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Topic:Seasonal Variation
Seasonal variation in horses refers to the physiological and behavioral changes that occur in response to different seasons. These changes can affect various aspects of equine biology, including coat growth, reproductive cycles, and metabolic processes. Horses may exhibit alterations in their feeding patterns, activity levels, and thermoregulation as they adapt to environmental shifts such as temperature fluctuations and daylight length. Understanding these seasonal adaptations is important for managing horse health and welfare. This page aggregates peer-reviewed research studies and scholarly articles that explore the mechanisms, effects, and implications of seasonal variation on equine physiology and behavior.
Responses of guinea-pig lung parenchymal strips to tracheobronchial lavage fluid from horses affected with summer pasture-associated obstructive pulmonary disease. The response of parenchymal strips from guinea-pig lungs to tracheobronchial lavage fluid (TBLF) collected from 8 normal horses and from 8 affected with summer pasture-associated obstructive pulmonary disease (SPAOPD) was determined. TBLF was collected during the summer (July) and winter (February) seasons. The serum/TBLF urea nitrogen ratio was used to standardize the mediator concentration in the TBLF. Four strips were used from each guinea-pig. The first strip did not receive any antagonist and served as the control. The second, third and fourth strips received antagonists of PGE2, LTD4 and...
A case-control study of grass sickness (equine dysautonomia) in the United Kingdom. A case-control study was performed to investigate the epidemiology of grass sickness in the United Kingdom from 1992 to 1995. Data were collected by means of postal questionnaire when cases of grass sickness were identified. Sets of three questionnaires were posted to owners of, or veterinary surgeons attending, cases of grass sickness, with a request to provide information on the case, on one healthy animal on the same premises as the case and on another healthy animal on other premises. Controls were matched to cases by date of onset. After univariate analyses, the probability of grass sickn...
Testicular inhibin in the stallion: cellular source and seasonal changes in its secretion. The cellular localization of inhibin alpha, betaA, and betaB subunits, 3beta-hydroxysteroid dehydrogenase (3beta-HSD), and cytochrome P450 aromatase (aromatase) in stallion testes was investigated. In addition, detailed seasonal changes in circulating immunoreactive (ir)-inhibin were investigated in correlation with testosterone, estradiol, LH, and FSH. Inhibin alpha subunit-positive staining was observed in Sertoli cells, and more clearly positive staining was noted in Leydig cells. Inhibin betaA and betaB subunits were also stained in both types of cells. Immunoreactivity of 3beta-HSD and ar...
Relationship between plasma testosterone concentrations and age, breeding season and harem size in Misaki feral horses. Jugular vein blood samples were collected from 23 young and sexual mature feral stallions to examine the relationship between plasma testosterone concentration and age, breeding season or harem size. Testosterone concentration increased with the age of the stallions until they formed their own harems, at about 4 to 6 years old. Seasonal variations in testosterone concentrations were observed, and found to be significantly higher (P<0.001) throughout the breeding season than non-breeding season, from 3 years of age. Testosterone levels were correlated with harem size for individual stallions...
Inhibin secretion in the stallion. To examine the physiological role of inhibin in the stallion, a heterologous radioimmunoassay (RIA) based on a bovine RIA was validated and used to measure immunoreactive (ir)-inhibin concentrations in plasma and testicular homogenates. The bioactivity of equine testicular inhibin was also examined using an assay for suppression of FSH secretion from rat anterior pituitary cells. In addition, to identify the cell responsible for secreting testicular inhibin, the localisation of inhibin in the testis was investigated by an immunohistochemical method using a polyclonal antibody against (Tyr30)-p...
Influence of melatonin and oestradiol on the opioidergic regulation of LH and prolactin release in pony mares. The aim of this study was to investigate the influence of oestradiol, melatonin and season on the opioid regulation of LH and prolactin release. Effects of the opioid antagonist naloxone (0.5 mg/kg) on LH and prolactin secretion were determined in ovariectomized pony mares. In experiment 1, mares in January (n = 6) were pretreated with oestradiol benzoate (5 micrograms/kg) for 20 days. In experiment 2, beginning in May, mares (n = 7) received melatonin (15 mg) for 15 days and subsequently a combination of melatonin plus oestradiol for 20 days. In experiment 3, beginning in May, mares (n = 6) w...
