Cobalamin deficiency

Jörg Steiner, med.vet, Dr.med.vet., PhD, DACVIM, DECVIM -CA, AGAF
Professor and Dr. Mark Morris Chair of Small Animal Gastroenterology and Nutrition
Director of the Gastrointestinal Laboratory
Dept. VSCS, Texas A&M University, College Station, TX
Posted on 2018-08-09 in Gastrointestinal & Internal Medicine

Cobalamin (vitamin B12) deficiency occurs commonly in both dogs and cats with chronic gastrointestinal disease and it is widely accepted that patients may not respond favorably to treatment of the underlying condition unless they are also treated with cobalamin supplementation. Traditionally, cobalamin has been supplemented by parenteral administration, but recent data would suggest that oral cobalamin supplementation may be as efficacious as parenteral supplementation. Given the ease of oral administration this mode of supplementation would be much preferable over the parenteral route.

Cobalamin function

Cobalamin refers to a group of compounds that are exclusively derived from bacterial sources. The biologically active forms of this vitamin are methylcobalamin (required for methylgroup transfers) and adenosylcobalamin (required for adenosyl-group transfers), but there are other molecules that belong to this group of vitamins, such as hydroxocobalamin or cyanocobalamin. Cyanocobalamin does not occur naturally, but is manufactured by bacterial fermentation for treatment of cobalamin deficiency. Cobalamin has important functions in amino acid metabolism and DNA synthesis.

Cobalamin is an essential cofactor for several enzyme systems in many mammalian cells. The first enzyme system, methylmalonyl-CoA mutase, is located in the mitochondria and plays a crucial role in the transformation of propionyl-CoA to succinyl-CoA. Thus, cobalamin plays a major role in the metabolism of several amino acids.

Cobalamin is also important in the transformation of the sulfur-containing amino acids methionine and cysteine. Homocysteine is an intermediary amino acid that is formed from methionine and is not found in the diet. The transformation of homocysteine to methionine is linked to another metabolically crucial process, the generation of the biologically active tetrahydrofolate from N 5 -methyltetrahydrofolate. Simplistically, the cobalamin-dependant enzyme methionine synthase transfers a methyl group from N 5 -methyltetrahydrofolate to homocysteine, which results in tetrahydrofolate and methionine. Thus, this enzyme not only plays a role in the transformation of sulfur-containing amino acids, but may be even more important in the generation of the biologically active tetrahydrofolate, which is involved in the synthesis of both purines and pyrimidines and thus a crucial step in DNA and RNA synthesis.

Cobalamin absorption

Dietary cobalamin is tightly bound to dietary animal-derived protein. In the stomach, dietary protein is partially digested by pepsin and HCl and cobalamin is released. However, cobalamin immediately binds to a transporter protein called haptocorrin or R-protein. Haptocorrin is mostly synthesized and secreted by the gastric mucosa. Haptocorrin in turn is digested by pancreatic proteases in the small intestine. Free cobalamin binds to intrinsic factor. In humans, intrinsic factor is mostly synthesized and secreted by the parietal cells of the gastric mucosa, but there is good evidence that in dogs and cats most of intrinsic factor is synthesized and secreted by pancreatic acinar cells. Cobalamin/intrinsic factor complexes are absorbed by a complex receptor in the microvillus pits of the apical brush border membrane of the ileal enterocytes. Thus, the absorption of cobalamin is an extremely complex system that relies on a multitude of factors and processes. As cobalamin is being absorbed into the intestinal epithelial cells, it dissociates from intrinsic factor and free cobalamin is released into the circulation, where most of it binds to yet another protein, transcobalamin II. The main storage compartments for cobalamin in the body are the liver and the kidney, which maintain serum cobalamin concentrations by releasing cobalamin whenever needed.

Cobalamin deficiency

The most common causes of cobalamin deficiency in dogs and cats are chronic and severe distal or diffuse small intestinal disease and EPI. In addition, short-bowel syndrome, an exclusively vegetarian or vegan diet, or hereditary cobalamin deficiency are less common causes of cobalamin deficiency.

