The chemistry of ammonia in water is central to its toxicity. Total ammonia nitrogen exists in equilibrium between an ionized form, the ammonium ion, and a far more dangerous un-ionized form, NH3, which diffuses readily across the gills and skin. The balance between the two forms is strongly pH dependent: at pH 6 or 7 ammonia is mostly ionized, while at pH 12 or above virtually every molecule becomes un-ionized. The research team exploited this relationship carefully. Using ammonium chloride from Merck as the toxicant source, they calculated the exact salt doses needed to produce target concentrations of un-ionized NH3 at the controlled laboratory conditions of pH 7.8 and 28 degrees Celsius, applying equilibrium equations that link the pKa of ammonia to water temperature. Five nominal concentrations were selected, spanning 0.1 to 10 milligrams of NH3 per litre, corresponding to ammonium chloride doses from 0.0074 to 0.7404 grams per litre.

Crucially, the authors designed the exposure regime to mimic reality rather than a textbook acute test. In natural water bodies and un-aerated culture ponds, ammonia does not arrive as a single pulse; it accumulates daily from sewage outfalls, fertilizer leaching and decomposing feed. Accordingly, adult wild-type zebrafish, 2.5 to 3.5 centimetres long and weighing 250 to 300 milligrams, were acclimatized for twelve days and then exposed repeatedly: ammonium chloride was added every 24 hours for four consecutive days, so that by the fourth day the cumulative dose in the lowest tank reached 0.4 milligrams per litre of NH3 and higher tanks received proportionally more. The tanks held ten fish each in ten litres of seasoned municipal water, were covered with mesh, and were deliberately left without artificial aeration to reproduce the oxygen-limited conditions typical of stagnant ponds. Bioavailable ammonia was verified every 18 hours using Nessler’s colorimetric method, confirming that the toxicant genuinely accumulated despite its volatility; in the 1 milligram per litre tank, measured NH3 climbed from 0.926 to 4.027 milligrams per litre across the four days.

The lethality results were sobering. Probit analysis of the mortality data yielded a median lethal concentration, LC50, of just 0.89 milligrams of NH3 per litre under this repeated-exposure, low-oxygen regime. Kaplan-Meier survival curves told the same story graphically: cumulative survival in the 1 milligram per litre group remained above 0.6 at 72 hours but fell below 0.4 by 96 hours, while all fish exposed to 5 milligrams per litre died within 48 hours and the 10 milligram per litre group was wiped out within 24 hours. No mortality occurred in the control or in the two lowest exposure groups. These values contrast sharply with earlier reports of LC50 figures around 360 milligrams of ammonium for zebrafish embryos and roughly 311 milligrams for adults, a difference the authors attribute to the combination of continuous contamination, the more permeable un-ionized form, and oxygen deprivation. In short, fish in poorly aerated, repeatedly contaminated water are far more fragile than single-dose laboratory benchmarks suggest.

Water chemistry shifted in parallel with the toxicant. Dissolved oxygen dropped in every ammonia-exposed tank, from a control range of 3.4 to 3.7 milligrams per litre down to as low as 1.5 milligrams per litre in contaminated tanks, while pH rose steadily with each daily dose. This matters because hypoxia and elevated pH both push the ammonia equilibrium toward the un-ionized, tissue-permeable form, creating a self-reinforcing spiral of toxicity. Previous work on Japanese flounder has shown that hypoxic conditions amplify ammonia-induced gill damage, reactive oxygen species production and blood cell loss, and the present findings are consistent with that mechanistic picture playing out in a small tropical model fish.

The behavioural component of the study is where the work becomes genuinely novel. The researchers selected seven well-characterized zebrafish behaviours, shoaling, playing, courtship, resting, feeding, aggression and fighting, and stress movement, and quantified them with unusual rigour. Each day was divided into four 90-minute observation windows, two before and two after the afternoon toxicant dose, and each window was subdivided into eighteen 5-minute segments. For every segment the team counted how many individuals displayed each behaviour, producing a cumulative number of individuals, or CNI, score that captures both the breadth and the persistence of each activity. This dense sampling allowed the authors to track behavioural trajectories across the full 96 hours and to separate genuine toxicant effects from normal diurnal rhythms.

