Kidneys & Excretion
Fig. 20. Details of nephrons, blood supply and loop of Henle. (Image by Hill, R.W., Gordon A.W., and Anderson, M. “Kidneys and Excretion,” Animal Physiology [2016]: 4th ed, 789.)
Every day an animal’s body works hard to maintain homeostasis, ensuring the body is functioning at equilibrium, or as close to it as possible. Body fluids are constantly fluctuating throughout the day, which means they can easily be shifted away from a normal range. Any mismatch between the rates lost and gained can result in body fluids that are too concentrated or too dilute (Hill, 2016, 779).
According to Little and McMahon (2012, 1), the mammalian kidney is a vital organ that eliminates nitrogenous waste, maintains the volume, composition, and pressure of the blood, and density of the bones. It consists of tubules, called nephrons, which are attached to long segments called the “loops of Henle.” These loops are structured in a way that allows for the development of hyperosmotic urine, which is more
concentrated than blood plasma. A detailed look at nephrons, blood supply, and the loop of Henle can be seen in figure 20.
Urine is drawn from the blood plasma, and as such its concentration affects blood concentration. It is used to eliminate sodium, potassium, nitrogen, and more from the body, helping to maintain a steady blood pH level. Anything that disrupts the elimination of urine therefore disrupts the homeostasis of the body, which can be catastrophic.
A study by Yoong et al., (2018, 867), found that Aonyx cinereus under human care are more prone to Urolithiasis than those in the wild. As defined by the Blacks Veterinary Dictionary (2015), urolithiasis is the “formation of calculi (stones), or of a crystalline sand-like deposit, in the urinary system.” As these uroliths form, they can cause blockages of the ureter, extreme pain, and even bacterial or viral infections. If left untreated, this can result in death. Development of this type of blockage can also occur in the kidney and is referred to as nephrolithiasis.
A study by Higbie et al., (2014, 691) shows a 10-year-old captive male, with a history of bilateral nephrolithiasis. He came into the clinic with lethargy, dehydration, and fluid present in the abdomen. An ultrasound (figure 22) showed stones in the ureter and the left kidney. Additionally, the renal pelvis contained a large amount of purulent material. This otter underwent antibiotic therapy, surgery, and was declared clinically healthy again around two months later.
To demonstrate the severity of this condition, figures 22 and 23 show its presence in a 7-year-old male small-clawed otter. He was brought into a veterinary clinic with severe dehydration and anorexia. Serum biochemistry showed increases in blood urea nitrogen, creatinine, inorganic phosphorus, and ammonia. More than 30 calculi were present in both kidneys, ureters, and urinary bladder. Despite stone removal, the otter was unable to recover and died 7 days after surgery (Aziz, 2020).
Fig. 21. Ultrasound of the left kidney. Renal pelvis (P) is dilated, white arrows indicate acoustic shadowing consistent with calculi. (Image by Higbie et al. “Nephrectomy in an Asian Small-clawed Otter (Amblonyx cinereus) With Pyelonephritis and Hydronephrosis Secondary to Ureteral Obstruction,” Journal of Zoo and Wildlife Medicine, [2014]: Vol. 3, 692.)
Overall, from the reviewed knowledge on kidney function and the importance of excretion to maintain proper homeostasis within the body, it is obvious that there is an issue with A. cinereus in captivity. It is believed that this may have something to do with their diet (see nutrition) in captivity vs. that of the wild, however further study on this is required.
Fig. 22. Kidney Stone in a Small-clawed Otter. (Image by Aziz, N. “Case II: 2926 (JPC 4118632),” Smithsonian Conservation Biology Institute, [2020]: Joint Pathology Conference 14.