A recent study has shown that 82% of dogs with EPI were cobalamin deficient. Similar studies in cats have shown that most, if not all, cats with EPI are cobalamin deficient. As discussed above, intrinsic factor in dogs and cats is mostly supplied by pancreatic acinar cells. Therefore dogs and cats with EPI may lack enough intrinsic factor for cobalamin absorption. The lack of pancreatic proteases and the alteration of the small intestinal microbiota may also play minor roles.

Dogs and cats with severe and long-standing small intestinal disease involving the ileum may also show cobalamin deficiency. In one study, 49 of 80 cats (61%) with chronic signs of gastrointestinal disease had cobalamin deficiency, as evidenced by a subnormal serum cobalamin concentration. It is interesting to note that there is one study from the UK that would suggest that cobalamin deficiency is much less common in cats in the UK than in the USA. However, there are other reports from the UK that would suggest that cobalamin deficiency does occur frequently in cats with gastrointestinal disease in the UK. These differences between studies are interesting as they point to differences in measuring serum cobalamin concentrations in dogs and cats with different assays. There appear to be considerable cobalamin stores, but it is unclear where most of the cobalamin stores are located in dogs or cats, but as in humans it takes a considerable amount of time for these body stores to be depleted if an insufficient amount of cobalamin is being absorbed.

Dogs and cats with short bowel syndrome are typically cobalamin deficient because cobalamin absorption is exclusively limited to the ileum and removal of the ileum will thus lead to cobalamin deficiency. Also, vegetarian or vegan diets do not contain cobalamin unless they have been supplemented with that vitamin. Dogs and cats fed these diets exclusively and who are not receiving any vitamin supplementation will develop cobalamin deficiency. Hereditary cobalamin deficiency has been recorded in a few dog breeds, including the Giant Schnauzer, Beagle, Border Collie, Australian Shepherd, and Chinese Shar Pei. Recently, a region of chromosome 13 has been identified that cosegregates with cobalamin deficiency in the Chinese Shar Pei, but the actual gene causing the disease has not yet been identified.

Most dogs and cats with cobalamin deficiency only show clinical signs of gastrointestinal disease, which could either be a cause or the effect of cobalamin deficiency. Other clinical signs include weight loss, central neuropathies, peripheral neuropathies, or immunodeficiencies. In a recent case report a Border Collie with selective cobalamin deficiency was described. The dog presented with hyperammonemic encephalopathy and fully responded to cobalamin supplementation. In another case report a juvenile Beagle presented with failure to gain weight, lethargy, intermittent vomiting, seizures, anemia, and leucopenia. This dog also fully responded to treatment with cobalamin supplementation. In a separate case report, a 4-year old cat presented with severe encephalopathy and was diagnosed with an organic acidemia and cobalamin deficiency. Interestingly, in contrast to the Border Collie mentioned above, this cat had a normal plasma ammonia concentration.

Diagnosis of cobalamin deficiency

A definitive diagnosis of cobalamin deficiency can be challenging. Clinical signs are ultimately caused by cobalamin deficiency on a cellular level. However, the cellular cobalamin status is difficult to assess. Serum cobalamin concentration has been traditionally measured to help assess cobalamin status, but some patients with cobalamin deficiency on a cellular level do not always have severely decreased serum cobalamin concentrations. Thus, in order to avoid missing patients with cobalamin deficiency, cobalamin supplementation should be considered even when serum cobalamin concentration is low normal. Several assays for the measurement of serum concentrations of cobalamin in humans are available. In order to be used in dogs and cats, these assays designed for use in humans must be validated for use in dogs and cats. The GI Lab at Texas A&M University has analytically validated an automated chemiluminescence assay designed for the measurement of cobalamin concentrations in humans for use in dogs and cats. A reference range for serum cobalamin concentration in dogs and cats was established. Reference ranges are not transferrable between labs and each lab should establish their own reference range.

