March 15, 2018

Does Heat Treatment Aid Heartworm Detection?

Is it diagnostically useful to heat-treat serum samples when testing for the presence of heartworm antigen? A recent study sought to answer that question.
By JoAnna Pendergrass, DVM
In veterinary medicine, accurate heartworm antigen detection helps not only to diagnose patients with heartworms but also to determine the effectiveness of heartworm treatment. Because antigen-antibody complexes are believed to block antigen detection and cause false-negative results, breaking down this complex has been hypothesized to increase levels of circulating heartworm antigen and improve detection.

Historically, commercial heartworm tests employed a dissociation step to break down the antigen-antibody complex. Over time, the increasing accuracy and sensitivity of heartworm tests rendered this step unnecessary. In 2014, a study on feline heartworm disease suggested that heat-treating serum samples can break down this complex. Yet, whether this heat treatment is useful and effective remains debatable.

Recently, a research team from Bayer Animal Health and the University of Georgia College of Veterinary Medicine evaluated the diagnostic usefulness of heat-treating serum samples during heartworm treatment with Advantage Multi and doxycycline in dogs. Their findings were reported in Parasites & Vectors.

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Infection and Testing
Sixteen dogs with no previous macrocyclic lactone exposure were experimentally infected with heartworms through surgical transplantation of adult heartworms into the jugular vein. The dogs were then divided equally into 2 groups:
  • Control
  • Treatment: 30 days of twice-daily 10 mg/kg doxycycline and 10 months of once-monthly Advantage Multi for Dogs

At predetermined time points, serum samples were collected and tested for heartworm antigens with the DiroCHEK Heartworm Antigen Test Kit (Zoetis), before and after heat treatment. A spectrophotometer was used to measure optical density values to determine heartworm positivity or negativity.

At the study’s end (10 months posttreatment), all dogs were euthanized and necropsy was performed to visually identify adult heartworms.

Results
All dogs were heartworm-negative before surgery and heartworm-positive at day 0 (first day of treatment). All control dogs remained heartworm-positive for the study’s duration, while 5 of 8 treated dogs were heartworm-negative at the study’s end.

Interestingly, for 3 dogs in the treatment group, researchers observed the conversion of serum samples from heartworm-negative before heat treatment to heartworm-positive after heat treatment on several study days. These conversion events, the researchers noted, could have represented detection of either residual heartworm antigen released after heartworm death (rather than live adult heartworms) or off-target antigens that were denatured with heat treatment; such off-target detection after heat-treatment could lead to false positives.

The end-of-study heartworm test results correlated with necropsy findings, indicating 100% accuracy of the heartworm test. The 5 treated dogs that were heartworm negative did not have adult heartworms at necropsy, while heartworms were found in all dogs that were heartworm positive. Notably, the 3 treated dogs with conversion events had no heartworms at necropsy, suggesting that heat-treating serum samples may not accurately detect live adult heartworm infections in dogs.

Throughout the study, optical density values were higher in heat-treated than non–heat-treated serum samples.

Diagnostic Implications
Given the 100% accuracy of the heartworm test, regardless of whether serum samples were heat-treated, the investigators concluded that “the use of heat-treatment in this study did not provide any unique or valuable data in the detection of viable heartworm infection post-treatment.”

 
Dr. Pendergrass received her Doctor of Veterinary Medicine degree from the Virginia-Maryland College of Veterinary Medicine. Following veterinary school, she completed a postdoctoral fellowship at Emory University’s Yerkes National Primate Research Center. Dr. Pendergrass is the founder and owner of JPen Communications, a medical communications company.

 

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