Temporal Change in Infection of Philometroides seriolae (Nematoda: Philometridae) in the Definitive Host, Seriola quinqueradiata

Abstract

Philometroides seriolae infection was monitored monthly over a 16-mo period in a single population of wild Japanese amberjack Seriola quinqueradiata, estimated to be 2–5 years old that maintained in a sea cage where no new infection could occur. The overall prevalence and mean intensity of infection was 65.0% (78/120) and 6.2 ± 5.5 worms per infected fish (1–26), respectively. Female nematodes, ranging from 2.0 to 41.3 cm in body length, were unevenly distributed within the skeletal muscle, with a higher proportion located in the dorsal (62.7%) and posterior (45.4%) regions. Some mature worms exited the host, while others remained in the muscle. Parasite remnants including dead worms, scars and empty cavities where nematode had resided, were observed in most of the fish examined. Live nematodes collected in July and August 2020 consisted of two size classes. In the following months, the larger worms disappeared, while smaller ones continued to grow, reaching lengths of up to 30 cm by June of the following year. These findings suggest that P. seriolae remains in Japanese amberjack for at least 1.5 year and that infection likely occurs through ingestion of small paratenic fish hosts.