September 2024

A sunfish's silent struggle

​A sunfish (Lepomis spp.) kept in a freshwater aquarium at a zoological collection in Texas was found deceased with bloody lesions across its body. The ~13,000 gallon community aquarium housed sunfish, gar, large-mouth bass, catfish, and turtles, many of which were locally acquired. These small but grossly visible organisms were removed from a section of gill on necropsy and sent to the diagnostic lab for identification.

Thanks to Rachel Busselman PhD, DVM Class of 2025 and her supervisor Dr.
Gui Verocai DVM, MSc, PhD, DACVM (Parasitology) at Texas A&M for contributing to this case.
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​Images 1 & 2: Several specimens of varying sizes, with dorsal and ventral views

​Images 3 & 4: Close-ups of ventrum featuring suction cups, either on dissecting microscope (3) or compound microscope at 10x magnification (4).

What's your diagnosis?

​Argulus sp. or fish lice.  They are crustacean ectoparasites of fish not uncommonly found to infest freshwater fish. They have a jelly-like appearance, two compound eyes, and a disc-shaped wide carapace covering most of their rounded body. All life stages are parasitic and use a stylet to pierce their fish host, secrete digestive enzymes, and feed on body fluids. Adults are approximately 3-7mm in length and 2-4mm wide and have paired ventral suckers near their mouth to help attach to the fish. Females leave the host to lay rows of eggs on vegetation and hard surfaces in the environment before returning to the host to feed.
 
Infestations on fish can cause local inflammation at sites of attachment, anemia, scale loss, behavior changes (e.g., reduced feeding, rubbing on surfaces), and secondary bacterial infections at the attachment sites. Because of their rapid reproduction rate, infestations can quickly cause morbidity and mortality in captive environments. Argulus spp. can also serve as a mechanical vector or intermediate host for several fish diseases, including spring viremia of carp and several nematode species.
 
Treatment can be challenging and must target each life stage throughout their life cycle, which may take 30 to 60 days. Treatment options include multi-doses of organophosphates, potassium permanganate, or chitin synthesis inhibitors including pesticides such as diflubenzuron.