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Heartworm Disease in Dogs, Cats, and Ferrets

(Dirofilarosis, Dirofilariasis)

By

Clarke Atkins

, DVM, DACVIM, College of Veterinary Medicine, North Carolina State University

Last full review/revision Aug 2020 | Content last modified Sep 2020
Topic Resources

Heartworm disease (dirofilarosis), produced by Dirofilaria immitis, primarily affects the pulmonary arteries, producing inflammation, vascular dysfunction, and pulmonary hypertension. Secondarily, this results in right-side cardiac pressure overload and progressive pulmonary insufficiency. Tertiary problems include aberrant worm migration, caval syndrome, coagulopathies (DIC), and heart failure. Most infected dogs are asymptomatic, but the most common clinical signs are weight loss, exercise intolerance, cough, and labored breathing. The most important therapy is prevention, which is done with macrocyclic lactones. Clearing a heartworm infection typically requires an organic arsenic compound, melarsomine, administered intramuscularly in three injections. Chronic use of certain macrocyclic lactones with doxycycline has also been effective when melarsomine cannot be used or is unavailable.

Commonly affected species:

  • Dogs

  • Coyotes

Heartworm (HW) disease is caused by the filarial organism Dirofilaria immitis. At least 70 species of mosquitoes can serve as intermediate hosts; Aedes, Anopheles, and Culex are the most common genera acting as vectors. Patent infections are possible in numerous wild and companion animal species. Wild animal reservoirs include wolves, coyotes, foxes, California gray seals, sea lions, and raccoons. In companion animals, HW infection is diagnosed primarily in dogs and less commonly in cats and ferrets. HW disease has been reported commonly in most countries with temperate, semitropical, or tropical climates, including the USA, Canada, Australia, Latin America, and southern Europe. In companion animals, infection risk is greatest in dogs and cats housed outdoors. Although any dog or cat, indoor or outdoor, is capable of being infected, most infections are diagnosed in medium- to large-sized, 3- to 8-year-old dogs living outside in endemic areas.

Infected mosquitoes can transmit HW infections to people, but there are no reports of such infections becoming patent. Maturation of the infective larvae may progress to the point where they reach the lungs, become encapsulated, and die. The dead larvae precipitate granulomatous reactions called “coin lesions,” which are visible with thoracic radiography and significant because they mimic lung cancer.

HW infection rates in other companion animals such as ferrets and cats are lower but tend to parallel those in dogs in the same geographic region. No age predilection has been reported in ferrets or cats, but male cats have been reported to be more susceptible than females. Indoor and outdoor ferrets and cats can be infected. Other infections in cats, such as those caused by the feline leukemia virus or feline immunodeficiency virus, are not predisposing factors.

Life Cycle of Heartworms in Dogs, Cats, and Ferrets

Mosquito vector species acquire microfilariae (a neonatal larval stage) while feeding on an infected host. Once ingested by the mosquito, microfilariae develop into the first larval stage (L1). They then molt into the second larval stage (L2) and again to the infective third stage (L3) within the mosquito in ~1–4 weeks, depending on environmental temperatures. This development phase requires the shortest time (10–14 days) when the average ambient temperature is >81°F (27°C) and the relative humidity is 80%. When mature, the infective larvae migrate to the labium of the mosquito. As the mosquito feeds, the infective larvae erupt through the tip of the labium, and with a small amount of hemolymph, onto the host’s skin. The larvae migrate into the bite wound, beginning the intra-mammalian phase of the life cycle. A typical Aedes mosquito can survive the complete development of <10 larvae per mosquito.

In canids and other susceptible hosts, infective larvae (L3) molt into a fourth stage (L4) in 3–12 days. After remaining in the subcutaneous tissue, abdomen, and thorax for ~2 months, L4 undergo their final molt at day 50–70 into young adults, arriving in the heart and pulmonary arteries ~70–120 days after initial infection. Only 2.5–4 cm in length on arrival, worms rapidly grow within the pulmonary vasculature to adult worms (males ~15 cm long, females ~25 cm). When juvenile heartworms first reach the lungs, blood flow forces them into the more distal small pulmonary arteries of the caudal lung lobes. As the parasites grow, they occupy larger and larger pulmonary arteries, occasionally moving into the right ventricle and even the atrium when the worm burden is high. Gravid females produce microfilariae as early as 6 months after infection but more typically at 7–9 months after infection.

Microfilariae are detectable in most infected canids (~80%) not receiving macrolide prophylaxis but only occasionally in those dogs placed on preventive after having been infected. The number of circulating microfilariae does not correlate well to the adult female HW burden. Adult worms typically live 3–5 years, whereas microfilariae may survive for up to 2 years in a dog.

Most dogs are highly susceptible to HW infection, and a majority of (an average of 56%) experimentally administered infective larvae (L3) develop into adults. Ferrets and cats are susceptible hosts, but the infection success rate is low (an average of 6% in cats and 40% in ferrets). In cats, the adult burden is often only one to three worms. It appears that early death of juvenile worms on arrival at the pulmonary vasculature is largely responsible for the heartworm-associated respiratory disease (HARD) syndrome in cats. HARD does not require maturation of heartworms but is due to the body’s response to the dying/dead immature heartworms. When maturation does occur, adult worm survival in cats is typically not longer than 2–3 years. In all animals capable of being infected, aberrant larval migration may occur, resulting in parasitic lesions in the CNS, eye, scrotum, peritoneal cavity, systemic arterial system, and in visceral and subcutaneous sites.

Pathogenesis of Heartworm Disease in Dogs, Cats, and Ferrets

The severity of cardiopulmonary pathology in dogs is determined by:

  • worm numbers and health

  • host immune response

  • duration of infection

  • host activity level

Live, adult heartworms cause direct mechanical trauma, and other suspected factors (eg, antigens and excretions) are thought to directly irritate or to stimulate the hosts’ immune system. This damages vessel intima, leading to proliferative endarteritis and perivascular cuffing with inflammatory cells, including infiltration of high numbers of eosinophils. Live worms seem to have an immunosuppressive effect; however, the presence of dead worms leads to more severe vascular reactions and subsequent lung pathology, even in areas of the lung not directly contacting the dead heartworms. Longterm infections, due to all of the factors noted (ie, direct irritation, worm death, and immune response) result in chronic lesions and subsequent scarring.