Serum or urine methylmalonic acid (MMA) concentration can also be used as an indicator of cobalamin status. Cobalamin deficiency leads to accumulation of MMA and thus concentrations of MMA are often dramatically increased in the serum or urine of patients with cobalamin deficiency. Serum MMA concentrations have been shown to be increased in cats with cobalamin deficiency and have been shown to decrease with cobalamin supplementation. Also, recently dogs with severely decreased serum cobalamin concentrations were shown to have increased serum MMA concentrations. Interestingly, several dogs with low-normal serum cobalamin concentrations were also shown to have increased serum MMA concentrations, demonstrating that a severely decreased serum cobalamin concentration is not optimally sensitive for the diagnosis of cobalamin deficiency on a cellular level and that a cut-off value for cobalamin supplementation should be chosen that is in the low-normal reference range. This is especially true if one considers that cobalamin supplementation is minimally invasive, safe, and relatively cheap. As suggested by these data, measurement of serum MMA concentration may be a better diagnostic test for cobalamin deficiency than serum cobalamin concentration. However, measurement of MMA concentration in serum or urine is technically involved and expensive. Thus, MMA is currently not routinely assessed in patients evaluated for cobalamin deficiency.

Thus, the only routinely available diagnostic tool to assess cobalamin status in dogs and cats is serum cobalamin concentration, which should be evaluated in every dog and cat with chronic signs of gastrointestinal disease or with clinical signs compatible with cobalamin deficiency that cannot be attributed to other conditions (i.e., unexplained immunodeficiencies, anemias, neuropathies).

Cobalamin supplementation

Patients with severe cobalamin deficiency often do not respond to therapy of the underlying gastrointestinal disorder unless or until cobalamin is being supplemented. As mentioned, patients with low-normal serum cobalamin concentrations should be considered for cobalamin supplementation as measurement of serum cobalamin concentration may not be optimally sensitive for the diagnosis of cobalamin deficiency and there is no indication that oversupplementation of cobalamin leads to complications. The most common form of cobalamin used for supplementation is cyanocobalamin, but hydroxocobalamin or methylcobalamin can also be used in patients that don’t respond to cyanocobalamin supplementation (most of these patients will also fail to respond to other forms of cobalamin) or those that appear to have side effects to supplementation with cyanocobalamin (side effects from cyanocobalamin administration have never been definitively demonstrated in either dogs or cats). Traditionally, the standard route of cobalamin application is by parenteral administration. This is because cobalamin deficiency has been shown to lead to cobalamin malabsorption in the ileum. However, there are recent data that show that oral supplementation may be as efficacious as parenteral supplementation. Dosing schedules for parenteral supplementation have been described. As for parenteral administration, dosing for oral supplementation is empiric with 250 µg of cyanocobalamin being administered orally once a day in cats or in dogs up to 10 kg BW, 500 µg in dogs weighing over 10 kg but less than 20 kg, and 1000 µg in dogs weighing more than 20 kg. Daily supplementation is required and after a 6-8 week period one should discontinue supplementation for about a week and recheck serum cobalamin concentration.

In one large retrospective study of 51 client-owned dogs with low-normal or subnormal serum cobalamin concentrations patients were supplemented with oral cyanocobalamin (2501000 µg cobalamin orally once a day) and serum cobalamin concentrations increased in all of the dogs. Interestingly, not all patients had the same underlying cause of cobalamin deficiency, suggesting that the cause of cobalamin deficiency may not play a role in determining the success of oral supplementation. Similarly, a recent study looked at oral cobalamin supplementation in geriatric cats, only some of which were actually cobalamin deficient. Similarly to the study in dogs serum cobalamin concentrations increased significantly with oral supplementation. Also, more recently a small retrospective study in 16 cats with chronic enteropathy or intestinal lymphoma and low or low-normal serum cobalamin concentrations showed dramatic increases in serum cobalamin concentrations in all 16 cats. Further prospective studies are still ongoing.