Wow, you were rather fortunate! (But I am also sure you did significant work researching which funders to approach, because those three are very good choices for the kinds of funding you were seeking.) There are only a few sources of funding specific to animal welfare that I'm aware of, so it is highly competitive work. I think in your position the first thing I would do is contact those current funders and tell them what you've said here. Novartis at least might have some ideas about funding for pharmaceuticals (the heartwork meds), I would guess.
Be sure and check the link for the Fund for Animals' info on animal welfare funders! I'm including it with this response. Having been funded already by other funders should provide some comfort to future funders, in that you know how to manage grants and can show that you've put money to excellent use.
You might also want to explore closer to home -- try some of the local sources such as community foundations and private foundations in your area, especially if you work in a few particular geographic areas or if your heartworm problems seem to affect less than all of your working area.
Congratulations on your success so far, and may you find plenty of ways to further your goals here on the board! (I'm a member of a similar group, though we work with cats).