From: West Greenwich Animal Hosptial Library
By: K. Dana Brown, DVM
Heartworm Disease
Heartworms are long, white, parasitic worms, which live in the heart and lungs of infected dogs. The worms affect the dog’s cardiovascular system in two ways: by making the lungs and arteries less flexible, and by making it harder for the heart to pump blood by clogging the vessels around the heart. Without treatment, heartworm disease will eventually cause heart failure and death. This could take months or even years to occur. Some symptoms of heartworm disease are coughing, difficulty breathing, sluggishness, weight loss, poor hair coat, or a dog may have no symptoms at all.
Heartworm infection is transmitted by mosquitoes. The cycle begins when a mosquito bites a dog that already has heartworm disease. The mosquito ingests heartworm larvae from the blood of the infected dog. When the mosquito bites another dog, these are deposited into the bloodstream. The larvae then migrate through the bloodstream, eventually reaching the heart and lungs within six months. The heartworms mature in the heart and the female heartworms release larvae into the bloodstream of the newly infected dog, continuing the cycle.
Heartworm Prevention
Heartworm disease is nearly 100 % preventable. Heartworm prevention is given once per month in the form of a tasty chewable treat, which kills any larvae that are circulating in the blood. We recommend giving heartworm prevention all year long, not just during the traditionally warmer months of the year. Once a dog has heartworm disease, the pill used for prevention will not work to treat the disease and may even be harmful. This is the reason for doing a heartworm test before beginning prevention on a dog that has never been on prevention or has been off it for a period of time.
Heartworm Treatment
Before beginning heartworm treatment, dogs begin a month of the antibiotic, doxycycline. Doxycycline is thought to kill a bacterium which is infecting the heartworm and causing some of the inflammation and clinical signs of heartworm disease.
Exercise restriction is very important from this point onward. (I’ll explain why just a little later.)
After one month of doxycycline, dogs are admitted to the hospital to begin taking the monthly heartworm prevention to kill immature worms. Dogs need to be observed closely for the first eight to twelve hours after we begin the monthly heartworm prevention. The rapid death of large numbers of microfilariae (baby heartworms) can cause lethargy, lack of appetite, salivation, vomiting, defection, pale mucous membranes, increased heart rate, and very rarely circulatory collapse. Additional monthly doses of heartworm preventative are unlikely to cause any clinical signs because the most of the baby heartworms will have been killed by the initial dose of medication.
When the dog is admitted for the first heartworm pill, we will need to run some blood tests. A complete blood count (CBC) will provide us with evidence of anemia or infection, and a chemistry profile will alert us to any underlying liver or kidney disease or other abnormalities. We radiograph the dog’s chest at this visit to make sure the lungs are clear and there is no evidence of severe heart disease. We must be sure that a dog can withstand the treatment and be able to clean up the adult heartworms once we kill them. Dogs with very advanced heartworm disease will have their heartworms removed manually at a referral facility under anesthesia. If the radiographs and blood work look good, then we schedule the heartworm treatment for the adult worms. Dogs remain on
monthly oral heartworm prevention until this appointment can be scheduled. Ideally, a dog would receive 3 months of heartworm prevention prior to the first injection to kill adult heartworms.
In order to kill adult heartworms, an insecticide must be injected into the dog. This medication is called Immiticide, and only one company produces it. This company holds the patent for the medication and it is very expensive.
Since the adult heartworms are living in a clump in the animal’s heart, killing all of them at once could dislodge a large number of dead worms into the bloodstream. This clump of dead worms could become lodged in the animal’s lungs or brain. For this reason, the treatment is given in three injections. The idea is to kill some of the worms and have the body clean them up, kill some more worms and have the body clean up, etc. To reduce the risk of dislodging a large clump of worms, the dog is to remain very calm and quiet at home between injections.
Typical schedule for Heartworm Treatment
* On the day the dog is diagnosed with heartworm
disease – Begin one month of the oral antibiotic,
doxycycline. Schedule appointment in one
month to begin monthly heartworm
prevention. Begin restricted activity.
* On day of first admission – Blood draw for CBC/Chemistry profile, physical exam, administer first heartworm pill and observe for the remainder of the day. Schedule the first injection to kill adult
heartworms for about 3 months from today. Begin monthly heartworm prevention. Restrict activity.
* Second hospital Admission – Physical
exam, chest radiograph, one Immiticide injection in lower back. Stay in hospital and be observed for the day. Rimadyl given as needed for pain. Schedule second and final heartworm injections for 1 month from today. Restrict activity.
* Third hospital Admission – Physical exam, two Immiticide injections in lower back 24 hours apart.
Dogs can stay overnight if it is more convenient for the owner, but remember, there is no one here
after business hours. Restrict activity for one additional month. Continue on monthly heartworm
prevention; slowly resume normal activity.
* Test for adult heartworms at yearly exam as long as it is at least 6 months post treatment.
Side effects / Risks of Treatment
Heartworm treatment is about 90% effective against adult heartworms. The largest potential risk associated with heartworm treatment is the dead worms lodging someplace that they should not. We worry most about the worms lodging in the lungs or brain and causing problems. Mobile worms are the reason for the required restricted activity after each treatment. Changes in the heart and lungs due to heartworm disease are partially reversible after the heartworms are eliminated. About 30% of dogs experience pain and swelling at the injection site. Rimadyl is generally given to decrease pain and swelling. Less than 20% of dogs become lethargic, have a decreased appetite, or develop a fever. A very small number of dogs (less than 6 %) have congested lungs, vomiting, diarrhea, trouble breathing, salivating, panting, or coughing blood.