Active dogs tend to more often develop pulmonary hypertension than inactive dogs for any given worm burden. Frequent exertion increases pulmonary arterial pathology and pulmonary artery resistance (with resultant pulmonary hypertension) and thereby may precipitate overt clinical signs, including congestive heart failure (CHF). High worm burdens are most often the result of infections acquired from numerous mosquito exposures. High exposures in young, naive dogs in temperate climates can result in severe infections, possibly precipitating vena cava (caval) syndrome the year after. In general, because of the worm size and smaller dimensions of the pulmonary vasculature, small dogs do not tolerate infection or treatment as well as large dogs.

HW-associated inflammatory mediators that induce immune responses in the lungs and kidneys (eg, immune complex glomerulonephritis) cause vasoconstriction and possibly bronchoconstriction. Leakage of plasma and inflammatory mediators from small vessels and capillaries causes parenchymal lung inflammation and mild, noncardiogenic edema formation. Pulmonary artery disease compromises vascular compliance, and this, with reduced ability to adequately vasodilate, results in increased flow velocity, especially with exertion, and resultant shear stresses further damage the endothelium. The process of endothelial damage, vascular dysfunction, increased flow velocity, and local ischemia is a vicious cycle. Inflammation with ischemia can result in irreversible interstitial fibrosis.

Pulmonary arterial pathology in cats and ferrets is similar to that in dogs, although the small arteries develop more severe muscular hypertrophy. Despite this, pulmonary hypertension with CHF is less common in cats than in dogs or ferrets. Arterial thrombi, thromboemboli, and living or dead worms become lodged within pulmonary arteries or arterioles, resulting in vascular remodeling with transient or permanent, complete or partial, obstruction. In cats, parenchymal changes associated with dead heartworms differ from those observed in dogs and ferrets. Rather than type I alveolar cell damage, as found in dogs, cats develop type II alveolar cell hyperplasia, which can act as a significant barrier to oxygenation. Most significantly, because of restricted pulmonary vascular capacity and subsequent pathology, ferrets and cats are more likely than dogs to die as a result of HW infection.

Wolbachia

The role of the endosymbiotic bacteria Wolbachia pipiens, which live intracellularly within the filarid parasite, is still being determined. However, these bacteria have been implicated in the pathogenesis of filarial diseases, possibly through endotoxin production. Furthermore, studies have demonstrated that a primary surface protein of Wolbachia (WSP) induces a specific IgG response in hosts infected by D immitis. For veterinarians, the most important aspect of Wolbachia is its symbiotic relation with D immitis. This bacterium is necessary for normal maturation, reproduction, and infectivity of the heartworm. If Wolbachia are eradicated, the heartworm gradually dies, after first becoming sterile. This can be accomplished with doxycycline therapy, which has become an important part of the armamentarium against heartworms.

Clinical Findings of Heartworm Disease in Dogs, Cats, and Ferrets

In dogs, heartworm infection is ideally identified by serologic testing before onset of clinical signs; however, at the earliest, HW antigenemia and microfilaremia do not appear until ~5 and 6.5 months after infection, respectively. When dogs do not receive preventive medication and are not appropriately tested, infection and disease progress undetected. Clinical signs of HW infection include:

  • cough

  • exercise intolerance

  • unthriftiness

  • cyanosis

  • dyspnea

  • hemoptysis

  • syncope

  • collapse

  • ascites (right-side CHF)

The frequency and severity of clinical signs correlate to lung pathology and level of animal activity. Signs are often not observed in sedentary dogs, even though the worm burden may be relatively high. Infected dogs experiencing a dramatic increase in activity, such as during hunting seasons, may develop overt clinical signs. Likewise, worm death and thromboemboli precipitate clinical signs.

Dogs 5–7 years old are at higher risk of having a heavy worm burden, presumably because of increased time of exposure and opportunity for disease development. Other concurrent health factors (eg, concurrent cardiopulmonary or other organ system disease) affect risk assessment. The degree to which exercise can and will be restricted during the recovery period is another important consideration. Infected dogs are classified from I to IV, based on severity of clinical signs (See table: Stages of Heartworm Infection in Dogs). Class IV includes dogs with caval syndrome, in which retrograde migration of worms into the right ventricle and atrium and caudal and cranial vena cavae precipitates tricuspid valve insufficiency on top of severe pulmonary hypertension. The resulting low-output heart failure, hemolysis with pigmenturia, anemia, and hepatorenal dysfunction merge to produce an often terminal crisis.

Table
icon

Stages of Heartworm Infection in Dogs

Stage

Signs

I

Minimal, if any, signs and none evident to owner

II

Mild to moderate signs of HWD, most often including cough

III

Severe signs of HWD, including respiratory signs, weight loss, exercise intolerance, and even heart failure

IV

Caval syndrome

Clinical Findings in Cats and Ferrets

Infected cats may be asymptomatic or exhibit intermittent coughing, dyspnea, heart failure, vomiting, lethargy, anorexia, or weight loss See table: Diagnostic Tests, Clinical Signs, and Treatment for Heartworms in Dogs, Cats, and Ferrets. When evident, signs usually develop during two phases of the HW life cycle: 1) the arrival of juvenile worms in the pulmonary vasculature ~3–4 months after infection, and 2) death of adult heartworms. The early signs are associated with an acute vascular and parenchymal inflammatory response to the newly arriving young worms and the subsequent death of many or all of these juveniles. This initial phase is often misdiagnosed as asthma or allergic bronchitis. However, this is now considered to be HARD; for this discussion, the term HARD refers to signs of immature HW infection, whereas HWD in cats refers to mature infection. Antigen tests in such cats are negative (measured antigens are associated with mature female worms) during the early eosinophilic pneumonitis syndrome, although antibody tests typically are positive. Although not yet well characterized, it is believed that clinical signs often resolve and may not reappear or may be silent for months. HARD has been postulated to contribute to longterm lung damage, but proof is lacking. Cats harboring mature worms (HWD) may exhibit intermittent vomiting, lethargy, coughing, episodic dyspnea, and CHF, manifested by pleural effusion. Death of even one adult heartworm can lead to acute respiratory distress and shock, which may be acutely fatal and appears to be the consequence of pulmonary thrombosis and/or anaphylactic-like shock.