Age, breed, sex and seasonality as risk factors for equine laminitis. A case-control study was conducted at the Texas Veterinary Medical Center between January 1, 1986 and December 31, 1991. Logistic regression was used to assess age, breed, sex, and seasonality as risk factors for equine laminitis. There were 70 acute cases, 183 chronic cases, and 779 controls. No statistical association was found between age, breed, sex, or seasonality and the diagnosis of acute laminitis. For chronic cases, the estimated odds ratio was statistically significant for age (OR = 1.05, 95% CI (1.02, 1.08)) and for the diagnosis of laminitis in the third quarter of the year (OR = 2...
Use of statistical models to evaluate racing performance in thoroughbreds. To develop a statistical model to evaluate the influence of specific parameters on racing performance in Thoroughbreds. Methods: Survey. Methods: Racing records of Thoroughbreds performing in Louisiana from 1981 to 1985. Methods: Race results for 20 randomly selected days from 5 racetracks during 5 years were analyzed, using regression analysis. Results: The most influential parameter was distance raced. There were significant differences in racing performance among horses at different tracks. At the tracks examined, fast racing surfaces resulted in significantly faster finish times than good ...
Seasonal rhythm of semen characteristics of a Brazilian breed (“Mangalarga”) stallion. An attempt has been made to define semen seasonality in a horse in the Southern Hemisphere. Repeated measurements of three variables in the semen were made for 36 months (Jan/90-Dec/92) in a 21-year old "Mangalarga" stallion living under natural photoperiod and temperature conditions in a farm situated in São José do Rio Pardo, São Paulo, Brazil (latitude 21 degrees) 36'S; longitude 46 degrees 53' W). The horse fed on natural pasture and a nutritionally balanced feed twice a day (11:00 and 17:00 h). Water and mineral supplement were available ad libitum. Semen was collected almost daily by ...
[The effect of the season and sexual stress on the concentration of testosterone and estradiol-17beta in the seminal plasma of stallions]. Semen from three stallions was collected weekly for six months (December through May) to determine semen parameters and hormones (testosterone and estradiol-17 beta) in seminal plasma. Once a month three ejaculates were collected at intervals of one hour and examined accordingly. Testosterone and estradiol-17 beta were also determined in peripheral blood plasma (V. jugularis) collected twice a week. Semen parameters (volume, gel-free volume and sperm concentration) were clearly influenced by season. The testosterone concentration in peripheral blood plasma was lowest during December whereas in...
Seasonal variation in the vector competence of Culex tarsalis (Diptera:Culicidae) from the Coachella Valley of California for western equine encephalomyelitis and St. Louis encephalitis viruses. The vector competence of Culex tarsalis Coquillett from the Coachella Valley of California for western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses was monitored monthly from February to November 1993. The concentration of WEE virus required to infect 50% of the females increased during summer coincidentally with ambient temperature and was highest during July. Transmission rates of WEE virus were high during March, low during May-June, and high again during July-September. Females expressed both mesenteronal escape and salivary gland barriers limiting WEE virus diss...
Seasonality and freezability vs routine parameters in stallion semen. The fertilizing ability of stallion semen was analyzed using fresh and frozen samples, obtained before (June-July) or during (October-November) the breeding season. Thirty ejaculates obtained from 4 stallions were used. The analysis comprises routine seminogram; ATP concentration (Comhaire et al., 1983); subjective and objective motility and sperm velocity (Makler, 1980). Freezing was done following the technique of Martin et al. (1979). Sperm velocity, ATP content and objective motility in ejaculates of subjective motility >50% show values of 14.0 + or - 0.84 mu m s(-1); 4.8 + or - 2.7x10(...
Chronic obstructive pulmonary disease in horses in Louisiana. To evaluate chronic obstructive pulmonary disease of horses in Louisiana by assessing the signalment, history, environmental factors, clinical signs, and treatment of such horses. Methods: Epidemiologic mail survey. Methods: 83 of 240 veterinarians contacted by mail agreed to take part in the survey. Veterinarians contacted were listed as mixed-animal or equine practitioners in the 1991/1992 directory of the Louisiana Veterinary Medical Association or had submitted a specimen from a horse to the Louisiana Veterinary Medical Diagnostic Laboratory within the past 2 years. Methods: The survey con...
Serum progesterone levels in mares in winter and during transitional periods. Weekly blood samples were collected for serum progesterone determination from 68 mares during the period from September 1990 to June 1991. A total of 78% (53/68) of the mares became anoestrous: 88% (15/17) of mares with foals and 75% (38/51) of dry mares (p = 0.323). Six mares of the 15 which continued cycling showed persistent corpus luteum for 77 days on average (range 42-106 days). Anoestrus began between September and February, most frequently in December, and ended in January to June, most frequently in March. Mares with foal at foot became anoestrous in 2 separate groups, in September an...
