Hi -- I'm sorry no one has responded to your question before now. Here is a quote from a very good article (https://aaep.org/sites/default/files/Guidelines/PigeonFeverGuidelines52713.pdf) about pigeon fever: "Once horses are recovered and there is no drainage from abscesses, no precautions should be needed to reduce the risk these horses pose for spread of infection."

However, the premise where your 4-yr.-old mare is/was has to be considered contaminated. Your filly could contract the disease at any time on this premise. The risk of transmission is higher during biting fly season, but will still exist to some extent year-round because the bacterium can persist in soil and on surfaces for months (see the article above). 

If at least 8 weeks have passed since the mare got the disease and the filly hasn't shown signs of problems, the filly probably won't become infected from the mare. She could still contract the disease by getting a small scratch or wound contaminated with the pigeon fever bacterium, though. You could deeply bed or put gravel on areas with exposed dirt on your property to decrease the risk of contact with contaminated dirt. Also, disinfect any surfaces you can. Sand away wood splinters and paint wood surfaces with a non-toxic paint to reduce the chance of your horse getting a tiny prick from a contaminated wood splinter. Continue excellent insect control using repellents, year-round if necessary.

Because biting flies can spread this problem, this disease can be an issue for a "neighborhood" of horses. Your horses are probably not a source of infection for other horses at this point, but the fact your mare got the problem should alert other horse owners nearby the disease is in the area. When there are "outbreaks" of pigeon fever, veterinarians usually do a good job alerting their equine clients about the situation. Certainly contact your veterinarian if you haven't done so already.

Best wishes,