None whatsoever. Heartworms are blood parasites that are spread by mosquitoes. When an infected mosquito bites a cat or dog, microscopic larvae are injected into the bloodstream. Over several months, the larvae mature into worms that live in the heart and the arteries leading out of the heart.
Cats and dogs suffering from heartworm infestation may show symptoms of heart failure such as coughing, shortness of breath and decreased exercise tolerance. Vomiting is a common symptom in cats.
Fortunately, heartworms cannot spread directly from one pet to another. A mosquito must be involved in the process. What’s more, heartworm larvae must develop in the mosquito for 12 – 19 days before they can infest another pet. Therefore, even if a mosquito bites the foster dog and then bites one of your dogs, your dogs won’t be at increased risk.
Finally, you state that your dogs are on a heartworm preventative. If you use a high-quality preventative as directed, it should afford excellent protection against the parasite regardless of other circumstances. In other words, you don’t need to worry.