The results read like a progressive shutdown of normal fish life. In control tanks, playing dominated the behavioural repertoire, accounting for over 40 percent of afternoon activity and rising to about half of all behaviour in the evening. Under sub-acute exposure of 0.1 and 0.5 milligrams per litre, playfulness declined gradually, from roughly 60 percent to around 30 percent of the behavioural regime. At 1 milligram per litre and above, the collapse was dramatic: evening playfulness that stood at 65.6 percent after the first exposure fell to a mere 1.7 percent after three consecutive days, and by the fourth day stress movements, hyperactive and zig-zag swimming followed by listless floating near the surface, occupied 95 to 100 percent of all observed behaviour. Feeding, which ranged from 12 to 22 percent in controls, dropped below 7 percent in every exposed group, and aggression, normally a marker of social hierarchy, faded from moderate levels to essentially zero at the highest doses as fish became immobilized.

Statistical validation came from Mann-Whitney U tests comparing behaviour distributions across groups, plotted as dot plots with medians in GraphPad Prism. Shoaling, playing, feeding, aggression and stress movement all emerged as significant indicators of ammoniacal stress by the fourth day, with playing showing a highly significant drop at 1 and 5 milligrams per litre, p below 0.001, and stress movement showing the mirror-image significant rise. Interestingly, courtship and resting behaviours, which in controls followed a clear evening rhythm, were suppressed so early and so completely by ammonia that the authors deprioritized them as indicators. The team also noted an appetite-loss signature consistent with hypothalamic signalling and cortisol-driven stress responses documented in earlier zebrafish studies, and an energy-conservation logic in which toxicant-exposed fish abandon costly locomotion to preserve metabolism for basic survival functions.

The practical implications extend well beyond the laboratory. In intensive and semi-intensive aquaculture, undigested protein from overfeeding and fish faeces continuously generate ammonia inside closed systems, and in natural lentic water bodies industrial effluents, agricultural runoff and organic waste do the same, often without any mechanical aeration. The study demonstrates that under exactly those conditions, the tolerance threshold of a sensitive freshwater fish falls to levels below 1 milligram per litre of un-ionized ammonia, far lower than conventional single-dose toxicity values would suggest. Because the behavioural shifts, declining play and shoaling, reduced feeding, fading aggression and rising stress movement, appear progressively as contamination moves from sub-acute to acute, farmers and field biologists could in principle read these signs in real time and intervene before mass mortality occurs, simply by observing the fish.

The authors are careful to frame this as a foundation rather than a conclusion. The behavioural expression observed here is the outward face of neurophysiological change, and the team points toward future work linking the observed pattern shifts to tissue damage, neuroendocrine and haematological alterations, and genetic markers of ammonia stress. Such mechanistic follow-up could turn the behavioural matrix constructed in this study, which maps how each behaviour rises or falls with concentration and exposure duration, into a predictive tool for environmental monitoring. For now, the message is clear and actionable: in oxygen-poor, repeatedly contaminated water, ammonia is deadlier than standard toxicity tables imply, and the fish will tell you so, through their behaviour, long before they die.

Subject of Research: Ammonia toxicity tolerance and behavioural stress responses in zebrafish as a freshwater model

Article Title: Ammonia toxicity tolerance and behavioural response in Zebrafish

Article References: Mondal, N., Kaur, J., & Ghosh, A. (2025). Ammonia toxicity tolerance and behavioural response in Zebrafish. Discover Toxicology, 2(1), Article 13. https://doi.org/10.1007/s44339-025-00028-8

Image Credits: AI Generated

DOI: 10.1007/s44339-025-00028-8

Keywords: zebrafish, ammonia toxicity, LC50, un-ionized ammonia, behavioural indicators, aquaculture, water pollution, ecotoxicology, stress movement, dissolved oxygen, Kaplan-Meier survival, freshwater fish

The research, led by Marianne Lappalainen of the Royal Veterinary College in London together with colleagues from referral centers across the United Kingdom, France, Belgium, Ireland, Switzerland, Spain, and Italy, took a retrospective approach. The team contacted veterinary oncology services across Europe and combed their medical records for dogs in which advanced imaging reviewed by a board-certified radiologist had been combined with cytological or histological confirmation of a primary intrapelvic lipoma. Crucially, the investigators excluded any mass that had merely spread into the pelvis from elsewhere, ensuring that every case represented a tumor that truly originated within the pelvic canal. Because the condition is so rare, the authors set no time limit on case selection, gathering examples across an open-ended window. Thirteen dogs made the final cut.