Ferrets, more so than cats, mimic canine HW infection in terms of clinical signs. The large parasite:host body weight ratio dictates that ferrets (and cats) develop clinical signs with relatively small worm burdens. Ferrets with HW disease may demonstrate one or more of the following:

  • weight loss

  • fatigue

  • cough

  • rapid and/or labored breathing

  • heart murmur

  • distended and pulsatile jugular veins (CHF)

  • gray and cold mucous membranes

  • ascites

  • pleural effusion

  • fainting

  • sudden death

Table
icon

Diagnostic Tests, Clinical Signs, and Treatment for Heartworms in Dogs, Cats, and Ferrets

Host

–-–----------------Diagnosis (Test Utility)---------------------

Clinical Signs

Prevention

------------Treatment------------

Microfilaria

Ag or Ab Test

Thoracic Radiographs

Echocardiography

Adults

Microfilariae

Dog

****

***** (Ag)

***

MPA, PA, RHE, PTE, PIE

**

Respiratory, weight loss, exercise intolerance, heart failure

Ivermectin, milbemycin, selamectin, moxidectin

Melarsomine, doxycycline, symptomatic treatment

Moxidectina, ivermectin + doxycycline, milbemycin

Cat

*

** (Ag)

**** (Ab)#

**

PA, PIE

****

Respiratory, vomiting, sudden death, heart failure

Ivermectin, milbemycin, selamectin, moxidectin

Symptomatic treatment;, doxycycline + ML, worm extraction

NA

Ferret

*

*** (Ag)

*

RH

***

Heart failure, respiratory, sudden death, heart failure

Topical imidacloprid-moxidectin product labeled for cats and ferrets

This product should be administered monthly at the recommended minimum dosage of imidacloprid (20 mg/kg), and moxidectin (2 mg/kg), by topical administration, regardless of indoor/outdoor status.

Symptomatic treatment (including corticosteroids) Possibly topical imidacloprid-moxidectin labeled for cats and ferrets

NA

aImidacloprid-moxidectin is the only approved microfilaricide.

* = rarely useful; ** = somewhat useful; *** = moderately useful; **** = often useful; ***** = excellent test; Ag = antigen; Ab = antibody; # = Ab-positive does not prove mature infection, only exposure/abbreviated infection; MPA = main pulmonary artery enlargement; PA = pulmonary artery enlargement; PTE = pulmonary thrombus/thromboembolus; PIE = pulmonary infiltrate with eosinophils; ML = macrocyclic lactone (macrolide); RH = right heart enlargement; NA = not applicable

Adapted from McCall, JW. Clin Tech Small Anim Pract. 1998;13(2):112.

Diagnosis of Heartworm Disease in Dogs, Cats, and Ferrets

  • Serology - antigen test for cats and dogs, antibody test for cats

  • Detection of microfilariae - direct smear, modified Knott test, millipore filter technique

Heartworm Detection in Dogs

The antigen detection test is the preferred diagnostic method for routine screening or when seeking verification of a suspected HW infection. Antigen testing is the most sensitive and specific diagnostic method available to veterinary practitioners. Microfilaria testing is limited by the fact that ~20% of infected dogs are amicrofilaremic. This percentage is even higher for dogs infected with adult heartworms and that are consistently administered monthly macrolide prophylaxis, because this kills microfilariae and induces embryostasis in mature female dirofilariae.

Timing of antigen testing is critical. A predetection period must be considered, because these tests detect only adult, female worms. This takes into account the time from exposure to seroconversion (ie, positive antigen test result). A reasonable interval is 7 months after last possible exposure. There is no value in testing a dog for antigen or microfilariae before ~7 months of age. To ensure that a previously acquired infection does not exist in these young dogs, they should be tested 6–7 months after beginning HW prophylaxis. For dogs >7 months old at first presentation and not on HW prophylaxis, testing should be performed at that time and 7–12 months after initiating preventive therapy. Subsequently, annual antigen detection tests are recommended.

The terminology for HW antigen tests is changing, with some experts replacing the word "negative" 'with “below detectable limits” to underscore the possibility that HW-infected pets with a negative test may convert to positive as worms mature or that small-burden female infections may test inappropriately negative.

The level of antigenemia is directly related to the number of mature female worms present. Most dogs harboring more than two adult female worms will test positive with most available tests. For low-burden suspects, commercial laboratory-based microwell titer tests are the most sensitive. There is, however, no test that can accurately determine worm burden. Testing for microfilariae may be useful as an adjunctive test in suspect cases that have negative antigen test results. In addition, antigen-testing heat-treated serum can change a false-negative to a positive result. Heating breaks down complexes of the antigen being tested for and antibodies (blocking antibodies) formed against the parasite antigens. This frees antigen, making it detectable.

Microfilaria testing is no longer the primary means of testing for HW. Nevertheless, it has value because a small percentage of HW-infected dogs are antigen-negative and microfilaria-positive, allowing an otherwise missed diagnosis to be made. It is of extreme importance to check for microfilariae when there is a positive antigen test, due to the need to understand microfilarial burden and hence risk of reaction to first macrocyclic lactone administration and so that, if present, microfilariae can be eliminated so that risk of resistance development is not impacted during therapy with macrocyclic lactones.

In dogs, echocardiography is relatively unimportant as a diagnostic tool, although it can allow assessment of cardiac damage and performance. Visualization of worms in the right heart and vena cava is associated with high-burden infection with or without caval syndrome. Severe, chronic pulmonary hypertension causes right ventricular hypertrophy, septal flattening, underloading of the left heart, and high-velocity tricuspid and pulmonic regurgitation. The ECG of infected dogs is usually normal. However, right ventricular hypertrophy patterns are seen when there is severe, chronic pulmonary hypertension, often associated with overt or impending right-side CHF (ascites). Cardiac rhythm disturbances are usually absent or mild, but atrial fibrillation is an occasional serious complication in dogs.

