Measurement of 25- hydroxycholecalciferol in captive grey parrots (Psittacus
erithacus).
Michael Stanford BVSc MRCVS
Birch Heath Veterinary Clinic
Birch Heath Road,
Tarporley,
Cheshire. CW6 9UU
Hypocalcaemia is a common syndrome
in African grey parrots (Psittacus erithacus) in captivity although the aetiology is still unconfirmed. It is expressed
clinically by hypocalcaemic seizures, poor breeding performance and osteodystrophy (Rosskopf et al, 1985; Hochleithner 1989).
Vitamin D deficiencies are common in poultry kept indoors in a UV deficient environment with insufficient dietary vitamin
D (Bernard et al. 1989) and a similar situation might be expected to occur with captive psittacines. Seed based diets contain
low levels of calcium and vitamin D3 and these are traditionally fed to African grey parrots in captivity. It has
been postulated that this contributes to a nutritional secondary hyperparathyroidism (Klasing 1998). A deficiency of ultraviolet
light in the 285-315nm range (UVb) range may also be implicated in the aetiology of the disease leading to a functional vitamin
D3 deficiency. The vitamin D3 metabolism of birds has been extensively reviewed (Taylor and Dacke, 1984, Bentley 1998). It
has been established that the domestic chicken secretes 7-dehydrocholesterol (provitamin D) onto the featherless skin of the
skin and feet (Tian et al 1994). Conversion of the provitamin D to cholecalciferol (vitamin D3) occurs by an ultraviolet light
dependent isomerisation reaction. Cholecalciferol is a sterol prohormone which is subsequently activated by a 2 stage hydroxylation
process.
Cholecalciferol is initially metabolised to 25 hydroxycholecalciferol in
the liver. Synthesis of 25 hydroxycholecalciferol is regulated by product inhibition. 25 hydroxycholecalciferol is transported
to the kidney via carrier proteins and converted to either 1, 25 dihydroxycholecalciferol or 24, 25 dihydroxycholecalciferol,
the active metabolites of cholecalciferol in the domestic fowl. Serum concentration of 25 hydroxycholecalciferol is considered
the most reliable measure of the vitamin D status of an individual due to its long half-life compared with other vitamin D
metabolites (Hollis et al.1999). Traditionally radio immunoassays (RIA) have been used to assay 25 hydroxycholecalciferol
but more recently enzyme immunoassays (EIA) have become available with the advantages of both convenience and economy. Forty
African grey parrots were randomly selected from a group of 100. The group are all adult African grey parrots of known sex
that have been fed on an unsupplemented seed diet for the previous 12 months prior to the start of the study. As part of their
annual health checks faecal samples were taken from each bird for parasitology and gram stain examination. Blood samples taken
from each bird were subject to routine haematological and biochemical tests including circovirus, polyoma and chlamydophilia
PCR. Each bird was examined clinically and by laparoscopy. On the basis of these tests only birds with
no evidence of clinical disease were included in the study.
The group were blood
sampled under isoflurane anaesthesia with the informed consent of the owner for 25 hydroxycholecalciferol. The blood testing
was performed outside the breeding season to minimise the effects of both oestrogen and seasonality on both vitamin D3 and
calcium levels (Bentley 1998). ). All the blood samples were taken into heparin and immediately cooled to -70 C prior to analysis.
The IDS OCTEIA 25 hydroxycholecalciferol assay (IDS Ltd. 10 Didcot Way,
Tyne and Wear, Newcastle upon Tyne) was used in this study for the quantitative determination of 25 hydroxycholecalciferol.
Each sample was assayed in duplicate with no significant difference between assay results for the same sample.
The results are shown in figure 1. 25 hydroxycholecalciferol was consistently recovered from
blood samples taken from the group of 40 healthy African grey parrots using the enzyme immunoassay. The results show a wide
variation in the level of 25 hydroxycholecalciferol with a range between 7.2-380nmol/l. The mean (n=40) was 116.52nmol/l with
a standard deviation of 126.70 nmol/l. A study in captive green iguanas revealed a similar wide variation in 25 hydroxycholecalciferol
(Mitchell et al 2002). A simple explanation for this would be that different birds were subject to varying levels of unfiltered
UVb light. A further study is investigating the effects of UVb on 25 hydroxycholecalciferol on the same group of birds.
In the laying hen 25 hydroxycholecalciferol would not be expected to fall below 26nmol/l and
would normally be above 50nmol/l (Dacke G.C. 2000). Although no normal ranges are available for psittacines at the present
time 16/40 birds had 25 hydroxycholecalciferol levels below 50nmol/. Chronic deficiency of vitamin D would be expected to
lead to hypocalcaemia and secondary hyperparathyroidism. This would potentially have significant consequences for the African
grey parrot known to suffer from hypocalcaemia and related disorders of calcium metabolism. In addition vitamin D has been
found to have a profound effect on the immune system (Aslam et al. 1998) Further studies are ongoing using this 25 hydroxycholecalciferol
assay to derive an adequate level of vitamin D in complete psittacine diets under different ultraviolet light regimes. The
25 hydroxycholecalciferol enzyme immunoassay has practical uses with other species known to suffer from disorders of calcium
metabolism such as iguana iguana.
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