Maintaining osmotic balance with an aglomerular kidney

The gulf toadfish, Opsanus beta, is a marine teleost fish with an aglomerular kidney that is highly specialized to conserve water. Despite this adaptation, toadfish have the ability to survive when in dilute hypoosmotic seawater environments. The objectives of this study were to determine the joint role of the kidney and intestine in maintaining osmotic and ionic balance and to investigate whether toadfish take advantage of their urea production ability and use urea as an osmolyte. Toadfish were gradually acclimated to different salinities (0.5, 2.5, 5, 10, 15, 22, 33, 50 and 70 ppt (1.5%, 7.5%, 15%, 30%, 45%, 67%, 100%, 151% and 212% seawater)) and muscle tissue, urine, blood and intestinal fluids were analyzed for ion and in some cases urea concentration. The renal and intestinal ionoregulatory processes of toadfish responded to changes in salinity and when gradually acclimated, toadfish maintain a relatively constant plasma osmolality at environmental salinities of 5 to 50 ppt. However, at salinities lower (2.5 ppt) or higher (70 ppt) than this range, a significant deviation from resting plasma and urine osmolality as well as changes in muscle water content was measured, suggesting osmoregulatory difficulties at these salinities. The renal system compensates for dilute seawater by reducing Na+ reabsorption by the bladder, which allowed excess water to be excreted. In the case of hypersalinity, Na+ reabsorption was increased, which resulted in a conservation of water and the concentration of Mg2+, Cl-, SO(4)2- and urea. A similar pattern was observed within the gastrointestinal system. Notably, Mg2+, HCO3- and SO4(2-) were the dominant ions in the intestinal fluid under control and hypersaline conditions due to the absorption of Na+, Cl- and water. When exposed to dilute seawater conditions, the absorption of Na+ was greatly reduced which likely increased water elimination. As a result of decreased environmental levels and a reduction in drinking rate, Mg2+ and SO4(2-) in intestinal fluids under hypoosmotic conditions were greatly reduced. While urea did play a minor role in renal osmoregulation, toadfish appear to preferentially regulate Na+ and to some extend Cl- in urine and intestinal fluids.     

Cytopathological observations on renal tubule epithelium cells in common carp Cyprinus carpio under Trypanoplasma borreli (Protozoa: Kinetoplastida) infection

Cytological alterations in renal tubule epithelium cells of carp Cyprinus carpio infected with the blood flagellate Trypanoplasma borreli Laveran & Mesnil, 1901 were investigated during the course of a laboratory infection of a highly susceptible carp line. With the development of the parasitaemia, a hyperplasia of the interstitial renal tissue was induced, which resulted in a tubulus necrosis. Cytological changes were already seen in tubulus epithelium cells on Day 7 post injection (PI) of the parasite. The basilar invaginations of the cells fragmented and a swelling of mitochondria was noted. With increasing parasitaemia, on Days 14 and 21 PI, these changes progressed up to the loss of the basilar invagination and high amplitude swellings of mitochondria and deterioration of their internal membrane structures. Cells of the distal tubule segment reacted earlier and more rapidly than cells of the proximal tubule. The cytological alterations suggested a loss of function of the epithelum cells, which most likely resulted in impaired ionic and osmotic regulation of T. borreli-infected fishes. Our findings indicate that in response to the proliferation of the interstitial renal tissue cell structures of the renal tubule cells are altered quickly and in a progressive manner.     

Studies on renal excretion of potassium in the dik-dik antelope

1. In a study on the renal handling of potassium by the dik-dik antelope, plasma and urine samples were analysed for potassium, sodium and creatinine concentrations and osmolality during dehydration and intra-ruminal loading of potassium solutions. 2. The fractional excretion of potassium was 0.64 during the control period and rose up to as high as 2.3 during potassium loading. Urinary osmolality and potassium concentration decreased as the urine volume increased but the total amounts of potassium excreted were independent of urine volume. 3. Potassium loading led to a steady increase in its urinary excretion but a decrease in plasma potassium concentration was observed. This observation casts doubt on the hypothesis that alterations in potassium intake produce parallel alterations in plasma potassium concentration (which supposedly stimulates or depresses potassium excretion) and thereby maintain potassium homeostasis. 4. A possible alternative signal for increased potassium excretion following increased intake is discussed.     