Heartworm Detection in Cats and Ferrets

The diagnosis of HW infection in cats is based on historical and physical findings, index of suspicion, thoracic radiographs, echocardiography, and serologic test results. Cats may develop a positive antigen test 7–8 months after L3 inoculation. However, antigen tests alone are considered too unreliable (insensitive, missing 25%–50% of mature infections) as the initial screening test for cats. This occurs with unisex (all male) infections, infections with insufficient numbers of mature females to be detectable, and in cats with HARD. Cats with HARD remain antigen negative, if no adult worms develop. Cats with mature infections are also occasionally found to be temporarily negative, if tested before detectable antigenemia develops. Nevertheless, the antigen test is strongly recommended in cats in which HW infection is suspected.

Antibodies to heartworms, produced by 90% of infected cats, often appear by 2–3 months after L3 infection and are generally present by 5 months. However, antibodies can persist for several months after worm death. Also, antibodies induced by larvae can persist in aborted infections and after macrolide prophylaxis has been instituted, killing the early larval stages. Thus, a positive antibody test indicates infection by HW larval stages, and possibly HARD, but not necessarily of a mature infection. In conjunction with other provocative findings, antibody seropositivity is useful in making a clinical diagnosis of HW infection in cats, and it certainly identifies cats at risk. False-positive results from cross-reactivity with other parasites have not been seen. A negative antibody test indicates ≥90% probability of the absence of mature infection. Microfilariae are rarely detected (<10%) in cats, regardless of method of detection. Annual screening of cats is not necessary but may yield information for concerned cat owners. For this purpose, the antibody test is preferred in that it detects cats with heartworms and those at risk. The antigen test is not appropriate for screening in cats because of its low sensitivity.

In cats, worms can often be imaged using echocardiography. This is because of the relative sizes of the heartworm(s) and the right heart and pulmonary arterial system of cats. Heartworms, particularly the females, are long enough to occupy the pulmonary arteries as well as the right heart, where they can be easily imaged. Parallel hyperechoic lines, produced by the HW cuticle, may be seen in the right heart and pulmonary arteries. Echocardiography is more important in cats than in dogs because of the increased difficulty of diagnosis in cats (low antigen test sensitivity and low antibody test specificity for mature infection) and the relatively high sensitivity of the test in experienced hands.

In ferrets, commercial antigen tests have detected HW antigen in experimental infection as early as 5 months after infection and are effective in clinical situations. False-negative results may occur, especially in species that harbor lower worm burdens (cats and ferrets). Furthermore, although microfilaria testing is only rarely helpful, adult worms can often be seen with echocardiography and nonselective angiography.

Ancillary Tests in Cats, Dogs, and Ferrets

In addition to antigen, antibody, and microfilaria tests in cats and dogs, a CBC, chemistry profile, urinalysis, and particularly, thoracic radiographs are sometimes indicated. Laboratory data are often normal. Eosinophilia and basophilia alone or together may occur in dirofilariasis. Eosinophilia is most often seen at the time that stage 5 (young adult) larvae arrive in the pulmonary arteries. Subsequently, eosinophil counts vary but are usually high in dogs with immune-mediated occult infections, especially if eosinophilic pneumonitis develops (<10% of total infections). Anemia in HW-infected dogs occurs due to chronic inflammation (usually mild) and due to hemolysis (more severe) seen with the complications of DIC and caval syndrome.

Hyperglobulinemia, due to antigenic stimulation, may be present in dogs and cats. Hypoalbuminemia in dogs can be associated with proteinuria in severe immune-complex glomerulonephritis or with severe emaciation/cardiac cachexia. Serum ALT and alkaline phosphatase are occasionally increased but do not correlate well with abnormal liver function, efficacy of adulticide treatment, or risk of drug toxicity. Urinalysis may reveal proteinuria that can be quantitated by a urine protein:creatinine ratio. Occasionally, severe glomerulonephritis can lead to hypoalbuminemia and nephrotic syndrome. Dogs with hypoalbuminemia, secondary to glomerular disease, also lose antithrombin III and are at risk of thromboembolic disease. Hemoglobinuria is associated with caval syndrome and occurs when RBCs are lysed in the circulation.

In dogs, thoracic radiography provides the most information on disease severity and is particularly important in patients with clinical signs. High-risk infections are characterized by a large main pulmonary artery segment and dilated, tortuous caudal lobar pulmonary arteries. Right ventricular enlargement may also be seen and, along with enlarged pulmonary arteries, is indicative of pulmonary hypertension. With pulmonary thromboembolism and pulmonary infiltrate with eosinophils (pneumonitis), ill-defined parenchymal infiltrates surround the caudal lobar arteries, typically most severe in the right caudal lobe.

In cats, cardiac changes and pulmonary hypertension are less common. In ~50% of infected cats, caudal lobar arteries are larger than the corresponding vein and >1.6 times the diameter of the ninth rib at the ninth intercostal space. Patchy parenchymal infiltrates may also be present in cats with respiratory signs. The main pulmonary artery segment usually is not visible because of its relatively midline location.

In ferrets, radiographs can demonstrate cardiac and pulmonary arterial changes compatible with HW disease. In addition, adult worms can often be seen with echocardiography and nonselective angiography.

Treatment of Heartworm Disease in Dogs

  • Doxycycline, split-dose melarsomine, and exercise restriction is most effective

  • Non-arsenical protocols using ivermectin can be used in cases of melarsomine failure or significant financial constraints

Pretreatment Evaluation

The extent of the preadulticide evaluation varies, depending on the clinical status of the dog, the likelihood of coexisting diseases that may affect the outcome of treatment, the owner's ability to restrict the dog's exercise, and cost considerations. Clinical laboratory data should be collected selectively to complement information obtained from a thorough history, physical examination, antigen and microfilaria tests, and often, thoracic radiography.

