Allen G. McDowell; Andrew S. Kane; Renate Reimschuessel
Aquatic Pathobiology Center, University of Maryland, Baltimore, MD, USA; Center for Veterinary Medicine, Food and Drug Administration, Laurel, MD, USA
Approximately 40,000 juvenile largemouth bass, Micropterus salmoides, were maintained in a 45,000 liters recirculating raceway in a
Maryland aquaculture facility. The system was initially setup in May 1998, and the first shipment of 20,000 fish arrived on July 8, 1998. Preliminary indications
of a pathogenic disease within the population were observed in early September 1998 with a marked increase in mortality and signs of swollen abdomens and redness
around the head and operculum. Samples of brain, kidney, and liver tissue were taken on October 5, 1998 revealing the presence of Aeromonas hydrophila. The
fish were then treated with oxytetracycline for fourteen days. Following this time, the remaining 20,000 individuals were divided up into two separate systems
similar to the one previously described. For a brief period, the mortality rate decreased, suggesting the treatment had been successful. However, a high mortality
rate ensued and continued through December of the same year, with the peak mortality of 500 fish per day occurring between the end of November and early December.
Materials and Methods
Twenty juvenile largemouth bass from a Maryland aquaculture facility were delivered for gross examination to the Aquatic Pathobiology Center
in November 1998. Ten additional fish from the facility's supplier were also obtained. Necropsy examinations were performed and tissues preserved for histology in
10% neutral buffered formalin. Tissues of seventeen fish from the aquaculture facility and six additional fish from the facility's supplier were embedded in
paraffin and 6 micron sections were stained with hematoxylin and eosin.
Twenty juvenile largemouth bass, ranging from 15 to 28 grams in weight and from 103 to 122 mm in length, were examined. Five of these were
moribund; the others were clinically asymptomatic. The moribund fish were emaciated, had pale gills, and the distal portions of the kidneys were very enlarged,
with some areas measuring 0.8-1.0 cm thick compared to 0.3 cm in a normal kidney. The enlarged kidneys were a pale pinkish tan. In several fish, impression smears
of the enlarged portions of the kidneys revealed several golden oval-shaped eggs, approximately 50 μm, in the collecting ducts and within the kidney
parenchyma. In a few of the larger collecting ducts, live trematodes (measuring up to 200 μm in length) were observed. A haptor with multiple hooklets was
observed on the posterior end of the organisms. There was approximately 50% incidence of these parasites in the clinically asymptomatic fish from the aquaculture
facility. Further, a 50% incidence of the parasite was also observed in the ten clinically asymptomatic fish submitted from the supplier. Again, enlarged kidneys
were observed containing similar shaped and sized trematodes and trematode eggs. The extent of damage did not appear to be as significant as with the fish from
the aquaculture facility. There were no signs of any type of parasitic infection in other organs examined.
Of the seventeen specimens submitted for histology, fourteen provided evidence of a chronic parasitic infection in the trunk kidneys. Five
of these fish were moribund at the time of sacrifice; kidneys of these fish had extensive tubular necrosis. The tubular epithelium in some nephrons was
hypertrophic and basophilic, indicating an attempt to repair the chronic injury. Many tubules were also dilated, some containing blood cells. Glomerular injury
was also quite prominent, with a proliferation of the glomerular basement membrane in addition to a hyaline substance in the mesangium. Acute and massive
hemorrhage in the interstitium and within the glomeruli was present in kidneys of all the moribund fish. Trematode eggs were observed in moderate to high levels
in the smaller collecting ducts upstream in the kidney. Some were also observed in the interstitium as well as in the perirenal tissues. It appeared that juvenile
trematodes were present in the larger collecting ducts, while the adult trematodes were observed in the ureters. The collecting ducts and ureters were surrounded
by an infiltrate of macrophages and lymphocytes with fibroblastic proliferation.
The kidneys of the clinically asymptomatic fish had evidence of chronic renal injury with multiple granulomata containing eggs in the
interstitial tissues and fibrosis around some of the collecting ducts containing eggs or parasites. There was little glomerular damage.
Histological samples of the other organs appeared unremarkable with the exception of the liver in one moribund fish. This liver section
contained areas of necrosis and bacterial infection, most likely due to Aeromonas hydrophila, which had been cultured from other moribund fish. Edema was
also seen in muscle tissue from the moribund specimens, suggesting osmoregulatory failure.
Specimens from three of the six clinically asymptomatic source fish provided evidence for a parasitic infection in the kidneys. Inflammation,
necrosis, and fibrosis were observed within the kidney tissue but to a lesser extent than the fish from the aquaculture facility. Trematodes and eggs were again
found inside the collecting ducts.
The trematode species discussed in this case most closely resembles Acolpentron ureteroecetes, which was first reported by Fischthal
and Allison1. No other cases have been documented since the 1940's. This species is unusual in that it is reported to infect the ureters of basses.
Since it is a monogenetic trematode, it requires only one host in its life cycle. Thus monoculture of bass in recirculating systems provides the species with
ideal conditions for reproduction2. The eggs of this species hatch after 6-9 days and the adults can reach a size of almost a millimeter in
Based on our observations in this report the renal damage caused by this trematode species can be severe. In large numbers the parasite is
able to cause significant damage with the haptor as well as feeding on the surrounding tissue. The eggs also appear to cause injury to the surrounding tissue when
migrating into the collecting ducts with its spine or hooklet. This condition predisposes the host to opportunistic pathogens such as Aeromonas
Since the recirculating system that the bass were maintained in had never been used for any other purpose, and the parasites were observed in
samples from the supplier, it is safe to assume that the trematodes came with the fish from the supplier. As intensive aquaculture of largemouth bass is
increasing this parasite may become a more common clinical finding. At this time there is no approved effective treatment for these internal trematodes.
1. Fischthal JH, LH Allison. 1940. Acolpentron ureteroecetes n. g., n. sp. a Monogenetic Trematode from the Ureters of Black
Basses. Journal of Parasitology 27:34-35.
2. Thoney DA, WJ Hargis Jr. 1991. Monogenea (platyhelminths) as hazards for fish in confinement. Annual Review of Fish