Use of an antigen detection assay to determine mortality of Dirofilaria immitis after thiacetarsamide therapy

The use of an antigen detection enzyme immunoassay (EIA) to determine the post-treatment infection status of 16 dogs naturally infected with Dirofilaria immitis was investigated. Dogs were treated with thiacetarsamide at a dose rate of 12mg/4.5kg twice daily for 2 days, bled at regular intervals and necropsied 9 weeks later. The infection status of all dogs at necropsy was compared to the ratios of optical density (OD) values from the EIA using fresh plasma samples (day 60/day 0 = R60) and dogs were divided into 2 groups. Using the R60 ratios, those dogs with fewer than 2 live adult worms or immature worms at necropsy ("cleared" dogs) could be differentiated with 95% confidence from those dogs with more than 1 live adult worm ("non-cleared" dogs). Changes in the average OD values from the plasma of "cleared" dogs and "non-cleared" dogs were similar up to 46 days after treatment but diverged significantly thereafter. The efficacy of thiacetarsamide was 50% if all worms were considered and 75% if the presence of immature worms was ignored. The benefits of antigen detection assays for diagnosis and improved patient assessment and the use of an R60 ratio to assess the efficacy of adulticides such as thiacetarsamide are discussed in relation to their practical significance for clinicians.