The affected dogs were, in keeping with what is known about lipomas in general, middle-aged to older animals. The median age at diagnosis was nine years and five months, with a range from roughly seven and a half to twelve years, and the median body weight was 24.45 kilograms. English Springer Spaniels topped the breed list with three of the thirteen cases, followed by crossbreeds and Labrador crosses; single cases appeared in Labrador Retrievers, Standard Poodles, English Cocker Spaniels, West Highland White Terriers, Golden Retrievers, and Smooth Collies. Interestingly, males slightly outnumbered females in this cohort, with a male-to-female ratio of four to three—a finding that runs counter to earlier UK primary-care studies suggesting that neutered females carry a higher risk of lipomas overall. The authors caution that the small sample size may explain the discrepancy.

The clinical picture was dominated by the bowel. Tenesmus—persistent, straining attempts to defecate—was recorded in nine of the thirteen dogs, or 69.2 percent. Altered stool shape, the classic signature of a mass compressing the colon and rectum into a narrowed channel, appeared in six dogs (46.15 percent), while dyschezia, painful or difficult defecation, affected four dogs (30.7 percent). Two dogs struggled to urinate, and one presented with constipation and vomiting. Remarkably, one lipoma was discovered incidentally during a routine rectal examination in a dog whose owners had noticed nothing wrong. For the rest, clinical signs had been present for a median of just twenty-one days before diagnosis, ranging from five to 180 days—a shorter interval than the seven and a half months reported in earlier work on benign and low-grade intrapelvic tumors, though individual dogs in this series did endure signs for up to six months.

Computed tomography emerged as the undisputed diagnostic hero of the study. Every dog underwent a CT scan, and in twelve of the thirteen cases (92.3 percent) the mass displayed the hallmark fat attenuation that essentially announces a lipoma on the scan. The thirteenth dog’s mass showed a mixture of fat and soft-tissue attenuation, yet histopathology subsequently confirmed it as adipose tissue all the same. In one case the CT report described the mass as infiltrative, a detail that would later prove prognostically important. On cross-sectional images, the tumors showed a striking positional preference: of the ten dogs in which the location was described, nine had lipomas on the left side of the pelvic canal, shoving the colon and rectum to the right. Where measurements permitted, the team calculated tumor volumes using the ellipsoid formula, finding a median volume of 161.2 cubic centimeters and dimensions ranging from about 5 by 3 by 5 centimeters in a Springer Spaniel to a remarkable 6 by 3 by 23 centimeters in a Labrador Retriever.

The imaging modality matters because the pelvic bones themselves conspire against simpler tools. Abdominal radiography and ultrasonography, the workhorses of general practice, struggle to visualize masses in this region because the surrounding bone obscures detail. Ultrasound can suggest a lipoma when it reveals an oval, well-defined, encapsulated mass with internal striations, hyperechogenicity, and a telltale absence of blood flow on color Doppler examination, while liposarcomas tend to appear heterogeneous with increased vascularity and invasion of nearby structures. But CT provides something neither can: precise information about a mass’s size, extent, and relationship to surrounding organs, which is indispensable for surgical planning. In human medicine, magnetic resonance imaging is often preferred for its superior soft-tissue contrast, but in veterinary oncology MRI remains rarely used for staging or surgical planning due to time and cost. The study’s findings suggest that for these patients, a CT scan alone can deliver a confident radiological diagnosis, potentially eliminating the need for preoperative cytohistopathology in some cases.

