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Heartworm disease primarily affects the cardiopulmonary system and the severity and extent of lesions depends several factors. These include the number and location of adult worms^{merck, fera}, the duration of infection, and the level of activity of the host^merck. Parasites in the pulmonary arteries causes mechanical irritation, leading to endothelial damage, proliferation of the intima and perivascular cuffing with inflammatory cells. This results in narrowing and occlusion of the vessels which in turn causes pulmonary hypertension. A combination of pulmonary hypertension and inflammatory mediators can lead in an increase in the permeability of pulmonary vessels, giving periarterial oedema and intersitial and alveolar infiltrates. Eventually, irreversible interstitial fibrosis arises.  

Heartworm disease primarily affects the cardiopulmonary system and the severity and extent of lesions depends several factors. These include the number and location of adult worms^{merck, fera}, the duration of infection, and the level of activity of the host^merck. Parasites in the pulmonary arteries causes mechanical irritation, leading to endothelial damage, proliferation of the intima and perivascular cuffing with inflammatory cells. This results in narrowing and occlusion of the vessels which in turn causes pulmonary hypertension. A combination of pulmonary hypertension and inflammatory mediators can lead in an increase in the permeability of pulmonary vessels, giving periarterial oedema and intersitial and alveolar infiltrates. Eventually, irreversible interstitial fibrosis arises.  

 

Sequelae to heartworm infection include pulmonary thromboembolism, which can either occur due to the death and metastasis of adult worms, or due to platelet aggregation induced by the parasite. In severe cases, live nematodes can migrate to the right ventricle, right atrium and caudal vena cava. The resulting incompetence of the tricuspid valve, augmented by concurrent pulmonary hypertension, leasds to signs of right-sided heart failure. Flow of erythrocytes through the mass of parasites formed can also cause haemolysis and thus haemoglobinaemia. This combination of acute right-sided heart failure and intravascular haemolysis is referred to as "caval syndrome", which in severe cases can also be characterised by thromboembolic events and disseminated intravascular coagulation. Due to the smaller numbers of adult worms, caval syndrome is less common in cats^fara.

Pulmonary thromboembolism, due to platelet aggregation induces by the parasis or in response to the deat (spontaneous or induced by adultidicat treatment) of adult worms is another possible sequela of heartworm disease.

 

In some severe cases, worms can migreat to the right ventricle, right artrium and caudal vena cava. This retrografe migration induces incompetence of the tricuspid valve which, in association with concurrent pulmonary hypertension, is responsible for the clinical signs of right-sides heart failure (e.g. jugular distension, liver congestion, ascites). In addition, red blood cell memranes may rupture as the vells flow through the mass of parasits, causing haemolysis and haemoglobinaemia. The concomitant presence of acture right-sided heart failure and intravascular haemolysis is referred to as caval syndrome. Severe cases of caval syndrome can also be characterised by the present of adult worms in the caudal vena cava, thromboembolic events and disseminated intravascular coagulation. Caval syndrom is less common in cats due to the lighter woem burden.

In cats, hertwoms disease generally induces a diffuse plumonary infiltrate and signs of eosinophilis pneumonia. The death od adult worms may cause acute pulmonary embolism with severe haemorrhage and oedema of the affected lobe. Occasionally, immature nematodes cna migrate to sites other than the heart and plumonary areties, causing ectopic infection. Localisation of D. immitis has been repored in the eye, CNS, systemic arteries and subcutaneous tissues. Ectopic infections are more commonly seen in cats than in dogs, suggesting that the parasite is not well adapted to feline hosts.

In cats, hertwoms disease generally induces a diffuse plumonary infiltrate and signs of eosinophilis pneumonia. The death od adult worms may cause acute pulmonary embolism with severe haemorrhage and oedema of the affected lobe. Occasionally, immature nematodes cna migrate to sites other than the heart and plumonary areties, causing ectopic infection. Localisation of D. immitis has been repored in the eye, CNS, systemic arteries and subcutaneous tissues. Ectopic infections are more commonly seen in cats than in dogs, suggesting that the parasite is not well adapted to feline hosts.