Plasma melatonin in the horse: measurements in natural photoperiod and in acutely extended darkness throughout the year. Plasma melatonin was measured at the winter and summer solstices and the autumn and spring equinoxes in four mares held under natural conditions at 35 degrees S. At all seasons the onset of the nightly elevated melatonin was coincident with or after the time of sunset and the melatonin offset after the time of sunrise. The duration of elevated melatonin was not different from the duration of natural scotophase for each season, with the duration of elevated melatonin longer in winter than the other seasons. Immediately following each 24 hr sampling two mares were resampled in acutely extended d...
Photic headshaking in the horse: 7 cases. Seven horses with headshaking are described. No physical abnormalities were detected in any of the cases. Six of these horses had onset of clinical signs in the spring. The role of light was assessed by application of a blindfold or dark grey lens to the eyes, covering the eyes with a face mask and observing the horse in total darkness outdoors. Cessation of headshaking was observed with blindfolding (5/5 horses), night darkness outdoors (4/4 horses) and use of grey lenses (2/3 horses). Outdoor behaviour suggested efforts to avoid light in 4/4 cases. The photic sneeze in man is suggested as a ...
The prevalence and epidemiology of gastrointestinal parasites of horses in Victoria, Australia. A quantitative post mortem study of 150 horses from Victoria was conducted to determine the prevalence and epidemiology of gastrointestinal parasites. A total of 42 species of metazoan parasite was found. The following species of non-cyathostome parasite were found (% prevalence): Trichostrongylus axei (51%); Habronema muscae (13%); H. majus (2%); Draschia megastoma (5%); Gastreophilus intestinalis (81%); G. nasalis (29%); Parascaris equorum (5%); Anoplocephala perfoliata (29%); Fasciola hepatica (0.7%); Oxyuris equi (7%); Strongylu vulgaris (23%); S. edentatus (23%); S. equinus (3%); Crateros...
Epidemiological risk factors associated with a diagnosis of clinical cyathostomiasis in the horse. Multiple logistic regression was used to assess epidemiological risk factors associated with the diagnosis of cyathostomiasis in 87 cases of chronic diarrhoea in the horse. Age, season and the period since last receiving anthelmintics were identified as important risk factors using chi-square and two-sample t test analyses, whereas access to grazing, shared grazing with other horses and recurrence of signs were only weakly associated with a diagnosis of cyathostomiasis. Multivariate analysis of the parameters using logistic regression was performed. The final model included age, season and tim...
Involvement of endogenous opioids in the regulation of LH and testosterone release in the male horse. To investigate the involvement of endogenous opioids in the regulation of gonadotrophin release in male horses, effects of the opioid antagonist naloxone (0.5 mg kg-1 i.v.) on plasma LH and testosterone concentrations and the possible influence of season and of gonadal steroids were investigated. To determine quantitative as well as qualitative changes in gonadotrophin release, LH concentrations were measured by radioimmunoassay and by an in vitro bioassay. Experiments were performed in May, August and December. In stallions, basal LH secretion in May and August was significantly higher than i...
Localization of aromatase in equine Leydig cells. Stallion testes secrete large amounts of estrogens, but the cellular location of the enzyme that converts androgens to estrogens, cytochrome P450 aromatase, has not been determined. The goal of the present study was to immunocytochemically localize stallion testicular aromatase using a polyclonal antibody generated against human placental cytochrome P450 aromatase. Testes were obtained from 12 stallions from 2 to 23 years of age, during both the breeding and non-breeding seasons. Immunoreactivity was confined to the Leydig cells in all testes examined. No immunostaining was observed in the Ser...
Effects of the opioid antagonist naloxone on release of luteinizing hormone in mares during the anovulatory season. To investigate an involvement of endogenous opioids in the regulation of circannual changes in reproductive activity, effects of the opioid antagonist naloxone on the concentration of immunoreactive and bioactive luteinizing hormone (LH) in plasma were measured in mares during the anovulatory season. Naloxone (0.5 mg/kg i.v.) caused a significant increase (P < 0.05) in immunoreactive as well as bioactive LH concentration in plasma. The amplitude of the increase in LH concentrations measured with an in vitro bioassay was more pronounced than the amplitude of the increase in LH secretion dete...
