Related Experiment Video
Updated: Dec 18, 2025
Topical Application Bioassay to Quantify Insecticide Toxicity for Mosquitoes and Fruit Flies
Published on: January 19, 2022
Using population genetics to examine relationships of Dirofilaria immitis based on both macrocyclic
Julie Sanchez1, Guha Dharmarajan2, Melissa M George1
1University of Georgia College of Veterinary Medicine, Department of Infectious Diseases, Athens, GA, United States.
Abstract:
Prevention of infection with canine heartworm (Dirofilaria immitis) is based on the compliant administration of macrocyclic lactone (ML) drugs. Resistance to ML drugs is well documented in D. immitis; however, there remains a paucity of information on the spatial distribution and prevalence of resistant isolates. This project aims to improve understanding of ML-resistance by using a population genetic approach. We developed a large panel of microsatellite loci and identified 12 novel highly polymorphic markers. These 12, and five previously published markers were used to screen pools of microfilariae from 16 confirmed drug-susceptible, 25 confirmed drug-resistant, and from 10 suspected drug-resistant field isolates. In isolates where microfilarial suppression testing indicated resistance, Spatial Principal Component Analysis (sPCoA), Neighbor Joining Trees and Bayesian clustering all revealed high genetic similarity between pre- and post-treatment samples. Somewhat surprisingly, the Neighbor Joining tree and sPCoA generated using pairwise Nei's distances did not reveal clustering for resistant isolates, nor did it reveal state-level geographic clustering from samples collected in Georgia, Louisiana or Mississippi. In contrast, Discriminant Analysis of Principle Components was able to discriminate between susceptible, suspected-resistant and resistant samples. However, no resistance-associated markers were detected, and this clustering was driven by the combined effects of multiple alleles across multiple loci. Additionally, we measured unexpectedly large genetic distances between different passages of laboratory strains that originated from the same source infection. This finding strongly suggests that the genetic makeup of laboratory isolates can change substantially with each passage, likely due to genetic bottlenecking. Taken together, these data suggest greater than expected genetic variability in the resistant isolates, and in D. immitis overall. Our results also suggest that microsatellite genotyping lacks the sensitivity to detect a specific genetic signature for resistance. Future investigations using genomic analyses will be required to elucidate the genetic relationships of ML-resistant isolates.
Insights
Canine heartworm (Dirofilaria immitis) drug resistance shows high genetic variability, but microsatellite markers lack sensitivity to pinpoint specific resistance genes. Further genomic studies are needed to understand resistant isolate genetics.
Area of Science:
- Veterinary Parasitology
- Population Genetics
- Molecular Entomology
Background:
- Macrocyclic lactone (ML) drugs are crucial for preventing canine heartworm (Dirofilaria immitis) infections.
- Drug resistance in D. immitis is a growing concern, yet its spatial distribution and prevalence remain poorly understood.
- Understanding the genetic basis of ML resistance is essential for effective disease control.
Purpose of the Study:
- To investigate the population genetics of macrocyclic lactone-resistant Dirofilaria immitis isolates.
- To identify potential genetic markers associated with ML resistance in D. immitis.
- To assess the genetic diversity and geographic distribution of resistant and susceptible D. immitis populations.
Main Methods:
- Development and application of a large panel of novel and established microsatellite markers.
- Screening of microfilariae from drug-susceptible, resistant, and suspected-resistant field isolates.
- Utilizing population genetic analyses including Spatial Principal Component Analysis (sPCoA), Neighbor Joining Trees, Bayesian clustering, and Discriminant Analysis of Principle Components (DAPC).
Main Results:
- High genetic similarity was observed between pre- and post-treatment samples in resistant isolates.
- Microsatellite analysis did not reveal distinct geographic clustering or specific resistance-associated markers.
- Discriminant Analysis of Principle Components (DAPC) could differentiate between susceptible and resistant samples, driven by multi-locus allele effects.
- Unexpectedly large genetic distances were found between laboratory strain passages, indicating potential genetic bottlenecking.
Conclusions:
- Dirofilaria immitis exhibits greater genetic variability than previously expected, particularly in resistant isolates.
- Current microsatellite genotyping methods may lack the sensitivity to detect specific genetic signatures of ML resistance.
- Genomic analyses are necessary for a comprehensive understanding of the genetic relationships within ML-resistant D. immitis populations.
