The growth hormone (GH) secretagogue activity of variable dosages of clonidine (16.5, 50, 150, and 450 microgram/kg of body weight), given orally mixed with the daily food ration, was evaluated in young and old dogs. Significant (P < 0.05) increase in plasma GH concentration was detected at all dosages tested in young dogs and in response to all but the lowest dose tested in the old dogs fed the clonidine-containing diet. Old dogs had plasma GH concentration that exceeded that of young dogs when higher doses of clonidine were used. A clonidine (100 microgram/kg)-supplemented diet was fed to middle-aged dogs twice daily for 30 days. Significant (P < 0.01) increase of plasma GH concentration was observed on the first day of the feeding trial, but was undetectable by day 30. After feeding the clonidine-enhanced diet for 30 days, the effects on thymic morphology were variable, and there was no effect on plasma thymulin titer. Clonidine-fed dogs had significantly increased lymphocyte blastogenic responsiveness to mitogens, compared with that of control dogs, when evaluated as stimulation index.

A case-control study was done to identify factors associated with the development of equine degenerative myeloencephalopathy (EDM). Questionnaires were mailed to the owners of 146 horses admitted to the New York State College of Veterinary Medicine between November 1978 and June 1987 and diagnosed as having EDM by histologic examination. Questionnaires also were sent to owners of 402 clinically normal horses admitted to the college during the same period. Data were compared between the EDM-affected and control groups (56 and 179 questionnaires returned, respectively). Risk factors identified included the use of insecticide applied to foals, exposure of foals to wood preservatives, and foals frequently spending time on dirt lots while outside. Foals spending time outside on green pastures was a protective factor. Foals from dams that had had an EDM-affected foal were at higher risk of developing EDM than were foals from other dams.

Hematologic and serum biochemical values were determined in blood samples from 217 donkeys (Equus asinus). Donkeys were classified on the basis of size, sex, age, and whether they were domestic or feral. Parametric (mean +/- 2 SD) and nonparametric (2.5th to 97.5th percentile) reference ranges were calculated for each analyte. For all donkeys, 26 of 46 analytes significantly departed from gaussian distribution. Serum lactate dehydrogenase activity in miniature donkeys was higher than that in other donkeys. Differential leukocyte counts in feral donkeys differed from those in other types in ways that suggested that the former had smaller parasite loads or experienced greater stress. Erythrocyte, lymphocyte, and platelet counts and fibrinogen, glucose, inorganic phosphorus, and potassium concentrations decreased with age. Eosinophil counts, mean corpuscular volume, and plasma protein, serum protein, and serum globulin concentrations increased with age. Female donkeys had significantly (P < 0.05) higher mean corpuscular hemoglobin concentration and leukocyte and neutrophil counts than did male donkeys. Mean corpuscular hemoglobin increased with age, and females had higher values than did males of all age groups. An interaction between age and sex was observed for alkaline phosphatase activity, with a trend for decreased activity with age.

Blood samples were obtained from 20 bison (Bison bison) from a ranch in northern lower Michigan, as well as from 20 free-ranging bison of the same sex and similar age from the Badlands National Park in South Dakota. Hematologic and serum biochemical values were determined. The values were comparable in both groups, except for those for BUN, aspartate transaminase, and phosphorus, which were significantly (P < 0.001) higher in the ranched bison than in the free-ranging bison. These differences were attributed to nutritional effects. Impact of age on blood characteristics was assessed in the ranched bison only by comparing values from calves weighing less than 185 kg with those from bison weighing more than 185 kg. Calves had significantly (P < 0.001) higher values for phosphorus and RBC counts and lower total protein values than adults. Adult bison had higher eosinophil and neutrophil counts with lower numbers of lymphocytes, suggestive of a stress leukogram, whereas calves had the typical bovine neutrophil:lymphocyte ratio.

The directional (chemotactic) and random migration activities of neutrophils from cows and newborn and 2-week-old calves were determined by use of the chemotaxis-under-agarose assay. Blood samples were stored for 2, 24, or 48 hours and at 4 or 25 C before testing. During the assay, cells were incubated at 17, 27, or 37 C. The assay was found suitable for testing the directional and random migration activities of neutrophils from cattle. Directional migration of neutrophils was diminished (P < 0.05) when cells were incubated at 17 or 27 C, compared with data from incubation at 37 C. Random migration of neutrophils was unaffected by test incubation temperature. Significant (P < 0.05) differences were found between cows and calves regarding the percentage number and viability and the directional and random migration activities of neutrophils. Neutrophils from cows were adversely affected to a greater extent by prolonged sample storage times or low storage temperature than were neutrophils from calves. Results indicate that a sample storage time of up to 24 hours, a sample storage temperature of 25 C, and a test incubation temperature of 37 C provided optimal conditions for testing the migratory activities of neutrophils from cattle.

