Heartworm Disease in Dogs: Prevention, Diagnosis, and Treatment

Etiology and Pathogen Biology

Heartworm disease in dogs is caused by the filarial nematode Dirofilaria immitis, a member of the family Onchocercidae [1]. The adult parasites reside within the pulmonary arteries and right ventricle of the definitive canine host, where they can reach lengths of 15 to 30 cm [2]. The parasite requires an intermediate host, mosquitoes of the genera Aedes, Culex, and Anopheles, for transmission [3, 4]. Molecular characterization of D. immitis isolates has revealed population genomic structures suggesting an ancient origin in canids, with subsequent global dissemination facilitated by anthropogenic movement of domestic dogs [1].

The endosymbiotic bacterium Wolbachia plays a critical role in the biology and pathogenesis of D. immitis [5]. Wolbachia is essential for larval development, embryogenesis, and adult worm survival [5]. The release of Wolbachia antigens during worm death contributes to the inflammatory response observed in treated dogs [5]. A novel glutamate-gated chloride channel subunit (GLC-2) has been isolated from D. immitis, providing a molecular target for macrocyclic lactone (ML) anthelmintics [6].

Life Cycle

The life cycle of D. immitis is complex and requires both a mosquito vector and a canine host. Adult female worms in the pulmonary arteries produce microfilariae (first-stage larvae, L1) that circulate in the peripheral blood [7]. When a mosquito takes a blood meal from a microfilaremic dog, it ingests microfilariae [3]. Within the mosquito, the larvae develop through two molts to the infective third stage (L3) over a period of 10 to 14 days, depending on ambient temperature [4]. The L3 larvae migrate to the mosquito's mouthparts and are deposited onto the skin of a new canine host during subsequent feeding [3].

In the dog, the L3 larvae penetrate the skin through the bite wound and molt to the fourth stage (L4) within 1 to 12 days [8]. The L4 larvae migrate through subcutaneous tissues and muscle, molting to the fifth stage (L5, immature adult) approximately 50 to 70 days post-infection [8]. The immature adults enter the venous circulation and are carried to the pulmonary arteries, where they reach sexual maturity and begin producing microfilariae approximately 6 to 7 months after initial infection [8]. The prepatent period, from infection to detectable microfilariae in blood, is typically 6 to 7 months [9].

Epidemiology and Transmission Dynamics

Canine heartworm disease has a global distribution, with endemic foci on every continent except Antarctica [10, 11]. Prevalence rates vary significantly by geographic region, climate, and local mosquito vector populations [12, 11]. In humid coastal zones, epidemiological predictors include higher mean annual temperatures, increased precipitation, and the presence of competent vector species [11]. Molecular screening of mosquitoes has confirmed the presence of D. immitis DNA in vector populations, providing a direct measure of transmission risk [13, 4].

Seroprevalence studies in domestic dogs from various regions have reported infection rates ranging from less than 1% to over 30% in high-risk areas [12, 14, 15, 16]. In Sri Lanka, subclinical infections of D. immitis have been documented, suggesting a silent reservoir of infection that complicates control efforts [9]. Similarly, studies in Lebanon, Pakistan, Ecuador, and Brazil have identified substantial prevalence rates, often with co-infections with other vector-borne pathogens [12, 15, 17, 2]. The emergence of D. immitis in previously non-endemic areas, such as Prince Edward Island in Canada, highlights the expanding geographic range of this parasite, likely driven by climate change and increased movement of infected reservoir hosts [18, 10].

Clinical Signs and Pathophysiology

The clinical manifestations of heartworm disease are directly related to the number of adult worms, the duration of infection, and the host's inflammatory response [19]. The primary pathophysiologic mechanism is pulmonary endarteritis, caused by the physical presence of adult worms in the pulmonary arteries [19]. This leads to intimal proliferation, villous endarteritis, and thrombosis, resulting in increased pulmonary vascular resistance and pulmonary hypertension [19].

Classification of Disease Severity

Disease severity is classified into four classes, as outlined in Table 1.

