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Five Things to Know About Heartworms

1. Heartworms are carried by mosquitoes. The mosquito bites your pet and transfers a heartworm larva to your pet. 

2. Even indoor pets get heartworms.

3. By the time your pet has any visible signs of heartworms, permanent damage has already been done to the heart and lungs.

4. Dogs as young as six months of age can already have adult heartworms in the heart and lungs.

5. Dogs as young as 18 months of age can die from heartworms.


Both dogs and cats can be infested with Dirofilaria Immitis, the dog heartworm, but cats are more resistant than dogs. Heartworms generally start to cause problems when they are still in juvenile form in the tissue of the lungs. In cats, the most common signs of heartworm infestation are asthma-like breathing problems and sudden death. They rarely have any other symptoms. There is no approved treatment for heartworms in cats, so it is best to keep your cat on a prescription monthly heartworm preventative. Because there is no treatment for heartworms in cats, and because testing for heartworms in cats is complicated, we rarely test for heartworms in cats. 


Dogs are the preferred host of heartworms. In the dog, Dirofilaria immitis can grow up to 13 inches long and is thick like a strand of spaghetti pasta. This long, thick worm swims around in the blood vessels of the lungs and chambers of the heart. As it bangs around it causes inflammation and scars. The worms can also get tangled in the valves of the heart and cause severe sudden disease. It takes up to 6 months from the time the mosquito drops the larvae in the skin of the dog for the heartworm to grow into an adult in the blood vessels of the heart and lungs. Once the larvae reach the lungs, they become resistant to the preventatives. It takes 3 to 4 months for the larvae to reach the lungs. You’ve heard the old saying “An ounce of prevention is worth a pound of cure”, this is certainly the truth with heartworms. Once your dog has adult heartworms in the heart and lungs, a medication called melarsomine must be used to kill the adult worms. One round of melarsomine treatment costs as much as up to seven years of heartworm prevention. 


The best way to prevent Heartworm disease in dogs is to start your dog on a prescription heartworm preventative BEFORE 12 weeks of age.


By the time you see symptoms of heartworm disease at home, the worms have already done a lot of damage. That is why we require yearly heartworm testing. Preventatives will stop up to 98% of heartworm larvae, but there is always that small chance that your pet didn’t digest the medication well or spit it out when you were not looking. 


A simple blood test uses three drops of your dog’s blood to check for a protein produced in the pregnant female heartworm. If the test is positive, a second test can be sent to an outside laboratory to confirm whether or not there are heartworms present. If your dog has heartworms, treatment can be performed to remove the adult heartworms. For dogs that have stage 1 heartworm infestation (no outward symptoms), treatment tends to be easier on them and they usually make a full recovery. 


Dogs with stage 2 heartworm disease (outward symptoms such as coughing, shortness of breath, or reduced energy levels) or stage 3 heartworm disease (right-sided heart failure resulting in a fluid-filled belly, weight loss, coughing) will require additional medications and treatment and may need medications for the rest of their life. Stage 4 heartworm disease (complete cardiovascular collapse and end-stage heart failure or shock and collapse caused by caval syndrome) rarely survives long. 


Please call today to schedule an appointment to have your pet tested for heartworms and please keep your pet on a monthly heartworm preventative.

Dr. Hoffpauir, Alvin Animal Clinic: 281-585-5183

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