An in vitro bioassay for measuring anthelmintic susceptibility in Dirofilaria immitis

Abstract

The canine heartworm (Dirofilaria immitis) is a widespread parasitic filarial nematode of veterinary importance. Anthelmintics of the macrocyclic lactone (ML) drug class have been widely and effectively used in chemoprophylaxis, but an increased number of lack of efficacy reports, in which dogs develop mature infection despite consistent preventive dosing, has raised concerns that heartworms are developing resistance. The objective of this study was to optimize an in vitro bioassay for measuring the susceptibility of heartworm populations to ML drugs and, using this assay, to compare the dose-responses of susceptible and suspected-resistant isolates. We developed and used a larval migration inhibition assay (LMIA) in which the ability of third-stage larvae to migrate through a 25-µm sieve following incubation in drug was measured. Fitting the dose-response data from the LMIA to a nonlinear regression model, we observed reproducible results, but found no differences in the IC50 values of susceptible and suspected-resistant isolates.