Two important variables known to directly influence the probability of posttreatment thromboembolic complications and treatment outcome are the extent of concurrent pulmonary vascular disease and the current worm burden. Assessment of cardiopulmonary status is indispensable for evaluating a dog's prognosis. Pulmonary thromboembolic complications after adulticide treatment are most likely to occur in heavily infected dogs already exhibiting clinical and radiographic signs of severe pulmonary vascular disease, especially when severe pulmonary hypertension and CHF are present. There is currently no effective way to determine worm burden other than direct visualization echocardiographically. Most cases do not warrant this test.

Before adulticide therapy, HW-infected dogs are assessed and rated for risk of postadulticide thromboembolism. Dogs can be categorized as follows:

  • Low risk of thromboembolic complications, light worm burden, and no evidence of parenchymal and/or pulmonary vascular lesions

  • High risk of thromboembolic complications

Dogs in the low-risk category would ideally fulfill the following conditions: young, with no clinical signs, normal thoracic radiographs, a low level of circulating antigen or a negative antigen test with circulating microfilariae, no worms visualized by echocardiography, no concurrent disease, and with owners capable of completely restricting exercise. The low-risk group would also include dogs having previously undergone adulticidal therapy but that remain antigen positive (presumed low worm burden). Dogs with near-normal thoracic radiographs may develop severe thromboembolic disease, occurring most often when exercise is not restricted. Dogs at high risk of thromboembolic complications include those with signs related to HW infection (eg, coughing, dyspnea, ascites), abnormal thoracic radiographs, high level of circulating antigen, worms visualized by echocardiography, concurrent disease, and little or no possibility that the owners will restrict exercise.

After evaluation, risk assessment, and financial consideration, an adulticidal treatment is chosen. These approaches (see Table: Guide to Choosing Heartworm Therapeutic Protocol), listed in order of highest to lowest, regarding safety, efficacy, duration of therapy, and cost, include:

  • laboratory and x-ray workup, doxycycline, split-dose (3) melarsomine, exercise restriction

  • doxycycline and split-dose (3) melarsomine, exercise restriction

  • split-dose (3) melarsomine, confinement

  • 2-dose melarsomine, strict confinement

  • non-arsenical adulticide: preventive dosage ivermectin or moxidectin (twice weekly for 6 months) + doxycycline, exercise reduction

  • non-arsenical adulticide ("slow-kill"): preventive dosage ivermectin or moxidectin - not advised

Pretreatment Stabilization

High-risk dogs should be stabilized before melarsomine administration. Adulticidal therapy often precipitates worsening of pulmonary and/or cardiac signs as worms die. Stabilizing treatment includes cage confinement, oxygen, corticosteroids, and doxycycline for 1–2 months before initiation of the split-dose melarsomine treatment protocol. The use of doxycycline and the split-dose protocol lessens the adverse reaction to dying worms.

Dogs with right-side CHF should be treated with:

  • furosemide (1–2 mg/kg, twice daily)

  • the inodilator, pimobendan (0.25 mg/kg, twice daily

  • an angiotensin-converting enzyme (ACE) inhibitor such as enalapril (0.5 mg/kg/day, increased to 0.5 mg/kg, twice daily, after 1 week pending renal function test results)

  • moderate dietary sodium restriction

  • abdominal paracentesis, as needed

Sildenafil can be used initially at 1 mg/kg, three times daily, as a pulmonary vasodilator. Caution is warranted with this and other vasodilators to avoid the adverse effect of systemic hypotension. Adulticidal therapy should be delayed indefinitely in dogs with CHF.

Caval syndrome results from worms migrating retrograde to the right atrium and great veins and is usually the result of a precipitous fall in cardiac output, as might occur with pulmonary thrombosis. Severe pulmonary hypertension is then complicated by worm-induced tricuspid valve leakage, hemolysis, and damage to the liver and kidneys.

In caval syndrome, removal of worms from the right atrium and orifice of the tricuspid valve is typically necessary to save the life of the dog. This may be accomplished by using light sedation, local anesthesia, and either rigid or flexible alligator forceps or an intravascular retrieval snare, introduced preferentially via the right external jugular vein. With fluoroscopic guidance, if available, the instrument should continue to be passed until worms can no longer be retrieved. Immediately after a successful operation, the clinical signs should lessen or disappear. Fluid therapy may be necessary in critically ill, hypovolemic dogs to restore hemodynamic and renal function. After full recovery from surgery, adulticidal therapy is undertaken to eliminate remaining worms. Particular care should be taken if many worms are still visible echocardiographically.

Arsenical (Melarsomine) Adulticidal Therapy

The only approved heartworm adulticide is melarsomine dihydrochloride, which is variably effective against mature (adult) and immature heartworms of both sexes, with male worms being more susceptible. Melarsomine is given at 2.5 mg/kg, deep IM in the belly of the epaxial (lumbar) musculature in the area of the third to fifth lumbar vertebrae, using a 22-gauge needle (1 in. long for dogs <10 kg or 1.5 in. for dogs >10 kg). Pressure at the injection site is applied and maintained for 5 minutes to prevent drug migration.

Approximately one-third of dogs will exhibit local pain, swelling, soreness with movement, or rarely, sterile abscessation at the injection site. Local fibrosis is not uncommon (and is the reason for targeting the belly of the epaxial musculature). In standard use, the procedure is repeated on the opposite side 24 hours later for dogs at low risk of treatment complications. However, to reduce the danger of thromboembolism, a two-phase (also termed “split-dose” or "three-dose” method) treatment is highly recommended for at-risk dogs and, indeed, for all patients, unless cost considerations prohibit this approach. Using this protocol, a single injection of melarsomine is given, followed by two injections 24 hours apart, after an interval of at least 30 days. The American Heartworm Society recommends this three-dose alternative regimen, regardless of the stage of disease or risk category.

Exercise restriction is essential once treatment is started to minimize the risk of pulmonary thromboembolism due to dead and dying adult worms. Further benefits accrue with the addition of doxycycline therapy.

