Temporal Pattern of Circulating Antigens and Antibody Responses in Cats Experimentally Infected with Dirofilaria immitis

Abstract

Heartworm disease in cats has been attributed to immature adult heartworms reaching the pulmonary arteries approximately 2.5 -4 months post infection and the presence of mature adult worms in the cardiopulmonary system approximately 6 months after infection. The arrival and death of immature heartworms causes significant lung pathology, a condition referred to as heartwormassociated respiratory disease (HARD). To evaluate the humoral immune response to Dirofilaria immitis, 12 cats were each infected by subcutaneous injection of 100 third-stage larvae. Postinfection serum samples were collected weekly for 4 weeks, every other week for the following 4 weeks, then monthly to day 270 for evaluation of antibodies to D. immitis recombinant antigens HWAg-1 and HWAg-2 and circulating heartworm antigen (HWAg) using the PetChek ® HTWM PF Test Kit (IDEXX Laboratories). Necropsies were performed 278 -299 days post infection for collection of adult worms. Eleven cats were HWAg-1 antibody-positive 68 days post-infection, and the remaining cat became positive by day 140. Eleven cats were positive for HWAg-2 antibody 42 -84 days post infection; all 11 remained HWAg-2 antibodypositive through day 270. Circulating HWAg was detected in 10 cats, 2 by day 140 and 8 others by day 168. The 2 antigen-negative cats had no adult worms at necropsy. This study demonstrates that decomposition of immature adult heartworms can result in detectible levels of circulating antigen prior to sexual maturation. In this experimental Original Article S92EndoparasitEs model, D. immitis antigen was detectable in cats at time points (days 140 and 168) associated with HARD from dying immature adult heartworms.