Table 1. Classification of Canine Heartworm Disease Severity

Clinicopathologic variables correlate with disease severity [19]. Dogs with higher worm burdens exhibit more pronounced hematologic abnormalities, including thrombocytopenia, anemia, and eosinophilia [19, 20]. Thrombocytopenia in heartworm-infected dogs may be associated with platelet surface-associated immunoglobulin positivity, suggesting an immune-mediated component [20]. Acute phase protein concentrations, such as haptoglobin, are elevated in microfilaremic dogs compared to amicrofilaremic dogs, reflecting ongoing systemic inflammation [21].

Chronic cough and exercise intolerance are the most commonly reported clinical signs [19]. In advanced cases, right-sided congestive heart failure develops, characterized by ascites, hepatomegaly, and jugular distension [19]. Caval syndrome, a life-threatening complication, occurs when a large mass of adult worms obstructs blood flow through the tricuspid valve and right atrium [22]. Rare presentations include the expectoration of adult worms in bronchial mucus following treatment [23]. Concurrent conditions, such as gastric dilatation and volvulus, have been reported in dogs with situs inversus and heartworm disease, though this represents an unusual comorbidity [22].

Diagnosis

Accurate diagnosis of heartworm disease relies on a combination of antigen testing, microfilarial detection, and imaging modalities [24, 25]. The diagnostic approach must account for the prepatent period, the potential for occult infections (amicrofilaremic infections), and the presence of antigen-antibody complexes [25].

Antigen Testing

Commercial enzyme-linked immunosorbent assays (ELISAs) for the detection of D. immitis adult female worm antigens are the primary screening tools in clinical practice [25]. These tests detect circulating antigen, which is produced by adult female worms [25]. The sensitivity of antigen tests is high for infections with one or more adult female worms, but sensitivity decreases in infections with only male worms or very low worm burdens [25]. Point-of-care antigen tests have demonstrated high relative accuracy for ruling in heartworm infection in clinically suspected dogs when evaluated using Bayesian latent class modeling [25].

Microfilarial Detection

Detection of microfilariae in peripheral blood confirms the presence of adult worms and indicates that the dog is a potential reservoir for transmission [24]. The modified Knott's test is the reference method for microfilarial detection and allows for differentiation between D. immitis and D. repens based on morphometric criteria [24, 26]. The modified Knott's test involves centrifugation of blood fixed in formalin, followed by staining of the sediment with methylene blue or Giemsa [24]. Novel point-of-care tests for the detection of D. immitis and D. repens have been developed and show comparative performance to the modified Knott's test [24].

Molecular Diagnostics

Polymerase chain reaction (PCR) assays targeting the cytochrome c oxidase subunit I (COI) gene provide species-level identification of D. immitis [27, 2]. Loop-mediated isothermal amplification (LAMP) assays targeting the COI gene offer high sensitivity and specificity for epidemiological studies and can be performed with minimal laboratory infrastructure [27]. Droplet digital PCR (ddPCR) assays have been developed for the rapid detection of molecular markers associated with macrocyclic lactone resistance and susceptibility in D. immitis [28]. These assays can quantify the frequency of resistance-associated single nucleotide polymorphisms (SNPs) in the P-glycoprotein and glutamate-gated chloride channel genes [28].

Imaging

Thoracic radiography is used to assess the severity of pulmonary arterial disease and to monitor response to therapy [22]. Characteristic radiographic findings include enlargement of the main pulmonary artery segment, tortuous and blunted peripheral pulmonary arteries, and right ventricular enlargement [22]. Echocardiography can directly visualize adult worms in the pulmonary arteries and right heart chambers, particularly in cases of high worm burden or caval syndrome [22].

