Using droplet digital PCR for the detection of hco-acr-8b levamisole resistance marker in H. contortus

The response of Anisakis simplex (s. s.) to anthelmintics - Specific changes in xenobiotic metabolic processes

Anisakiasis is a parasitic disease transmitted through the consumption of raw or undercooked fish and cephalopods that are infected with larvae of Anisakis simplex (sensu stricto) or Anisakis pegreffii. The purpose of this study was to investigate how A. simplex (s. s.) responds to the influence of anthelmintics such as ivermectin (IVM) and pyrantel (PYR). In vitro experiments were conducted using larvae at two developmental stages of A. simplex (s. s.) (L3 and L4) obtained from Baltic herring (Clupea harengus membras). Larvae were cultured with different concentrations of IVM or PYR (1.56, 3.125, and 6.25 μg/mL) for various durations (3, 6, 9, and 12 h) under anaerobic conditions (37 °C, 5% CO2). The gene expression of actin, ABC transporter, antioxidant enzymes, γ-aminobutyric acid receptors, and nicotinic acetylcholine receptors, as well as the oxidative status were analyzed. The results showed that A. simplex (s. s.) L3 stage had lower mobility when cultured with PYR compared to IVM. The analysis of relative gene expression revealed significant differences in the mRNA level of ABC transporters after treatment with IVM and PYR, compared to the control group. Similar patterns were observed in the gene expression of antioxidant enzymes in response to both drugs. Furthermore, the total antioxidant capacity (TAC) and glutathione S-transferase (GST) activity were higher in the treatment groups than in the control group. These findings suggest a relationship between the expression of the studied genes, including those related to oxidative metabolism, and the effectiveness of the tested drugs.

Understanding anthelmintic resistance in livestock using "omics" approaches

Widespread and improper use of various anthelmintics, genetic, and epidemiological factors has resulted in anthelmintic-resistant (AR) helminth populations in livestock. This is currently quite common globally in different livestock animals including sheep, goats, and cattle to gastrointestinal nematode (GIN) infections. Therefore, the mechanisms underlying AR in parasitic worm species have been the subject of ample research to tackle this challenge. Current and emerging technologies in the disciplines of genomics, transcriptomics, metabolomics, and proteomics in livestock species have advanced the understanding of the intricate molecular AR mechanisms in many major parasites. The technologies have improved the identification of possible biomarkers of resistant parasites, the ability to find actual causative genes, regulatory networks, and pathways of parasites governing the AR development including the dynamics of helminth infection and host-parasite infections. In this review, various "omics"-driven technologies including genome scan, candidate gene, quantitative trait loci, transcriptomic, proteomic, and metabolomic approaches have been described to understand AR of parasites of veterinary importance. Also, challenges and future prospects of these "omics" approaches are also discussed.

Digital PCR: modern solution to parasite diagnostics and population trait genetics

The use of polymerase chain reaction (PCR)-based diagnostic approaches has steadily increased in the field of parasitology in recent decades. The most recent large-scale technological modification of the PCR formula, also known as third-generation PCR, came in the form of digital PCR (dPCR). Currently, the most common form of dPCR on the market is digital droplet PCR (ddPCR). Unlike quantitative real-time PCR (qPCR), the digital format allows for highly sensitive, absolute quantification of nucleic acid targets and does not require external standards to be included in the developed assays. Dividing each sample into thousands of compartments and using statistical models also eliminates the need for technical replicates. With unprecedented sensitivity and enforcement of binary endpoint reactions, ddPCR not only allows the use of tiny sample volumes (especially important when working with limited amounts of DNA) but also minimises the impact of variations in amplification efficiency and the presence of inhibitors. As ddPCR is characterised by excellent features such as high throughput, sensitivity and robust quantification, it is widely used as a diagnostic tool in clinical microbiology. Due to recent advances, both the theoretical background and the practical, current applications related to the quantification of nucleic acids of eukaryotic parasites need to be updated. In this review, we present the basics of this technology (particularly useful for new users) and consolidate recent advances in the field with a focus on applications to the study of helminths and protozoan parasites.

The presence and relative frequency detection of the levamisole-resistance-associated S168T substitution in hco-acr-8 in Haemonchus contortus

Parasitic sheep nematodes, among which Haemonchus contortus is often considered to be the most clinically important, exact a significant toll on the animals, not least because of their capacity to evolve drug resistance. Despite decades of research, our understanding of the mechanism of resistance to compounds such as levamisole is fairly limited, which therefore constrains our ability to develop sensitive and efficient molecular diagnostic tools for rapid and accurate resistance detection in field settings. Herein, we investigated the presence and frequency of the newly reported, levamisole-resistance-associated, mutation, yielding a S168T substitution in exon 4 of hco-acr-8, in six different phenotypically described isolates (three susceptible and three resistant), three Swedish field isolates and eight larvae culture samples, the latter two of which originated on farms where levamisole showed complete parasite elimination. For this purpose, we created both an allele-specific and droplet digital PCR approaches and found the mutated allele to be present only in the Kokstad isolate, whereas the other five as well as both the Swedish isolates and larvae cultures displayed only the non-mutated, serine-encoding, allele. While the finding of only the non-mutated allele in the phenotypically susceptible and Swedish isolate and larvae culture samples seemed sensible, we speculate that for the other two phenotypically resistant isolates, different (perhaps secondary) variants are responsible for conferring the resistance to levamisole phenotype, given the polygenic nature of levamisole resistance. All in all, despite the limited number of samples tested here, the mutation causing the S168T substitution in hco-acr-8 represents a plausible levamisole resistance-associated variant in, at least, some isolates of H. contortus.

Allele specific PCR for a major marker of levamisole resistance in Haemonchus contortus

Haemonchus contortus is a haematophagous parasitic nematode that infects small ruminants and causes significant animal health concerns and economic losses within the livestock industry on a global scale. Treatment primarily depends on broad-spectrum anthelmintics, however, resistance is established or rapidly emerging against all major drug classes. Levamisole (LEV) remains an important treatment option for parasite control, as resistance to LEV is less prevalent than to members of other major classes of anthelmintics. LEV is an acetylcholine receptor (AChR) agonist that, when bound, results in paralysis of the worm. Numerous studies implicated the AChR sub-unit, ACR-8, in LEV sensitivity and in particular, the presence of a truncated acr-8 transcript or a deletion in the acr-8 locus in some resistant isolates. Recently, a single non-synonymous SNP in acr-8 conferring a serine-to-threonine substitution (S168T) was identified that was strongly associated with LEV resistance. Here, we investigate the role of genetic variation at the acr-8 locus in a controlled genetic cross between the LEV susceptible MHco3(ISE) and LEV resistant MHco18(UGA2004) isolates of H. contortus. Using single worm PCR assays, we found that the presence of S168T was strongly associated with LEV resistance in the parental isolates and F3 progeny of the genetic cross surviving LEV treatment. We developed and optimised an allele-specific PCR assay for the detection of S168T and validated the assay using laboratory isolates and field samples that were phenotyped for LEV resistance. In the LEV-resistant field population, a high proportion (>75%) of L₃ encoded the S168T variant, whereas the variant was absent in the susceptible isolates studied. These data further support the potential role of acr-8 S168T in LEV resistance, with the allele-specific PCR providing an important step towards establishing a sensitive molecular diagnostic test for LEV resistance.