A Rickettsia-like Organism Found in the Blue-eyed Plecostomus, Panaque suttonii
Patricia M. Dennis, BS, MSL; Lester Khoo, VMD; Gregory A. Lewbart, MS,
North Carolina State University, College of Veterinary Medicine,
This report describes a rickettsia-like organism (RLO) in the tissues
of the blue-eyed plecostomus, Panaque suttoni. Although there are limited reports of
rickettsia-like organisms in fishes (sea raised salmon), this represents the first known
reported case of an RLO infection in a tropical freshwater fish. Both histopathology and
cytopathology revealed numerous, small, discrete, basophilic organisms within the cytoplasm
of the macrophages in the spleen, heart, kidney, and sometimes liver as well as within the
cytoplasm of circulating monocytes.
Blue-eyed plecostomus are popular fish among dedicated fresh water
aquarists. These fish are imported from South America and often arrive in the United States
in poor condition. High mortality occurs within the three week period following
transcontinental shipping. Surviving fish are often anorectic, cachectic and frequently
succumb to secondary bacterial infections.
Two moribund blue-eyed plecostomus from Columbia were examined in an
attempt to determine the cause of morbidity. Blood samples were drawn from both fish by
intracardiac puncture. The samples were centrifuged and cytological preparations of the
buffy coats were made and stained with Giemsa (Wohlbach's modification) (Sheenan and
Hrapchak, 1980). The fish were euthanized with an overdose of tricane methanesulfonate
(300mg/L) and tissue samples were fixed in 10% neutral buffered formalin and in Trump's
fixative. Large numbers of intracytoplasmic organisms resembling rickettsia were found in
the cells of the mononuclear phagocytic system.
Examination of the buffy coat smears revealed moderate numbers of
monocytes filled with numerous small (0.5um or less) basophilic intracytoplasmic organisms.
These organisms appeared to replace the foamy cytoplasm of the monocytes and peripherally
displaced the nucleus. Viewing the organisms at higher magnification revealed that many of
the organisms had a dumb-bell shape.
Histopathological examination of tissues showed that the most
significant changes were in the spleen, heart and kidney. There was a diffuse, severe
granulomatous splenitis, pericarditis, and a mild to moderate granulomatous interstitial
nephritis. The normal splenic architecture was replaced by islands and sheets of large,
round to polyhedral, vacuolated cells that are separated by mats of pale eosinophilic
fibrillar material. These vacuolated macrophages were markedly distended by small basophilic
organisms (with H&E stain; Sheenan, 1980) within the vacuoles. These organisms were
negative with Gram stain (Raphael, et al., 1976) and Fite's acid fast stain (tuna, 1968).
Results of the Macchiavello's stain (Brancroft and Stevens, 1982) and Pinkerton's stain
(tuna, 1968) for rickettsia were equivocal, with some organisms staining bright red
(positive) and others blue. Large numbers of similarly affected macrophages were found
peripheral to the epicardium and interspersed within the pericardial stroma. Individual
macrophages or small aggregates containing the organisms were scattered within the renal
interstitium, along with occasional small tufts of fibrin only a few Kupffer cells in the
liver were found to contain the small basophilic organisms.
Transmission electron microscopy revealed macrophages within the renal
interstitium that contained organisms and had characteristics suggesting metabolic activity.
These cells were characterized by moderate numbers of mitochondria and had prominent
interdigitating membranes. Degenerative changes such as swollen mitochondrial membranes,
small vacuoles and sometimes karyorrhectic nuclei were occasionally associated with these
The organisms within the macrophages occurred singly or in groups of
four in the center of membrane bound vacuoles which had a thin outer membrane followed by a
thicker, electron-lucent layer with a fine lacy network. The organisms had an adherent thick
trilaminar cell wall. Within the cell wall there was a 0.07 to 0.09 um thick electron dense
layer with an irregular poorly delineated inner border subjacent to the trilaminar membrane.
Electron dense areas were found in the center of the organisms and occasionally
electron-lucent vacuoles connected by fibrillar strands were seen. Several organisms
appeared to be degenerate and consisted of roughly concentric rings of electron dense
Few reports of RLOs in fishes can be found in the literature, and most
of these reports are of infections in marine salmonids (Fryer, Lannan, Garces, Larenas and
Smith, 1990; Garces, Larenas, Smith, Sandino, Lannan and Fryer, 1991; Branson and Nieto
Diaz-Munoz, 1991; Rodger and Drinan, 1993; Speare and Armstrong, 1992). There is one report
of RLO in dragonets, Callionymus 1yra L. (Davis, 1986) and another in Tetrodon fahaka
(Mohamed, 1939). All of these reports indicate that significant pathological lesions are
caused by the presence of the rickettsial organisms.
The histopathology seen in these fish differed somewhat from the
histopathology reported with salmonid rickettsia. No vasculitis or thrombosis was seen in
the blue-eyed plecostomus but these lesions have been reported in salmonid rickettsial
disease (Turnbull, 1993; Speare and Armstrong, 1992; Branson and Nieto Diaz-Munoz,
Lesions similar to those seen with salmonid rickettsial disease included
the granulomatous inflammation in the spleen, kidney and pericardium. Necrosis and
effacement of the hematopoietic elements seen in these fish have also been reported in
salmonid rickettsial septicemia. Staining characteristics of the organisms in the
plecostomus were similar to those that have been reported for rickettsia in salmonids, that
is, Gram stain, acid fast and Macchiavello's negative and staining blue with Giemsa and
hematoxylin and eosin.
The morphological and staining characteristics of the organisms found
in the mononuclear phagocytic cells of these fish are consistent with descriptions of
rickettsia identified in other fishes. It is possible that infection with this
rickettsia-like organism is responsible for the morbidity seen in these fish. Further
research will include attempts to isolate the organism and determine its clinical
The authors would like to express their thanks to Dr. John Fryer, Dr.
Michael Levy, and Dr. Ed Noga for their advice and assistance.
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