Fish lice (Argulus spp) are irritants that cause significant morbidity (mortality in heavy infestations) in fish and act as mechanical vectors for spring viremia of carp (SVC), a fatal disease that has resulted in significant economic losses. Historically, chemical eradication of Argulus spp has been difficult, as treatments are unapproved, hazardous, or ineffective. Lufenuron, a benzoylurea pesticide that inhibits chitin biosynthesis, prevents development of ectoparasites with chitin exoskeletons.

In this novel case report, 12 koi (Cyprinus carpio) fish in a 3800-L pond developed progressive, dark-brown spots; Argulus spp infestation was confirmed, and off-label lufenuron (Program, ah.novartis.com) treatment was initiated at 409.8 mg/3785 L of water (~0.1 mg/L) q7d for 5 weeks. The owner observed significant decline in lice numbers with no apparent adverse effects to the fish after 4 days. Ten days into treatment, the owner reported no lice; 30 days after treatment, veterinarians confirmed no remaining lice. With strict monitoring, this koi collection was still considered lice free a year after treatment. While lufenuron can successfully treat lice in pond fish, it may harm wild crustaceans in the environment, so care must be taken to avoid contamination.

Commentary
This study illustrated the necessity of thinking outside the box when treating nontraditional species, such as using lufenuron, a common parasiticide in other companion species, off-label as an ectoparasiticide in a koi pond. However, this medication must be used carefully because of its potential to kill beneficial and/or desired organisms in the enclosure and environment (should treated water spill into groundwater); more than the case at hand should be considered in all situations.—Adolf K. Maas, DVM, DABVP (Reptile & Amphibian)

Source
The use of lufenuron to treat fish lice (Argulus spp) in koi (Cyprinus carpio). Meyer J, Hensel P, Mejia-Fava J, et al. J EXOTIC PET MED 22:65-69, 2013.