Abstract
Cats differ from the dog and other species in many ways. For example, diabetes mellitus (DM) as a syndrome in the cat has several notable differences from the dog. This manuscript will address some of the ways in which feline DM differs from canine DM or is otherwise unique.
Classification of DM in the cat
The two major classifications of DM are type 1, also known as IDDM (insulin dependent DM), juvenile onset DM, or non-ketosis prone DM, and type 2 or NIDDM (non-insulin dependent DM), adult onset DM, or ketosis prone DM. Type 1 DM refers to a diabetic state brought about because of a loss of insulin production and secretion by the β-islet cells. Because these patients are insulinopenic they require insulin injections as part of their treatment. Type 2 DM refers to a diabetic state brought about because of insulin resistance, generally through a loss of insulin receptor numbers or affinity. This renders the insulin that diabetics do have less effective and, at least initially, these patients have hyperinsulinemia as well as hyperglycaemia. Treatment for these patients focuses on increasing the efficiency of insulin utilisation and production. Additionally, type 2 DM can progress to type 1 DM.
In people type 2 is the most common cause of DM, whereas our small animal patients usually have type 1 DM. In dogs DM is almost exclusively type 1, with rare cases of type 2. In contrast, the cat has a much higher (at least 20%) incidence of type 2 DM. The actual incidence of type 2 DM in the cat is unknown and is controversial. The author believes that, in the cat type 1 (IDDM) is the most common classification of DM, however, a significant percentage of cats do have type 2 (NIDDM). Other authors believe type 2 DM is the most common form of the disease in the cat. Irrespective of what percentage of diabetic cats have type 2 diabetes, this issue is major and relates directly to the many other differences between diabetic cats and dogs as noted below.
Our clients, being people are most familiar with type 2 DM and therefore often inquire about treatment with diet, exercise, and pills (oral hypoglycaemics). As veterinary practitioners our clinical experience for treatment revolves around insulin usage, because our patients are usually type 1 diabetics. This difference is important to make clear for the client. That being said, the type 2 feline diabetic patient raises some interesting possibilities with regard to alternative treatments, treatment outcomes, and etiology.
Etiology of DM
The cause of DM is multifactorial and is perhaps better defined in people than the cat or dog. Nevertheless, pathology of the pancreas of small animals is variable and undoubtedly reflects the multifactorial etiology of DM. In the pancreas of diabetic cats, islet-specific amyloidosis, β-cell vacuolation and degeneration, are often noted. However, other cats do not have any of these changes but instead have a reduction in the number of pancreatic islets and/or insulin containing cells. Pancreatitis, though previously thought to be uncommon in the cat, was found in 51% of diabetic cats at the time of necropsy in one study. The principle constituent of amyloid isolated from the pancreatic tissue of humans and cats is islet-amyloid polypeptide (IAPP), or amylin. IAPP has been identified within β-cell secretory granules of both species and is co-released with insulin. Evidence suggests that IAPP can antagonise insulin actions and therefore itself might be hypersecreted in NIDDM. In this way it may contribute to progression of type 1 DM
In contrast dogs have very little if any amyloid deposition in the islets. Genetic predispositions have been suggested through familial associations and by pedigree analysis of Keeshonds. Environmental factors such as viral infections, chemicals, and chronically stressful situations may also have a role in etiology of DM. Other important considerations are immune-mediated destruction of the islets as well as pancreatitis
The following table indicating possible causes of DM in cats and dogs helps to emphasis the difference between the two species as well as the multifactorial etiology of this disease (from Feldman EC, Nelson RW (1996) Diabetes mellitus. In: Canine and feline endocrinology and reproduction. 2nd ed. Feldman EC and Nelson RW (eds): Philadelphia, WB Saunders, p 340).
Potential factors involved in the etiopathogenesis of diabetes mellitus
Signalment
This is a disease of middle-aged to older cats. While there are no reported breed dispositions, males are about two times more commonly affected than are females. In contrast, some dog breeds are over represented (Keeshonds, Pulik, Carin Terriers, and Miniature Pinchers) or under represented (Cocker Spaniels, German Shepherd's, Collies, Pekingese, Rottweilers, and Boxers) and the disease occurs more frequently in females.
What is glucose toxicity?