Effects of dehydration,rehydration, and hyperhydration in the lactating and non-lactating black Moroccan goat

The effects of water deprivation, rehydration and hyperhydration were investigated in the black Moroccan goat (Capra hircus). Mean daily water intake was 46 ± 5 ml/kg in lactating and 36 ± 4 ml/kg in non-lactating black Moroccan goats, and milk production 21 ± 1 ml/kg. Mean urine excretion was 8 ± 2 ml/kg body weight in both groups, and the daily water losses via evaporation and feces were estimated at 23 ± 3 ml/kg during lactation and 28 ± 4 ml/kg during non-lactation. Forty-eight hours of water deprivation caused a body weight loss of 9% and 6% in lactating and non-lactating goats, respectively, and a drop of 28% in milk production with only a slight decrease in food intake. After rehydration, the elevated plasma osmolality as well as Na and total protein concentrations returned to basal values within 2–3 hr, indicating a rapid absorption of the ingested water, but urine excretion did not increase. After hyperhydration (10% of body weight), 46% of the load was excreted by the kidneys within 6 hr. In conclusion, black Moroccan goats have a low water turnover, and they can retain water upon rehydration but not store excess water after hyperhydration.     

Role of urea in the postprandial urine concentration cycle of the insectivorous bat Antrozous pallidus

Insectivorous bats, which feed once daily, produce maximally concentrated urine only after feeding. The role of urea as an osmolyte in this process was investigated in pallid bats (Antrozous pallidus) in the laboratory. Following a 24-h fast, plasma and urine were sampled before and 2 h after feeding in postprandial (PP) animals and before and 2 h after similar treatment without feeding in nonfed (NF) animals. Food consumption by PP animals and handling of NF animals had no effect on blood water content as measured by hematocrit and plasma oncotic pressure. Food consumption increased both plasma osmolality (P(osm)) and plasma urea (P(urea)) by as much as 15%. Food consumption also increased urine osmolality (U(osm)) and urine urea (U(urea)) by 50-100%. Feeding increased U(osm) regardless of changes in P(osm), and elevation of U(osm) resulted primarily from increased U(urea). In NF bats, P(osm) and P(urea) were unchanged, while U(osm) and U(urea) increased by as much as 25%. Again, increased U(osm) resulted primarily from increased U(urea). The PP urine concentration cycle of pallid bats resulted from increased urea excretion in response to apparent rapid urea synthesis. Bats rapidly metabolized protein and excreted urea following feeding when body water was most plentiful.     

Synthesis of superoxide dismutase derivative that specifically accumulates in renal proximal tubule cells.

Protection of tissues from oxygen toxicity is one of the major prerequisites to aerobic life. Since a wide variety of xenobiotics with prooxidant activity is excreted by the kidney, renal tubule cells should be protected from hazardous oxygen species. Because intravenously injected Cu/Zn-type superoxide dismutase (SOD) is rapidly excreted in the urine in its intact form, effective dismutation of superoxide radicals cannot be achieved in vivo by intravenously administered SOD. To scavenge superoxide radicals and inhibit their toxic effects in and around renal tubule cells, a hexamethylene-diamine (AH)-conjugated SOD (AH-SOD) was synthesized. When injected intravenously into the rat, (125)I-labeled AH-SOD disappeared from the circulation with a half-life of 3 min and accumulated in the kidney. After 30 min of administration, more than 80% of the radioactivity derived from AH-SOD was found to localize in the kidney without being excreted in the urine. Immunohistochemical examination revealed that, 60 min after administration, the major part of AH-SOD localized in renal proximal tubule cells. Kinetic analysis using right-side-out-oriented renal brush border vesicles revealed that AH-SOD bound to their membrane surface by some mechanism which was inhibited by AH but not by heparin and albumin. These results indicated that AH-SOD rapidly underwent renal glomerular filtration, bound to apical plasma membranes of proximal tubule cells, and localized in these cells for a fairly long time without being excreted in the urine. Thus, AH-SOD might permit studies on the role of superoxide radicals in and around renal proximal tubule cells.     

Resetting of osmoreceptor response as cause of hyponatraemia in acute idiopathic polyneuritis.

In a patient with hyponatraemia associated with acute idiopathic polyneuritis plasma concentrations of antidiuretic hormone increased when hypertonic saline was infused intravenously, and urine osmolality rose concomitantly. A water load was excreted normally, while the plasma remained extremely hypo-osmolal. It is concluded that osmoregulation was functioning normally but was set abnormally low, possibly owing to a disturbance of the peripheral volume receptors.     

