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Cardiopulmonary dirofilariasis

ISSN 2398-2942

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Synonym(s): Canine heartworm disease


  • Cause: infection with nematode parasite,Dirofilaria immitis Dirofilaria immitis.
  • Signs: variable with no pathognomonic signs. If present, vary from subclinical to caval syndrome and death.
  • Diagnosis: detection of circulating antigens or microfilariae; identification of adult filariae by echocardiography or necrospy.
  • Treatment: melarsomine as an adulticide with supportive treatment.
  • Prevention: macrocyclic lactones. Disease can be easily prevented.
  • Prognosis: good to guarded depending on severity of infection.
Print off the owner factsheet on Heartworm disease Heartworm disease to give to your client.

Presenting signs

  • Often subclinical.
  • Exercise intolerance.
  • Coughing.
  • Dyspnea.
  • Ascites in severe cases.
  • Syncope Syncope.
  • Intravascular hemolysis with caval syndrome Caval Syndrome.

Acute presentation

Geographic incidence

  • D immitisinfections described in all continents except Antartica.
  • Adequate temperature and humidity necessary to support viable mosquito population and sustain sufficient heat to allow maturation of ingested microfilariae to infective, third-stage larvae (L3) within vector.
    • Reported throughout USA, endemic in 49 states.
    • Growing prevalence reported in Mexico, Puerto Rico and the Caribbean.
    • Well established in Southern Canada, Australia, Africa and Japan.
    • Endemic in Southern Europe with scattered reports from Greece, Turkey and some eastern European countries.
    • Most of S America.
    • 70 + spp of mosquitoes capable of supporting development of larvae to infective stage (L3). Approximately 12 spp are major vectors.

Age predisposition

  • Antigenemia and microfilaremia appear 5 - 6.5 m post-infection, at the earliest.
  • Clinically affected animals can be encountered within 7-8 months of infection.
  • Animals <6 months of age cannot develop clinical signs of HW disease.

Breed/Species predisposition

  • None, all dogs exposed to mosquito bites may be infected.

Public health considerations

  • Human infection infrequent.
  • Encapsulated pulmonary dirofilaria infections have been diagnosed in humans. Usually recognized by radiographic examinations Dirofilariasis: human lung lesions.

Cost considerations

  • Adulticide treatments and hospitalization can be expensive.
  • Prophylaxis is inexpensive.



  • Dirofilaria immitis, large nematode parasite usually located in pulmonary arteries Dirofilariasis: villous endoarteritis.

Predisposing factors

  • Infection common where parasite is endemic and if patients are not receiving chemoprophylactics.
  • Travel to endemic areas (eg for vacation) without chemoprophylaxis.


  • Clinical disease depends on:
    • Number of worms present.
    • Reactivity of the dog to the filarial and wolbachial antigens.
    • Size of dog.
    • Activity of dog.
    • Number of dead worms present.
  • After migration into the pulmonary vasculature, filariae causes mechanical damage to walls of pulmonary arteries.
  • Vascular immune reaction is stimulated by either verminous antigens or wolbachial antigens.
  • Endothelial cells are separated and appear inflamed. WBCs accumulate and intima thickens.
  • Endothelium grows villous-like structures made of smooth muscle and intima Dirofilariasis: villous endoarteritis.
  • Heartworms cause lining of heart and pulmonary arteries to become rough and disrupt blood flow.
Pulmonary hypertension Pulmonary Arterial Hypertension (PHT)
  • Increase of peripheral resistance of pulmonary arteries occurs due to intimal and medial proliferative pathology.
  • Usually mild or moderate in most cases. Cases with right-sided CHF often have severe pulmonary hypertension.
  • Can also be caused by:
    • Vasoconstriction which can be due to inflammatory reaction, attempts to compensate local hypoxia, reduction of modulation of smooth muscle tone caused by parasite metabolites.
    • Anatomical obstruction of vessels by reduction of arterial lumen due to villous proliferations, emboli caused by live or dead parasites or even microfilariae, secondary thrombi due to endothelial damage and parasite tissue.
Cardiac lesions
  • Pulmonary hypertension initially induces dilation of right ventricle with a compensatory hypertrophy of the myocardium.
  • Right-sided congestive heart failure in severe cases.
  • Presence of parasite inside heart can trigger mechanical valvular damage resulting in tricuspid regurgitation.
Caval syndrome Caval Syndrome
  • Caused by massive infections of worms obstructing right ventricle or vena cava, causing intravascular hemolysis, right-sided congestive heart failure and death.
  • Tricuspid valve insufficiency due to mechanical interference by worms.
Lesions in the pulmonary parenchyma
  • Increased vascular permeability expands inflammatory reaction to alveolar or interstitial perivascular tissue.
  • Vascular and perivascular inflammatory reaction is amplified during adulticide treatment.
  • Epithelial cells type I affected with a consequent fibrosis and reduction of exchange of gases and capillary fragility.
  • Experimental inoculation of extracts of adult parasites induce a grave shock apparently with immunological basis.
  • Also induced with death of large numbers of microfilariae (usually >10,000/ml).
Lesions caused by microfilariae
  • Can induce immune mediated pulmonary pathology.
Lesions in other organs
  • Antigen-antibody complexes often cause protein-losing nephropathy (reversible).