The phenomenon of glucose toxicity (GT) is important for the veterinarian to understand and makes for a better understanding of DM in the cat. In the face of chronic hyperglycaemia, β-islet cells down regulate insulin secretion. In other words in response to hyperglycaemia the islet cells lose their ability to produce and secrete insulin, and DM results. Fortunately this toxicity, if caught early enough, is reversible and with correction of hyperglycaemia to euglycaemia normal insulin secretion can be restored. This is in contrast to dogs where a phenomenon of β-cell burnout (exhaustion) is more likely to occur. In this situation, as the β-cells are stimulated to secrete more and more insulin, they become ‘exhausted’ and permanently lose their ability to secrete insulin. Therefore even when euglycaemia can be restored in the dog, the pancreas will usually not regain its ability to produce and secrete insulin. In this case DM is permanent and insulin dependent (type 1).
This glucose-induced, sometimes reversible, insulinopenia is responsible in large part for difficulties in distinguishing type 1 from type 2 DM in the cat. With type 1 DM, measurement of insulin should reflect a deficient or undetectable amount. With type 2 DM insulin concentrations are expected to be increased but may be normal or low. Because of glucose toxicity, in the cat, both type 1 and 2 would have insulinopenia, thus, distinguishing between the two is difficult prospectively Glucose toxicity may, in part, explain some cases of transient DM.
Transient DM in cats: How do we manage it?
Can we predict which cats Will experience it?
The exact incidence of transient DM in the cat is unknown but is usually estimated at or below 20%. It is an important issue from three perspectives: first, it may influence the client's decision to pursue therapy, it may influence the course of treatment, and lastly, when cats lose their need for insulin (ie regain the ability of the pancreas to secrete insulin), the risk of hypoglycaemia is greatly increased.
Some client's that would otherwise consider euthanasia for their cat, may move forward with treatment of DM (understanding that the need for treatment may disappear), and discover that it is not so hard to manage DM after all. The possibility of transient diabetes in the patient may influence whether insulin or oral hypoglycaemics are used for initial treatment. It is important that owners periodically monitor urine glucoses or that glucose curves be done because cats may lose their need for insulin (unknown to the owner who continues insulin therapy) and develop severe or fatal hypoglycaemia (see later section hypoglycaemia unawareness).
Cats with type 2 DM would be most likely to have transient DM but it can be impossible to distinguish type 1 from type 2 DM in the cat. Candidates include obese cats, cats receiving progestages or steroids, and cats with hyperadrenocorticism. As obese cats lose weight in concert with adequate glycaemic control ‘glucose toxicity’ would be reversed and insulin secretion restored. Care must always be taken in cats with weight reduction because hepatic lipidosis is common. The author usually recommends that the owner spot check urine glucose at home and call if there is a decreasing trend in urine glucose concentration or a negative result. In this way impending hypoglycaemia is adverted.
In contrast to the cat, DM is very rarely reversible in the dog. The circumstances are usually secondary to increased progesterone. In the dog progesterone induces an increase in growth hormone secretion. Growth hormone is diabetogenic and in excess can lead to DM. If caught early enough (before pancreatic β-cell exhaustion) this DM is easily reversed. It is often characterised by high insulin requirements and no ketosis. Progesterone may be increased iatrogenically through treatment with drugs to prevent oestrous or to modify behaviour. Another situation occurs in intact bitches during the long diestrous phase that is characterised by high progesterone. Treatment is accomplished by reducing progesterone (thereby GH) with ovarioysterectomy or drug withdrawal. An important part of treatment however is preventing hypoglycaemia that quickly results.
Why do we consider use of oral hypoglycaemics in cats but not dogs? Do They really work?
Usually when we think of oral hypoglycaemic drugs in veterinary medicine we think of the sulfonylurea, glipizide. The drug works primarily by increasing pancreatic insulin secretion and enhancing β-cell responsiveness to glucose. Therefore, glipizide will not work in type 1 diabetics (remember in type 1 diabetics, the pancreas cannot produce insulin). It is only the population of type 2 diabetics that might respond to treatment with glipizide. However, as previously noted, it can be impossible to distinguish type 1 from type 2 DM in the cat. Other classes of oral hypoglycaemics are commonly used in humans but experience with them in the cat is limited. Clearly further investigation is warranted and these other drugs are discussed below.
