Vitamin B12 (cobalamin) is essential for energy and protein metabolism in mammals. Vitamin B12 is a co-factor for two enzymes: methyl malonyl coenzyme-A mutase and methionine synthase; the former catalyses a step in the gluconeogenic pathway involving the conversion of propionate to succinic acid and the latter catalyses the conversion of homocysteine to methionine (Grace et al., 2010). Mature sheep and cattle require cobalt (Co) for the synthesis of vitamin B12 by rumen microorganisms; cobalamin is absorbed in the small intestine, is stored mainly in the liver, and is excreted in faeces, milk, and urine. However, very young, pre-ruminant livestock require preformed vitamin B12 in milk, milk replacers, or supplements (Grace & Knowles, 2012; Suttle, 2010). Cobalt is only useful to ruminants if incorporated into cobalamin, and cobalamin deficiency is always secondary to cobalt deficiency.
Cobalt/vitamin B12 deficiency impairs critical metabolic pathways. Under grazing conditions, sheep are more susceptible to Co/B12 deficiencies than cattle (Grace & Knowles, 2012; Suttle, 2010). In Co-deficient areas of Australia, such as coastal areas, calcium-rich or sandy soils producing pasture containing < 0.10 mg Co/kg DM, in young animals, and in mature animals on high energy diets containing < 0.16 mg Co/kg DM, dietary Co or injectable cobalamin supplementation are recommended to prevent or treat deficiencies (Freer et al., 2007; Suttle, 2010).
Two commercially available forms of injectable cobalamin, hydroxocobalamin (OHB12) and cyanocobalamin (CNB12), are registered for use in livestock. Cyanocobalamin is not the naturally occurring form of cobalamin in the animal's body; a reductive decyanation reaction is required to remove the cyanide group from CNB12 (Kim et al., 2008; Jeong & Kim, 2011) and only once the cyanide group is removed, CNB12 is converted to OHB12. Hydroxocobalamin is a common intermediate form, which is then transformed into methylcobalamin or deoxyadenosylcobalamin (5-deoxyadenosylcobalamin), the physiologically active forms or coenzymes of vitamin B12 (McDowell, 2000; Kim et al., 2008; Paul & Brady, 2017).
Although CNB12 is widely used in pharmaceutical preparations because of its chemical stability, OHB12 is better retained after parenteral administration than CNB12 (McDowell, 2000). A recent study (Gonzalez-Rivas et al., 2021) demonstrated that in non-Co/cobalamin deficient cattle given an equivalent dose of OHB12 or CNB12 (28 μg/ kg BW) a single injection of OHB12 was effective in increasing the level of cobalamin in the blood in the first 24 hours after injection, and this increase was maintained in the liver for at least 28 days, while CNB12 was unable to raise liver cobalamin reserves relative to basal levels at any time point. These results agreed with previous studies conducted on sheep and humans. Sheep studies have demonstrated the superiority of a single dose of OHB12 over CNB12 in maintaining high levels of cobalamin in plasma and liver (Judson et al., 2002). Judson (1996) showed that lambs given either 2 mg OHB12 or 2 mg CNB12 excreted about 45% and 70% of the respective doses within 6 hours of treatment and that OHB12 was more effective in raising liver cobalamin reserves than CNB12. This proves that OHB12 is the preferred cobalamin analogue to be used in sheep. Human studies have shown a sustained rise in plasma cobalamin levels and lower urinary excretion of cobalamin after the intramuscular injection of OHB12 than after the injection of CNB12 (Herbert et al., 1963). Most recently, Paul & Brady (2017) concluded that supplementation using OHB12 is preferred instead of CNB12 due to its superior bioavailability and safety in humans. It is widely accepted, therefore, that CNB12 has lower bioavailability, rapid mobilisation from the injection site, and fast renal clearance resulting in a shorter duration of activity compared with OHB12 (Hall et al., 1984; Judson, 1996; McDowell, 2000; Judson et al., 2002; Paul & Brady, 2017).
Weekly injections of CNB12 alone or in combination with butaphosphan, folic acid or other actives are commonly recommended for pigs, horses, dogs and dairy cows (Akins et al., 2013; Duplessis et al., 2017; Girard & Matte, 2005; Preynat et al., 2009; Weerathilake et al., 2019). However, scientific studies on the use of CNB12 in cattle do not report the duration of the activity of CNB12. To date, there is no existing scientific information that indicates that a single dose of CNB12 is effective in improving B12 status of ruminants over time. Therefore, it is suggested that livestock advisors and veterinarians consider this when recommending a single dose of CNB12 injection as a primary input to overcome or prevent Co deficiency in ruminants.