Heartworms in Dogs

Frequently Asked Questions About Canine Heartworm Disease 

What is heartworm disease?

Heartworm disease is caused by a parasitic worm that lives in the heart and lungs of dogs. The worms cause damage to the heart and lungs that over time can lead to illness and even death. Wild species such as foxes and coyotes are considered important carriers of the disease.

How do dogs become infested with heartworms?

Heartworms are transmitted by mosquitoes, which is why the risk of heartworm disease in dogs is high in Louisiana. Even dogs that spend most of their time indoors are susceptible. Adult heartworms living in infested dogs or wild canines produce microscopic baby worms called microfilaria. When a mosquito bites and takes a blood meal from an infested animal, it picks up these baby worms, which change into infective larvae in the mosquito. When the infested mosquito bites another dog, the infective larvae get into the bloodstream and cause infestation in that dog.

What are symptoms of heartworm disease in dogs?

Dogs with early heartworm infestation may not show any symptoms. Dogs with early symptoms may tire easily, especially during exercise, and might have a soft, deep cough. If not treated, this progresses to rapid breathing and weight loss. In severe cases dogs might have fainting spells. Sudden death is also possible.

How do I know if my dog has heartworms?

Heartworm disease is a serious, progressive disease, and the earlier it is detected, the better the chances of recovery. Even if your dog appears perfectly healthy, it should be tested at least once a year. Your veterinarian can test your dog with just a small blood sample. If your dog tests positive, then x-rays of the heart and other blood tests may be ordered.

How can I prevent my dog from getting heartworms?

It is recommended that heartworm prevention be administered year-round in Louisiana in order to prevent heartworm disease. Several products are available, and some are also effective against intestinal worms and fleas. Heartworm prevention products require a prescription from your veterinarian. A veterinarian-client/patient relationship must exist in order for you to receive either a prescription or preventative product for your dog. This relationship is formed while the veterinarian is examining your dog and discussing heartworm disease and prevention with you.

My dog has been on heartworm preventive and still tested positive for heartworms. How did that happen?

Your dog can become infested with heartworms in a number of ways while receiving preventative medications. Just one skipped or delayed dose can let an infestation take hold, particularly in highly endemic areas. Endemic areas are usually hot, humid environments with lots of mosquitoes, like Louisiana. If topical products are not placed directly on the skin, or if the dog spits out a pill unnoticed, then an infestation can result.

If you are sure the preventive products have been administered on time and properly, then heartworm resistance to the preventative medication may be involved. The extent and reason for the development of resistance is still not fully understood, and more research is needed. It appears to be a problem mostly in the Mississippi Delta, and cases have been reported in Louisiana.

If we have heartworms resistant to preventive products in Louisiana, why should I continue to give them to my dog?

The current preventative products available are still effective more than 95 percent of the time, as long as the administration guidelines are followed. Even if a few resistant worms get through, most of the worms will still be killed, and the infestation will be much smaller so treatment will be more successful and have less risks.

The pharmaceutical companies that produce licensed heartworm preventatives continue to stand behind their products. In some instances, they will cover the cost for heartworm treatment if the infection is the result of product failure or resistance and there is evidence that the product was being administered according to the manufacturer’s recommendations. The primary care veterinarian must provide documentation that yearly heartworm tests were performed and enough of the product was purchased from a veterinary clinic to cover year-round protection. The companies will NOT honor the product if purchased online because quality control from the companies to online pharmacies cannot be documented.

My dog tested positive for heartworms. What’s next?

Several avenues of treatment are available for dogs that test positive for heartworms. Your veterinarian can discuss these treatments so together you can decide what is best for your dog

In summary

  • Get your dog tested for heartworms by your veterinarian at least once a year.
  • Keep your dog on heartworm preventives year-round as directed by your veterinarian.
  • Administer products exactly as directed.

For more information, including heartworm disease in cats, visit www.heartwormsociety.org


Nancy Welborn, DVM

Assistant Professor, LSU School of Veterinary Medicine

Diana Coulon, DVM

LSU AgCenter Biotechnology Laboratory

Christine B. Navarre, DVM

Extension Veterinarian, LSU AgCenter

Professor, School of Animal Sciences

William B. Richardson, LSU Vice President for Agriculture

Louisiana State University Agricultural Center

Louisiana Agricultural Experiment Station

Louisiana Cooperative Extension Service

LSU College of Agriculture

The LSU AgCenter and LSU provide equal opportunities in programs and employment.