The current ideal approach to adulticidal therapy is to give doxycycline (10 mg/kg, twice daily, for 30 days; reduced to 5 mg/kg if not tolerated) and HW preventive, at the standard preventive dosage and frequency (monthly or every 6 months). After 2 months, adulticidal injections (melarsomine at 2.5 mg/kg, IM) are initiated as the dog's condition allows. Daily corticosteroids, using a tapering dosage, may also be administered during this period to reduce pulmonary inflammatory lesions from dying worms and from melarsomine. Although exercise is minimized from the day of diagnosis, cage rest must be enforced from the day of each initial injection for 4–6 weeks. If the dog's condition allows, melarsomine injections are repeated in 1 month (2 injections 24 hours apart), with the same regimen of prescribed exercise restriction. If, after the first injection, the dog displays significant pulmonary damage from the resultant worm death, the second and third injections can be withheld indefinitely.

Dogs with high worm burdens are at risk of severe respiratory complications. Because only ~50% of heartworms are destroyed after the first injection and because worm antigenic burden has been reduced by the wolbachiostat, doxycycline, the cumulative impact of worm emboli on severely diseased pulmonary arteries and lungs is reduced. This approach destroys a higher percentage of adult heartworms than the standard two-dose protocol. For the utility and advisability of various therapeutic protocols, see Table: Guide to Choosing Heartworm Therapeutic Protocol.

Table
icon

Guide to Choosing Heartworm Therapeutic Protocol

Protocol

Advantages

Disadvantages

Utilitya

1

Split dose (3 injections), melarsomine and doxycycline; thoracic radiographs, CBC, UA, chemistry panel, coagulation profile

↑ efficacy, ↓ risk of PTE, safety of phased worm kill, no resistance concern

Cost $$$$; exercise restriction for 2 months

Appropriate for all; best approach for severe HW disease

2

Standard dose (2 injections), melarsomine and doxycycline; thoracic radiographs, CBC, UA, chemistry panel, coagulation profile

↓ cost, ↓ risk of PTE (vs standard dose), exercise restriction only 1 month, no resistance concern

Cost $$$; ↑ risk of PTE (vs split dose); ↓ kill efficacy (vs split dose)

Appropriate when financial constraints and mild to moderate HW disease

3

Standard dose (2 injections), melarsomine

↓ cost; exercise restriction only 1 mo, no resistance concern, easier for shelters

Cost $$; ↑ risk of PTE (vs standard dose); ↓ kill efficacy (vs split dose); cage rest imperative

Appropriate when financial constraints and mild HW disease

4

Non-arsenical adulticidal therapy: oral ivermectin, topical moxidectin/imidacloprid, or oral moxidectin, at preventive dosage, with doxycycline

↓ cost, no injections, no hospitalization, shorter treatment duration than slow kill protocol

Cost $; ↑ risk of resistance, not approved by AHS, ~12-month course, lung disease progression, time of HW death unknown

Appropriate when there are severe financial or other constraints, lack of melarsomine availability, or failure of melarsomine treatment to clear dog previously

5

Slow kill non-arsenical adulticidal therapy with ivermectin, topical moxidectin/imidacloprid, or oral moxidectin, at preventive dosage

Inexpensive; no injections

↑ risk of resistance, 30-month course, lung disease progression, time of HW death unknown

Should be avoided; MUST clear microfilariae

a None of these is appropriate for initial management of caval syndrome but may be used to complete therapy after worm removal.

UA = urinalysis; PTE = pulmonary thromboembolism; AHS = American Heartworm Society; $ = relative cost

Doxycycline in Adulticidal Therapy

Doxycycline has become an important part of treatment of HW infection in dogs. Through its negative action on Wolbachia, it provides benefits to the canid host and works to the detriment of D immitis. Doxycycline is indicated for preadulticide therapy (at 10 mg/kg, twice daily, for 30 days, or 5 mg/kg, twice daily, if not tolerated) in HW-infected dogs. It is given in conjunction with ivermectin at the preventive dosage (6–12 mcg/kg/month). This combination reduces the severity of lung injury after adulticidal therapy, probably through reducing the amount of Wolbachia antigen and the proteins released from the HW uterus as the bacteria die and the uterus degenerates. Doxycycline at this dosage hastens worm death when a “slow-kill” approach is used, thereby presumably reducing the negative impact of worms on the host. Doxycycline with a macrocyclic lactone also clears the host of microfilariae (in both resistant and nonresistant infections). Therefore, in dogs undergoing slow-kill treatment, this combination decreases risk of macrolide resistance, which is a concern in the slow-kill method using ivermectin alone. Doxycycline is advocated in treating dogs with HW infection regardless of severity classification or protocol.

The American Heartworm Society recommends administration of prophylactic doses of macrolides for 2 months before administration of melarsomine, with the first dose given concurrently with the first dose of doxycycline (day 1) and a second dose given after the end of the doxycycline treatment (day 30). A third dose is then given concurrently with the first dose of melarsomine (day 60). Macrolide administration is continued monthly thereafter at the preventive dosage. The rationale for this approach is to eliminate susceptible migrating D immitis larvae and to allow nonsusceptible 2–4 month old larvae to age to a point at which they are more susceptible to melarsomine. This approach of a 2-month pretreatment with macrolides has become less compelling with the recent knowledge that doxycycline kills developing larvae (L3>L4>young adults) when administered at 10 mg/kg, twice daily, for 30 days, thereby closing the gap during which developing larvae are not susceptible to melarsomine treatment. However, the delayed institution of melarsomine for 2 months still makes sense. Though unproven, the degeneration of the HW, which starts at inception of doxycycline therapy, is likely not maximized at one month but is more advanced with another 30 days' time for worm degradation. This should further reduce the reaction of the host to dying parasites.

