What is Heartworm disease and how does it affect your pet? Heartworm disease is a serious and often fatal disease, and the state of Texas has the highest percentage of heartworm positive dogs. Heartworms and transmitted from mosquitoes and are a year-round threat in Texas. At Southlake Crossing Animal Clinic, we offer many types of oral and topical heartworm preventatives. Your pet should be on heartworm preventative year-round. Should your dog ever become heartworm positive, we have the safest and most advanced heartworm treatments available.   Heartworm treatment can be very hard on an animal so it is always in your pets better interest to be on the preventative.