Diagnostic Algorithm

The following Mermaid diagram illustrates a recommended diagnostic workflow for canine heartworm disease.

flowchart TD
    A[Clinical suspicion or annual screening], > B{Antigen test}
    B, >|Positive| C{Microfilarial test}
    B, >|Negative| D[Consider prepatent period or low worm burden]
    C, >|Positive| E[Confirm D. immitis infection]
    C, >|Negative| F[Occult infection: consider PCR or repeat antigen test after heat treatment]
    D, > G[Repeat antigen test in 3-4 months if high risk]
    E, > H[Classify disease severity: radiography, echocardiography, clinicopathology]
    H, > I[Initiate treatment protocol]
    F, > H

Treatment

Treatment of canine heartworm disease involves adulticide therapy to kill adult worms, microfilaricide therapy to eliminate circulating microfilariae, and supportive care to manage complications [29, 30]. The standard adulticide protocol has historically relied on melarsomine dihydrochloride, an arsenical compound administered via deep intramuscular injection [30]. However, concerns regarding adverse effects and the emergence of ML-resistant isolates have prompted investigation into alternative protocols [31, 29, 30].

Adulticide Therapy

Melarsomine dihydrochloride remains the only approved adulticide in many regions [30]. The standard protocol involves two injections of melarsomine (2.5 mg/kg) administered 24 hours apart, followed by a third injection one month later [30]. This protocol achieves greater than 90% efficacy against adult worms [30]. A systematic review and meta-analysis of non-arsenical adulticide protocols using moxidectin and doxycycline has been conducted [29]. The combination of monthly moxidectin (a macrocyclic lactone) and daily doxycycline (targeting Wolbachia) over a 6 to 9 month period has demonstrated efficacy in reducing adult worm burden, though it is less effective than melarsomine [29]. The doxycycline component depletes Wolbachia, rendering adult worms sterile and more susceptible to the effects of moxidectin [5, 29].

Microfilaricide Therapy

Following adulticide therapy, or in cases where adulticide is not immediately indicated, microfilaricide therapy is necessary to eliminate circulating microfilariae and prevent transmission [30]. Macrocyclic lactones, including ivermectin, moxidectin, and selamectin, are effective microfilaricides [30]. Administration of a microfilaricide can cause a rapid die-off of microfilariae, potentially leading to anaphylactic or shock-like reactions; therefore, careful monitoring is required [30].

Management of Macrocyclic Lactone Resistance

The emergence of ML-resistant D. immitis isolates poses a significant challenge to both prevention and treatment [31, 30, 28]. Resistance is associated with SNPs in genes encoding P-glycoproteins and glutamate-gated chloride channels [28]. Comparative efficacy studies have demonstrated that some ML-based combination products retain efficacy against resistant isolates, while others show reduced efficacy [31]. The use of quadruplex ddPCR assays allows for the detection and quantification of resistance-associated alleles in field isolates, facilitating surveillance and informed product selection [28].

Supportive Care and Prognosis

Exercise restriction is critical during and after adulticide therapy to reduce the risk of pulmonary thromboembolism as dead worms are cleared from the pulmonary arteries [30]. Corticosteroids may be used to manage inflammatory reactions [30]. The prognosis for dogs with Class 1 or Class 2 disease is excellent with appropriate treatment [19]. Dogs with Class 3 disease have a guarded prognosis, and dogs with caval syndrome (Class 4) have a poor prognosis, with high mortality even with surgical intervention [22].

Prevention

Prevention of heartworm disease relies on the consistent administration of macrocyclic lactone preventives, which kill tissue-stage larvae (L3 and L4) before they reach the pulmonary arteries [32, 33]. Monthly administration is recommended in endemic areas, with year-round administration advised in regions where transmission is possible throughout the year [32].

Macrocyclic Lactone Preventives

Ivermectin, milbemycin oxime, moxidectin, and selamectin are the primary MLs used for heartworm prevention [32, 33]. These compounds bind to glutamate-gated chloride channels in nematode neurons and muscle cells, causing hyperpolarization and paralysis of the parasite [6]. Sustained-release formulations of ivermectin have demonstrated efficacy in preventing heartworm infection in endemic areas [32].

