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Heartworm Disease in Dogs and Cats

Heartworm disease is caused by infection with the parasitic worm dirofilaria immitis. This disease most commonly affects dogs of all ages, but has been reported rarely in cats (scroll down to see heartworm disease 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 warm, sub-tropical to tropical areas where mosquitos persist all or most of the year are particularly at risk. In the State of Florida, for example, heartworm disease is an serous endemic problem.

The heartworm positive canine patients often present with no symptoms at all and the disease is discovered on routine testing regularly performed on dogs 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 exercise intolerance, syncope (fainting spells), trouble breathing, free fluid in the abdomen (known as Ascites), or acute sudden death.

Treatment for heartworm disease is by a series of 3 injections with an arsenic based drug called melarsamine, commercially available in a product called Immiticide. This has been dubbed the “fast kill” approach and it is the gold standard for treating heartworm disease in dogs. It is essential to understand that Immiticide specifically kills advanced stages of heartworm larvae and adults and is not the same as monthly preventive heartworm medications.  Monthly heartworm preventive medications are successful in killing only the microscopic infective larvae that live in the gut of the mosquito called microfilaria.

Fast-kill heartworm treatment in dogs per the latest guidelines of the American Heartworm Society is far more involved than just Immiticide injections in order to maximize the safety of treatment and treatment efficacy and minimize adverse side effects.  Below are the basic diagnostic and treatment guidelines, as approach to treatment is very much dependent on the patient’s clinical condition at the time of diagnosis.

1.) Diagnostic testing – the first step in approaching a treatment strategy is establishing a baseline of blood work (complete blood count and biochemistry profile) and urinalysis.  This establishes a patient’s safety for treatment, as well as established baseline values in the event of treatment complications.

Chest x-rays enable the veterinarian to check the heart and lungs for signs of heart and/or lung changes that may have resulted from heartworm infection.  If there is the presence of a heart murmur, it is advisable to also have an electrocardiogram (ultrasound of the heart) to evaluate heart chamber wall thickness, heart contractility, and flow of blood through the heart.

2.) 1-3 months treatment with a macrolytic lactone – Examples of macrolytic lactones are heartworm preventative medication compound classes known as avermectins and milbemycins.  Most veterinarians will go with an ivermectin preventative such as Heartgard given that it is the oldest heartworm preventative compound in the veterinary industry and less likely to contribute to the increasing issue of heartworm preventive resistance.

By treating with a macrolytic compound prior to adulticide therapy, the load of infective larvae (microfilaria) is significantly reduced, reducing the potential for a severe allergic reaction to occur as the result of their mass death in the circulatory system.  By reducing this potential, the the overall safety of the melarsamine adulticide treatment.

Also, the effectiveness of adulticide therapy for the more juvenile forms of heartworm under 4 months can be questionable, so preventive medication that is more effective in killing these juvenile forms may help to prevent the resultant treatment gap that could potentially result in treatment failure if only adulticide is administered.

3.) 4 weeks of doxycycline therapy – Heartworms are known to harbor bacteria within a a bacterial genus called Rickettsiales.  These bacteria are released into the bloodstream as heartworms die and can cause blood born infection leading to a deadly condition known as sepsis.  Doxycycline has strong activity against Rickettsiales genus bacteria, so twice daily oral therapy with doxycycline dosed at 10 mg/kg for 4 weeks prior to the first adulticide injection is recommended.

4.) Prednisone therapy – Prednisone is a steroid that has powerful anti-inflammatory properties.  These properties help to reduce the formation of dangerous blood clots within blood vessels as microfilaria and adult worms die.  Prednisone also significantly reduces the potential for an allergic reaction during the macrolytic lactone induced kill off of microfilaria in the early phases of treatment.  The typical oral regimen of prednisone starting around the first dose of macrolytic lactone is 0.5 mg/kg dosed twice daily for one week, once daily for another week, then tapered back to every other day dosing for one week.