Effect of daily hyperhydration on fluid-electrolyte changes in endurance-trained volunteers during prolonged restriction of muscular activity

The aim of this investigation was to evaluate the effect of a daily intake of fluid and salt supplementation on fluid and electrolyte losses in endurance-trained volunteers during prolonged restriction of muscular activity (hypokinesia). The studies were performed on 30 long-distance runners aged 23–26 who had a peak oxygen uptake of 65.5 mL/kg/min and had taken 13.8 km/d on average prior to their participation in the study. The volunteers were divided into three groups: The volunteers in the first group were placed under normal ambulatory conditions (control subjects), the second group of volunteers subjected to hypokinesia alone (hypokinetic subjects), and the third group of volunteers was submitted to HK and consumed daily 0.1 g sodium chloride (NaCl)/kg body wt and 26 mL water/kg body wt (hyperhydrated subjects). The second and third group of volunteers were kept under an average of 2.7 km/d for 364 d. During the pre-experimental period of 60 d and during the experimental period of 364 d sodium, potassium, calcium, and magnesium in urine and plasma were determined. Blood was also assayed for osmolality, hemoglobin, hematocrit, plasma volume, plasma renin activity and plasma aldosterone. Mean arterial blood pressure was also determined. In the hyperhydrated volunteers plasma volume and arterial blood pressure increased, whereas plasma osmolality, plasma renin activity, plasma aldosterone, hematocrit, hemoglobin concentration, and urinary excretion and concentrations of electrolytes in plasma decreased. In the hypokinetic volunteers, plasma volume and arterial blood pressure decreased significantly, whereas hematocrit values, hemoglobin concenfration, plasma osmolality, plasma renin activity, plasma aldosterone, and electrolytes in urine and plasma increased significantly during the experimental period. It was concluded that chronic hyperhydration may be used in minimizing fluid and electrolyte losses in endurance-trained volunteers during prolonged restriction of muscular activity.     

Physiological and biochemical responses to dietary protein in the omnivore passerine Zonotrichia capensis (Emberizidae)

We studied the physiological, biochemical and morphological responses of the omnivore sparrow Zonotrichia capensis, a small opportunistic passerine from Central Chile acclimated to high- and low-protein diets. After 4 weeks of acclimation to 30% (high-protein group) or 7% (low-protein group) dietary casein, we collected urine and plasma for nitrogen waste production and osmometry analysis, and measured gross renal morphology. Plasma osmolality and hematocrit were not significantly affected by dietary treatment, but urine osmolality was higher in the high-protein group than in the low-protein group. Kidney and heart masses were higher in animals acclimated to the high-protein diet. Mean total nitrogen waste was significantly higher in the high-protein group, but the proportions of nitrogen excreted as uric acid, urea and ammonia were unaffected by diet. Our data suggest that the response of Z. capensis to an increase in dietary protein content is through greater amounts of total nitrogen excretion and hypertrophy of kidney structures, without any modification of the proportion of excretory compounds.     

Water balance and arginine vasotocin in the cocooning frog Cyclorana platycephala (hylidae)

It is well established that forming a cocoon, for frog species capable of doing so, markedly reduces evaporative water loss; however, the capacity of cocooned frogs to maintain hydration during extended estivation is not well understood. The combined effects of long-term estivation and water loss were examined in the cocoon-forming species Cyclorana platycephala by assessing the hydration state of the frogs throughout a 15-mo estivation period. Frogs lost mass throughout the 15-mo period to a maximum of 36%+/-6.5% of their initial standard mass. Plasma osmolality reached maximal levels by the ninth month of estivation at 487 mOsm kg(-1) and then remained stable to the fifteenth month of estivation. Urine osmolality continued to increase to the fifteenth month of estivation, at which point plasma and urine concentrations were isosmotic. The use of bladder water to counter losses from circulation was indicated by the relatively slow rate of increase in plasma osmolality with mass loss and the progressive increase in urine osmolality. For estivating frogs, evidence was found for a possible threshold relationship between plasma osmolality and plasma arginine vasotocin (AVT) concentration. After estivation, plasma AVT concentrations decreased markedly after 15-mo estivators were placed in water for 2 h, suggesting that high levels of AVT may not be integral to rapid rehydration in this species.     

