Heartworm disease is caused by infection with the parasitic worm difilaria. This disease most commonly affects dogs of all ages, but has been reported rarely in cats. Heartworm disease is spread by infective larvae that are transmitted by the bite of an infected mosquito. The larvae mature, live, and reproduce in the pulmonary artery and right ventricle of the heart. The movement and space occupying effects of the worms cause scarring, thickening of the pulmonary artery and other affiliated vessels, as well as scarring and enlargement of the right ventricle of the heart. This combination predisposes the patient to acute death from clot formation or often results in right sided heart failure. Heartworm disease occurs in all areas where mosquitos persist at least part of the year, placing dogs nearly nationwide at risk. Dogs that live in sub-tropical to tropical areas where mosquitos persist year round are particularly at risk. In the State of Florida, for example, hearworm disease is an endemic problem.
The heartworm positive patient often presents with no symptoms at all and is discovered on routine testing regularly performed at the yearly visit. The reason for this is that, unless clinical disease results from clot formation or allergic reaction, it can take several years before chronic changes are observed. Clinical disease is most commonly found in patients who’s owners who do not participate in yearly visits, heartworm preventative medication, or vaccines; or, in stray dogs. Patients showing clinical disease present with symptoms including chronic non-productive cough, lethargy and excercise intolerance, syncope (fainting spells), trouble breathing, free fluid in the abdomen (known as ascites), or acute sudden death.
Treatment for heartworm disease is by injections of an arsenic – like drug called Melarsamine. This has been dubbed that “fast kill” approach. Given the toxic nature of routine blood screening is necessary before treatment. Treatment is in most cases successful, although complications arising from toxicity of the treating drug or reactions to the dead worms sometimes occur. Pre-screening labwork, including bloodwork, chest x-rays and EKG should therefore be performed prior to treatment to make certain the patient is a good candidate for treatment. Treatment is always most successful when the disease is caught early, before clinical disease has set in. Therefore, if your dog has never been tested for heartworm disease before, or, more than one year has elapsed since the last test, the dog should be tested A.S.A.P
In cases where the patient is not determined to be a good candidate for Melarsamine treatment as the result of labwork abnormalities, or the client simply cannot afford treatment, then an approach called the “slow kill” method can be applied. The slow kill method is based on the 5 year life span of adult heartworms.Given this 5 year life span, application of a monthly heartworm preventive, which prevent heartworm disease by killing immature and infective larvae, should render the exisintg adult heartworms unable to reproduce. Unable to produce new generations of heartworms, existing heartworm numbers do not increase and after their 5 year lifespan is up, the patient should be clear of worms.
The main drawbacks to this slow kill aproach are
1.) An initial kill off of mass quantities of infective larvae following the first few treatments can lead to a dangerous allergic reaction.
2.) Even though adult worms are rendered unable to reproduce, they still continue to exert their damage to the heart muscle, blood vessels, and lungs, potentially leading to heart disease, vascular disease, and chronic airway disease.
3.) Adult heartworms will continue to keep patient prone to clot formation.
4.) Missed or late heartworm preventive treatments can prolong the duration of heartworm disease.
Given all of these drawbacks, the fast kill approach as described above is the gold standard of treatment, with slow kill only considered under special circumstances. Also, given the potential for complication with the slow kill approach, a pet owner should not ever try to implement a slow kill protocol on his/her own. Any heartworm treatment must be approached under the careful supervision of a licensed veterinarian.
Thankfullly, heartworm disease is easily prevented by administration of an oral monthly medication. A number of effective products are available through your veterinarian, including: Heartgard, Interceptor, Trefexis, Advantage Multi, Revolution, and Sentinel. Yearly heartworm testing is always recommended as, while the preventative treatments are very effective, nothing is 100 %.
Roger L. Welton, DVM
Founder and Chief Editor, Web-DVM.net
President, Maybeck Animal Hospital
Article updated 10/30/2012