Hypophosphatemic Rickets in the Llama

Brad Smith, DVM, PhD
College of Veterinary Medicine, Oregon State University

For many of us the term RICKETS brings up memories of long forgotten high school and college biology classes and a dim memory of something to do with bones. When pressed we might be able to tell you that it also has something to do with weak bones and vitamin D and - oh -  something to do with sunlight.  The genesis of these stories was the recognition decades ago that sunlight plays a key role in the formation of vitamin D which in turn plays a central role in the development of strong bones.

The story of vitamin D and rickets in the llama and alpaca began with the recognition of a significant bone development problem in rapidly growing crias by Dr. Murray Fowler more than 10 years ago.  He observed that an unusually high percentage of crias had limb deformities.  He noted that effected animals tended to lie down more than herd mates, often had a shifting leg lameness, and frequently appeared to be reluctant to move. When investigated more carefully, he concluded that the cause of these problems  was rickets - a disease characterized by inappropriate formation and maintenance of bone structure. The lameness and reluctance to move was due to the pain caused by the weakened, damaged, or deformed growth plates in the legs.

Identifying the exact cause of the problem proved to be more challenging, however, since normal bone development requires the appropriate interplay of calcium, phosphorus, and vitamin D and inappropriate concentrations of any one of these components can result in rickets.  Once Dr. Fowler had identified the clinical cause of the leg deformities and lameness, i.e. rickets, he moved backwards to identify the source of the problem- specifically calcium, phosphorus, or vitamin D deficiencies.  The next part of the puzzle began to resolve when he measured serum phosphorus (P) concentrations  and found them to be uniformly low in clinically effected animals. He concluded that since the crias were still nursing, they must be receiving adequate calcium in the milk and ruled out insufficient calcium as a cause.  He also assumed that since most of the crias were housed out of doors that vitamin D concentrations were probably sufficient and therefore concluded that insufficient P in the diet was the most likely cause of the problem.  Based on these observations he concluded that the problem was most appropriately characterized as - HYPOPHOSPHATEMIC RICKETS.

The next clue in the story came when he examined multiple sources of supplemental dietary P to address the problem.  Some of the products were mono and disodium phosphate and a commercial product called P-16.  The interesting observation was that while the mono and disodium phosphate had a significantly greater amount of P per unit weight than did the P-16, the P-16 product was much more effective in helping animals with clinical rickets.

About this time, Dr. Pat Long started talking to Dr. Bob Van Saun and myself about some of the clinical problems he was having with poor-doing crias with limb deformities.  As the problem was being discussed Bob looked at the label on the P-16 and noticed that it also contained a significant amount of Vitamin D.  He also ran a series of forage analyses and estimated total P intake for the crias.  He concluded that unless the P requirements of the llama were markedly different than all other domestic livestock species, that there was likely adequate dietary P for normal bone growth and development.

The story continued to evolve when Bob and I began a pilot project funded by the WVLA to look at blood concentrations of calcium (Ca), P, and vitamin D (Vit D) in clinically effected and age and sex paired uneffected animals. The results, published in the J Amer Vet Med Assoc, identified several important issues concerning the problems, specifically: a) the problem was significantly more common in Fall born crias, b) vitamin D concentrations were very low in affected crias, and c) P concentrations were low (confirming earlier results). The upshot of the work was a recognition that Vitamin D, and not P, was the key player in our understanding of the problem.

A related problem was a recognition that we did not have an adequate understanding of normal P concentrations as a function of age in the llama.  With support from the WVLA we established normal mineral concentrations using a sample population of 270 carefully prescreened animals. The results for P revealed another important part of the puzzle.  Normal P concentrations in the young animal are markedly higher than in the mature llama with P concentrations in crias often in the 7 - 9 mg/dl range, values far higher than in the adult.  The practical result of the research (published in Am. J. Vet Res) was a recognition that crias previously classified as normal due to the use of normal ranges appropriate for adults, actually had  low serum P concentrations.

The next step in unraveling the mystery came with an investigation of the seasonal incidence of the problem.  We noticed that the problem was more common in Fall vs Spring crias leading us to hypothesize that sunlight availability was having an effect on vitamin D and P

A very elaborate WVLF funded study examined the interactions of age, sex, and season on alterations in serum Ca, P, and Vit D concentrations. The interesting observation was the very dramatic seasonal change in vitamin D concentrations, specifically the amplitude of the seasonal Vit D change with crias shifting from Vit D concentrations of < 50 nMol/l during the winter months to > 150 nMol/l during the late summer and early Fall.  The magnitude of this seasonal shift is far greater than has been observed in other species.  The magnitude of the P shift is also illustrated with P concentrations changing from peak concentrations of > 8 mg/dl during the summer and late Fall to about 5 mg/dl during the winter.  The important results of this study were: a) Vit D shifts radically as a function of season in the llama, b) Fall born crias will be experiencing their greatest surge in growth and need for vitamin D during the months when vitamin D concentrations are the lowest.   These results indicate that the Fall crias are particularly susceptible to hypophosphatemic rickets.

With the recognition that insufficient vitamin D is the underlying cause of rickets in the camelids, that Vit D and P concentrations vary dramatically as a function of time of year, and that Fall crias are the most susceptible group for developing clinical bone problems, the next WVLF funded study examined Vit D supplementation. The project examined the effects of varying dosages of Vit D administered by injection on blood Vit D, P, and Ca concentrations.  The study concluded that a dosage of 1000 international units per pound of body weight would boost Vit D and P concentrations into an optimal range for at least 2 to 3 months.  Based on our research our current recommendation is administration of 1000 IU (units) of vitamin D/# of body weight by injection twice during the winter.  We have typically supplemented animals in our herd during November/December and again in February/March.  With the shift to this program the incidence of angular limb deformities and other growth problems has diminished markedly.  Additional studies not described here have also looked at oral supplementation and preliminary examination of  the risk associated with excessive Vit D administration.

Summary: The most significant results to date have been: a) hypophosphatemic rickets is due to insufficient serum Vit D concentrations, b) P concentrations for crias are markedly higher than for adult camelids, c) serum Vit D and P concentrations vary dramatically as a function of season, d) Fall crias are more susceptible than Spring crias to the development of rickets, and e) supplementation of Vit D at a rate of 1000 IU/# by injection twice during the winter provides appropriate Vit D and P concentrations.  As a final note of caution, if you are planning on supplementing your llamas with Vit D during the winter, work with your local vet concerning the choice of Vit D products, help with injections if necessary, and confirmation that your dosages are correct since Vit D has the potential for causing serious problems if administered at too high a dosage.  Finally, we want to thank the WVLF for its generous support of llama research at Oregon State University.

Reproduced from www.orst.edu with permission of Susan Tornquist. Copyright © Oregon State University