Iron storage disease (ISD) is now recognized as a serious clinical disorder acquired by two species of browsing rhinoceroses, the African black (Diceros bicornis) and the Asian Sumatran (Dicerorhinus sumatrensis) rhinoceroses, when displaced from their natural habitats. The most complete knowledge of ISD comes from studies of the black rhinoceros, but the Asian species is also at risk. Sumatran rhinoceroses housed in traditional zoological settings outside of range countries have suffered significant morbidity and mortality potentially related to ISD induced by diet and/or other confinement conditions. With so few animals in captivity, very little information exists on iron loading in the Sumatran rhinoceros. To better characterize the problem, we retrospectively compared captive management conditions of Sumatran rhinoceroses housed under traditional zoological care with those in two native sanctuary environments. In general, zoo rhinoceroses are offered a paucity of plants and browse species compared with their sanctuary and wild counterparts managed in native rainforest habitats. Iron analyte levels and limited histopathologic observations in these populations suggest variable tendencies to overload iron, dependent upon differences in managed diet and individual food preferences. More detailed investigation of these markedly dissimilar ex situ populations is warranted to better understand the role of nutrition and other conditions affecting iron loading in browser rhinoceroses.

Phenolic compounds are bioactive chemicals found in all vascular plants but are difficult to characterize and quantify, and comparative analyses on these compounds are challenging due to chemical structure complexity and inconsistent laboratory methodologies employed historically. These chemicals can elicit beneficial or toxic effects in consumers, depending on the compound, dose and the species of the consumer. In particular, plant phenolic compounds such as tannins can reduce the utilization of iron in mammalian and avian consumers. Multiple zoo-managed wild animal species are sensitive to iron overload, and these species tend to be offered diets higher in iron than most of the plant browse consumed by these animals in the wild and in captivity. Furthermore, these animals likely consume diets higher in polyphenols in the wild as compared with in managed settings. Thus, in addition to reducing dietary iron concentrations in captivity, supplementing diets with phenolic compounds capable of safely chelating iron in the intestinal lumen may reduce the incidence of iron overload in these animal species. It is recommended to investigate various sources and types of phenolic compounds for use in diets intended for iron-sensitive species. Candidate compounds should be screened both in vitro and in vivo using model species to reduce the risk of toxicity in target species. In particular, it would be important to assess potential compounds in terms of 1) biological activity including iron-binding capacity, 2) accessibility, 3) palatability, and 4) physiological effects on the consumer, including changes in nutritional and antioxidant statuses.

Iron overload disorder afflicts captive rhinoceros but has not been documented in the wild. The specific cause for the disorder has not been identified but is likely associated with diet and management. Compared with wild counterparts, captive rhinoceros eat diets containing more iron, have greater fat stores, and exercise less. It has been suggested that the problem may be linked to development of insulin resistance in the captive population. Given that controlled experiments with sufficient numbers of rhinoceros are logistically not possible, an equine model was used to look for a relationship between iron status and insulin resistance; the nutritional requirements of horses are used as a guide for rhinoceros, because they have similar gastrointestinal tracts. Sixteen horses were tested to determine blood insulin responses to an oral drench of dextrose (0.25 g/kg bodyweight) and a meal of pelleted corn (1.5 g/kg bodyweight). Fasting blood samples were taken 30 and 0 min before administration. Further blood samples were taken every 30 min for 4 hr after administration to determine peak insulin and total area under the insulin curve (AUC). Fasting samples were tested for serum ferritin concentrations. Correlations were determined between ferritin and peak insulin concentrations and insulin AUC after administration of oral dextrose and pelleted corn. The strongest correlation was between ferritin and insulin AUC after dextrose administration (r=0.61; P=0.01) followed by AUC after feeding a meal of pelleted corn (r = 0.60; P = 0.01), with the correlation for peak insulin being 0.53 (P = 0.03) after dextrose administration and 0.56 (P = 0.02) after pelleted corn. When evaluating responses by gender, a significant correlation existed only for females, influenced by one insulin resistant individual. These data suggest a potential link between insulin resistance and body stores of iron and also suggest that approaches to reduce the susceptibility to insulin resistance should be incorporated into management of captive browsing rhinoceros.

