Pseudocapillaria tomentosa Infections in Laboratory Larval and Adult Zebrafish (Danio rerio): Development and Advances in an In Vivo Anthelmintic Drug Discovery Model.

Parasite resistance is an increasing problem in livestock and companion animals. Developing new drug discovery models may improve the identification of novel anthelmintic drugs which will reduce parasite infections. Adult zebrafish (Danio rerio) have been previously used as a model for anthelmintic drug discovery by infecting them with the gastrointestinal nematode Pseudocapillaria tomentosa. Using larval zebrafish will increase assay sensitivity and throughput because this in vivo platform evaluates host parasite interactions and is conducted in multi-well plates. To develop this assay, this study focuses on 1) evaluating infections in 5-30 days post fertilization (dpf) fish, 2) validating the assay with a known anthelmintic used in domestic animals, and 3) documenting the growth and development of P. tomentosa. 30 dpf fish had the most robust infections in multi-well plates and the best survival compared to younger larvae. Assay sensitivity was evaluated by aqueously exposing 30 dpf infected zebrafish to emamectin benzoate (a macrocyclic lactone), which successfully reduced infection intensity. In vitro larvae hatched from eggs, larval, and adult zebrafish (n=488) were used to document P. tomentosa development from 1-37 days post exposure. A change point analysis (CPA) predicted the ecdysis (molting) points as follows (mm): L1/L2 = 0.220, L2/L3 = 0.571, L3/L4 = 1.174, and L4/L5 = 1.584. Identifying the worm molts sizes enables the inclusion of developmental endpoints within trials. Overall, this study will increase the sensitivity of anthelmintic drug discovery because the validated high throughput larval zebrafish model accounts for drug interactions between the host and parasite.