Treatment Protocol for Parasitic Infection by Diplozoon in Grass Carp and Silver Carp

2026-06-05

Cultured species: silver carp and bighead carp

Cultivation area: 2,500 mu

Water quality parameters: nitrite 0, ammonia nitrogen 0.1, pH 8.8

I. Disease Description

In affected fish, the eyeballs may turn milky white or develop white spots; in severe cases, the lens can detach. 1. After infection with Diplozoon or bacterial pathogens, failure to treat promptly can result in complete whitening of the eyeball, commonly referred to as cataracts. 2. Upon examination, small, clearly visible white spots are observed within the eyeball, and microscopic analysis reveals a shoe‑sole‑shaped parasite lodged in the lens. 3. In dying fish, the eyes may become blind or exhibit cloudy, opaque corneas.

As shown in the figure:

II. Handling Plan

Oral administration: Praziquantel (calculated based on fish body weight) plus Chlorobenzonil hydrochloride powder (also calculated based on fish body weight), mixed into the feed and administered orally for 7 days, with two consecutive treatment courses.

III. Feedback on Results

After one course of oral administration of the formulated feed, netting and microscopic examination revealed a reduction in the number of double‑eyed flukes in the fish’s eyes. Half a month later, microscopic analysis showed that the parasites within the lens had virtually disappeared, and the fish returned to normal condition.

Diplozoon is a widely prevalent parasitic disease that occurs particularly frequently in areas inhabited by gulls and snails of the genus *Lymnaea*. It can infect a variety of fish species, including silver carp, bighead carp, blunt snout bream, and grass carp. Given that grass carp winter syndrome is characterized by typical symptoms such as redness and proptosis of the eyeballs, Diplozoon is suspected to be a potential contributing factor. During the season when Diplozoon is most common: 1. Trichlorfon can be applied in bait‑feeding platforms in hanging bags, providing some preventive efficacy against various parasites. 2. Remove weeds along pond edges, and consider co‑culturing black carp to help control snail populations. 3. Implement effective bird‑control measures to break transmission pathways and reduce the incidence of the disease.

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