Acute respiratory distress in wild caught round goby (Neogobius melanostomus)
Heindrich N. Snyman, Michelle Wodzak, Kerri Nielsen, Deborah Pakes
Animal Health Laboratory, University of Guelph, Kemptville, ON (Snyman); University of Toronto, Scarborough, ON (Pakes, Wodzak, Nielsen)
AHL Newsletter 2019;23(4):11-12.
In mid-July, 30 adult wild round goby (Neogobius melanostomus) were collected from Lake Erie along the shore of Kingsville, ON. These fish were destined to establish a research colony at a local University. At the research facility, fish were housed in groups of 6-7 individuals in re-circulating 20 gallon tanks with aerators, rocky substrate, and PVC tubes for hides. Water temperatures were maintained at 20-24°C and fish were exposed to a ~ 1% salt bath upon intake. Shortly after introduction to the tanks, fish started to exhibit rapid breathing, irregular darting behavior, and clustering around the intake flow pipe. This was followed by progressive worsening of the respiratory distress and a decrease in appetite ultimately culminating in the mortality of 5 fish ~ 1 week after initial introduction. Water quality parameters were within normal limits (ammonia levels were 0-0.25 ppm, nitrite levels <1ppm, and water pH 7.8-8.2). Three fish were fixed whole in formalin and submitted to the Animal Health Laboratory for analysis.
Histological evaluation revealed widespread and marked hyperplasia of the lamellar gill epithelium as well as the epithelium lining branchial and oral cavity in all three fish with extensive blunting and fusion of secondary lamellae. Scattered throughout the affected foci were numerous irregular, round to oval, 50 to 200 µm diameter, subepithelial ciliated protozoal cysts that contained prominent 1-2 µm thick pale outer eosinophilic hyaline walls. Cysts contained a single large 20 × 120 µm crescent-shaped, deeply basophilic eccentric macronucleus and abundant finely granular to vacuolated basophilic cytoplasm with large amounts of phagocytized erythrocytes and cellular debris. The size, histomorphology, and particularly the subepithelial localization are highly characteristic of Ichthyophthirius multifiliis.
I. multifiliis is a well-known ciliated protozoal fish pathogen and causes the characteristic fish disease referred to as “ich” or “white spot disease”. It is most common in ornamental tropical freshwater fish but can affect any freshwater fish species. Along with temperature, stress plays a major role in epidemics and the associated stress of handling, capture, and shipping likely played a significant role in the initiation of disease in this colony. Mortality rates are highest in warmer water (15 - 25°C) but significant disease can still occur in temperatures as low as 10°C.
Treatment of this pathogen is often very problematic as it exists as both free-living (tomont) and parasitic (trophont and theront) life stages. Briefly, the ich trophont (feeding stage) feeds in a capsule formed in the skin or gill epithelium which forms the characteristic “white spots” that lends this disease its name. After feeding, the parasite breaks through the epithelium and is free in the water column during which time it forms a capsule and divides (tomont stage). The tomont capsule is very sticky and can adhere to nets, substrate, plants, etc. Tomites subsequently break through the capsule and form motile, infective theronts which then reinfect the gill epithelium. Disease is further complicated by a particularly rapid and prolific rate of replication with the whole life-cycle being completed in as little as 3-8 days at optimum temperatures of 23-24°C and with a single tissue trophont being able to produce up to 2000 re-infecting theronts.
Figure 1. A Histology of gills with widespread lamellar epithelial hyperplasia with lamellar fusion (asterisks). B Sub-epithelial ciliated protozoal cysts (arrows) contain a characteristic crescent-shaped macronucleus (asterisks) with abundant phagocytized erythrocytic and cellular debris.
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