the brochure may be easy to ignore.
My approach would be to make it personal and make it real. First, get as many people to help you. If you don't have any help, call some of your donors and tell them what you are doing, give them a chance to meet you, see your facilities, and spend a few hours or a day helping with your marketing campaign. Set a date and ask people to bring food and supplies if they can to save as much money as possible.
Have a tour and everyone picks an animal to write about.If people have digital cameras, use them if you have some way to cheaply print them out, otherwise just post them on your web site. Use paper and pen or a computer, it doesn't matter. Pick a donor name and write them a personal note about the animal you want to tell them about. The letters should write themselves, but if you can mention something about the donor like past donations, the area they live in, anything it will help drive home the fact that these are real people and real animals that need a little help. If you don't know what to write, describe the room full of people writing letters for the animals.
If you have a bigger mailing list than people to help, that's not a problem because most people you contact this way will be more likely to respond then if you just send a form letter or brochure. Plan to get together again from time to time and work on your mailing list until you contact everyone. Your response rate should be better and your campaign more effetive.
I hope this helps.