Rickets has been seen as being a problem, mainly in cria, but also in older animals. It has been put down to the fact that during winter, vitamin D generation is reduced in temperate parts of the world because of lower intensity light and shorter days than would be experienced in native countries. The solution has been to give susceptible animals vitamin D, or multivitamin, injections periodically throughout the winter months. Some people have seen signs of abscesses from repeated injections and prefer fortnightly pastes, but the concept of intermittent treatment has been well accepted.
Is this necessary? Wouldn’t daily intake, as part of the normal diet, be as effective and more easily controlled?
To answer this we need to look more closely at what role vitamins, and vitamin D in particular, have in the body.
A vitamin is defined as an essential nutrient that the animal cannot synthesise in sufficient quantities, but requires in small amounts to avoid deficiency symptoms. They vary in their function; having effects on vision and fertility (Carotenoids, including vitamin A), combatting oxidation (Tocopherols including vitamin E & vitamin C), having a supportive role in a vast range of enzyme mediated biochemical pathways (B vitamins,) and hormonal-type roles like vitamin D.
Vitamin D is known to be involved in the process of bone development and therefore presumed to play a part in calcium and phosphorus metabolism, because bone is basically a calcium phosphate – this is wrong on two counts.
Firstly, the role of vitamin D is to increase the absorption of calcium from the diet by improving the permeability of the gut wall to calcium. It does this by controlling the production of a calcium binding protein that transports calcium across the gut and induces its absorption into bone. Secondly, it is not vitamin D but its metabolite 1,25-Dihydroxycholecaciferol (DHCC) that induces the calcium binding protein and bone resorption. 1,25-DHCC also moderates the resorption of phosphorus from the kidney tubules and so its role in calcium/phosphorus balance and bone function is essential.
There are around 10 forms of vitamin D; vitamin D2 & D3 being the most important. Of these, vitamin D3 is the prime precursor of 1,25-DHCC and is likely to be given to livestock as an additive.
In addition to dietary sources there are two pro-vitamins and one 7-dehydrocholesterol (DHC), which are found in the fatty tissues of the skin. Under sufficient UV light 7-DHC is converted to vitamin D3 and it is this pathway that “converts” sunlight into vitamin D. Where UV light is restricted (temperate regions in winter, housed animals etc.) the 7-DHC activation is insufficient to complement dietary sources and deficiency symptoms occur.
Natural dietary sources of vitamin D are limited to animal tissue and some sun dried forages and so the feeding of a supplement is necessary for alpacas of all ages.
The question is: during winter, is it better to inject or increase the daily intake of vitamin D Vitamin D3 can be stored in the lipid layers of the body and can be slowly released; this makes injectable sources attractive as it can be converted into the active form over months. Research has shown that over wintering injections of 1,000 – 1,500 IU/kg body weight in adult alpacas was adequate to combat vitamin D inadequacy. The results for both cria and adults gave 12 to 16 week’s cover – so 2 injections per year are probably required.
Where some owners prefer to use a vitamin paste, and so avoid the possibility of the negative aspects of injections, there has been a concern that vitamin D will be degraded in the fermentation chambers and so prove to be ineffective. It was assumed that microbial activity in the fore-stomach does degrade fat soluble vitamins and so large quantities need to be introduced in the paste for it to be effective. This was based on the apparently low levels, especially of vitamin E, circulating in plasma. However, recent work has shown this not to be the case. Using radioactive tracers in two forms of vitamin D (D2 & D3) and one of vitamin E, in single high doses, as would be used in dosing alpacas, researchers found no degradation in the rumen of dairy cows. Although not alpacas, the similarity of rumen and fore gut microbes strongly indicate fat soluble vitamin stability is not affected in camelids.
If pastes are an acceptable alternative to injections as there appears to be no loss of vitamin in the fore gut, why not simply increase dietary levels on a daily basis during the shorter days of winter?
The recommended daily requirement for alpacas is 30iu/kg body weight. This is based on the observation that bioavailability of vitamin D is lower than in other animals. However, assuming a 60 kg adult that is 1800 IU per day, which is about twice the amount given by injections, when they are administered. Interestingly, oral doses give about the same daily amount so it would seem that bioavailability of vitamin D is not an issue.
It was understood that vitamin D was absorbed by passive diffusion. This means that bile salts produced small droplets of fat in which the vitamin dissolved and was carried across the gut cell walls by one of three methods that did not require an energy transfer system. More recently researchers have determined that there are also active transport mechanisms involved and so bioavailability is higher than first thought. The implication is that daily dietary supplementation is probably more efficient than previously thought.
Practically, it is likely that, during most of the year in the UK, Vitamin D3 generation in the skin complements dietary input and the injections cover day-light shortfall, then increasing dietary intake over winter by a similar amount would be sensible.
This value would be 900iu per day, based on injected or paste levels and frequencies, although this amount would include an overage to compensate for the half-life of the stored product.
When feeding Camelibra NG-2 at the recommended maintenance levels, a 60kg alpaca would receive 900iu per day of vitamin D3, leaving 900iu to be supplied by sunlight. Over winter the “missing” amount from sunlight that an injection supplies can alternatively be supplied by increasing the amount of Camelibra from 1g to 2g per kg live weight per day. This would have added benefits in nutritional terms; most herbivores link feeding to day length and may have lower levels during winter. Shortfalls in forage intake could be compensated with higher levels of balancer. This would include enhanced levels of other fat-soluble vitamins, omega-3 fatty acids and other nutrients that may help offset negative effects of poor environmental conditions.
There is no definitive data on the bio-availability of vitamin D3, but the evidence of active transport and so less competition with vitamins A & E (as was once thought) implies that its value is high. Increasing feeding level over the winter months, with its added benefits of extra nutrients, could well be a cost effective, simple alternative to injections or pastes.