Combination Products Including Flea Control

Many preventive products are formulated as combination tablets that include an ML for heartworm prevention and an additional agent for flea and tick control. The term dog heartworm and flea pill commonly refers to these oral combination products. Examples of such combinations include products containing moxidectin plus sarolaner and pyrantel, or moxidectin plus afoxolaner and pyrantel [31]. A novel chewable tablet containing lotilaner, moxidectin, praziquantel, and pyrantel has demonstrated efficacy for the prevention of heartworm disease, providing broad-spectrum coverage against heartworm, fleas, ticks, and intestinal parasites [33]. These combination products simplify preventive care by addressing multiple parasite classes with a single monthly dose [31, 33].

Compliance and Resistance Prevention

The most common cause of heartworm prevention failure is inconsistent administration of preventives [30]. Client education regarding the importance of year-round, uninterrupted prophylaxis is essential [30]. The emergence of ML-resistant D. immitis isolates underscores the need for compliance and for the use of products with proven efficacy against local parasite populations [31, 28]. Molecular surveillance for resistance markers using ddPCR can guide product selection in regions where resistance is suspected [28].

Prognosis

The prognosis for dogs with heartworm disease is highly dependent on the stage of disease at diagnosis and the adherence to treatment and exercise restriction protocols [19]. Dogs diagnosed early (Class 1 or 2) and treated appropriately have an excellent prognosis, with most returning to normal activity levels [19]. Dogs with advanced disease (Class 3) require intensive management and have a more guarded prognosis [19]. Caval syndrome carries a grave prognosis, with survival rates below 50% even with surgical intervention [22]. Prevention remains the most effective strategy for ensuring long-term health and avoiding the morbidity and mortality associated with this disease [32, 33].

References

[1] Power RI, Abdullah S, Walden HS, et al. Population genomics reveals an ancient origin of heartworms in canids. Commun Biol. 2026. https://pubmed.ncbi.nlm.nih.gov/41559308/

[2] Moreira SMK, Moreira AVC, Uquillas CAM, et al. Morpho-molecular identification of heartworms (Dirofilaria immitis) in domestic dogs in the Sucre canton, Ecuador. Parasitol Int. 2026. https://pubmed.ncbi.nlm.nih.gov/41453721/ *** Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.

[3] Amarasinghe S, Ranasinghe K, Rodrigo W, et al. First molecular characterization of Dirofilaria vector species and the distribution of canine dirofilariasis in Gampaha district, Sri Lanka. Front Cell Infect Microbiol. 2026. https://pubmed.ncbi.nlm.nih.gov/42064220/

[4] Hammond NG, Todaro A, Fairbanks KA, et al. Mosquito (Diptera: Culicidae) surveillance for Dirofilaria immitis (Rhabditida: Onchocercidae) using a zoo as a focus for operational detection in central Utah. J Med Entomol. 2026. https://pubmed.ncbi.nlm.nih.gov/41723581/

[5] Loku Gamage N, Ranasinghe K, Rodrigo W. Molecular Characterization of Wolbachia Endosymbionts and Their Association With Canine Dirofilariasis in Colombo District, Sri Lanka. J Parasitol Res. 2026. https://pubmed.ncbi.nlm.nih.gov/42328691/

[6] Nichols J, Forrester SG. Isolation and characterization of a novel glutamate-gated chloride channel subunit (GLC-2) from the canine heartworm Dirofilaria immitis. Mol Biochem Parasitol. 2026. https://pubmed.ncbi.nlm.nih.gov/41581768/

[7] Roya GM, Yagoob G, Bahram AT. Seasonal study of Blood Parasites: DirofilariaImmitis and Dipetalonema Reconditum in the Guard Dogs of Tabriz city, Iran. Arch Razi Inst. 2025. https://pubmed.ncbi.nlm.nih.gov/41769291/

[8] Nakhale M, Hess JA, Oliver E, et al. Development of Dirofilaria immitis adult worms in NSG mice, detection of parasite-derived microRNA and comparative analysis of laboratory isolates. Sci Rep. 2026. https://pubmed.ncbi.nlm.nih.gov/41617897/

