| Popis: |
The aim of this study was to determine the minimum single, intra-duodenal dose of tunicamycins that would significantly alter serum biochemistry (liver enzymes, albumin, total protein and total bilirubin) but would not induce overt clinical signs of the disease. This dose would subsequently be used as a challenge dose in the mucosal vaccination experiments. Six- to 12-month-old fine wool Merino wethers were given single doses of tunicamycins dissolved in 0.05 M aqueous NaOH at 12.5, 18.75, 25, 31.25, 37.5, 50, 62.5, 100, 150, 200, 250 or 1000 µg TMs/kg body weight. After 16 days or when clinical signs were observed, the sheep were euthanized and liver samples were taken for histopathology. Blood from each animal were collected from the jugular vein at 08.00-09.00 h each day and immediately prior to euthanasia for biochemical examination. Sheep that received doses less than 150 µg TMs/kg bw showed no evidence of neurological signs. At 150 µg TMs/kg bw, one sheep from the group showed episodes of moderate to severe tremor and ataxia after stimulation on Day 3 and was later euthanized. One case of mild neurological signs was seen following a dose of 200 µg TMs/kg bw; however, full recovery was seen once stimulation had ceased and no further periods of neurological signs were presented. A dose of 250 µg TMs/kg bw caused periods of lateral recumbency and convulsions in one sheep at Day 2.5 post-dosing and moderate stimulation-initiated tremor and ataxia in another at Day 4.5 post-dosing. All sheep given 1000 µg TMs/kg bw presented spontaneous, repeated episodes of lateral recumbency, severe tremor and convulsions on Day 3. While there were statistically significant differences between the levels of some serum liver enzymes and haematology parameters even between lower dose groups, the relevance of these differences is difficult to judge without further studies. The levels remained within normal ranges expected for sheep and the apparent differences persisted throughout the 16 days. Histopathological assessment revealed liver changes characteristic of tunicaminyluracil toxicity in sheep exposed to a single, intra-duodenal dose of 1000 µg TMs/kg bw. Diffuse mild lipidosis and hydrophic degeneration of hepatocytes were observed along with occasional hepatocytes that were undergoing coagulative necrosis accompanied by cytoplasmic changes. The results of this experiment indicate that further studies need to be conducted to establish the relevance of lower dosage results, and to therefore establish whether serum liver enzymes or haematological parameters are of any value in detecting subclinical toxicity resulting from intraduodenal administration of tunicamycins. However, whilst the data indicated that a study of the efficacy of mucosal vaccination is best assessed by challenging with doses of tunicamycins that cause overt clinical signs, regular assessment of LDH and TBIL during a challenge study may yet reveal a lessened impact at lower doses following vaccination. |
