Ocular lesions of 6-aminonicotinamide toxicosis in rabbits

6-Aminonicotinamide, an antimetabolite of nicotinamide, given by intraperitoneal injection produced diarrhea, ascending paresis/paralysis, death, and bilateral ocular alterations in both sexes of New Zealand white and Dutch belted rabbits. Ocular vascular lesions consisted of iridal congestion with iridal hemorrhage and associated acute iritis and aqueous flare in some rabbits. Cytoplasmic swelling, vacuolation, and loss of staining affinity that represented hydropic change were present in both layers of ciliary epithelium and the inner layer of iridal epithelium. Cells in the outer layer of the iridal processes and the ciliary ridge, were most severely affected. Vacuoles were also present in the retinal pigment epithelium and scattered throughout the outer plexiform layer of the retina with a few in the inner and outer nuclear layers. Ocular alterations were prevented by simultaneous administration of nicotinamide and their development appeared related to nicotinamide deficiency. No ocular alterations were caused by nicotinamide administration alone.     

Ultrastructural ocular lesions of 6-aminonicotinamide toxicosis in rabbits

6-Aminonicotinamide, given by intraperitoneal injection to male and female Dutch belted rabbits, produced swelling and vacuolation of ciliary and iridal epithelium plus vacuolation of the retinal pigment epithelial and outer plexiform layers of the retina. By transmission electron microscopy, inner and outer ciliary epithelial cells and inner iridal epithelial cells contained numerous coalescing, membrane-bound vacuoles of the cytocavitary network. These vacuoles were viewed as numerous interconnecting, intracytoplasmic cavities in scanning electron micrographs. Swelling of vacuolated epithelial cells and the presence of fibrin and proteinaceous fluid in the ciliary stroma resulted in thickening of the anterior ciliary processes with the formation of surface alterations detectable by scanning electron microscopy. In transmission electron micrographs the vacuoles in the retinal pigment epithelium were large, electron-lucent spaces and the vacuoles in the outer plexiform layer of the retina appeared to be intracytoplasmic spaces in axons of photoreceptor cells. Distention of cytocavitary structures has been reported in glial cells of animals given 6-aminonicotinamide and this change was apparently due to alterations in ion and water movement across cellular membranes that resulted in intracellular edema.     

Lesions in the cat spinal cord following local injections of 6-aminonicotinamide.

Lesions were made in the spinal cord of cats by means of local injections of 6-aminonicotinamide. The concentration needed to induce primary demyelination also caused extensive axonal death. In the early stages following injection there was evidence of astrocyte and oligodendrocyte destruction and with longer survival times most of the surviving demyelinated axons were remyelinated by Schwann cells. This experimental system indicates that Schwann cell remyelination of central axons follows astrocyte destruction, but it was not considered a suitable model system for the study of cellular relationships in remyelination because of the extensive concomitant axonal damage.     

The toxicity of Datura stramonium (thorn apple) to horses

Meal contaminated by Datura stramonium seeds at the rate of 0.5% by weight was fed to two horses. Both horses showed clinical signs of depression, anorexia, weight loss, rapid heart and respiration rates, mydriasis, polyuria, polydipsia and diarrhoea. Both recovered with treatment. Maize screenings contaminated by the seeds had been used in the manufacture of the meal.     

Conditioning taste aversions to locoweed (Oxytropis sericea) in horses

Locoweed (Oxytropis sericea) is a serious poisoning problem for horses grazing on infested rangelands in the western United States. Our objectives were to determine 1) whether lithium chloride or apomorphine would condition aversions to palatable foods, and at what doses, and 2) whether horses could be averted to fresh locoweed in a pen and grazing situation. Apomorphine was not an acceptable aversive agent because at the dose required to condition an aversion (> or = 0.17 mg/kg BW), apomorphine induced unacceptable behavioral effects. Lithium chloride given via stomach tube at 190 mg/kg BW conditioned strong and persistent aversions to palatable feeds with minor signs of distress. Pen and grazing tests were conducted in Colorado to determine if horses could be averted to fresh locoweed. Pen tests indicated that most horses (5/6) were completely averted from locoweed. Treated horses ate 34 g of fresh locoweed compared to 135 g for controls (P < 0.01) during three pen tests when offered 150 g per test. One horse (T) in the treatment group ate locoweed each time it was offered in the pen, but ate no locoweed while grazing. In the grazing trial, control horses averaged 8.6% of bites of locoweed (P < 0.01) during the grazing portion of the study, whereas treated horses averaged <0.5%. One treated horse (S) accounted for all consumption; he consumed 15% of his bites as locoweed in a grazing bout on d 2 of the field study. Thereafter, he was dosed a second time with lithium chloride and ate no locoweed in the subsequent 5 d. Three of six horses required two pairings of lithium chloride with fresh locoweed to condition a complete aversion. The results of this study indicate that horses can be averted from locoweed using lithium chloride as an aversive agent, and this may provide a management tool to reduce the risk of intoxication for horses grazing locoweed-infested rangeland.     