A study was conducted to develop valid estimates of lymphocyte count (LC; cells per microliter) of individual, clinically normal dairy cattle. Estimated weighted regression was used on repeated measures of individual LC to examine 6 models predicting LC as a function of age in cattle not infected with bovine leukemia virus. The generalized growth curve model of analysis of variance was used to estimate intercepts, slopes, and prediction limits for the models and to compare the LC-to-age relationship between Holstein and Guernsey breeds. The best-fitting model (P = 0.0001) with the narrowest prediction interval was LC = 4,414.4 - 84.6X, where X = (age - 48) if age less than or equal to 48 months, and X = 0 if age > 48 months, and 163.6 and 8.1 are the SE of the estimates, respectively. Upper one-sided 95%-predicted normal LC tended to be higher than estimates derived from traditional hematologic keys that use confidence limits of mean LC. Difference was not found in the LC-to-age relationship between the Holstein and Guernsey cattle (P = 0.67). Results of this study provided estimates of normal LC that are more specific in diagnosing lymphocytosis in individual cattle.

The effect of age and training status on the pharmacokinetics of flunixin meglumine was evaluated in 16 Thoroughbreds. Horses were assigned to 1 of 3 groups on the basis of age and training status: group A (n = 6), horses in active training and less than or equal 5 years old; group B (n = 5), horses out of training for a minimum of 6 weeks and less than or equal to 5 years old; and group C (n = 5), horses out of training for at least 2 years and greater than or equal to 9 years old. After administration of 500 mg of flunixin meglumine IV, multiple serum and urine samples were obtained over 24 hours and assayed for flunixin by high-performance liquid chromatography. Although the mean distribution rate constant and volume of distribution were similar for the 3 groups, mean total body clearance and elimination rate constant were significantly (P < 0.05) greater and half-life significantly (P < 0.01) less in groups A and B, compared with group C. Differences in pharmacokinetic values were not observed between the horses in groups A and B. In addition, the changes in clearance, elimination rate constant, and half-life of flunixin were found to significantly (P < 0.05) correlate with age. The results of this investigation indicated that age, but not training status, influences disposition of flunixin meglumine in Thoroughbreds.

The antiviral activity of recombinant DNA-derived bovine alpha 1-1 interferon on an established swine testicular cell line and primary testicular cell cultures derived from swine of various ages (2 days, 3 weeks, and 5 weeks) was determined. Bovine interferon induced a dose-dependent increase in 2-5A synthetase in testicular cells, regardless of the source of the cells. Furthermore, interferon inhibited replication of vesicular stomatitis virus to an equivalent extent in all testicular cell cultures. The results indicate that 2-5A synthetase is a reliable marker of interferon activity in swine testicular cell cultures and that the induction of 2-5A synthetase and antiviral effects of recombinant bovine interferon in primary testicular cell cultures are not dependent on the age of the donor animal.

Spinal-evoked potentials were recorded from 2 litters of clinically normal mixed-breed dogs between 35 and 300 days of age. Summated responses to tibial nerve stimulation were recorded from percutaneous needle electrodes placed at L7-S1, L4-5, T13-L1, C7-T1, and the cisterna cerebellomedullaris. The ulnar nerve was stimulated with recordings at C7-T1 and the cisterna cerebellomedullaris. Amplitudes did not change significantly with age, but were significantly (P < 0.05) different between various recording sites. On day 35, segmental and overall (L7-cisterna cerebellomedullaris) conduction velocities were less than half of the adult values. Spinal cord conduction velocities increased with age, reaching adult values at approximately 9 months of age. It was determined that quadratic equations best predicted the conduction velocities during maturation.

Cartilage retention strengths of laryngoplasty prostheses were compared in larynges of 2-, 3-, and 4-year-old horses, using doubled polyester and expanded polytetrafluoroethylene prostheses. Bilateral laryngoplasties were performed on each of 15 (seven 2-year-old, two 3-year-old, and six 4-year-old) larynges, which were collected at an abbatoir. Prostheses were secured to a mechanical testing machine, and tension causing arytenoid cartilage abduction was applied, until total failure of the cartilage or prosthesis resulted. Tension caused cricoid cartilage failure in 1 specimen, and muscular process cartilage failure in the remainder. There was no significant effect of age, prosthetic material, or side of prosthesis placement on cartilage retention of the prostheses. Additionally, frequency of multiple load-displacement peaks, indicating partial muscular process failure, was not affected by age or prosthetic material variables.