Overview

Occult Heartworm Disease refers to the presence of D. immitis in the pulmonary arteries but with no circulating microfilaria (amicrofilaremic).

1. Prevalence:
   • accounts for 15-25% of infected dogs
2. Etiology :
   • prepatent infections
   • senile worms
   • unisexual infection
   • drug-induced sterility of adult worms
   • immune-mediated reaction with antibody causing death of microfilariae and suppressing the production of microfilariae from female worms
3. Mechanism of disease:
   • In some individuals, a high antibody level results in the destruction of microfilaria within the pulmonary capillaries as they are released from the adult. This results in Occult Heartworm Disease. These cases usually are associated with the greatest degree of pulmonary pathology and most severe clinical signs. A hypersensitivity reaction with so-called allergic pneumonitis may result due to a marked increase in capillary permeability with mixed alveolar and interstitial disease. So-called pulmonary granulomatosis may occur. This is suggested by the radiographic finding of lung lobe consolidation.