[9] Ranahewa DK, Dangolla A, Rajakaruna RS, et al. Subclinical Infections of Babesia and Dirofilaria in Dogs Presented to a Veterinary Teaching Hospital: Evidence for a Silent Reservoir of Infection in Sri Lanka. Acta Parasitol. 2026. https://pubmed.ncbi.nlm.nih.gov/42113286/

[10] Aguilar-Elena R, Rodríguez-Escolar I, Collado-Cuadrado M, et al. Global Research Trends in Emerging Zoonosis Due to (the Filarial Nematode) Dirofilaria repens (1955-2025): A Bibliometric Analysis of a Climate-Driven Expansion. Pathogens. 2026. https://pubmed.ncbi.nlm.nih.gov/42075713/

[11] Monteiro ACMP, Ribeiro CM, Fehlberg HF, et al. Emergence of Dirofilaria immitis in humid coastal zones: Epidemiological predictors and molecular characterization. Vet J. 2026. https://pubmed.ncbi.nlm.nih.gov/41990946/

[12] Khalife S, El Safadi D. Canine vector-borne diseases in Lebanon: Unveiling prevalence trends and risk factors for public health and disease control. Vet Parasitol Reg Stud Reports. 2026. https://pubmed.ncbi.nlm.nih.gov/42150803/

[13] Glinšek Biškup U, Knap N, Korva M, et al. Molecular screening of mosquitoes for filarioid helminths in Slovenia. Parasit Vectors. 2026. https://pubmed.ncbi.nlm.nih.gov/42083038/

[14] Julca LA, Salas-Fajardo MY, Guevara S, et al. Seroprevalence of zoonotic vector-borne pathogens in domestic dogs from rural areas in northern Peru. Top Companion Anim Med. 2026. https://pubmed.ncbi.nlm.nih.gov/42035833/

[15] Safdar I, Ur Rehman S, Roman U, et al. Serological and molecular detection of Dirofilaria immitis in pet dogs of Lahore, Pakistan. Ann Parasitol. 2026. https://pubmed.ncbi.nlm.nih.gov/41881496/

[16] Martínez-Durán D, Mujika M, Morales M, et al. Vector-borne pathogens in Spanish greyhounds from Central Spain: Prevalence and hematobiochemical findings. Vet Parasitol Reg Stud Reports. 2026. https://pubmed.ncbi.nlm.nih.gov/41741046/

[17] Zanfagnini LG, Chocobar MLE, Schmidt EMS, et al. Unveiling filariid infections in dogs living in the Western Amazon, Brazil. Comp Immunol Microbiol Infect Dis. 2026. https://pubmed.ncbi.nlm.nih.gov/41818949/

[18] Leaman LJ, Graham KF, Jones MEB, et al. First reported case of Dirofilaria immitis in a coyote (Canis latrans) from Prince Edward Island. Can Vet J. 2026. https://pubmed.ncbi.nlm.nih.gov/42266338/

[19] Kim M, Seo M, Cho J, et al. Clinicopathologic variables according to disease severity in dogs with heartworm disease. BMC Vet Res. 2026. https://pubmed.ncbi.nlm.nih.gov/42152057/

[20] Boontuboon W, Sakcamduang W, Osathanon R, et al. Evaluation of platelet surface-associated immunoglobulin positivity and its association with hematologic findings and vector-borne pathogens in thrombocytopenic dogs. J Vet Intern Med. 2026. https://pubmed.ncbi.nlm.nih.gov/41789552/

[21] Chocobar MLE, Eckersall DP, Panarese R, et al. Comparison of Haptoglobin Concentrations Between Microfilaremic and Amicrofilaremic Dogs Infected by Dirofilaria immitis. Vet Clin Pathol. 2026. https://pubmed.ncbi.nlm.nih.gov/41612543/

[22] Stokowski S, Steuri SK, Lux C, et al. Gastric Dilatation and Volvulus and Heartworm Disease in a Dog With Situs Inversus. Vet Radiol Ultrasound. 2026. https://pubmed.ncbi.nlm.nih.gov/42011803/

[23] Nogueira LLDC, Araújo BVS, Antunes JMAP, et al. Blood bronchial mucus with Dirofilaria immitis adult worms after the treatment with doxycycline and moxidectin: a rare case presentation. Parasitology. 2026. https://pubmed.ncbi.nlm.nih.gov/41549761/

