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A Rare Case of Kidney Disease - May Case of the Month

A three year old Thoroughbred gelding was presented to Veresdale Equine Veterinary Services for investigation of lethargy, depression and inappetance. The horse had been in the owner’s possession for six months and, until a few days ago, had been bright and active with a good appetite. Four days prior to presentation the horse became inappetant and began spending a lot of time in his shelter. He would occasionally graze the grass but was not interested in hard feed. Consequently the horse dropped condition over this time.

From questioning the owner there had been no recent change in diet, the horse was up to date with his worming regime, and he had a dental three months previously suggesting that none of these factors were responsible for the horse’s inappetance. A quick check of the teeth confirmed that no major dental problems had arisen in the last three months. The horse had not exhibited any signs of colic. The physical examination findings were within normal limits. The heart rate was only 28 beats per minute, which is within the normal range, but is only usually that low in fit racehorses. However, the significance of this finding was not clear at the time. Otherwise, the horse’s temperature, gut sounds, respiratory sounds and colour were all normal.

As the history and physical examination findings had not revealed the cause of the horse’s problem, blood was collected for submission to the laboratory for analysis. The haematology was unremarkable; the horse was not anaemic or dehydrated, the white cell count and fibrinogen were normal (indicators of inflammation or infection), and there were no other abnormalities. The biochemistry revealed some significant changes. The most concerning abnormality was a very low bicarbonate value of 11 mmol/L (normal range 26-34 mmol/L) indicating the horse was severely acidotic. Also, the chloride concentration was moderately increased at 112 mmol/L (normal range 96-104 mmol/L). There were no other significant abnormalities present on the biochemistry.

Based upon the non-specific clinical signs of lethargy, depression and inappetance, and the severe abnormalities on the biochemistry profile, a diagnosis of renal tubular acidosis (RTA) was made. The treatment of RTA involves correcting the bicarbonate deficiency. Most horses affected by RTA also have low potassium levels. In this case the potassium concentration on the biochemistry profile was normal. However, the horse was also treated with potassium as a precaution. Low potassium can cause bradycardia (low heart rate), and the heart rate was noted to be lower than expected on the physical examination. The high chloride concentration reduces as the bicarbonate concentration increases in response to treatment. A formula is used to calculate how much bicarbonate must be administered to correct the deficit. The replacement bicarbonate can be administered by the intravenous route or orally. Correction via the intravenous route is quicker and can be more easily regulated, however, it requires the horse to be in hospital for close monitoring and regular blood tests, making this a more expensive option. Treating the horse via the oral route is much less expensive; however, it takes longer to correct the deficit and may not be as successful. The materials needed to treat the horse with oral supplementation can be bought at the supermarket; in this case we used baking soda (sodium bicarbonate) and lo salt (potassium chloride). The appropriate amount of the two ingredients are measured out according to how much is required, they are mixed with a small amount of water to form a paste, and are syringed into the horse’s mouth, over the tongue, ensuring the horse swallows it. Horses can also be treated orally in hospital, and in this case the treatment would be administered via a nasogastric tube to ensure the horse received the full amount and could not spit any out!

In this case the treatment was administered over a period of two days. The reason being that oral supplementation can cause diarrhoea, which can worsen some of the abnormalities such as acidosis, so to avoid this, the dose was divided into six portions, and three portions a day were given for two days.

Three days after commencing treatment another blood sample was submitted to the laboratory to assess any change in the biochemistry. There had only been a small increase in the bicarbonate concentration to 12 mmol/L, and the chloride concentration had decreased to 109 mmol/L. However, the most important change in the case was that the horse’s appetite had improved significantly.

The horse underwent another two treatments with bicarbonate and potassium. No further blood tests were conducted due to financial constraints. Instead, the horse’s clinical status was used to assess the response to treatment.

Renal tubular acidosis affects humans as well as horses. Much more is known about the disease in humans, where it is recognised to occur secondary to a variety of conditions, such as kidney disease, liver disease and autoimmune diseases. In horses, however, all reported cases appear to have no underlying cause as no evidence of other kidney disease, liver disease or other primary problem (e.g. ingestion of toxins) has been revealed.

The function of the kidney is to filter the blood and reabsorb all the useful components. The waste is excreted in the urine. The kidney is able to reabsorb components of the filtrate by setting up concentration gradients. If you can think back to high school level science, molecules move from areas of high concentration to areas of low concentration. Once the kidney has filtered the blood, the solution (filtrate) will have high concentrations of elements of the blood, such as bicarbonate. The kidney itself has low concentrations of the elements that must be reabsorbed, so the elements move from the filtrate into the kidney, and then back into the blood. In RTA the kidney loses the ability to reabsorb some important elements, such as bicarbonate, as it fails to maintain a low concentration of bicarbonate within the kidney, disrupting the concentration gradient. As it is not reabsorbed and does not go back into the blood, the concentration of bicarbonate in the blood decreases. By supplementing the horse with bicarbonate and increasing the concentration in the blood, the kidney is able to re-establish the concentration gradient and begins functioning normally.

Horses that have had the disease once are no more likely than unaffected horses to suffer from it again. The key to successful treatment of this condition is recognition of it in the first place, as it is relatively rare and the presenting clinical signs are very vague. However, a good physical examination, which rules out other possible causes of some of the abnormalities on the blood results, combined with the particular abnormalities of the blood that occur with RTA lead to the diagnosis. If left untreated, RTA can be fatal. However, treatment is relatively straight forward and the prognosis is very favourable. In this case, the horse has made a full recovery and is back to his normal self.

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