The vitamin A and vitamin E status of horses raised in Alberta and Saskatchewan. The purpose of the study was to determine normal baseline levels of vitamin A and vitamin E in clinically normal horses under typical field conditions in Saskatchewan and Alberta. Heparinized blood samples were collected from approximately 400 clinically healthy horses selected from 24 locations in Alberta and Saskatchewan during a two-year period. For each horse, historical information including feed type, vitamin supplementation, time of year, sex, and age were recorded. From each blood sample, the plasma vitamin A (all-transretinol) and vitamin E (alpha-tocopherol) levels were measured usin...
Follicular and FSH responses to parturition during the anovulatory season in mares. The ovaries of periparturient pony mares (n=9 to 16 parturitions per month for January to April) were scanned ultrasonically on the day of parturition, while those of postpartum and control mares (n=12) were examined at least twice weekly. Four mares had apparent lactational anovulation (incidence, 7%) that corrected spontaneously (1 mare) or within 14 d after the weaning of foals on August 10 (3 mares). All but 2 of the postpartum ovulations occurred after April 29; that is, parturition did not effectively stimulate ovulation in ponies foaling during the anovulatory season. Mean diameter of t...
Genetics of racing performance in the Japanese Thoroughbred horse:: II. Environmental variation of racing time on turf and dirt tracks and the influence of sex, age, and weight carried on racing time. Data collected by the Japan Racing Association (JRA) were individual horse racing times at eight racecourses (Hakodate, Fukushima, Niigata, Tokyo, Nakayama, Chukyo, Kyoto and Hanshin) and at five distances (1000 m, 1200 m, 1400 m, 1600 m, and 1800 m) from 1982 to 1990. Important sources of variation in racing time were examined using a nested model and expressing the variance components as percentages of the total on both turf and dirt. At all racecourses and at all distances where races were on both turf and dirt, racing times were less on turf than dirt. Differences were from 2.09s to 3.91s ...
Haematological parameters of the Polish Primitive Horses. Average values of chosen haematological parameters have been described in a population of 90 clinically normal Polish Primitive Horses. The comparison of the results with values given in literature for horses as a whole and for other breeds enabled us to state that Polish Primitive Horses' blood is characterized by relatively low values of RBC, PCV, segmented neutrophils and monocytes, and high percentage of lymphocytes. It was demonstrated that values of some blood parameters of examined animals are similar to typical results obtained for primitive and cold-blooded horses. Moreover, it was pr...
Changes in the hypothalamic-hypophyseal axis of mares in relation to the winter solstice. In mares, the amount of gonadotrophin-releasing hormone (GnRH) is low in the hypothalamus during seasonal anoestrus, but by early spring, concentrations of GnRH are high. The timing of this response was characterized more precisely by determining concentrations of GnRH in hypothalamic tissue collected immediately before and at various times after the winter solstice (22 December 1986). Ovaries, pituitary gland, hypothalamus and a blood sample were collected from six groups of mares (6-12 mares per group) at death, 1 week before day of the winter solstice and 1, 2, 3 and 12 weeks afterwards. No...
Seasonal effects on seminal quality, plasma hormone concentrations, and GnRH-induced LH response in fertile and subfertile stallions. Seasonal effects on hormonal and seminal parameters in subfertile stallions have not been well documented and could provide information that is needed to understand the underlying endocrine mechanisms associated with testicular dysfunction. Such information may be useful in developing diagnostic tools to identify those stallions who are candidates for treatment. This investigation characterizes and compares the effects of season on endocrine function and seminal quality in fertile and subfertile stallions. Eight fertile and six subfertile stallions between the ages of 5 and 18 years were injec...
Endocrine and testicular changes associated with season, artificial photoperiod, and the peri-pubertal period in stallions. The seasonal reproductive cycle of stallions is characterized by an annual regression and recrudescence in testicular function and concentrations of LH, FSH, and testosterone in serum. Maximum reproductive capacity occurs during the increasing day lengths of spring and summer. The annual cycle in LH secretion may reflect a seasonally associated and photosensitive reduction and replenishment in pituitary content of LH. Similar to other seasonal breeders, it appears that stallions may possess an endogenous circannual rhythm in reproductive function that is subject to manipulation by altering the...
Risk factors associated with injuries in thoroughbred horses. A case-control study was conducted on Thoroughbred horses to identify factors associated with the risk of breakdown on racetracks. A total of 310 cases (breakdowns) were identified from the Horse Identification Department records kept by the chief examining veterinarian of New York Racing Association. For each case, two control horses were selected randomly from the Daily Racing Form Inc. records. Multiple logistic regression analysis was used to identify and quantify the risk of factors associated with breakdown, while simultaneously controlling for the effect of other putative factors. Facto...
