You should assume that all animals including your ferret are at risk of heartworms especially if they go outside or come in contact with mosquitoes. Even if you don’t think your area has a bad mosquito problem you should still protect your ferret by having him tested for heartworms and then placed on a monthly heartworm prevention program. Mosquitoes are the primary transmitter of heartworm disease. They carry it from infected animals to non-infected animals via bites. The larvae can take 10 to 14 days in the bloodstream to become effective. After that, the works can grow quickly and invade the heart and other organs. They can become so big or so numerous that they actually block the blood flow. The disease is severe and not always treatable. Make sure your ferret is on prevention medication and always have your veterinarians Sugar Land, TX check him.