We don't cover costs or provide meds for owned animals - it's caused hurt feelings a couple of times, but....
a) That's not our animal, so the treatments our supervising vet allows us to give to our rescue animals can't be given to a foster's personal pets;
b) Huge liability for you. Covering the cost of a round of doxy once or twice is pretty minor, but what happens when their pet gets a more serious injury or illness? Depending on where they take the pet for treatment, that can be hundreds to thousands of dollars that they may expect you to cover if you've previously been willing to cover the minor stuff.
We also heavily caution people to keep foster pets separated from foster pets in our printed materials and manuals. Many of them don't, but we make it clear that these pets have unknown histories and may be in an incubation phase of an illness on intake.
I personally had my cat attacked by a foster dog when I was a newbie foster and didn't know better. Spent probably $2500 to save her life and definitely got no help from the organization I was fostering for. At the time I thought it was kind of crappy but also understood that I had signed a contract and that it was my fault. I think how the fosters receive the info is all about how well it's handled and explained.
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Ryan Barker
SFC virginia
Fredericksburg VA
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