Hemochromatosis in bottlenose dolphins (Tursiops truncatus) is associated with high postprandial plasma insulin levels, suggestive of insulin resistance. In humans, insulin resistance is associated with liver pathologies, including excessive iron deposition and nonalcoholic fatty liver disease. Dolphin liver tissues, in addition to excessive iron storage, were evaluated for other pathologies supportive of underlying insulin resistance. Archived liver tissues collected postmortem during 1985–2010 from 18 dolphins (median age 27.9 yr, range 0.7–51.4) that were part of the Navy Marine Mammal Program's managed collection were assessed for the presence and severity of hemosiderin deposition, fatty liver disease, and hepatitis. Demographics, clinical pathology values, and percentage weight loss were compared among dolphins with and without these changes. Twelve (66.7%) dolphins had mild to moderate hemosiderin deposition, 7 (38.9%) had mild to severe fatty liver disease, and 11 (61.1%) had mild to moderate hepatitis. Of the 12 dolphins with hemosiderosis, deposition occurred in the Kupffer cells among 11 (91.7%). Dolphins with fatty liver disease were more likely to have higher postprandial serum hyperglycemia (>140 mg/dl), leukocytosis (>11,000 cells/μl), and hyperglobulinemia (>3.5 g/dl). Unlike in many nonhuman terrestrial animals, fatty liver disease was not associated with rapid weight loss or hypoglycemia. Interestingly, there were no significant associations among dolphins with hemosiderosis, fatty liver disease, and hepatitis. This study supports that both hemochromatosis and fatty liver disease were present in the dolphin study population, and histopathology and clinical pathology among these animals suggest a nonhereditary, metabolic etiology.

Serum samples collected from 20 black rhinoceros (Diceros bicornis) were analyzed for iron values from six different areas in South Africa. In addition, biochemical profiles were performed on individual samples. Comparisons of iron values from free-ranging black rhinoceros and from 28 free-ranging white rhinoceros (Ceratotherium simum) were conducted by location and age. Among the free-ranging black rhinoceros, samples were compared from different regions to a set of samples from black rhinoceros that had been captured and held in bomas. Serum iron levels were not significantly different (P = 0.55) among the three locations with more than one animal (medians 5.57, 5.70, 6.47 ppm), but the median value from the boma group was significantly lower (2.91 ppm; P = 0.042), contrary to previous studies. Similar to reports in captive black rhinos, serum iron levels appeared to show a trend toward increasing values between subadult and adult animals, although differences were not statistically significant among black rhinoceros. Comparison of serum iron levels between free-ranging black and white rhinoceros showed significantly higher median value in black rhinoceros (5.73 ppm) versus white rhinoceros (3.38 ppm, P = 0.001). Other significant differences (P < 0.05) in biochemical values between species included lower median aspartate aminotransferase (37 versus 76.5 U/L), higher copper (1.50 versus 1.34 ppm), higher zinc (1.36 versus 0.37 ppm), lower total protein (8.0 versus 10.35 g/dL), higher gamma glutamyltransferase (13.0 versus 12.5 U/L), and lower globulin (6.6 versus 7.6 g/dL) in black rhinoceros. Further investigations should be conducted to examine the role of age, location, and time in boma confinement on iron values in South African rhinoceros to understand iron metabolism in these species.

Excessive burden of iron, or iron storage disease (ISD), has been reported in a large variety of captive mammal species, including browsing rhinoceroses; tapirs; fruit bats; lemurs; marmosets and some other primates; sugar gliders; hyraxes; some rodents and lagomorphs; dolphins; and some carnivores; including procyonids and pinnipeds. This report collates the comparative evidence for species' susceptibility, recognizing that the data for mammal species are limited. Differences reported in the occurrence of ISD between facilities, or within facilities over periods that span management changes, have been reported in individual cases but are underused in ISD research. Given the species composition, the hypothesis that evolutionary adaptations to the iron content and availability in the natural diet determine a species' susceptibility to ISD (in the face of deviating iron content and availability in diets offered in captivity) seems plausible in many cases. But exceptions, and additional species putatively susceptible based on this rationale, should be investigated. Whereas screening for ISD should be routine in zoo animal necropsy, screening of live individuals may be implemented for valuable species, to decide on therapeutic measures such as chelator application or phlebotomy. Whatever the reasons for ISD susceptibility, reducing dietary iron levels to maintenance requirements of the species in question seems to be a logical, preventive measure.

Although excessive iron storage in black rhinoceros (Diceros bicornis) has been a cause for continuous concern over the last four decades and differences in the iron content of diet items fed in captivity and in the wild have been documented, no reports exist on the iron content of the total diet ingested by free-ranging animals. Here, the results of field studies using backtracking to record the ingested diets of black rhinoceros from three habitats across three seasons are reported. Levels of iron and of condensed tannins, which might reduce iron availability, averaged at 91 ± 41 ppm dry matter and 3.0 ± 1.0% dry matter, respectively, across all habitats and seasons. Although geographic and seasonal variation was significant, these differences are of a much lower magnitude than differences between the averages of these diets and those fed to black rhinoceros in captivity. The results can provide guidelines for the iron content of diets designed for black rhinoceros and suggest that the effect of tannins in these species should be further investigated.

