Ichthyoparasites

Ichthyoparasites are organisms that live on or inside fish, deriving nutrients at the host’s expense. They include ectoparasites that inhabit the skin, gills, fins, or external surfaces, and endoparasites that occur within internal organs, tissues, or the digestive tract. They affect wild and farmed fish and can influence morbidity, mortality, and commercial value, particularly in aquaculture where high stocking densities facilitate transmission.

Parasites of fish span several groups. Protozoan parasites such as Ichthyophthirius multifiliis (the “ich”), Trichodina spp., and various myxosporeans cause skin, gill, or systemic infections. Monogeneans (e.g., Gyrodactylus and Dactylogyrus) are common gill and skin ectoparasites with direct life cycles. Crustacean ectoparasites include sea lice (Caligus, Lepeophtheirus) and branchiurans such as Argulus. Endoparasites include nematodes (Camallanus spp.), cestodes (Bothriocephalus sp.), trematodes (Clinostomum, Allocreadium), acanthocephalans (Acanthocephalus), and myxozoans affecting muscle, kidney, or intestine.

Life cycles vary by group. Many monogeneans and trichodinids have direct life cycles, with free-swimming larval stages that infect new hosts. Others require intermediate hosts such as mollusks or crustaceans, or more complex cycles involving multiple hosts. Transmission occurs through contaminated water, direct contact, ingestion of infected prey, or during spawning.

Diagnosis relies on clinical signs and laboratory tests: observation of external parasites on skin and gills, microscopy of skin scrapes and gill biopsies, fecal analyses, and molecular assays. Management includes biosecurity, quarantine of new stock, proper water quality and stocking density, routine health monitoring, and targeted antiparasitic treatments appropriate to the parasite and host species. Resistance and environmental considerations necessitate integrated parasite management.