All thirteen dogs underwent surgery, via either a perineal approach in two cases or a caudal ventral celiotomy in seven; the surgical reports for four dogs were unavailable for review. No intraoperative complications were reported anywhere in the cohort, and the only perioperative problem was a single wound dehiscence two days after surgery, which was repaired without further incident. Pathology confirmed the benign nature of the tumors: all initial histopathology reports described well-differentiated adipocytes, the microscopic signature of a lipoma. Yet the very blandness of these tumors creates a diagnostic trap, because lipomas histologically resemble normal subcutaneous fat and often lack clear borders, making surgical margins difficult or impossible to evaluate. Margins went unreported in twelve of the dogs, a gap the authors attribute to the difficulty of distinguishing neoplastic from normal adipocytes, the benign expansile nature of the growths, or inadequate sampling.

The outcomes were largely encouraging, with one sobering caveat. Four dogs remain alive with a median follow-up of 507 days after diagnosis. Five dogs were euthanized, but only one of those deaths was attributed to the lipoma itself; the median time to death among the four with known dates was 889 days, ranging up to 1,740 days. Kaplan–Meier analysis, which accounts for the heavily censored data produced by long survival and lost follow-up, yielded an overall survival time of 1,020 days—nearly three years. The caveat concerns recurrence: three dogs (23.1 percent) experienced regrowth at a median of 220 days after their initial surgeries, a rate the authors note is uncomfortably close to figures reported for infiltrative lipomas, a more aggressive variant whose recurrence rates after excision range from 36 to 50 percent even with aggressive surgery. One recurrent case was confirmed histologically as an infiltrative lipoma, matching the earlier CT suspicion of infiltration.

Even more striking was the fate of one dog that recurred twice. Cytology at its third relapse suggested malignant transformation into a soft tissue sarcoma, and the dog was switched to metronomic chemotherapy—oral low-dose cyclophosphamide combined with the anti-inflammatory drug meloxicam. The authors weigh several explanations: an initial misdiagnosis, an original mixed tumor only partially characterized, or the development of sarcoma following repeated surgical injury, a phenomenon with tentative support in the literature linking tissue trauma to sarcoma formation. A fourth dog suspected of recurrence turned out to have intra-abdominal adhesions rather than tumor, though without necropsy the team could not definitively rule out regrowth. The episode underscores a broader point: not every returning symptom means the cancer is back.

For veterinary clinicians, the practical takeaways are concrete. A lipoma belongs on the differential diagnosis list for any intrapelvic mass in a dog, and CT imaging can reliably identify its fat content and map its relationship to the colon, rectum, and urinary tract. Surgical excision, though demanding specialized expertise that may exceed the capabilities of general practice, offers a favorable prognosis with low complication rates and survival approaching three years—provided owners and veterinarians remain vigilant for recurrence, particularly when the initial imaging hints at infiltrative behavior. The authors acknowledge the limits of their work: a small sample, retrospective design, referral-only population, and loss of follow-up in nearly a third of cases all constrain the statistics. Larger studies with longer follow-up, including cases of infiltrative lipoma, will be needed to refine the prognosis and clarify how different treatments shape outcomes. Until then, this rare and easily overlooked tumor has a documented playbook—and for most dogs, a genuinely good ending.

Subject of Research: Clinical presentation, CT diagnosis, surgical treatment, and outcomes of intrapelvic lipomas in dogs

Article Title: Presentation, treatment, and outcome of lipomas of the pelvic canal in dogs

Article References: Lappalainen, M., Gomez Fernandez, J., Béguin, J., Fournier, Q., Hayes, A., Bing, Y., Rodiño-Tilve, V., Bazelle, J., Borrego, J., & Desmas-Bazelle, I. (2025). Presentation, treatment, and outcome of lipomas of the pelvic canal in dogs. Veterinary Oncology, 2(1), Article 28. https://doi.org/10.1186/s44356-025-00044-1

Image Credits: AI Generated

DOI: 10.1186/s44356-025-00044-1

Keywords: lipoma, dogs, pelvic canal, computed tomography, veterinary oncology, surgical excision, tenesmus, dyschezia, infiltrative lipoma, recurrence, Kaplan-Meier survival, case series