5.) Melarsamine (Immiticide) injection –  Dosed at 2.5 mg/kg and is administered by deep intra-muscular injection in the large muscle groups on either side of the lumbar spine called the epaxial muscles.  A first injection is administered then one month later, two more injections are administered 24 hours apart for a total of 3 injections.

In cases where the patient is not determined to be a good candidate for melarsamine treatment as the result of lab work 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 3-5 year life span of adult heartworms.  Given this finite life span, application of an effective monthly heartworm preventive which prevent heartworm disease by killing immature and infective larvae, should render the existing adult heartworms unable to reproduce. Unable to produce new generations of heartworms, existing heartworm numbers should not increase and after their 3-5 year lifespan is up, the patient should be clear of worms.  A slow kill patient should not be considered clear of heartworm until there are two consecutive negative antigen tests 6 months apart.

Just as when a macrolytic lactone based preventive is administered in the fast kill approach to reduce microfilaria numbers prior to adulticide treatment, a similar tapering course of prednisone should be administered as the first dose of preventive is administered.

The slow kill approach is not recommended by the American Heartworm Society and is not advocated by most veterinarians.  It should be considered only as a last resort if/when the fast kill protocol is not possible.

The main drawbacks to this slow kill approach are:

1.) 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 inflammatory airway disease.
2.) Adult heartworms will continue to keep the patient prone to dangerous clot formation.  Deadly vascular events called a thromboembolus commonly occur in patients attempting the slow kill protocol.

4.) Missed or late heartworm preventive treatments can prolong the duration of heartworm disease.

5.) Normal quality of life with unrestricted play is dangerous for slow kill patients commonly results in the aforementioned complications.

Given all of these drawbacks, the fast kill approach as previously stated 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.

Thankfully, 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%.

Heartworm preventive medications have very high safety profiles with only micro-doses necessary to kill off infective microfilaria introduced by the the bite of mosquito carriers.  The much safer and less costly option for a dog is to receive timely, effective heartworm prevention.

Heartworm Disease In Cats

Heartworm is occasionally reported in cats.  Since cats are not the definitive host of the heartworm parasite, the parasite does not progress beyond juvenile larval stages so it does not colonize the heart chambers of the feline host.  Instead, larval forms can persist in a lung and heart tissues leading to chronic inflammatory airway disease in some cases.  Thus, it is standard protocol for feline patients presenting with symptoms of asthma to be tested for heartworm disease.

In addition to asthma, cats infected with heartworm disease are prone to life threatening clot formations.  These can lead to conditions such as saddle thrombus or acute sudden death.

Diagnosis and diagnostics for cats suspected to be infected with heartworm disease follow similar protocols as dogs.  A diagnostic data base of blood work (complete blood count and biochemistry profile) and urinalysis is run for assessment of general health and baseline values.  X-rays of the chest are also recommended to assess the condition of the heart and lungs.  If a heart murmur is present it is advisable to also have an electrocardiogram (ultrasound of the heart) to evaluate heart chamber wall thickness, heart contractility, and flow of blood through the heart.

Treatment for heartworm disease in cats is not possible, as the killing the larvae is far too dangerous.   In asymptomatic cats, it is recommended to test once every 6 months for heartworm antigen.  Many cats will show negative within 6-18 months, as the life span of juvenile forms of heartworm are much shorter than the adult forms.

In cats showing signs of inflammatory respiratory disease, it is advisable to treat cats with a tapering course of prednisone, a steroid with powerful anti-inflammatory properties.  These cases can be successfully managed with prednisone dosed at 2 mg/kg once daily, declining gradually to 0.5 mg/kg every other day by 2 weeks and then discontinued after an additional 2 weeks.  This can be repeated if clinical signs of respiratory disease persists.

Some veterinarians will treat with prednisone in this manner even in cats that are not symptomatic as a preventative measure.

Roger L. Welton, DVM
Founder and Chief Editor, Web-DVM.net
President, Maybeck Animal Hospital

Article updated 9/4/2017

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