Effects of fresh and seawater ingestion on osmoregulation in Atlantic bottlenose dolphins (Tursiops truncatus)

Bottlenose dolphins (Tursiops truncatus) are marine mammals with body water needs challenged by little access to fresh water and constant exposure to salt water. Osmoregulation has been studied in marine mammals for a century. Research assessing the effects of ingested fresh water or seawater in dolphins, however, has been limited to few animals and sampling times. Nine 16- to 25-h studies were conducted on eight adult dolphins to assess the hourly impact of fresh water, seawater, and seawater with protein ingestion on plasma and urine osmolality, urine flow rate (ufr), urinary and plasma solute concentrations, and solute clearance rates. Fresh water ingestion increased ufr. Fresh water ingestion also decreased plasma and urine osmolality, sodium and chloride urine concentrations, and solute excretion rates. Seawater ingestion resulted in increased ufr, sodium, chloride, and potassium urine concentrations, sodium excretion rates, and urine osmolality. Seawater with protein ingestion was associated with increased ufr, plasma osmolality, sodium excretion, and sodium, chloride, potassium, and urea urine concentrations. In conclusion, bottlenose dolphins appear to maintain water and plasma solute balance after ingesting fresh water or seawater by altering urine osmolality and solute clearance. Ingestion of protein with seawater appears to further push osmoregulation limits and urine solute concentrations in dolphins.     

Modulation of tyramine signaling by osmolality in an insect secretory epithelium

The control of water balance in multicellular organisms depends on absorptive and secretory processes across epithelia. This study concerns the effects of osmolality on the function of the Malpighian tubules (MTs), a major component of the insect excretory system. Previous work has shown that the biogenic amine tyramine increases transepithelial chloride conductance and urine secretion in Drosophila MTs. This study demonstrates that the response of MTs to tyramine, as measured by the depolarization of the transepithelial potential (TEP), is modulated by the osmolality of the surrounding medium. An increase in osmolality caused decreased tyramine sensitivity, whereas a decrease in osmolality resulted in increased tyramine sensitivity; changes in osmolality of ±20% resulted in a nearly 10-fold modulation of the response to 10 nM tyramine. The activity of another diuretic agent, leucokinin, was similarly sensitive to osmolality, suggesting that the modulation occurs downstream of the tyramine receptor. In response to continuous tyramine signaling, as likely occurs in vivo, the TEP oscillates, and an increase in osmolality lengthened the period of these oscillations. Increased osmolality also caused a decrease in the rate of urine production; this decrease was attenuated by the tyraminergic antagonist yohimbine. A model is proposed in which this modulation of tyramine signaling enhances the conservation of body water during dehydration stress. The modulation of ligand signaling is a novel effect of osmolality and may be a widespread mechanism through which epithelia respond to changes in their environment. Drosophila; Malpighian tubule; cell volume regulation; G protein-coupled receptor; biogenic amines     

Copper induced alterations of biochemical parameters in the gill and plasma of Oreochromis niloticus

The main objective of this study was to determine the effects of copper exposure on copper accumulated in branchial tissue, gill Na+/K+-ATPase activity and plasma Na+, Cl-, osmolality, protein, glucose and cortisol, in Oreochromis niloticus. Fish were experimentally exposed to 40 and 400 microg L(-1) of waterborne copper and sacrified after 0, 3, 7, 14 and 21 days. Copper accumulation and Na+/K+-ATPase activity were determined in branchial tissue, whereas osmolality, Na+, Cl-, protein, glucose and cortisol concentrations were measured in plasma samples. Gill copper accumulation increased linearly with exposure time and concentration, whereas gill Na+/K+-ATPase activity was maximally inhibited after 3 days of exposure and showed a significant negative correlation with copper tissue levels. Plasma Cl- values decreased with time of exposure but only at 400 microg L(-1) of copper. Plasma Na+, protein and osmolality decreased with exposure time at the highest copper concentration tested, whereas at 40 microg L(-1) of copper this effect was only observed after 21 days of exposure. Plasma glucose and cortisol levels increased in a dose and time dependent manner, while showing complex fluctuations during the intermediate exposure times. In conclusion, copper induces an early maximum inhibition of gill Na+/K+-ATPase activity in O. niloticus. The subsequent slow decrease in ion plasma levels was related to compensatory mechanisms involving a non-specific stress response that appeared overcome at long-term exposures.     

