Immunohistological observations on pulmonary tissues from dogs infected with Dirofilaria immitis.

Pulmonary tissues from non-infected dogs, naturally Dirofilaria immitis-infected dogs and experimentally infected puppies, selectively necropsied after infection, were assessed using peroxidase-antiperoxidase (PAP) staining technology. Sequential sections of pulmonary tissue were PAP stained with anti-fresh D. immitis serum, anti-paraffin processed D. immitis serum, anti-dog immunoglobulin (IgG, IgG Fc, IgM) sera and anti-dog complement (C3) serum, and examined for antibody, complement and for D. immitis antigen. The extent of alveolar septal thickening was positively correlated with infection status. Cellular infiltration was most evident surrounding obstructed areas where D. immitis were in situ. Antigenic material (microfilariae, eggs, fragmented filariae) labelled by PAP was identified in the pulmonary arteries, alveolar capillaries and alveolar septa. Deposits of complement and IgG, presumably associated with immune complex formation, were also observed in some of the infected dogs. Identification of antigen, antibodies and complement associated with alveolar septal pathology suggested that immune mechanisms were active in its development.