Vitamin D is a fat-soluble vitamin (meaning they are stored in the liver and fatty tissues) which is responsible for helping the cat maintain levels of calcium and phosphate in the body and promoting mineralisation the mineralisation of bones. When blood levels of calcium drop, vitamin D is activated, which helps the gut to absorb more calcium from food and reducing urinary calcium loss.
Vitamin D is produced in the fur (or skin in humans) upon exposure to sunlight and also ingested in small quantities via food. Foods containing vitamin D include fish, liver, dairy products and egg yolk.
There are two major forms of vitamin D:
Vitamin D toxicity (also known as hypervitaminosis) can occur in cats of any age, although kittens and young cats are at greater risk than adult cats.
How do cats develop vitamin D toxicosis?
Accidental ingestion of rat poison containing cholecalciferol (vitamin D3) is the most common cause of vitamin D toxicosis in cats.
Secondary poisoning. Eating a rodent who has ingested rat poison.
Over-supplementation of vitamins can also occur. Fish liver oils, in particular, can cause high levels of vitamin D. You should never give supplements to your cat without veterinary approval.
Ingestion of certain human medications can increase vitamin D levels.
Feeding an imbalanced diet containing foods rich in vitamin D such as liver and fish. These are fine in small quantities, but should not be given regularly.
The effects of vitamin D toxicosis can be from acute or chronic exposure. Young cats are more prone to developing vitamin D toxicosis, however, it can occur in cats of any age. Cats with predisposing conditions such as kidney failure or hyperparathyroidism are more susceptible to vitamin D toxicosis than cats with no illness.
What are the symptoms of vitamin D toxicosis?
The mechanism of vitamin D toxicoses relates to raised blood calcium (hypercalcemia) and high blood phosphate (hyperphosphatemia).
Hypercalcemia can lead to calcification of various soft tissues, particularly the kidneys, lungs, arterial walls, myocardium and the intestinal tract. Skeletal abnormalities and eventually renal failure, cardiac upset and GI upset.
Symptoms may vary depending on the level of toxicity. For example, acute vitamin D toxicity is more likely to occur if your cat has ingested rat poison, chronic toxicity if your cat has been fed an improper diet.
Common symptoms include:
- Increased thirst
- Increased urination
- Pain in bones
- Gastrointestinal or pulmonary hemorrhage (bleeding from the lungs)
- Muscle tremors
- Black, tarry feces which contain blood
Long-term complications of vitamin D toxicity can include kidney stones, kidney failure, abnormal heart rhythms and excessive bone loss, mineralisation of certain organs, such as the lungs can produce a chronic cough due to their inability to contract as they should.
How is vitamin D toxicity diagnosed?
Your veterinarian will perform a complete physical examination and obtain a medical history from you including questions on possible exposure to rodenticide or vitamin supplementation in the diet.
He may wish to perform the following tests:
Biochemical profile which may reveal hypercalcemia (high blood calcium levels), hyperphosphatemia (high blood phosphate levels), and azotemia (high urea levels in the blood). Elevated BUN (blood, urea, nitrogen) levels can indicate kidney failure.
Urinalysis to check for high levels of calcium in the urine.
X-Ray may reveal mineralisation of certain tissues and look for bone loss.
Echocardiogram (ECG) to evaluate the heart.
How is vitamin D toxicity treated?
Prompt veterinary attention is vital to remove as much vitamin D as possible and prevent further absorption. Your cat will need aggressive treatment, and will be required to stay in hospital for several days.
Treatment is aimed at lowering serum calcium levels, increasing urinary calcium excretion and decreasing absorption of calcium via the intestines. This may include:
- Induce vomiting for acute exposure to a rodenticide.
- Activated charcoal to prevent further absorption.
- Phosphate binders to decrease phosphate levels.
- Stop supplementation immediately.
- IV fluids to treat and correct dehydration if necessary.
- Reduce serum calcium by flushing it out of the body. This may be accomplished by diuresis (facilitation of increased urine output) with normal saline solution.
- Calcitonin is a hormone which is produced by the C cells of the thyroid gland when blood levels of calcium are elevated. It works by inhibiting the activity and formation of osteoclasts (cells in the bone which breaks down bone tissue). When calcium in the bone is broken down by the osteoclasts, it enters the bloodstream. Your veterinarian may prescribe an injectable synthetic form of this hormone to help bring down calcium levels.
- Furosemide is a loop diuretic (water tablet) which may be given to increase excretion of calcium via the kidneys.
- Prednisolone may be prescribed to reduce bone and intestinal absorption of calcium and increase kidney excretion.
- Obviously, if you have been giving your cat supplements, this should be discontinued.
Preventing vitamin D toxicity in cats:
Never give your cat supplements unless told to do so by a veterinarian.
Avoid the use of rodenticides if possible. Keep all poisons locked away from cats.
Feed a well-balanced diet. Home prepared is fine, but liver and fish should only be given occasionally and in small amounts.
Keep all medications and creams away from your cat. While most tend to be more discriminating than dogs, some cats are curious enough to eat medications they shouldn't, especially kittens.
Don't let your cats hunt. This is a hard one to control but there are many risks associated with hunting. A poisoned rat or mouse is going to be slower and easier prey than a healthy one.