Technics for the study of the renal excretion of free water in normal rats and those with experimental liver cirrhosis

Different techniques to measure free water excretion in rats, administered an oral water overload and with measurement of its ability to excrete it into the urine have been studied. When 30 or 50 ml/kg b.wt. were administered and the urine excreted in 3 h was collected, a decrease on the urinary osmolality (UOSM) was observed with respect to the baseline UOSM, which was similar in both overloads, although the percentage of the overload excreted was significantly greater with 50 ml/kg. However, the UOSM obtained was hypertonic as compared to plasma osmolality (POSM) indicating that this determination was not useful to study free water excretion. In a further study it was investigated if there was any period of time in which all the animals excreted hypotonic urine. However, results indicated that the period for excreting a maximally diluted urine was very variable in time. The best technique to study free water excretion in these animals was the collection of each spontaneously voided urine independently, to measure the minimal UOSM. When a 50 ml/kg water load was administered and the minimal UOSM was determined it was observed to be lower than POSM in all the animals indicating that this technique was useful to study this derangement in these animals.     

The effect of intravenous hypertonic saline infusion on renal function and vasopressin excretion in sheep.

Conscious Merino ewes were given an intravenous hypertonic sodium chloride load of 4 mmol.min-1 for 100 min. This resulted in increases in urine flow, sodium and potassium excretion and plasma sodium concentration and osmolality. Urinary vasopressin output and solute-free water reabsorption increased and plasma renin activity declined. Renal plasma flow and glomerular filtration rate (GFR) rose, as did the solute clearance. The change in urinary osmolality was related to the initial urine osmolality such that when the initial urine osmolality was high the urine became more dilute, and vice versa. Tubular sodium reabsorption increased but the fractional reabsorption rate fell. It is suggested that the increase in GFR was at least partly due to the increase in AVP and that the electrolyte loss can be accounted for by the increase in GFR without necessarily involving AVP or other hormonal effects at the tubular level.     

Prolonged Ethanol Ingestion Increases Renal AQP2 and AQP3 Expression in Adult Ratsand in Their Offspring

This study evaluates the effect of prolonged ethanol ingestion on the renal ability to concentrate urine. Suckling Wistar rats born to mothers given ethanol before and during gestation and suckling periods (ethanol-exposed offspring) were used and the results were compared with those obtained from offspring of dams given diets containing no ethanol. Comparisons were also made between progenitors with or without prolonged ethanol ingestion. Body and kidney weights; arginine-vasopressin (AVP) and aldosterone plasma levels; plasma, urine and renal papillary osmolality; urine outflow; kidney AQP2, AQP3 and AQP4 expression and diencephalon AVP mRNA expression were determined. As compared with control offspring, the ethanol-exposed offspring present i) lower body and kidney weights; ii) lower urine outflow; iii) higher renal AQP2 and AQP3 mRNA; iv) higher renal AQP2 protein content and v) higher urine and renal papillary osmolality. These changes were also observed in the ethanol-treated progenitors, although they were of smaller magnitude. Plasma osmolality, renal AQP4 mRNA, AVP plasma levels and diencephalon AVP mRNA expression were not affected by the ethanol treatment. Plasma levels of aldosterone were only significantly increased in the ethanol-exposed suckling rats. It is concluded that maternal ethanol ingestion before and during gestation and suckling periods affects the renal function of the offspring, up-regulating renal AQP2 expression by an AVP-independent mechanism. Ethanol-treated progenitors manifest similar renal changes, although of lesser magnitude than the offspring.     

An analogue computer simulation of cloacal resorption of salt and water from ureteral urine in birds

The transmural flow of NaCl and water occurring during the retrograde flow of ureteral urine into the coprodeum and large intestine of birds has been simulated by analogue computation. The purpose was to estimate whether a fraction of the urine (water) which in the dehydrated state is hyperosmotic to plasma can, in spite of this, be absorbed from the narrow space between the epithelium and the central faeces core. The values of urine flow, urine osmolality, osmotic permeability, net NaCl absorption rate, and solute-linked water flow determined by in vivo perfusion studies in the domestic fowl were used in the calculation. The cloacal sojourn of ureteral urine was found to result in a net water gain but at the expense of a hyperosmotic NaCl absorption. The model was further used to evaluate the quantitative influence of the system's parameters upon the fractional water absorption. This was found very sensitive to the urine osmolality, moderately sensitive to the urine flow and NaCl absorption rate and almost unaffected by the osmotic permeability of the coprodeum and large intestine within a reasonable physiological range. The change of the epithelial transport parameters from the normally hydrated to the dehydrated state resulted in a marked increase in water absorption.     