Published success with glipizide use in diabetic cats varies. A preliminary study of 20 cats with DM revealed a long-term response rate of 55%. Thirteen cats (65%) initially had a complete or partial response. Seven cats (35%) did not respond. Results from a larger more recent study showed less success with glipizide treatment. Of the 50 cats, seven (14%) responded completely, six (12%) were transiently diabetic, and 28 (56%) had no response. Three cats had an initial but not continued response. Long-term response rate was about 38%. A larger retrospective study of 104 cats had a long-term response rate of about 35%.
Side effects of glipizide administration include vomiting that is usually transient (15%), increased liver enzymes and icterus (10%), and hypoglycaemia (12–15%). Glipizide may eventually lose it effectiveness over time working for only days to several years. The true incidence of long-term success in the stable feline diabetic population is unknown as is the true incidence of eventual loss of efficacy of glipizide over time. Generally, unstable diabetics or those with concurrent illnesses are not good candidates for treatment with glipizide. The reader is referred elsewhere for treatment schedules and drug dosages.
Diet and exercise. How do I do that?
Diet and exercise may have a profound effect on glycaemic control. However, putting a diabetic cat on a diet, or even feeding them on a strict schedule, is easier said than done! There are several things to consider: (1) hepatic lipidosis, (2) ‘monster cat syndrome’, (3) hypoglycaemia, (4) fibre, carbohydrate and protein content of the diet, and (5) use of timed feeders or ad lib feeding.
Hepatic lipidosis is always a concern with weight reduction in an obese cat. To initiate weight loss in these patients, caloric intake should be limited to 70 to 75% of the energy needs for the cat's optimum weight. If weight loss is not needed then diets fed at a maintenance level of 60–70 kcal/kg/day are adequate.
‘Monster cat syndrome’ is the author's description of those cats that develop major behavioural problems in response to decreased feeding. Owner complaints range from stealing food from the garbage, cupboard, or your hand — you'd be amazed how quickly even an obese cat can move — cats that nip at their owner's ankles and defiant behaviour such as urinating and defecating on the bed, or any other inappropriate place. It seems as though some cats need to eat and/or have their stomachs full. Treats that the author has successfully used are carrots and string beans (frozen vegetables that were thawed). Even though cats are obligate carnivores, many cats will eat these high fibre, low calorie snacks. Benefits are improved behaviour and owner-pet relationship, continued weight loss, and by virtue of this weight loss, and perhaps even the increase of fibre in the diet, better glycaemic control.
Weight loss generally necessitates a reduction in insulin dosage. This is because of a reduction in calories consumed as well as decreased insulin resistance. Remember that obese diabetic cats may be type 2 diabetics or possibly transient diabetics so you want to plan for decreased insulin requirements. Have the owners measure periodic urine glucoses at home and/or perform glucose curves.
As far as the specific diet is concerned fibre, carbohydrate and protein requirements/recommendations for the diabetic cat are unknown. However, special considerations for this species include: the cat is an obligate carnivore, protein rather than carbohydrates stimulate insulin release, and glucose requirements are maintained from protein precursors (gluconeogenesis) rather than carbohydrate sources. Current dietary recommendations are for a low-carbohydrate, high protein diet. Increased fibre content may also be helpful.
Ideal feeding schedules for animals receiving twice a day insulin treatments would involve QID feedings, that is a meal before each insulin injection and one in the mid-afternoon and late evening. Of course there are alternatives when owners are unable/unwilling to adhere to this schedule. For example, one could use commercially available timed feeders, or if the cat is a nibbler, feed ad lib.
As for increasing exercise; going for a run or walk, throwing a ball in the park, or stick in the lake just isn't going to do the trick! But find the pets favourite fascination and see what is possible. For example, the author's personal favourite for her cat is the laser light (that way her cat, Sabs, can play while she rests in a chair). Another trick for the polyphagic cat is to hide treats in play toys or in hard-to-reach places so that exercise is required to obtain the food.
Are Their any other treatments That veterinarians can offer feline pet owners?
Yes, but the veterinarian should proceed cautiously as experiences with these modalities are limited and should only be used as adjunctive therapy. There are no substitutes for insulin if it is needed, but ways to improve glycaemic control may include the following: diet, exercise, and glipizide (covered above), or other hypoglycaemic agents that act to inhibit hepatic glucose production, diminish absorption of glucose from the intestine, or act as insulin-sensitising agents.