The toxicity of Datura stramonium (thorn apple) to horses.

Meal contaminated by Datura stramonium seeds at the rate of 0.5% by weight was fed to two horses. Both horses showed clinical signs of depression, anorexia, weight loss, rapi heart and respiration rates, mydriasis, polyuria, polydipsia and diarrhoea. Both recovered with treatment. Maize screenings contaminated by the seeds had been used in the manufacture of the meal.     

Pharmacokinetics of phenytoin (diphenylhydantoin) in horses

The pharmacokinetics of the anti-convulsant phenytoin were investigated in clinically healthy horses after oral (p.o.) and intravenous (i.v.) administration. A single dose of phenytoin (8.8 mg/kg body weight) was given i.v. as a bolus to nine horses and one horse received 13.2 mg/kg. A two-compartment open model was used to describe the disposition of phenytoin. Four of the horses that received an i.v. dose (three at 8.8 mg/kg and one at 13.2 mg/kg) were then given the same dose 3 days later by the oral route. Phenytoin achieved a peak concentration in serum within 1–4 h after p.o. administration and was poorly absorbed with a bioavailability of 34.5 ± 8.6%. Oral dosage regimens were calculated on the basis of a dosing interval of 8 h to provide average serum steady-state concentrations of 5 and 10 μg/ml for phenytoin.     

Behavioral and physiological responses of horses (Equus caballus) to head lowering

Horse trainers often report that lowering the height of a horse’s head so the poll is below the height of the withers can induce a calming effect during training. Four groups of horses were used in a 2-part study to investigate the behavioral and physiological effects of head lowering in horses. In Part 1, Group A had no experimental stimuli applied and horses in Group B were trained to lower their heads when presented with a specific stimulus by the handler. The stimulus for head lowering was the application of downward pressure on the headcollar via the lead rope until the horse lowered its head such that its lips were approximately at mid-cannon (third metacarpal) height, whereupon the pressure was released. The stimulus was applied again if the horse raised its head during the 300-second test period. In Part 2, Groups C and D were aroused until their heart rates exceeded 100 beats per minute (bpm). Group C had no further experimental stimuli applied whereas Group D lowered their heads as a response to the above stimulus for a period of 300 seconds. Repeated measures analysis showed that there was no difference between the heart rate of Groups A and B or Groups C and D but that the heart rate of Groups A and B were lower than Groups C and D during the 300-second post-arousal (P < 0.001). The horses in Groups A and B were more likely to contact the handler (P < 0.001), exhibit licking and chewing (P < 0.001), rest a hindleg (P < 0.001), and sniff the ground (P < 0.001) than those in Groups C and D. The number of stimuli required to maintain the head in a lowered position was greatest during the first 30 seconds (P = 0.012 and P < 0.001, Parts 1 and 2, respectively). The current study has shown that head lowering in horses does not influence cardiac responses, even after the horses had been aroused to have their heart rates above 100 bpm. Therefore, it is not a method that will aid in calming an aroused horse in training. Contrary to popular belief, there was no association with licking-and-chewing and head lowering, nor with these behaviors and response acquisition.     

Acquired resistance of horses to Amblyomma cajennense (Fabricius, 1787) ticks

Acquired immunity of horses to larvae, nymphs and adults of the Amblyomma cajennense tick was evaluated through three consecutive experimental infestations of tick-bite na?ve hosts. Data from these infestations were compared to those from field-sensitized horses and donkeys. It was observed that tick-bite na?ve horses developed a low level of resistance after two infestations as shown by a significant decrease in larval yield and a tendency for lower engorged weight of nymphs during third infestation. Ticks fed on field-sensitized horses had a similar biological performance to that observed on the third infestation of tick-bite na?ve horses but the mean engorged nymph weight was significantly lower than that of the first infestation from tick-bite na?ve horses. Donkeys presented the strongest resistance with significantly lower engorged weights of all instars and of the egg mass compared to the first infestation of tick-bite na?ve horses. Donkeys also displayed a significantly higher resistance than field-sensitized horses as demonstrated by significantly lower egg mass weights. Overall these results indicate that donkeys but not horses maintain a strong resistance to A. cajennense ticks. The importance of these findings in relation to vectoring of tick-borne diseases is discussed.     

Black grain mycetoma (maduromycosis) in horses

Black grain mycetoma occurred in two horses, one a cross-bred pony and the other a Thoroughbred racehorse. Lesions were limited to wart-like growths in one case and a deep-seated girth gall in the other. In both cases the granules in the affected tissues were similar in pigmentation and structure to those produced by Curvularia geniculata in dogs. Since all the specimens were received in formalin, the causative fungus could not be isolated. These are the first cases of maduromycosis in animals to be recorded in South Africa.