Gill trematodes (flukes) in wild-caught killifish (Fundulus heteroclitus)

Three wild caught killifish (Fundulus heteroclitus) on an Institutional Animal Care and Use Committee-approved protocol were found dead within 2 days after being received. The fish were housed in two separate aquaria. Aquarium water was evaluated, and pH, salinity, ammonia, nitrate, and nitrite levels were within acceptable parameters. Several remaining fish appeared to be slow-moving and were presented for necropsy. Multiple, scattered, ulcerated skin lesions (diameter, 1 to 5 mm) were noted at necropsy and were cultured. No pathogenic bacteria were isolated. Wet-mount samples of the gills revealed multiple cysts at the gill margins, each containing a motile organism. No other gill parasites were detected. A diagnosis of trematodiasis was made. The cysts were identified as encysted metacercariae of a digenetic trematode. We surmise that the large numbers of gill flukes combined with the stress of recent shipment likely caused the observed morbidity and mortality.