Many frugivorous avian species kept in captivity develop iron storage disease (ISD) as indicated by high concentrations of hepatic iron and hemosiderin deposits in hepatocytes or phagocytes. In several susceptible species fed diets containing moderate levels of iron, ISD develops because of an inability to match rates of iron absorption to tissue needs. Evidence suggests that the pathophysiologic basis of excess iron absorption is due to high levels of expression of divalent metal transporter-1 that transports iron into enterocytes in the proximal intestine, and ferroportin that exports iron to the circulation. The regulatory basis for this inability to sufficiently down-regulate iron absorption is unknown, but disruptions in the hepcidin–ferroportin axis are likely candidates based on recent research in humans and laboratory rodents. It is likely that ISD-susceptible avian species evolved on foods that were very low in bioavailable iron, so there was strong selection pressure for the efficient capture of the small amount of dietary iron but low selection pressure for preventing iron toxicities. Thus, the transporters and regulatory networks for iron absorption seem to be heavily skewed toward iron storage even when food items that are high in iron are consumed. Infections, trauma and neoplasias that trigger an acute phase response may exacerbate ISD in susceptible species and may be the primary cause in species that are normally resistant to ISD (i.e., those that are normally able to shut down intestinal iron absorption when iron stores are replete). The evolutionary basis that resulted in some avian species to be susceptible to ISD (e.g., dietary cause) seems to differ from many inherited ISD disorders in humans that are thought to have evolved to bolster protection against infectious diseases. However the evolutionary basis of ISD in other mammalian species might be more similar to that in ISD-susceptible avian species.

During the last decade, there have been remarkable advances in the understanding of iron homeostasis and its disorders. This review summarizes our presentation at the International Workshop on Iron Storage Disease in Black Rhinos that was held in Orlando, Florida, USA, from 23 to 26 February 2011, and it is directed to veterinarians and nutritional experts who treat rhinoceroses. This review summarizes the current knowledge in humans and mice regarding the physiology and molecular basis of iron overload, and it explores how it can be applied to the problem of iron overload in captive rhino populations.

During routine health screens for black rhinoceroses (Diceros bicornis minor) in a captive setting, serum iron and ferritin were analyzed as well as total iron binding capacity and total iron saturation. Trends for ferritin and percent iron saturation showed steady increases since 2003 in four of four animals (three males; one female) with two animals (one male; one female) consistently showing higher elevations over conspecifics. The historical diet had been comprised of a commercial or in-house complete pelleted feed; several species of fresh browse, Bermuda grass, alfalfa and timothy hays, as well as enrichment and training items (apples, carrots, sweet potatoes, and a small amount of leafy greens and vegetables). In 2009, one of the three male rhinoceroses showed a threefold increase in ferritin and concurrently exhibited clinical signs of lethargy, decreased appetite, and disinterest in training. The lone female showed a twofold increase; she also became reproductively acyclic in the prior year. The male was immobilized for examination and phlebotomy. During the same time period, a new version of the complete pelleted feed, with a reduced amount of iron, was introduced. Subsequent to the diet change, the male's ferritin levels have consistently declined, and the female started cycling again. Even with these corrective steps to reduce iron levels, levels of iron saturation remained high, and ferritin levels were still above 1,500 ng/ml. Therapeutic phlebotomy was instituted via a rigorous training program that allowed phlebotomies over a 30-min time frame. This was possible because of a long-term training program for the animals, consistent training personnel, routine collection of samples on a monthly basis, and general comfort level of the animals in the restraint chute. The results of this integrated approach showed some significant improvements and an overall positive impact on the animals.

Necropsies of two browser rhinoceroses, African black (Diceros bicornis) and Sumatran (Dicerorhinus sumatrensis), often reveal extensive iron-pigment deposition in various tissues. This condition (hemosiderosis) has not been observed in species that are natural grazers, African white (Ceratotherium simum) and Asian greater one-horned (Indian; Rhinoceros unicornis), nor in any species free ranging in the wild. The causes, clinical significance, and consequences of captivity-acquired hemosiderosis have remained controversial despite two decades of compelling evidence that iron tends to accumulate logarithmically in all members of affected species in proportion to periods of expatriation; total-body iron loads can reach 10-fold in less than 3 yr and eventually exceed reference ranges by two to three orders of magnitude; iron overburdens are accompanied by laboratory and histopathologic evidence of cellular injury, necrosis and other clinical consequences characteristic of chronic pathologic iron storage disorders (ISD) in humans and other species (hemochromatosis); and that ISD develops in many other exotic wildlife species displaced from their natural habitats. The historical evolution of evidence establishing the development of pathologic ISD in browser (but not in grazer) rhinoceroses and the possible relevance of ISD to other conditions affecting these two species will be reviewed. Evidence reviewed includes new as well as published data derived from quantitative measurements of iron analytes in sera and necropsy tissues and histopathologic evaluations of current and past necropsies of captive and free-ranging rhinoceroses of all four available species. The evolutionary, husbandry, and conservation implications of ISD in rhinoceroses are relevant to understanding ISD acquired by many other species of exotic wildlife when displaced from their natural environments.