  • Disease has insidious onset - from months to years after infection.
  • Adult worms can survive for 5-7 years.
  • Microfiliariae can survive several months - 2 years.


  • Transmitted by many spp of mosquitoes but mainly members of the family,Culicidae.
  • Microfilariae found in host blood.
  • Microfilariae ingested in blood-meal by mosquito, rapidly transform to L1, then moult to L2 and L3.
  • Adults, microfilariae, L3s Dirofilariasis: 3rd stage larva , L4s and young adults are found in dogs.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Bowman D D (2012) Heartworms, macrocyclic lactones, and the specter of resistance to prevention in the United States. Parasit Vectors 5, 138-147 PubMed.
  • Dingman P, Levy J K, Kramer L H, Johnson C M, Lappin M R, Greiner E C, Courtney C H, Tucker S J, Morchon R (2010) Association of Wolbachia with heartworm dsease in cats and dogs. Vet Parasitol 170, 50-50 PubMed.
  • Grandi G, Quintavalla C, Mavropoulou A, Genchi M, Gnudi G, Bertoni G, Kramer L (2010) A combination of doxycycline and ivermectin is adulticidal in dogs with naturally acquired heartworm disease (Dirofilaria immitis). Vet Parasitol 169, 347-351 PubMed.
  • Bazzocchi C, Mortarino M, Grandi G, Kramer L H, Genchi C, Bandi C, Genchi M, Sacchi L, McCall J W (2008) Combined ivermectin and doxycycline treatment has microfilaricidal and adulticidal activity against Dirofilaria immitis in experimentally infected dogs. Int J Parasitol 38, 1401-1410 PubMed.
  • Hoch H, Strickland K (2008) Canine and feline dirofilariasis: prophylaxis, treatment, and complications of treatment. Compend Contin Educ Vet 30, 146-151 PubMed.
  • Hoch H, Strickland K (2008) Canine and feline dirofilariasis: life cycle, pathophysiology, and diagnosis. Compend Contin Educ Vet 30, 133-140 PubMed.
  • Kramer L, Grandi G, Leoni M, Passeri B, McCall J, Genchi C, Mortarino M, Bazzocchi C (2008) Wolbachia and its influence on the pathology nad immunology of Dirofilaria immitis infection. Vet Parasitol 158, 191-195 PubMed.
  • McCall J W, Genchi C, Kramer L H, Guerrero J, Venco L (2008) Heartworm disease in animals and humans. Adv Parasitol 66, 193-285 PubMed.
  • Kramer L H, Tamarozzi F, Morchon R, Lopez-Belmonte J, Marcos-Atxutegi C, Martin-Pacho R, Simon F (2005) Immune response to and tissue localization of the Wolbachia surface protein (WSP) in dogs withh natural heartworm (Dirofilaria immitis) infection. Vet Immunol Immunopathol 106, 303-308 PubMed.
  • Strickland K N (1998) Canine and feline caval syndrome. Clin Tech Small Anim Pract 13, 88-95 PubMed.

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