Pre-treatment of canine plasma with heat, rather than acid, efficiently enhances Dirofilaria immitis antigen detection

Abstract

Background

The dissociation of antigen-antibody complexes has been utilized to enhance the accuracy of serological tests for infectious diseases, including Dirofilaria immitis. Currently, the antigen detected by available tests is primarily a glycoprotein found in the reproductive tract of female worms. However, this antigen can become inaccessible when bound to excessive circulating antibodies, leading to reduced test sensitivity and false-negative results. Acid and heat treatments of the sera or plasma have been established as reliable methods for inducing immune complex dissociation (ICD). Previous antigen testing for heartworm infection in dogs and cats has demonstrated that these treatments improve the diagnostic sensitivity without compromising specificity. This study aims to evaluate the performance of four distinct ICD methods in the detection of D. immitis antigen.

Methods

We utilized twofold serial dilutions of a well-characterized plasma (ranging from 1:2 to 1:4096) obtained from a D. immitis-infected dog to simulate the diverse antigen levels encountered in real-life infected dogs. The presence of antigen in the diluted samples, both without treatment and treated with four ICD protocols, was assessed in triplicate visually using DiroCHEK ^® by observing color changes. OD values were also obtained using the microplate reader SpectraMax® i Series-Spectramax Id3. A Factorial ANOVA test was conducted to compare the OD values between samples with and without treatments.

Results

The highest dilution at which color changes were observed was 1:128 for untreated samples and for samples subjected to acid treatments in ICD-3 and the hybrid ICD-4 protocol. In contrast, both heat treatment protocols (ICD-1 and ICD-2) exhibited color changes at a 512-fold dilution. The OD values in samples subjected to heat treatment were significantly higher than those in untreated samples, up to dilutions of 512-fold. Although OD values tended to be higher in samples subjected to acid treatment and the hybrid protocol compared to untreated samples up to a 128-fold dilution, this difference was not significant as the samples underwent further dilution.

Conclusions

Our findings affirm that heat treatments, rather than acid treatment, efficiently enhance the detection of D. immitis antigen by liberating the sequestered antigen from the immune complexes.