Plasma and urine levels of electrolytes, urea and steroid hormones involved in osmoregulation of cetaceans

Cetaceans are well adapted to their hyperosmotic environment by properly developed osmoregulatory ability. A question here is how they regulate water and mineral balances in marine habitats. In the present study, we determined blood and urine levels of various chemicals involved in osmoregulation, compared them with those in artiodactyls, and characterized the values in the whales. Blood and urine samples obtained from baleen whales of common minke (Balaenoptera acutorostrata), sei (B. borealis), and Bryde's whales (B. brydei), and toothed whales of sperm whales (Physeter macrocephalus) were analyzed for osmolality, major electrolytes, urea, steroid hormones and glucose. The urine osmolality and Na(+) concentrations in the cetaceans were much higher than those in the cattle. Furthermore, the cetaceans had 5 to 11-fold urea in plasma than the cattle, and 2 to 4-fold urea in urine. There were no significant difference in the plasma concentrations of corticosteroids between the cetaceans and the cattle. The present results indicate that the osmoregulatory parameters seem to be not affected by the reproductive stage and sex steroid hormones. The concentrations of urea in plasma and urine of the baleen whales were higher than those of the sperm whales, indicating a possibility that their osmoregulatory mechanisms may be correlated to their feeding habits. The present results suggest that cetaceans have unique osmoregulatory mechanisms by which they excrete strongly hypertonic urine to maintain fluid homeostasis in marine habitats.     

Homeostatic responses to water deprivation or hemorrhage in lactating and non-lactating Bedouin goats

Three lactating and three non-lactating black Bedouin goats were subjected to four days of water deprivation or to hemorrhage. Four days of water deprivation caused body wt losses of 32 and 23% and plasma volume losses of 30 and 34% in lactating and non-lactating goats respectively. Plasma osmolality increased 17 and 15% in lactating and non-lactating goats. Plasma arginine vasopressin concentration rose from about 5 pg/ml to a mean of 36 pg/ml. Plasma renin activity increased from about 0.7 ng/ml/hr to a mean of 3.45 ng/ml/hr in lactating and to 3.15 ng/ml/hr in non-lactating goats. At 4.5 hr post-rehydration plasma osmolality and plasma vasopressin concentration were back to normal in non-lactating, but still elevated in lactating goats. Plasma renin activity increased after rehydration. Rapid blood volume loss of 21-28% increased plasma vasopressin concentration to 16-35 pg/ml in non-lactating and to 70 or greater than 500 pg/ml in lactating goats. It is concluded that black Bedouin goats are well adapted to endure severe dehydration and rapid rehydration, but that they (especially lactating animals) react strongly to rapid volume depletion.     

Pathogenesis of acute viral disease induced in fish by carp interstitial nephritis and gill necrosis virus

A lethal disease of koi and common carp (species Cyprinus carpio) has afflicted many fish farms worldwide since 1998, causing severe financial losses. Morbidity and mortality are restricted to common carp and koi and appear in spring and autumn, when water temperatures are 18 to 28 degrees C. We have isolated the virus causing the disease from sick fish, propagated it in koi fin cell culture, and shown that virus from a single clone causes lethal disease in carp and koi upon infection. Intraperitoneal virus injection or bathing the fish in virus-containing water kills 85 to 100% of the fish within 7 to 21 days. This virus is similar to the previously reported koi herpesvirus; however, it has characteristics inconsistent with the herpesvirus family, and thus we have called it carp interstitial nephritis and gill necrosis virus. We examined the pathobiology of this disease in carp by using immunohistochemistry and PCR. We found large amounts of the virus in the kidneys of sick fish and smaller amounts in liver and brain. A rapid increase in the viral load in the kidneys was detected by using both immunofluorescence and semiquantitative PCR. Histological analyses of fish at various times after infection revealed signs of interstitial nephritis as early as 2 days postinfection, which increased in severity up to 10 days postinfection. There was severe gill disease evidenced by loss of villi with accompanying inflammation in the gill rakers. Minimal focal inflammation was noted in livers and brains. This report describes the etiology and pathology of a recently described viral agent in fish.