Overview
• Filarial nematode infection of dogs and cats – Dirofilaria immitis – adults (female worms up to 30 cm in length) live in pulmonary arteries causing respiratory, cardiac, and in some cases,
renal disease.
• Adults living in pulmonary artery cause lobar arterial enlargement, obstruction, and tortuosity causing pulmonary hypertension
and thrombosis.
• Severity of disease – directly related to the number of worms, host response, and duration of infection.
• Female adult worms release L1 microfilaria into circulation where they can live for up to 2 years; L1 infective stage
for the vector, mosquitoes, in which they develop to L3; mosquitoes inject L3 back into dog where they mature and migrate
to the heart. PPP = 6 to 7 months (dogs) – longer in cats.
• Prevalence – widespread throughout North America (even Alaska) but much more common in southern States - along the
Atlantic and Gulf coasts and Ohio and Mississippi River basins.
• 100% in of dogs not on prophylaxis may be infected in highly endemic regions.
• Low prevalence areas (Northern States) - pockets of infection usually where mosquito vector is common.
• Although D. immitis can infect man, infected dogs represent no direct zoonotic potential.
Signalment
• Mainly affects dogs 3 to 8 yrs old; all breeds but medium to large-breed dogs (those that spend a lot of time outdoors)
are most susceptible.
• Most infected dogs are asymptomatic (Class I or perhaps display an occasional cough) so most infections are picked
up during routine heartworm antigen screening during wellness checks.
Clinical signs
• Class I – dogs show no abnormal findings.
• Class II – exercise intolerance; cough; weight loss; pulmonary changes on thoracic radiography; blood worm might show
a mildly reduced PCV (20 – 25).
• Class III – exercise intolerance; cachexia; syncope; tachycardia; perhaps ascites due to right sides heart failure;
hepatomegaly; pulmonary and cardiac changes on thoracic radiography; hemoptysis may occur (suggests severe pulmonary thromboembolic);
blood work shows a PCV < 20.
• Vena cava syndrome (occurs when vena cava is obstructed by adult worms) can result in hemoglobinuria due to acute hemolytic
crisis.
Hematology/serum biochemistry/urinalysis
• CBC: anemia (Class II – mild; Class III – severe); eosinophilia and basophilia are a sensitive indicator of heartworm
disease when they occur together; leukocytosis and thrombocytopenia are often associated with severe thromboembolism.
• Serum biochemical profile and urinalysis: Hyperglobulinemia is not a consistent finding; proteinuria (immune complex
glomerulonephritis) is common in dogs with severe chronic infection; hemoglobinuria can occur during an acute hemolytic crisis
(vena cava syndrome).
