Plains viscachas (Lagostomus maximus) are large, nocturnal rodents that inhabit the pampas of Paraguay, Bolivia and Argentina. They are considered to feed on poor quality food in the wild: grass, forbs and bushes.1 Plains viscachas have been kept and bred at Zurich Zoo since 1964 and have shown the tendency to develop a type II diabetes mellitus, which was supposedly caused by inadequate feeding.5
Diet-induced diabetes is a well-known problem in many other rodent species like the tuco-tuco (Ctenomys talarum),6 degu (Octodon degus),4 and the fatty sand rat (Psammomys obesus).3 The aim of the present study was to examine the influence of two different feedings on body weight and blood/urine values with respect to a clinical manifestation of diabetes mellitus in this species.
To this end, one feeding trial with a high-fiber, low-energy feed (grass hay only—Trial A) consisting of 4.7, 44.9, and 74.2% crude protein (CP), crude fiber (CF), and neutral detergent fiber (NDF) concentrations on a dry matter basis, respectively, and one feeding trial with low-fiber, high-energy feed (grass hay, carrots and pellets [Kliba NAFAG, Kaiseraugust, Switzerland]-Trial B), consisting of 15.0–20.4, 17.9–27.3, 29.8–45.2% CP, CF, and NDF concentrations on a DM basis, respectively, were carried out. Each feed was fed to the same twelve plains viscachas (six males, six females) over a period of 2 weeks. Animals were weighed before and after each feeding period. At the end of each feeding period, animals were subjected to a short isoflurane anesthesia (Abbott, Cham, Switzerland, 2%; delivered by face mask), and blood was sampled by venipuncture of the vena femoralis or the vena coccygea. In addition, urine was sampled by applying manual pressure on the bladder. Blood samples were analyzed for glucose, insulin (analyzed by a Sensitive Rat Insulin RIA Kit [LINCO Research, St. Charles, MO, USA]), amylase and fructosamine as indicators of sugar metabolism. Triglycerides, cholesterol and lipase were analyzed by standard laboratory procedures as indicators of fat metabolism. Urinary glucose was evaluated by using a rapid strip test (Combur-Test®, Roche Diagnostics AG, Rotkreuz, Switzerland). Statistical analyses (t-test) were performed using the program SPSS 11.0 (SPSS, Chicago, IL, USA).
Trial A led to a significant decrease in body weight (from 2.98±1.22 kg to 2.80±1.17 kg; p<0.01) and Trial B to a significant increase in body weight (from 2.78±1.07 kg to 2.96±.17 kg; p<0.01). There were no significant differences in blood glucose, amylase, triglycerides, cholesterol or lipase. Fructosamine and insulin values differed significantly, with higher values at the end of Trial B (fructosamine from 226.83±38.14 µmol/l to 268.00±24.77 µmol/l, p<0.01 and insulin from 0.32±0.15 ng/ml to 0.46±0.24 ng/ml; p<0.05). An elevated urinary glucose value during Trial B could be seen in five animals.
In conclusion, it can be stated that a carbohydrate-based, high-energy diet bears a high risk for plains viscachas to develop a diabetes mellitus with the short-term consequence of glucosuria. The potential long-term consequences of diabetes mellitus (cataracts, hepatic lipidosis) have been described.2 Remarkably, initial clinical indications of developing diabetes, due to concentrate feeding, were observed in a very short time of only 2 weeks. Therefore, this species may prove valuable as an animal model for further diabetes research in the future. For the management of captive populations, a low-energy, high-fiber feeding regime appears warranted. However, the decrease in body weight on the hay-only diet during Trial A offers two different interpretations that should be investigated: either viscachas are not able to maintain body weight on a low-protein, high-fiber grass hay, as used during this study, or the animals may have stabilized their body weights on a lower level of this diet. In order to investigate these possibilities, further feeding studies, using a different hay source and longer feeding periods, with regular weighing intervals, are being conducted.
We thank the keepers at Zurich Zoo, Switzerland, for their care and management of the animals.
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