Heartworm Disease, Diagnosis, Treatment, Prevention.

Course Objectives – Things to Learn

  • To explain the Heartworm Life Cycle
  • Discuss the differences between the testing methods i.e. Smear, Filter, or Occult
  • Explain how an animal can be positive for heartworms after a heartworm treatment.
  • Explain the difference between the different heartworm preventatives. Interceptor, Sentinel, Heartgard, and Filaribits. Explain the risks, benefits and costs of each.
  • Be able to answer questions regarding what to do when a heartworm pill is missed or when the pet eats too many.
  • Why we recommend testing every six months.
  • Why we require testing every year for heartworms.
  • How does heartworm disease kill a pet?
  • How does the number of heartworms and size of the pet influence the treatments.
  • Be able to explain why a pet on preventative needs to be tested.
  • When to start on heartworm preventative.
  • What are the stages of a heartworm treatment and how does our treatment differ from others.
  • What is the most common symptom of heartworms.

Practical Objectives – Things to Do or See
(These must be witnessed and initialed and date be placed in blank by a current Supervisor)

  • Perform and observe all tests for heartworms: Occult _____ Smear______ Filter______
  • Fill out lab request forms for Occult Heartworm Test _____________
  • Observe a necropsy on a heartworm positive dog.________________
  • Discuss a heartworm treatment with a staff member _______________
  • Discuss a heartworm treatment with a Client___________________


Heartworms (Dirofilaria immitis) are nematode parasites which are transmitted by mosquitoes. All dogs, whether indoor or outdoor pets, are exposed to mosquitoes. The risk of heartworms in the Gulf Coast area is great. Eighty-five percent of the dogs not on preventive will contract heartworms. Heartworm infections are worldwide. Male dogs have a 4:1 greater risk for heartworms than females. Large breeds are more commonly affected than small breeds. Hair coat length is not a factor. Therefore, small indoor, long haired dogs are at risk as well as any outdoor dog. Heartworm disease affects dogs, cats, ferrets, sea lions, foxes, wolves, and other canids. Heartworms can infect humans. Infections in humans do not completely develop, but may cause a lesion in the lungs “coin sign” that may be mistaken for a tumor. Heartworm disease can be caused by only one infected mosquito bite.

Heartworm Life Cycle

Adult heartworms live in the main pulmonary artery. This artery exits the right ventricle of the heart and carries blood to each lung. This large artery is the home of the twelve inch long adult heartworms. The adult heartworms are swimming in the blood and are not attached to the heart or vessel wall. Heartworms consume nutrients from plasma and mate within the main pulmonary artery. The female heartworms give birth to live larva (L1) which are microscopic. The L1 larva circulate in the blood stream waiting for a mosquito to transport them to their new host. The L1 larva cannot complete their development into adult heartworms without first being passed through a mosquito. Therefore, these heartworm babies cannot grow up in the host dog in which they were born. A mosquito will pick up the circulating microfilaria (L1) from an infected dog during a blood meal. The gastric juices in the stomach of the mosquito stimulate the L1 larva to molt to the L2 stage. The L2 larva then migrates to the salivary glands of the mosquito where they molt to infective L3 larva. The process of molting within the mosquito takes 10 to 20 days. The migration of heartworm larva can be fatal to the mosquito if too many are ingested at once. The heartworm larva will continue their development after they have infected the new host dog. The L3 larvae are deposited into the salivary bleb that surrounds the mosquito’s mouthparts during feeding. After the beak of the mosquito is pulled out, the heartworm larva swim down the hole into the area under the skin (subcutaneous tissue) on the new host. The L3 larva will molt to L4 larva in 7 to 14 days, then to L5 larva in 45 to 60 days as further development continues. During this time the L4 larva travel through the tissues to the lungs. The L5 larva will penetrate the small arteries in the outer parts of the lungs on their way to the heart. These L5 larva will become the adult heartworms. They will grow and develop and move to progressively larger arteries of the lungs until they finally arrive in the main pulmonary artery. These heartworms will then continue to mature, mate and give birth to L1 larva. The L1 larvae (microfilaria) are carried in the blood throughout the body. The number of circulating microfilaria may vary during different times of the day. The number of circulating microfilaria is greatest during the morning and evening hours coinciding with mosquito activity.

heartworm life cycle

The concept of “Worms in the Heart” needs explanation to most clients. When a client thinks of worms they think of earthworms or worms in an apple. When they think of being infected with worms they think of being infested with worms. These points need to be articulated so the client will know that you understand what they are thinking. Then tell them that heartworms are not actually penetrating the heart muscle tissue causing holes like Swiss cheese. The heartworms are not even attached to the heart. They are only swimming in a large vessel exiting the heart. This vessel also has the slowest blood flow and is therefore easier for them to live there. It is really the disturbance to the blood flow by an allergic reaction in the lining of the blood vessels that causes heartworm disease.