Biodegradation of anthelmintics in soils: does prior exposure of soils to anthelmintics accelerate their dissipation?

Anthelmintics in the environment: Their occurrence, fate, and toxicity to non-target organisms

Anthelmintics are drugs used for the treatment and prevention of diseases caused by parasitic worms (helminths). While the importance of anthelmintics in human as well as in veterinary medicine is evident, they represent emerging contaminants of the environment. Human anthelmintics are mainly used in tropical and sub-tropical regions, while veterinary anthelmintics have become frequently-occurring environmental pollutants worldwide due to intensive agri- and aquaculture production. In the environment, anthelmintics are distributed in water and soil in relation to their structure and physicochemical properties. Consequently, they enter various organisms directly (e.g. plants, soil invertebrates, water animals) or indirectly through food-chain. Several anthelmintics elicit toxic effects in non-target species. Although new information has been made available, anthelmintics in ecosystems should be more thoroughly investigated to obtain complex knowledge on their impact in various environments. This review summarizes available information about the occurrence, behavior, and toxic effect of anthelmintics in environment. Several reasons why anthelmintics are dangerous contaminants are highlighted along with options to reduce contamination. Negative effects are also outlined.

Isolation of soil bacteria able to degrade the anthelminthic compound albendazole

Anthelmintic (AHs) veterinary drugs constitute major environmental contaminants. The use of AH-contaminated fecal material as manures in agricultural settings constitutes their main route of environmental dispersal. Once in soils, these compounds induce toxic effects to soil fauna and soil microbiota, both having a pivotal role in soil ecosystem functioning. Therefore, it is necessary to identify mitigation strategies to restrict the environmental dispersal of AHs. Bioaugmentation of AH-contaminated manures or soils with specialized microbial inocula constitutes a promising remediation strategy. In the present study, we aimed to isolate microorganisms able to actively transform the most widely used benzimidazole anthelminthic albendazole (ABZ). Enrichment cultures in minimal growth media inoculated with a soil known to exhibit rapid degradation of ABZ led to the isolation of two bacterial cultures able to actively degrade ABZ. Two oxidative products of ABZ, ABZSO and ABZSO2, were detected at low amounts along its degradation. This suggested that the oxidation of ABZ is not a major transformation process in the isolated bacteria which most probably use other biotic pathways to degrade ABZ leading to the formation of products not monitored in this study. Full length sequencing of their 16S rRNA gene and phylogenetic analysis assigned both strains to the genus Acinetobacter. The sequences were submitted in GeneBank NCBI, database with the accession numbers OP604271 to OP604273. Further studies will employ omic tools to identify the full transformation pathway and the associated genetic network of Acinetobacter isolates, information that will unlock the potential use of these isolates in the bioaugmentation of contaminated manures.

Interactions of anthelmintic veterinary drugs with the soil microbiota: Toxicity or enhanced biodegradation?

Anthelmintic (AH) compounds are used to control gastrointestinal nematodes (GINs) in livestock production. They are only partially metabolized in animals ending in animal excreta whose use as manures leads to AH dispersal in agricultural soils. Once in soil, AHs interact with soil microorganisms, with the outcome being either detrimental, or beneficial. We aimed to disentangle the mechanisms of these complex interactions. Two soils previously identified as « fast » or « slow», regarding the degradation of albendazole (ABZ), ivermectin (IVM), and eprinomectin (EPM), were subjected to repeated applications at two dose rates (1, 2 mg kg-1and 10, 20 mg kg-1). We hypothesized that this application scheme will lead to enhanced biodegradation in «fast » soils and accumulation and toxicity in «slow » soils. Repeated application of ABZ resulted in different transformation pathways in the two soils and a clear acceleration of its degradation in the «fast » soil only. In contrast residues of IVM and EPM accumulated in both soils. ABZ was the sole AH that induced a consistent reduction in the abundance of total fungi and crenarchaea. In addition, inhibition of nitrification and reduction in the abundance of ammonia-oxidizing bacteria (AOB) and archaea (AOA) by all AHs was observed, while commamox bacteria were less responsive. Amplicon sequencing analysis showed dose-depended shifts in the diversity of bacteria, fungi, and protists in response to AHs application. ABZ presented the most consistent effect on the abundance and diversity of most microbial groups. Our findings provide first evidence for the unexpected toxicity of AHs on key soil microbial groups that might have to be considered in a regulatory context.

Veterinary drug albendazole inhibits root colonization and symbiotic function of the arbuscular mycorrhizal fungus Rhizophagus irregularis

Arbuscular mycorrhizal fungi (AMF) are plant symbionts that have a pivotal role in maintaining soil fertility and nutrient cycling. However, these microsymbionts may be exposed to organic pollutants like pesticides or veterinary drugs known to occur in agricultural soils. Anthelminthics are veterinary drugs that reach soils through the application of contaminated manures in agricultural settings. Their presence might threaten the function of AMF, considered as sensitive indicators of the toxicity of agrochemicals to the soil microbiota. We determined the impact of the anthelminthic compounds albendazole and ivermectin on the establishment and functionality of the symbiosis between the model-legume Lotus japonicus and the AMF Rhizophagus irregularis. Our analyses revealed negative effects of albendazole on the development and functionality of arbuscules, the symbiotic organelle of AMF, at a concentration of 0.75 μg g-1. The impairment of the symbiotic function was verified by the reduced expression of genes SbtM1, PT4 and AMT2;2 involved in arbuscules formation, P and N uptake, and the lower phosphorus shoot content detected in the albendazole-treated plants. Our results provide first evidence for the toxicity of albendazole on the colonization capacity and function of R. irregularis at concentrations that may occur in agricultural soils systematically amended with drug-containing manures.

Albendazole from ovine excrements in soil and plants under real agricultural conditions: Distribution, persistence, and effects

Albendazole (ABZ), a broad-spectrum anthelmintic drug frequently used in livestock against parasitic worms (helminths), enters the environment mainly via faeces of treated animals left in the pastures or used as dung for field fertilization. To obtain information about the subsequent fate of ABZ, the distribution of ABZ and its metabolites in the soil around faeces along with uptake and effects in plants were monitored under real agricultural conditions. Sheep were treated with a recommended dose of ABZ; faeces were collected and used to fertilize fields with fodder plants. Soil samples (in two depths) and samples of two plants, clover (Trifolium pratense) and alfalfa (Medicago sativa), were collected at distances 0-75 cm from the faeces for 3 months after fertilization. The environmental samples were extracted using QuEChERS and LLE sample preparation procedures. The targeted analysis of ABZ and its metabolites was conducted by using the validated UHPLC-MS method. Two main ABZ metabolites, ABZ-sulfoxide (anthelmintically active) and ABZ-sulfone (inactive), persisted in soil (up to 25 cm from faeces) and in plants for three months when the experiment ended. In plants, ABZ metabolites were detected even 60 cm from the faeces and abiotic stress was observed in the central plants. The considerable distribution and persistence of ABZ metabolites in soil and plants amplify the negative environmental impact of ABZ documented in other studies.