Heartworm disease: Does Wolbachia change how we treat? (Proceedings)


Heartworm disease: Does Wolbachia change how we treat? (Proceedings)

Nov 01, 2010


     • 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.


     • 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).