[24] Sanders TL, Starnes A, Kelly MA, et al. Comparative performance of the novel, point-of-care Pluslife Mini Dock Dirofilaria immitis/Dirofilaria repens detection test with the modified Knott's test in dogs. Parasit Vectors. 2026. https://pubmed.ncbi.nlm.nih.gov/42104384/

[25] Atkinson PJ, Quimby C, Datt A, et al. Relative accuracy of point-of-care tests to rule-in heartworm infection in clinically suspected dogs using Bayesian latent class modelling. Prev Vet Med. 2026. https://pubmed.ncbi.nlm.nih.gov/41520422/

[26] Rajković M, Rajković V, Bogunović D. Dirofilaria immitis and Dirofilaria repens co-infection in a microfilaremic dog from Negotin, Eastern Serbia: Unusual localization of adult Dirofilaia repens in an abdominal hernia sac. Vet Parasitol Reg Stud Reports. 2026. https://pubmed.ncbi.nlm.nih.gov/41741045/

[27] Genc MG, Erol U, Sahın OF, et al. Application of COI-LAMP for Detection of Dirofilaria immitis with High Sensitivity and Specificity in Epidemiological Studies. Acta Parasitol. 2026. https://pubmed.ncbi.nlm.nih.gov/41801609/

[28] Kumar S, Lalicon IK, Prichard RK, et al. Development and evaluation of quadruplex droplet digital PCR assay for rapid detection of molecular markers associated with macrocyclic lactone resistance and susceptibility in Dirofilaria immitis. Int J Parasitol Drugs Drug Resist. 2026. https://pubmed.ncbi.nlm.nih.gov/41558254/

[29] Santiwattanatarm T, Sakcamduang W, Kongkaew C, et al. A systematic review and meta-analysis of non-arsenical adulticide protocols using moxidectin and doxycycline for the treatment of adult heartworm infection in dogs. Curr Res Parasitol Vector Borne Dis. 2026. https://pubmed.ncbi.nlm.nih.gov/42007372/

[30] Geary TG. Current issues in heartworm chemotherapy. Parasit Vectors. 2026. https://pubmed.ncbi.nlm.nih.gov/41851772/

[31] Rodriguez J, Jones S, Mahabir S, et al. Comparative efficacy of six monthly doses of Simparica Trio (sarolaner, moxidectin, and pyrantel chewable tablets) versus NexGard Plus (afoxolaner, moxidectin, and pyrantel chewable tablets) against a macrocyclic lactone-resistant Dirofilaria immitis isolate in dogs. Parasit Vectors. 2026. https://pubmed.ncbi.nlm.nih.gov/42087216/

[32] Genchi M, Venco L, Fozzer M, et al. Efficacy and safety of a sustained-release formulation of ivermectin (FILAPREV) in preventing heartworm infection (Dirofilaria immitis) in dogs in two endemic areas of Italy. Parasit Vectors. 2026. https://pubmed.ncbi.nlm.nih.gov/42087229/

[33] Young L, Reinemeyer CR, Abdelmoneim M, et al. Efficacy of a novel chewable tablet (Credelio Quattro) containing lotilaner, moxidectin, praziquantel, and pyrantel for the prevention of heartworm disease (Dirofilaria immitis) in dogs. Parasit Vectors. 2026. https://pubmed.ncbi.nlm.nih.gov/41943128/

[34] Szabó KÉ, Aresu L, Müller L, et al. Clinico-pathological and renal morphological findings in dogs naturally infected with Dirofilaria repens. BMC Vet Res. 2026. https://pubmed.ncbi.nlm.nih.gov/42069630/

[35] Malghem J, Vande Berg B, Lengelé B, et al. Dirofilaria repens Parasite: Review with Emphasis on Ultrasound Findings with Looking for Worm Mobility. J Belg Soc Radiol. 2026. https://pubmed.ncbi.nlm.nih.gov/41909165/