Your dog must be bitten by a female mosquito that is actively incubating microfilariae and the temperature and humidity need to be high enough for the L3 to make it through your dog’s skin (L2 won’t be passed to your dog). Many studies say this requires a steady daily temperature of at least 64°F, day and night for a month for this to happen. The veterinary college at Washington State University also notes that at 80°F the time to maturation would be about half that. If at any time, the temperature drops belos 57°F, the maturation cycle will fail and the female mosquito can no longer transmit heartworm to dogs.
There is no evidence of safe and effective use of topical corticosteroids in pregnant mothers. Therefore, they should be used only if clearly needed. Long term use and large applications of topical corticosteroids may cause birth defects in the unborn. It is not known whether topical corticosteroids enter breast milk. Therefore, caution must be exercised before using it in nursing mothers. Topical corticosteroids should not be applied to the breasts of nursing mothers unless the mothers instructed to do so by the physician.