Metformin, an agent that inhibits hepatic glucose output, has been associated with severe side effects when initially used in cats. However, it may be safe and effective when used in cats at lower doses. A published dose for the cat is 2–10 mg/kg given twice daily.
Acarbose, a drug that impairs glucose absorption from the intestine, is an alpha-glucosidase inhibitor. It decreases fibre digestion and consequently glucose production from the food. A published dose for the cat is 12.5–25 mg with meals. Side effects are more commonly noted at the higher dose and include semi-formed stool and diarrhoea. This drug should not be used alone or in normal or under weight cats.
Three insulin-sensitising agents have been used in the cat. One drug, a new class of oral hypoglycaemics that held promise for use in human type 2 diabetics, has recently been discontinued because of idiosyncratic hepatotoxicity (troglitazone). The transition metals vanadium and chromium are gaining renewed popularity in human medicine and are now being investigated for use in the cat. These metals are insulinomimetic or insulin-sensitising agents that work by bypassing the insulin receptor to directly stimulate glucose metabolism by the cell. This is an ideal adjunctive treatment for type 2 diabetics because these patients have decreased receptor numbers and or receptor affinity for insulin.
Studies have been done using vanadium in healthy or diabetic cats. Low doses of oral vanadium decreased blood glucose concentrations and alleviated clinical signs of DM in early type 2 diabetic cats. Serum fructosamine concentrations were also decreased compared to a placebo group. Side-affects include anorexia and vomiting initially but usually resolve after reinstitution of the vanadium therapy. Although a dose has been published for the use of chromium picolinate in the cat (200 mg/cat q 24 h in food), there are no published reports to substantiate its toxicity or efficacy.
When is glucosuria and hyperglycaemia not diagnostic for DM?
Epinephrine induced stress hyperglycaemia is common in the cat and can rise high enough to spill into the urine (vs glucocorticoid induced changes which are more common in the dog). Therefore, determination of DM in a cat may be difficult. When the diagnosis is questionable owners should follow urine glucoses at home in the pet's familiar environment.
Diabetic neuropathy
Peripheral neuropathy is a well-recognised complication of feline and human diabetes mellitus, but is very uncommon in the dog. The peripheral neuropathy usually involves the distal limbs with about 8% of diabetic cats having the classic clinical signs of plantigrade stance, progressive paraparesis, distal limb atrophy and pelvic limb hyporeflexia. Thoracic limb involvement is rare but may occur. Less severe clinical signs probably have a higher incidence of occurrence but may be detected through careful history and through physical examination. For example, the cat may have difficulty in jumping, pelvic limb abduction, distal weakness while standing, and inability to fully retract their claws.
Diagnosis is intuitive but may be confirmed with electrophysiology and peripheral nerve and muscle biopsy. Electrophysiological testing often reveals prominent demyelination at all levels of motor and sensory peripheral nerves and their corresponding nerve roots. There was splitting and ballooning of the myelin sheath noted on histopathology on nerve biopsies, while muscle biopsies had changes in both fibre types that were consistent with mild denervation. Thus demyelination appears to be the major peripheral nerve abnormality.
There is no specific treatment for this neuropathy except to more closely regulate the DM. Even if this is possible, improvement in peripheral nerve function will take several weeks to months, and rarely is complete.
Hypoglycaemia unawareness
This refers to a phenomenon where diabetics fail to respond physiologically to hypoglycaemia. In other words they don't recognise the hypoglycaemia because autonomic symptoms such as sweating, tremor, hunger, anxiety, and palpitations do not occur. Hypoglycaemia may be immediately life threatening and the body has many defence mechanisms to restore glucose to normal concentrations. This response primarily involves the counterregulatory hormones (epinephrine, glucagon, glucocorticoids and growth hormone), and a concomitant increase in the discharge of autonomic nervous system neurotransmitters (norepinephrine and acetylcholine).
Hypoglycaemia unawareness is more common in patients with frequent or persistent hypoglycaemia, which in turn is most likely in human diabetics when efforts are made to achieve normal glucose concentrations. With intensive therapy (or inappropriate therapy) and subsequent episodes of hypoglycaemia, there is believed to be central nervous system adaptation. This adaptation results in reduced counter regulatory hormone responses (eg, increased sensitivity to insulin) as well as diminished autonomic neurotransmitter release (eg, hypoglycaemia unawareness). The author believes this phenomenon occurs in our small animal practices and that it seems to be a much more common occurrence in the cat than dog.