Non-arsenical (Macrolide + Doxycycline) Adulticidal Therapy

Cases in which non-arsenical adulticidal therapy might be considered:

  • melarsomine shortage or lack of access

  • dog has already received 1 or more courses of melarsomine

  • owner concerns: risk, finances

  • overriding serious health issues

  • previous serious reaction to melarsomine

  • owners cannot or will not keep dog confined

  • shelters (financial and time constraints)

Although generally agreed that the only FDA-approved approach and the American Heartworm Society's recommendation for treating HW is ideal, financial and other concerns, as well as melarsomine availability, dictate the need for alternatives. Most have focused on the use of macrocyclic lactones in a "slow-kill" or "soft-kill" approach. This is controversial, largely because of the duration of therapy, reliance on patient compliance for years, ongoing host damage, and concern for resistance development. More recently, the addition of doxycycline has been shown to reduce the duration of therapy necessary for an ~95+% kill rate from ~2.5 to ~1 year. Furthermore, the combination of doxycycline plus moxidectin/imidacloprid at preventive dosage, given biweekly for the first 6 months, resulted in 96% HW-antigen negative status in an average of ~8 months. Ivermectin and doxycycline, used similarly, provided 78% negative tests at 300 days. This rate of HW antigen clearance is approximately that of the split-dose melarsomine protocol.

Post-adulticidal Therapy

After melarsomine injection(s), exercise must be restricted for 4–6 weeks to minimize pulmonary thromboembolic complications. Adverse effects of melarsomine are otherwise limited to local inflammation, cough, brief low-grade fever, and salivation. Hepatic and renal toxicity are seldom, if ever, seen.

Laboratory findings associated with adulticidal therapy may include:

  • inflammatory leukogram

  • thrombocytopenia

  • prolonged activated clotting time or prothrombin time

  • increase in serum CK

Local or disseminated intravascular coagulopathy may occur when platelet counts are <100,000/mcL. Treatment for severe thromboembolism should include oxygen, cage confinement, a corticosteroid at an anti-inflammatory dosage (eg, prednisone at 1 mg/kg/day, PO), and possibly, low-dose heparin (75–100 U/kg, SC, three times daily) for several days to 1 week. Severe lung injury is likely present if, after 24 hours of oxygen therapy, no improvement is noted and arterial partial pressures of oxygen remain <70 mm Hg.

The standard melarsomine protocol (two-dose, 24-hour treatment regimen) kills most adult worms, clearing 50%–85% of dogs, whereas the split-dose + doxycycline protocol appears to clear >95% of dogs. Antigen testing should be performed 8–12 months after the final dose of melarsomine. If a positive test result is obtained at this time, consideration can be given to abbreviated retreatment (two injections, 24 hours apart) or a nonarsenical approach with ivermectin or moxidectin/imidacloprid, at preventive dosages. This nonarsenical approach should be preceded by 30 days of doxycycline therapy (10 mg/kg, twice daily) because this minimizes the reaction to dead and dying worms, enhances the kill rate to ~1 year (vs 2.5 years with ivermectin alone) versus the standard slow-kill approach, and is thought to decrease the risk of resistance (see above). The standard "slow-kill" approach with ivermectin alone is against the current recommendations of the American Heartworm Society. Longterm use of macrolides alone to kill adult worms should be avoided because it allows pulmonary pathology to progress during the lengthy period in which worms are dying and being processed.

Microfilaricide Treatment

At specific preventive dosages, the macrolide preventive drugs are effective microfilaricides, although not approved by the FDA for this purpose. Adverse reactions may occur in dogs with high microfilarial counts, depending on the type of macrolide given. However, the microfilarial count is usually lower, and mild adverse reactions occur in ~10% of dogs. Most adverse reactions are limited to brief salivation and defecation, occurring within hours and lasting up to several hours.

Dogs, especially small dogs (<10 kg), with high microfilariae counts may develop tachycardia, tachypnea, pale mucous membranes, lethargy, retching, diarrhea, and even shock. Treatment includes an IV balanced electrolyte solution and a soluble corticosteroid. Recovery is usually rapid when treatment is administered quickly. Microfilarial tests are no longer routinely performed, and thus severe reactions are seldom expected.

Treatment specifically targeting circulating microfilariae has historically been undertaken 3–4 weeks after adulticide administration. The current practice is to start a macrocyclic lactone for prevention and microfilarial eradication at the time of diagnosis. Although all macrocyclic lactones have microfilaricidal activity and are the safest and most effective drugs available to clear microfilariae, this characteristic varies within the drug group. Only the combination topical product containing imidacloprid and moxidectin is FDA-approved as a microfilaricide. All macrocyclic lactones likely enjoy enhanced efficacy in this regard when accompanied by doxycycline. Livestock preparations of these drugs should not be used to achieve higher doses to obtain more rapid results. Performance of a microfilaria test is recommended at the time of diagnosis and 1–3 months after microfilaricidal therapy has begun.

Treatment of Heartworm Disease in Cats

There is currently neither a satisfactory nor approved treatment approach for heartworm infection in cats; therefore, all cats in regions endemic for canine HW disease should receive drug prophylaxis. Infections are likely more often lethal in cats than dogs, though a percentage of cats are thought to survive infection without demonstrable clinical signs. The lifespan of adult heartworms in cats is thought to be about 2 years, so spontaneous recovery is possible. Cats may remain asymptomatic, experience episodic vomiting and/or episodic dyspnea (resembling asthma), may die suddenly from either pulmonary thromboembolism or an anaphylactoid reaction, or rarely, develop CHF.

Because there is no safe or approved adulticide for cats, many are managed conservatively with restricted activity and corticosteroid therapy, such as prednisolone (1–2 mg/kg, PO, every 24–48 hours; dosage minimized as much as possible). Steroids reduce the severity of vomiting and respiratory signs. The hope is that episodes of pulmonary complications will not prove fatal as the worms die. Barring consecutive, additional infection, 25%–50% of cats may survive with this approach. Serial antigen and antibody testing (at intervals of 6–12 months) can be used to monitor status. Although there are no supportive data, administration of doxycycline (10 mg/kg, twice daily for 30 days) and ivermectin (24 mcg/kg/month) to an infected cat could be theorized to cause worm degradation and contracture, thereby lessening the potential for catastrophic consequences when the worms die. Of course, the macrocyclic lactone would also protect the cat from additional infection.

Surgical retrieval of worms from the right atrium, right ventricle, and vena cavae via jugular venotomy can be attempted in cats in which worms are detected by echocardiography. An endoscopic basket, snare, or horsehair brush can also be advanced via the right jugular vein under fluoroscopy. Cats in CHF have been cured by worm removal.

Treatment of Heartworm Disease in Ferrets

Treatment in ferrets is, likewise, difficult, because there is no approved agent for this purpose. Adulticidal therapies (thiacetarsemide and melarsomine) have resulted in ~50% mortality in ferrets. Moxidectin (injectable and topical formulations) has been widely thought to be adulticidal for heartworms in ferrets and is given at the same dosage and frequency as in dogs. Topical moxidectin and imidacloprid (combination), approved by the FDA for use in ferrets to prevent HW infection and to prevent and treat flea infestations, is a logical choice as an adulticidal macrocyclic lactone.

Prevention of Heartworm Disease in Dogs, Cats, and Ferrets

Heartworm infection is generally completely preventable with macrolide prophylaxis. Year-round prevention in dogs is advised, beginning at 6–8 weeks of age. No testing is necessary at this age, because the presence of mature female heartworms is required to produce a positive heartworm test (antigen or microfilaria). When prophylaxis is started after 7 months of age, an antigen test and a test for presence of microfilariae is recommended, followed by another antigen test 6–7 months later. This series of tests will help to avoid unnecessary delay in detecting subclinical infections as well as potential confusion concerning effectiveness of the preventive program, because it cannot be determined until the second test whether infection existed before beginning chemoprophylaxis.

Formulations of the macrolide (macrocyclic lactone) HW preventive molecules, ivermectin, milbemycin oxime, moxidectin, and selamectin, are safe and effective, as prescribed, for all breeds of dogs. Currently marketed products have additional chemicals and parasite spectra, including GI and ectoparasites:

  • ivermectin - hookworms

  • ivermectin/pyrantel pamoate - hookworms, roundworms

  • ivermectin/pyrantel pamoate/praziquantel - hookworms, roundworms, whipworms

  • milbemycin - hookworms, roundworms, whipworms

  • milbemycin/lufenuron - hookworms, roundworms, whipworms, flea-sterilization

  • milbemycin/lufenuron/praziquantel - hookworms, roundworms, whipworms, flea-sterilization, tapeworms

  • milbemycin/spinosad - hookworms, roundworms, whipworms, fleas

  • selamectin - fleas, ticks, ear mites, sarcoptic mites

  • moxidectin oral - hookworms (approved for HW prevention, but not currently marketed in the USA)

  • moxidectin injectable - hookworms

  • moxidectin/imidacloprid topical - hookworms, roundworms, adult fleas, microfilariae

At the approved dosage, milbemycin kills microfilariae quickly, and in the face of high microfilarial concentrations, a shock reaction may occur. Thus, milbemycin should not be administered without close monitoring and/or prophylactic pretreatment (steroids and/or antihistamine) as a preventive in dogs with high numbers of microfilariae. All macrocyclic lactones should be used with caution under these circumstances.

HW prevention is also recommended for all cats in endemic regions, regardless of housing status, because of the potential for severe consequences with infection. Performing microfilaria testing in cats before starting preventive therapy is not required, because cats have no or small numbers of microfilariae and, when present, are typically transient. Ivermectin for cats is safe and effective at 24 mcg/kg, PO, once monthly. At this dosage, the formulation is also effective against hookworms. Preventive treatment should be started in kittens at 6 weeks of age and continued lifelong.

Milbemycin oxime flavored tablets are approved for use in cats for HW prevention and control of hookworms, roundworms, and whipworms at a monthly dosage of ~1.3 mg/kg.

Selamectin is administered topically at a monthly dosage of ~6 mg/kg for HW prevention in cats and also kills adult fleas and prevents flea eggs from hatching for 1 month. It is indicated for treatment and control of ear mites, sarcoptic mange, , hookworms, and roundworms.

The newest feline HW preventive is the combination of eprinomectin (~0.12 mg/kg) and praziquantel (~2.0 mg/kg), applied topically once per month to prevent/treat HW, hookworms, roundworms, and tapeworms. This product is labeled for kittens >7weeks of age, whereas the others are approved for kittens >6 weeks of age.

A topical combined formulation of moxidectin and imidacloprid, administered at dosages of 1 mg/kg for moxidectin and ~10 mg/kg for imidacloprid, is effective against HW infection and flea infestations. Although all currently marketed preventives are likely effective in ferrets, only topical moxidectin/imidacloprid is approved by the FDA. Importantly, the preventive dosage for ferrets is the same as that for dogs, but not cats.

Resistance

Sporadic resistance of heartworms to the macrocyclic preventive class has been recognized since 2013. All the current molecules used to prevent heartworm disease have been implicated. However, some formulations (moxidectin/imidacloprid) appear to be more effective against some currently recognized resistant isolates than others. There have been isolates from 7 dogs with varying degrees of resistance. There is little evidence of spread outside of the Mississippi Delta region, where resistance was first recognized.

It is important to realize that the current preventives are effective in the vast majority of cases and should not be abandoned. Emphasis should be placed on owner compliance and year-round preventive therapy as well as on alternative methods of HW prevention, including topical and oral mosquito repellant/insecticides, indoor/screened housing, especially at night, and mosquito abatement programs. The role of "slow-kill" macrolide adulticidal therapy in the development of resistance has been suggested, and it should be avoided. If such therapy is unavoidable, it should absolutely be accompanied by 30 days of doxycycline treatment at the outset, with assurance that microfilariae are eradicated.

Key Points

  • Heartworm disease is completely preventable in most instances.

  • Although resistance to macrocyclic lactones is an important threat, its immediate concern is limited and localized to the Mississippi Delta region.

  • Some form of HW adulticidal treatment should be offered to all owners of HW-infected dogs, other than those with terminal illness or other definite contraindication to treatment.

  • Cats are at lesser risk than dogs and have a lower infection rate, yet they benefit from HW preventives because there are no effective treatments for infection.

  • Ferrets are also susceptible to HW, and treatment of infection is difficult, but there is an approved preventive medication combination.

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